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02-28-2015, 11:50 PM   #1
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Does FF vs APS-C affect amount of light?

I am confused by what I understand to be the advice here that using exactly the same settings (f-stop, ss, ISO, lens focal length) on a FF camera as an APS-C camera may result in different amounts of light landing on the sensor??

02-28-2015, 11:55 PM   #2
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The amount of light is the same, so the same exposure settings should be used. This animation shows the only difference there is between the two formats:

The Crop Factor Explained: An Animation - Tutorial Videos | PentaxForums.com

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03-01-2015, 02:05 AM - 2 Likes   #3
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And, please, let's not have another 20 page thread ending up with handbags at dawn from the usual suspects.
03-01-2015, 03:35 AM   #4
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QuoteOriginally posted by windhorse Quote
I am confused by what I understand to be the advice here that using exactly the same settings (f-stop, ss, ISO, lens focal length) on a FF camera as an APS-C camera may result in different amounts of light landing on the sensor??
The amount of light landing on the sensor, for a given f-ratio, also depends on the focal length.

In the thread you referenced, I assumed that whether one uses an APS-C camera or an FF camera, one wants to obtain the same framing (i.e., the same image content). This implies that the lens on the FF camera has a longer focal length. If you used the same focal length on both cameras, the FF camera would show a wider view.

One can calculate the focal length needed for the FF camera by multiplying the focal length of the lens on the APS-C camera with the factor 1.5 (the "crop factor" between the APS-C and the FF formats).

The amount of light landing on the sensor depends on the shutter speed and the aperture diameter. One can work out the aperture diameter by dividing the focal length by the f-ratio. For instance a 560mm lens at f/5.6 has an aperture diameter of 100mm.

We established before that the APS-C camera and the FF camera need lenses with different focal lengths. Let's assume we want the same shutter speed for both cameras to capture the same motion blur. If we want the same amount of light for both cameras then we need to make sure that the aperture diameters are the same. From this it follows that we cannot use the same f-ratio for both cameras. If we did, the aperture diameter of the lens on the FF camera would be 1.5 times as large as the aperture diameter of the lens on the APS-C camera (because the focal length of the lens on the FF camera is 1.5 times as large).

The above tells us that we need to stop down the lens on the FF camera by a factor of 1.5, if we want the same total amount of light. For instance, if the APS-C lens is stopped down to f/2.8 then the (longer) FF lens needs to be stopped down to ~f/4. This not only achieves the same total amount of light hitting both sensors, but also (by necessity) the same DOF.

QuoteOriginally posted by Adam Quote
The amount of light is the same, so the same exposure settings should be used.
With the same "exposure settings", you achieve the same exposure, i.e., the same amount of light per square millimetre. In other words, you achieve the same light intensity.

However, the same light intensity would create a different DOF and different noise levels.

In order to get the same DOF and the same noise levels, the total amount of light (the number of all photons) has to be the same, not the light intensity (the number of photons per square millimetre).

Hence, in order to get the same image from an FF camera/lens combination compared to an APS-C camera/lens combination, on the FF camera one
  • must use a longer lens (factor 1.5),
  • stop down more (factor 1.5),
  • use a higher ISO setting (factor 1.5*1.5).
The higher ISO setting will not negatively impact on the noise levels, because the FF sensor is larger and thus compensates for that (put very briefly).

BTW, there is one sentence in the "The Crop Factor Unmasked" article that isn't quite complete. The statement
"If you were to mount the 35mm lens on a Pentax Q, you would in fact get the same background blur as with the 35mm on the K-3..."
is only correct if you also crop the image from the K-3 to Q-dimensions. Otherwise, the K-3 images shows a much wider view (which is stated in the article) but also more DOF (by a factor of 3.7). Yes, cropping changes the DOF, as it is equivalent to using a longer focal length (apart from losing pixel resolution).

To the OP: You may want to also read my post to the "Full Frame: The Real Pros (and Cons?)" thread and perhaps even more from that thread.

In summary, there is no DOF or exposure disadvantage to using an FF camera, as you can just choose the respective settings (one can call them "equivalent" settings, but one does not need to).

