Lee_Photography wrote:
Let me see now. 70mm X 1.6 is 112mm and 300mm X 1.6 is 480mm

So 112 to 480 might be a closer description

Okay, it's a 70-200, but when you figure in the small adjustment for 18mp on the 1.6 crop, and 21 on the full frame (EOS 5D Mark II), the 100-300 equivalent is close enough. But the real question is do you also multiply the f stop by 1.6 as well (so f2,8 becomes f4.5)? I'm sure you must, but why is what has Michael and I going around and around.

I have said for years that when I mount my 70-200 f2.8 zoom on my EOS 7D (or any other Canon cropped sensor camera), it is like having a 100-300 f2.8 zoom in my bag. I now know this is incorrect, that the field of view is similar to having a 100-300 on my full frame EOS 5D Mark II (or any other Canon full frame sensor camera), but the f stop is equivalent to f4.5 on the full frame camera, not f2.8.

John
--
John Fisher
700 Euclid Avenue, Suite 110
Miami Beach, Florida 33139
(305) 534-9322
http://www.johnfisher.com

Grayscale Photo wrote:
The definition of an f-stop <snip>

None of those calculations change when different sensor sizes are used.

True, However- those calculations become less impactful in a world in which I can change ASA/ISO with the push of a button and the spin of a wheel.

ETA: Or if not less impactful at least re-valued

John Fisher wrote:
Or we can just set the camera on P mode and blast away, at which point we might as well be taking photos with an iPhone.

John
--
John Fisher
700 Euclid Avenue, Suite 110
Miami Beach, Florida 33139
(305) 534-9322
http://www.johnfisher.com

hmmmm.......are you aware that thousands of famous photographs were taken with the camera set on P yet were not shot with an iPhone?

descending chain wrote:
I would say don't give equivalents for either focal length or f-stop, but if you must, do both.  A lot of camera manufacturers give equivalents for only the focal length, with intent to mislead the consumer.  Gee, why does that 400 mm f/2.8 cost $12,000?  If I just buy this itsy-bitsy sensor, I can get a 400 mm equivalent that is f/2.8 for only $100.

And  you are into the heart of the matter. We do instinctively understand that those little point and shoot with their "equivalent 35-400 zoom range, at f2.8" are misleading. The equivalent zoom is correct if you are talking about field of view, but the f stop has to be multiplied as well (or the point and shoot would weigh 10 pounds!). The issue is that when we are talking about cameras which appear to have the same form factor (like the EOS 5D II and the EOS 7D) but different sized sensors, somehow the same math doesn't apply. We do say "My 70-200 f2.8 is equivalent to having a 100-300 f2.8 when I mount it on my cropped sensor EOS 7D." Well, no it's not. Any more than that point and shoot  has a 400mm f2.8 lens attached to it. The apparent field of view is correct, but the f stop has to be multiplied as well.

Interesting discussion, and it does drive Michael and I crazy (although for me it's a short trip!).

John
--
John Fisher
700 Euclid Avenue, Suite 110
Miami Beach, Florida 33139
(305) 534-9322
http://www.johnfisher.com

Zack Zoll wrote:
...
...
The only thing I can think of is that 'equivalent aperture' is referring to DOF, and is totally disregarding the impact of aperture on exposure. It seems like it's confusing one issue to make another one simpler.
...
...

For all practical purposes this is the case.

The "Equivalence wars" were really started by 35mm film size sensor vendors trying to differentiate their products as better on the basis of DOF control. Go to a site like dpreview to see this gone totally out of control - the FF fanboys posting the most artistically bereft photos with a tiny area in focus to make some sort of point.

I see it as being a pretty meaningless debate t be honest - because t always includes the weasel words "assuming an equivalently efficient sensor" - and they never ever are equivalent.

Zack Zoll wrote:
So since you guys are talking about this respectfully, maybe one of you can take a minute to explain something to me in very simple terms, as the oft-used engineering explanations mean nothing to me. And I suspect they mean nothing to some of the people that repeat them.

So I understand the idea of equivalent f-stops; a larger sensor gathers more light, and exposing a larger sensor with the same amount of light is 'like having more light from the lens.' The part I don't understand is how this has any practical application. If I take a lens off a D800 and mount it on a D7200 with all the same settings, I get roughly the same exposure. If I mount it onto a D3200 I get a darker exposure, but I understand that's a processing/ISO trick to hide a narrower highlight range.

So if I get roughly the same exposure on a FF and APS-C camera using the same lens, how do I have a different equivalent f-stop? As far as the amount of light actually falling on each square millimeter of sensor, both cameras are the same; the APS-C camera might have "twice as much light" or it might "use half as much light", but both sensors are equally exposed.

The only thing I can think of is that 'equivalent aperture' is referring to DOF, and is totally disregarding the impact of aperture on exposure. It seems like it's confusing one issue to make another one simpler.

Is  this correct? If not, could one of you fine gentlemen explain it in simple terms, using as few calculations as possible for me? And possibly a car or baseball analogy? I haven't heard one of those in a while

Again, you are at the heart of the matter. Does the smaller sensor (which by definition is collecting fewer photons since the lens projects the same image circle regardless of sensor size) produce an image when projected on your screen which is slightly darker if all things remained the same?

Now, they don't remain the same, and that gets into the whole ISO digital sensor thing. What I think happens is that in order to get the same image on your screen from the smaller sensor, the camera companies pump the signal so that ISO 100 appears to be the same on both full frame and cropped sensors. Pumping the signal of course leads to more noise, and we do notice that smaller sensors have more noise at the same ISO as larger sensors. Now, if they didn't pump the signal, the noise would be the same, but the brightness would fall off. I can get the brightness back, and keep the noise the same, if I increase the f stop on the smaller sensor camera. I know I've got some of this wrong, but the math does work.

Or as someone mentioned, we could just set the camera on P and take famous pictures. The problem with that is that probably 99% of the most famous pictures were taken before Program mode (or cell phones) had been invented. And in those days P really did stand for Professional (as in someone who know what the hell they were doing!). True that today.

John

PS: I think I'm getting somewhere with all of this now. My problem is that Michael keeps using film analogies, which are correct as far as they go. ISO 100 film is processed the same whether it is sheet film or 35mm (same chemicals, same temperatures, same time), and therefore the same grain size. But with digital sensors they have different processing engines for the same ISO 100 depending on the size of the sensor. So there really is no analog for comparing film to digital.

My head is starting to hurt again.
--
John Fisher
700 Euclid Avenue, Suite 110
Miami Beach, Florida 33139
(305) 534-9322
http://www.johnfisher.com

Grayscale Photo wrote:
The f-stop determines the amount of light energy (intensity) hitting each individual "pixel" of a sensor regardless of sensor size.

Only if the smaller sensor doesn't use smaller pixels.  The f-stop determines amount of light energy hitting each unit area.

J O H N  A L L A N wrote:
Just load a full-frame 35mm image in  Photoshop and crop out the middle and voila you have a image from a crop sensor - the focal length didn't change (nor did the fstop [brighter or darker] even if the field-of-view seems to mimic the other focal length.

THIS^^^^^^^^
It's explained perfectly and 100% accurate.
(at least the non-stricken part)

The only real variable is when you're trying to recompose the image in a crop sensor camera to match the image taken in a full frame camera (or vice versa).  To do so you'd have to change the focal length of the lens (zoom out on a crop sensor) which will change the depth of field. 

If you chose NOT to zoom out to match the full view of the full frame camera you can move the camera farther away from the subject until the field of view matches, but then you'd have to refocus the camera on the subject, which in turn changes the depth of field.

Cambridge in Colour wrote:
DEPTH OF FIELD REQUIREMENTS

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).


As an example calculation, if one wanted to reproduce the same perspective and depth of field on a full frame sensor as that attained using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one would need to use a 16 mm lens and an aperture of roughly f/18. Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6X crop factor — not possible with consumer lenses!