Hope all this helps.


Last edited by Class A; 03-01-2015 at 03:45 AM.
03-01-2015, 04:32 AM - 1 Like   #5
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And it begins. . . (sigh). . .
03-01-2015, 04:33 AM   #6
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I will say two things. First of all, you use exactly the same settings for your APS-C camera as for a full frame camera. If your camera meter gives you exposure settings of f4 iso 100 and 1/100 second shutter speed, you can use that on both cameras.

The difference that you will see has more to do with magnification. An APS-C sensor is smaller than a full frame sensor which means that to print at a given size, it has to be magnified more. This is not usually a problem in low iso situations, but when you get up into higher iso situations, you will see noise quicker than with full frame. In the end, it ends up being one stop different -- that is to say you see the same noise when shooting an APS-C camera at iso 800 that you do on a full frame camera at iso 1600.

I hope we don't get bogged down into an equivalence discussion as those tend to be confusing for many and frustrating.
03-01-2015, 04:48 AM   #7
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It's easier to explain this with a film camera than digital.

Your f-stop is pretty much the only thing that matters. If at f4 you need a 1/60 exposure time, that will be the same for a 6x7 camera at f4, a FF camera at f4, or an aps camera at f4 (assuming you are using the same film type in each). The amount of bokeh you get will differ (loads on the 6x7, less on FF, and less again in aps-c), but that's primarily a function of fstop vs the size of the negative. The same is also true of DOF (more on apsc, less on 6x7)

Now for digital...

If I double the area of a 24mp apsc sensor, I'd have a 48mp FF sensor. All things being equal, you'd need the same ISO, for the same f-stop, and the same shutter speed, when shooting in identical lighting conditions.

The noise per square cm of sensor would be the same, but it may be less visible on the 48mp image when they are viewed at the same size.

However, since FF cameras have had less than 48mp, this means that each pixel is slightly larger on the FF sensor, which translates to slightly less noise (meaning you can say, get away with ISO 6400 instead of ISO 3200, and have similar noise levels per square cm). The 645z has 50mp, but since it has a larger sensor, it also has low noise levels at high ISO.

At this point someone will inevitably mention depth of field and equivalence, and then the discussion will basically be over. It's godwins law of pentax forums.

The basic gist is that people who want dof will moan that you have to stop down to get the same image from FF. People who like bokeh don't care. The two will argue about that forever more....
03-01-2015, 04:51 AM   #8
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QuoteOriginally posted by Dartmoor Dave Quote
And it begins. . . (sigh). . .
I fail to see a constructive aspect in your post.

If such discussions annoy you, why don't you just ignore the thread?
Seems like an easy solution to me.

03-01-2015, 04:56 AM   #9
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QuoteOriginally posted by Class A Quote

Hence, in order to get the same image from an FF camera/lens combination compared to an APS-C camera/lens combination, on the FF camera one
...[*]stop down more (factor 1.5),[*]use a higher ISO setting (factor 1.5*1.5).[/LIST]
If you do the first, Class A, the FF image will be underexposed, if you do the second it will be overexposed!

This equivalence stuff, when not superficial, is just horribly wrong.
03-01-2015, 05:02 AM   #10
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QuoteOriginally posted by Rondec Quote
First of all, you use exactly the same settings for your APS-C camera as for a full frame camera.
You can use the same settings, but for what purpose?

You'll get a less noisy image from the FF camera with less DOF. Aleonx3 posted in the thread from which this thread was spawned that he does not want shallower DOF. Hence, he would be ill-advised to use the same settings, as you suggest.

The OP then mentioned that he does not want to "stop down". In the respective thread, I answered that although he is using a higher f-ratio (f-stop), he is not really "stopping down" compared to the APS-C camera. Using a higher f-ratio on the FF camera achieves the same conditions (no shallower DOF, no stopping down).

In summary, not using the same settings, is the way to go, unless one wants different images. And if one wants different images, why not use different settings on the APS-C camera in the first place?

QuoteOriginally posted by Rondec Quote
...that is to say you see the same noise when shooting an APS-C camera at iso 800 that you do on a full frame camera at iso 1600
Correct, but only when using an f-ratio that is 1.5 times as high as that on the APS-C camera.