LightDreams wrote:
I.E. The TOTAL amount of light added up over the entire sensor size just isn't that useful. Especially when compared to a single point on the sensor.  The sensor size isn't a factor (because it's perfectly relative) regarding the f-stop.

I like the way Simon Tindemans explains it, so I'll let him respond.
http://simon.tindemans.eu/essays/sensorsizes

"At a given level of technological sophistication, the noise performance is mostly determined by the total amount of light (the number of photons) that reaches the sensor."
"Scaling the focal length and f-number along with the sensor size conserves the field of view, the depth of field and the luminous flux."

J O H N  A L L A N wrote:
Just load a full-frame 35mm image in  Photoshop and crop out the middle and voila you have a image from a crop sensor - the focal length didn't change (nor did the fstop [brighter or darker] even if the field-of-view seems to mimic the other focal length.

SayCheeZ!  wrote:
THIS^^^^^^^^
It's explained perfectly and 100% accurate.
(at least the non-stricken part)

The only real variable is when you're trying to recompose the image in a crop sensor camera to match the image taken in a full frame camera (or vice versa).  To do so you'd have to change the focal length of the lens (zoom out on a crop sensor) which will change the depth of field. 

If you chose NOT to zoom out to match the full view of the full frame camera you can move the camera farther away from the subject until the field of view matches, but then you'd have to refocus the camera on the subject, which in turn changes the depth of field.

Actually with most current DSLR's this is not exactly correct. Current cropped sensor cameras (such as my EOS 7D) have much smaller pixels, and almost the same number of pixels on the sensor as my EOS 5DII. So cropping into the image of the 5DII will give you a picture with the same content, but not the same picture as the final image from the 7D is much (much) larger (20 mps) than the cropped image from the 5DII (approximately 8 mps). The final image is almost the same size, but fewer photons were available to produce the image, so brightness would be affected (except for the fact that Canon amplifies the signal before releasing the image from the sensor).

It's this amplifying of the signal that makes any analogy to film inaccurate. whether it is sheet film, or 35mm film, properly processed ISO 100 will have the same size grain on the film. However, the same image from a 24mp cropped sensor camera will have more noise than the same image from 24mp full frame sensor because they have to amplify the signal off the cropped sensor because the size of the sensor is smaller and and fewer photons are available for capture. (I guess the analogy to film would be they pushed the processing which would give you larger grain.)

"When you increase the ISO setting, you’re not really making it more sensitive to light, you’re simply amplifying the light values it’s managed to capture." - Digital Camera World

Clearly we know depth of field is affected when you take the same picture. Forget cropping in, you don't shoot big then crop. If you use the same focal length, say 200mm, with both a full frame and a cropped sensor camera, you must step back to get the same image with the cropped sensor camera and that does affect depth of field. The ghost of "digital zoom", so popular with early point and shoots doesn't apply here. Nikon has any number of cropped sensor and full frame cameras with exactly the same pixels so the reach is real, not digital. (Canon is slightly more confusing,  22mp full frame 5DIII and 20mp cropped sensor 7DII, but close enough.) And a limited depth of field is one of the reasons we buy expensive long lenses with large fixed apertures (particularly when shooting fashion). Fairly put, if you take a picture with a 200mm lens at f4 on a full frame camera, and then same picture at 200mm f4 with your cropped sensor camera, you will see more pronounced background blur with the full frame than with the cropped sensor and the image from the cropped sensor camera will be noisier even though both images were shot at the same ISO.

Woof, the more I get into this, the more I appreciate the full frame sensor cameras we now have available! And yes, it does seem like we are chasing butterflies, but it's understanding how these complex camera systems actually interact that allows us to predict the image outcome, thereby giving us more control. So,if I take a picture with a 300mm lens at f2.8 on a full frame camera, and compare it to a picture taken with a 200mm f2.8 lens on a cropped sensor camera, the field of view will be the same, but the cropped sensor camera will produce an image with greater depth of field (equivalent to shooting the full frame image at f4.5). And given all settings are the same, images produced with cropped sensor cameras will have more noise for a given ISO (assuming that the technology is roughly from the same generation of sensors).

Shhhh, don't tell anyone. I haven't shot an image for money since Wednesday so I  have had time on my hands, never a good thing. And I sold my full frame DSLR (EOS 5D Mark II) in preparation for the release of the new EOS 5Ds, so all I have in my bag are two cropped sensor EOS 7D's. I also found this video by Tony Northrup which goes into all of this in far more detail:


John
--
John Fisher
700 Euclid Avenue, Suite 110
Miami Beach, Florida 33139
(305) 534-9322
http://www.johnfisher.com

John Fisher wrote:
.
...
...
l. (Canon is slightly more confusing,  22mp full frame 5DIII and 20mp cropped sensor 7DII, but close enough.) And a limited depth of field is one of the reasons we buy expensive long lenses with large fixed apertures (particularly when shooting fashion).
....
....

BINGO!

When I said earlier that people go into some detailed and logical calculations and then make them all irrelevant with a hand wave over the sensors being different.....

Once you've acknowledged that the sensors are different - different pixel densities, efficiencies, pixl sizes etc then you're pretty much into the realms of "how many angels on the head of a pin".

John Fisher wrote:

Again, you are at the heart of the matter. Does the smaller sensor (which by definition is collecting fewer photons since the lens projects the same image circle regardless of sensor size) produce an image when projected on your screen which is slightly darker if all things remained the same?

Now, they don't remain the same, and that gets into the whole ISO digital sensor thing. What I think happens is that in order to get the same image on your screen from the smaller sensor, the camera companies pump the signal so that ISO 100 appears to be the same on both full frame and cropped sensors. Pumping the signal of course leads to more noise, and we do notice that smaller sensors have more noise at the same ISO as larger sensors. Now, if they didn't pump the signal, the noise would be the same, but the brightness would fall off. I can get the brightness back, and keep the noise the same, if I increase the f stop on the smaller sensor camera. I know I've got some of this wrong, but the math does work.

Or as someone mentioned, we could just set the camera on P and take famous pictures. The problem with that is that probably 99% of the most famous pictures were taken before Program mode (or cell phones) had been invented. And in those days P really did stand for Professional (as in someone who know what the hell they were doing!). True that today.

John

PS: I think I'm getting somewhere with all of this now. My problem is that Michael keeps using film analogies, which are correct as far as they go. ISO 100 film is processed the same whether it is sheet film or 35mm (same chemicals, same temperatures, same time), and therefore the same grain size. But with digital sensors they have different processing engines for the same ISO 100 depending on the size of the sensor. So there really is no analog for comparing film to digital.

My head is starting to hurt again.
--
John Fisher
700 Euclid Avenue, Suite 110
Miami Beach, Florida 33139
(305) 534-9322
http://www.johnfisher.com

The smaller needs fewer photons because it's a smaller surface. There shouldn't be any difference in the ISO or exposure between a FF and crop sensor that have identical designs besides their sizes.

descending chain wrote:
It is regarding DOF, but not exclusively.  There is also the matter of noise.  The same aperture used on a sensor with a crop factor of 2 results in one quarter the total light made available to the sensor, hence higher photon noise.

Yes, but the need is only one quarter because the sensor is smaller.

descending chain wrote:

I like the way Simon Tindemans explains it, so I'll let him respond.
http://simon.tindemans.eu/essays/sensorsizes

"At a given level of technological sophistication, the noise performance is mostly determined by the total amount of light (the number of photons) that reaches the sensor."
"Scaling the focal length and f-number along with the sensor size conserves the field of view, the depth of field and the luminous flux."

But that's a very different statement that what people here are saying. He's talking about noise performance, or IQ, not about the about of light needed for the correct exposure.

Take the 5D1 and the 5D3 and shoot them both at 3200. The noise performance of the 5D3 will be better than the noise performance of the 5D1, but the amount of light needed for the exposure is identical.