If you use the same settings with the exception of the ISO setting then the FF camera will produce a less noisy image with shallower DOF.
These are facts that have nothing to do with whether one likes the idea of "equivalence" or not.
03-01-2015, 05:06 AM   #11
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QuoteOriginally posted by Class A Quote
You can use the same settings, but for what purpose?

You'll get a less noisy image from the FF camera with less DOF. Aleonx3 posted in the thread from which this thread was spawned that he does not want narrower DOF. Hence, it would be ill-advised to use the same settings, as you suggest.

The OP then mentioned that he does not want to "stop down". In the respective thread, I answered that although he is using a higher f-ratio (f-stop), he is not really "stopping down" compared to the APS-C camera. Using a higher f-ratio on the FF camera achieves the same conditions (no shallower DOF, no stopping down).

In summary, not using the same settings, is the way to go, unless one wants different images. And if one wants different images, why not use different settings on the APS-C camera?


Correct, but only when using an f-ratio that is 1.5 times as high as that on the APS-C camera.

If you use the same settings with the exception of the ISO setting then the FF camera will produce a less noisy image with shallower DOF.
These are facts that have nothing to do with whether one likes the idea of "equivalence" or not.
The thing is that if you are shooting at low iso you don't care that you are getting a little more noise with APS-C. I have said before that depth of field is the least important thing in most photos and I believe that to be true. Noise level is only a little more important. The goal is usually to have a well exposed image and you can use different settings to get there. The fact that you could shoot f2.8 iso 200 and 1/100second on APS-C and could shoot f4 iso 400 and 1/100 second on full frame is beside the point. If one gives you the appropriate exposure, so will the other. The point is to have a well-exposed photo, isn't it? And if iso really was different from one camera to another, than it would be useless.
03-01-2015, 05:15 AM   #12
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QuoteOriginally posted by robthebloke Quote
The basic gist is that people who want dof will moan that you have to stop down to get the same image from FF. People who like bokeh don't care. The two will argue about that forever more....
What amount of background blur one prefers is a matter of taste.

The original question pertaining to which settings are needed to produce the same amount of light on sensors of different sizes, is not a matter of taste.

Many people understand that they need to use higher f-ratios on larger format cameras in order to achieve the same DOF as on smaller format cameras. Less people seem to understand that DOF and the total amount of light captured are intrinsically linked. Keeping the shutter speed constant, more total light means less DOF and less noise. Less total light means more DOF and more noise.

Using the same settings across different formats just produces the same "exposure" (i.e., same light intensity = amount of light per square millimetre). Keeping the "exposure" the same is useful if you always want to use the larger format to print bigger and don't want to introduce more visible noise for the same viewing distance. However, this "printing bigger" advantage is paid for with a price which is shallower DOF. As I said before aleonx3 didn't want shallower DOF, hence using the same settings would be wrong for him.
03-01-2015, 05:22 AM   #13
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QuoteOriginally posted by Rondec Quote
I have said before that depth of field is the least important thing in most photos and I believe that to be true.
I have no issue with you having this point of view.

Note, however, that Aleonx3 cares about his DOF. His subjects move quickly so he does not want to use shallower DOF. He felt that APS-C gave him an advantage in that respect and I just commented that this isn't true (because he could just use a higher f-ratio on FF with no penalty). That's all.
03-01-2015, 05:40 AM - 1 Like   #14
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The complexity of the equivalence argument is that there are at least 6 variables in a photo: shutter speed, aperture, sensitivity, focal length, sensor size and sensor technology . There are also at least 4 effects: exposure, motion blur, depth of field and noise. Adjusting any one of the variables has an effect on the result. They are all interrelated, so to try and compare them means that one will be chasing ones tail.

What exactly is equal or equivalent? Perhaps instead of trying to get the same from each system, rather exploit the advantages.
03-01-2015, 06:05 AM   #15
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A larger sensor has a larger image circle which naturally allows more light to be gathered by the sensor. That doesn't change the exposure in any significant way because it is the same light passing through the same (or similar) lens.
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