One of the reasons smaller sensors appear noisier is because they're enlarged relative to full frame and you'll see the noise more.

When you scale the f stop, you get some indication of how much more noise there will be, but that's got nothing to do with needing more light.

Zack Zoll wrote:
Okay, I think I follow now. Here's what I'm seeing in layman's terms - I realize that I am simplifying, but that is the point.

There are two reasons for 'equivalent aperture', neither of which are useful for most people. One is to describe the relatively poor light-gathering ability of a compact camera, and why it produces higher noise. This isn't useful to anyone but scholars and enginners. It's good to understand things, but no one will make a more informed purchasing decision based solely off this information, as the sort of person that is able to digest all this isn't likely to be buying a cheap pocket cam. Or if they are, they're buying it because it's cheap, and not because they think it will be as good as a larger camera. There is no confusion that needs to be cleared up.

I applaud those that want to learn, but it's about as useful to the average person as a study on the impact of the camera obscura on pre-Renassaince Dutch painting - and this is from a guy that has studied that. Not extensively, but enough to bore my students with a lecture here and there.

The other use is to describe the ability of a camera/lens combination to achieve a shallow DOF. This IS useful, but not in its current form. There are a ton of people that want a "professional blurry background", but don't understand the mechanics behind that, and have no desire to learn. For those people, an equivalent f/stop sort of makes sense. But maybe to avoid further confusion, we could roll everything together and use a new term, like 'defocusing factor', so that the DOF aspect of aperture doesn't get muddied up with the brightness aspect.

I think you mean "professional" blurry background.

John Fisher wrote:
However, the same image from a 24mp cropped sensor camera will have more noise than the same image from 24mp full frame sensor because they have to amplify the signal off the cropped sensor because the size of the sensor is smaller and and fewer photons are available for capture. (I guess the analogy to film would be they pushed the processing which would give you larger grain.)

That's not accurate. It's because of the size of the pixel.


The comparison between sensor sizes should not be made with identical number of MP. The FF will have more MP if they are the same sensor design.

The reason the field of view is cropped is because the sensors is cropped - a smaller cut off of the same original wafer, not a FF sensor compacted into a smaller space.

Michael Fryd wrote:
I think it is fair to say that John Fisher and I have very different views on this matter.  We have spent some time discussing it in person.  Let me chime in with some alternative points of view.

There are a number of issues bundled up in this discussion, and I prefer to address them individually.

The first issue is what is meant by "equivalent f/stop"?   There are lots of choices.   Do we mean the aperture that gives us the same depth of field?  Do we mean the aperture that gives us the same exposure?  Or do we mean the aperture that gives us the same total light on the frame?

Traditionally, photographic exposure is about getting enough photons on each square mm of film.  From the perspective of exposure, aperture and shutter speed are just ways of letting in more/less photons.

In terms of exposure, the actual diameter of the aperture is not as important as the ratio of the diameter to the focal length.   For instance f/2 means the diameter of the aperture is the focal length ("f") divided by 2.  On a 100mm f/2 means the diameter is 50mm.  With a 200mm lens, f/2 means the aperture is 100mm.    The reason we fixate on the ratio rather than the absolute diameter, is that it helps us get our exposure right.   All other things being equal, swap a 50mm lens at f/2 for a 200mm lens at f/2, and our film exposure stays the same.

Now let's compare a 35mm film camera to a 4 x 5 film camera.  We start with an 8 x 10 sheet of 100 ASA Plus-X film, and cut it into pieces.  We put a 4" x 5" piece into the 4 x 5 camera, and a much smaller piece into the 35mm camera.  We mount a 50mm lens on the 35mm camera, and a 180mm lens on the 4 x 5 camera.  This results in both cameras having similar fields of view.   We set both cameras lenses to f/16 and set both cameras to 1/100 second.

If you've been paying attention, the aperture on the 4 x 5 camera will be physically larger than the aperture on the 35mm camera (the 4x5 camera has a longer lens, so f/16 yields a larger number).  A larger physical opening means that more photons enter the lens, but the longer lens spreads them out more.  The result is that even though the longer lens allows in more photons, we get the same number of photons per sq mm on our film.  Thus our "exposure" is the same.

However, there are differences between the 4 x 5 negative and the 35mm one.  If we look at the image as a whole, the larger negative captured more photons (same number of photons per unit area, but a larger area).  There are a number of advantages to this.  If we are making a 16 by 20 print, the 4 x 5 negative will yield a print with less grain (noise).

Here comes the philosophical question - "is one negative more sensitive than the other?".  Remember, we needed fewer photons to expose the 35mm negative, but both negatives (large and small) were cut from the same piece of sheet film.

The answer is that it depends on what you mean by "sensitivity".    The larger negative requires more total photons for the same density.  To some people this means it is less sensitive.  To others both negatives require the same number of photons per unit area so they are equally sensitive.

Traditionally, we measure sensitivity in terms of photons needed per unit area.  This has the advantage that sensitivity is independent of frame size.   It is a philosophical question as to whether or not this is the best definition of "sensitivity".  On a practical level, looking at sensitivity per unit area has been quite useful.

Sensitivity is normally described by a number.  Over the years we have called this number "ASA", "ISO" or "DIN", depending on which standards organization was in favor.   Today, we use the "ISO" standard.

As a rule of thumb, ISO 100 means that you will get proper exposure when shooting in daylight at f/16 at 1/100 of a second.  By proper exposure I mean the developed film will have the proper density for the image.

We tend to think of sensitivity as being an inherent property of the film, but it is really a rating of the film/developing subsystem.  For instance Kodak Tri-X B&W film has a rating of ISO 400.   However this rating assumes that the film will be developed in a particular fashion.   Develop the film with different procedures, and the ISO rating of the film/developing system might be 200 or 800.


Now let's talk about digital.   Does it even make sense to talk about the sensitivity or ISO rating of a digital camera?

My position is "yes".   Although the underlying technologies are vastly different, the high level concept is the same -  How many photons per unit area do we need to get proper exposure?  With film, proper exposure is judged by the density of the film.  With digital, proper exposure is judged by the resulting numbers in the JPEG file.

With film, ISO is a rating of the film/developing subsystem.  With digital ISO is a rating of the sensor/hardware/firmware subsystem.  If f/16 at 1/60 gives us proper exposure at ISO 100, that should be true whether we are working with film or digital.

Philosophically, you can argue that it doesn't make sense to talk about ISO for a digital camera.  You can dissect the  underlying technology, and show that there is little similarity between film and digital.  From a practical matter talking ISO makes a lot of sense for digital.  If I am getting a good exposure with a particular combination of ISO, f/stop, and shutter speed, then it shouldn't matter whether the camera is digital or film, nor should it matter whether the film is 35mm, 8 x 10, or what the sensor size is.

The above is the traditional frame of reference.  In this frame a 70-200 f/2.8 lens is a 70-200 f/2.8 no matter what sort of camera it is on - 35mm film, APS film, crop factor digital, or full frame digital.   For marketing reasons, camera manufacturers talk about "equivalent focal length".  My suggestion is that, at most, this should be used to compare field of view.  One should not attempt to apply "equivalent focal length" to anything else.

Of course the above is how things look of you care about photons per unit area.   If you care about Depth of Field, then f/2 on a 50mm lens is not equivalent to f/2 on a 100mm lens.  In terms of Depth of Field f/2 on a 50mm lens is equivalent to f/4 on a 100mm lens (both have an aperture diameter of 25mm). 

If you care about total photons in the frame, then f/2.8 on a full frame camera is equivalent to f/4.5 on a 1.5 crop factor body.  Alternatively, if you care about total photons then f/2.8 on a full frame camera is equivalent to f/2.8 on the crop body, but 1/100 seconds on the full frame is equivalent to 1/262 seconds on a crop body.

Now we get into what we mean by equivalent seconds.  In terms of total photons we need to adjust the seconds.  In terms of motion stoping ability we need a different equivalence.  However in terms of photons per unit area 1/100 second is always 1/100 second.

The bottom line is that trying to make sense of equivalents leads to madness.  You need a different set of equivalents, depending on what you care about.

I suggest that one stick with the classics:

- Light sensitivity of a system is a measure of how it responds to photons per unit area.  This is independent of frame size, and is rated on the ISO scale

- A 100mm lens is a 100mm lens, no matter what frame size is being used.

- As special dispensation, one is allowed to use the term "equivalent focal length", but that use must be limited to comparing field of view, and no other aspects.

- One should keep in mind that the diameter of the aperture affects Depth of Field more than the ratio of focal length to diameter (also called the f/stop)

- One should be aware that at any particular ISO, larger frame sizes will receive more total photons than smaller frame sizes (same photons per unit area, but much larger area).   In terms of signal processing theory, this gives you more "signal" and less "noise".  The result is that larger sensors (or larger negative formats) yield less noise (grain) in a 16 by 20 print.


Note that all of these concepts apply whether or not the camera is digital or film.  Clearly the mechanisms for converting photons into the final print are different, but the overall concepts are the same.

That seems like a good description.

I can't think of any scenario where photons needed per unit area isn't more relevant than total photons.

The total number of photons needed is always going to be a function of photons needed per unit area. The only thing that changes the total photons needed is changing the total unit area. It's really two ways of saying the same thing, but one has a perspective that doesn't make any sense.

Mikey McMichaels wrote:
...
I can't think of any scenario where photons needed per unit area isn't more relevant than total photons.
...

Here's a scenario where total photons captured is more relevant:

As a general rule, if all other factors are equal, the more total photons you capture, the less noise you will see in your final image.  It is a useful point of view if you are trying to understand some of the signal to noise issues that affect the final print.

Looking at it this way, it is clear why higher ISO setting yield more noise (you are capturing fewer total photons).  It is also clear why larger sensors tend to have less noise (larger sensors capture more total photons).

Using this perspective we can determine how much we need to reduce ISO if we want to crop an image without increasing noise in the final print.

Imagine you are shooting with a full frame camera at ISO 3200.   You make a 16 by 20 print, and the noise level is just barely acceptable.   You take a small portion of the image  (about 1/4 the areas of the whole image) and blow it up to 16 by 20, but you find that the noise level is now a bit too high.

If you consider total photons captured, you can determine how many more photons you would need to capture in order for the total in the cropped portion to match the total in the full original frame.  If you want to get the same number of photons onto 1/4 the area, you need 4 times as many photons per unit area.  You dial your camera down to ISO 800 and reshoot.  You should find that the enlargement from the small area shot at ISO 800 has about the same noise as the original full image shot at ISO 3200.


So you see, there are different ways of looking at an issue.  Generally the best way depends on which aspect of the issue we are concerned about.  If we are concerned about exposure, we generally are concerned about photons per unit area.  If we are concerned about noise in the final print, we might care more about total photons captured.

Grayscale Photo wrote:
Most of this discussion has veered from the effects of smaller sensor sizes to the effects of smaller photosite sizes.  The two sizes are not always correlated and certainly not correlated in a linear relationship.

Although it may seem that way, it is all interrelated.

Consider two sensors of identical size and similar technologies.  One has 20 million photo sites, and one has 50 million.  Let's call the 20 million camera the "large pixel" camera, and the 50 million on the "small pixel" camera.

If we examine the images on our screen, mapping one sensor pixel to one screen pixel, we will see more noise in the pixels from the small pixel camera.  However, that is not the same as saying the small pixel camera produces images with more noise.

When we are looking at the final print, we really aren't concerned with noise at the pixel level, we are concerned with noise per sqare mm on the print.   Prints from both cameras will have about the same noise levels.  The difference is that the print from the small pixel camera will show a little more resolving power (detail).

Imagine a crop factor camera with 20 megapixels a full frame camera with 50 megapixels.  The individual photo sites for each camera are about the same size.  If photo site size was the important factor, then prints from both cameras would have equal noise levels.  In reality, the prints from the 50 megapixel camera will show less noise because we get a noise reduction by mapping individual sensor pixels to a smaller area on the print.


The reason that crop factor cameras produce noisier images really does boil down to the fact that smaller sensors capture fewer total photons.  Higher pixel counts (i.e. smaller pixels) give us more resolving power (ultimately limited by the noise in the sensor).  More total photons give us less noise.

Mikey McMichaels wrote:
The smaller needs fewer photons because it's a smaller surface. There shouldn't be any difference in the ISO or exposure between a FF and crop sensor that have identical designs besides their sizes.

The smaller sensor doesn't NEED fewer photos, it GETS fewer photons. So, the only way to get to a given brightness in the final image from a cropped sensor (relative to full frame) is to either boost the signal (and the noise), or to increase the number of photons hitting the sensor by opening up the aperture (increasing the f stop). And that is why that 70-200 f2.8 on your full frame sensor will APPEAR to be a 100-300 (field of view) f4.5 (depth of field and brightness/with same noise) on your cropped sensor camera.

Anyway, this makes sense to me.

John
--
John Fisher
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John Fisher wrote:

The smaller sensor doesn't NEED fewer photos, it GETS fewer photons. So, the only way to get to a given brightness in the final image from a cropped sensor (relative to full frame) is to either boost the signal (and the noise), or to increase the number of photons hitting the sensor by opening up the aperture (increasing the f stop). And that is why that 70-200 f2.8 on your full frame sensor will APPEAR to be a 100-300 (field of view) f4.5 (depth of field and brightness/with same noise) on your cropped sensor camera.

Anyway, this makes sense to me.

...

Your issue seems to be that smaller sensors get fewer total photons, therefore the smaller sensor acts like a larger sensor but at a smaller aperture.

What's so special about aperture?  Instead of talking about equivalent apertures why not talk about equivalent time?

If you're shooting 100mm, f/4 at 1/100 on a full frame, why not say those settings act like 150mm f/4 at 1/256 on the crop factor?  The crop factor results in the 100mm lens having a field of view like a 150mm on a full frame, and the crop factor also makes the 1/100 shutter speed gather the same number of photons as 1/256 would on a full frame.

Both points of view equally express the concept of differences in total light.     Of course both "equivalencies" break down if we try to use them for computing the ability to stop motion, or depth of field.

And that's the fundamental problem with equivalencies, you either need to accept that they only apply to one small aspect of the situation, or you need to go in "whole-hog" and essentially define a complete and consistent measurement system.   Generally it's easier to keep to actual measurements, and common units.


Getting past the "equivalent aperture" issue, there is an underlying concept - bigger sensors tend to capture more total photons, and this typically results in an image that has less noise than an image from a smaller sensor.   There is nothing new about this, and the concept is certainly not unique to digital.  Anyone who has shot multiple formats of film knows that larger film sizes result in prints with less grain. 


When it comes to film, no one talks about equivalent apertures, equivalent shutter speeds, or equivalent ISO speeds.  The interesting thing here is that Canon actually made an EOS crop factor film SLR (EOS IX 7).  It took the full line of EOS EF lenses.  It came with an EF 22-55mm f/4-5.6 USM lens.  The lens was not described as a 27-69mm "equivalent".  The manual did include a short section "About the Effective Field Angle of EF lenses".  The manual explains that the frame is smaller than a typical 35mm frame, and therefore the effective field angle of the lens is about what you would expect from a lens that was 1.25 times the focal length on a full frame camera.

It wasn't until digital SLR cameras came out that companies started this "effective focal length" nonsense.

The transition from film to digital has not changed the meanings of  aperture, shutter speed, ISO and focal length,

AVD AlphaDuctions wrote:
A lonely photon was checking out photosites looking for a photoshoot.  it got around pretty easily because it was travelling light...

LOL