Exposure and Equivalent Aperture? Or not?

sk66

sk66

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For anyone who doesn't know, "equivalent aperture" accounts for differences in DOF with different sensor sizes. Basically, to take the exact same image (FOV/DOF/SS) with a smaller sensor you have to open up the aperture and use a lower ISO (1 stop for APS, 2 stops for M4/3).

This requirement would seem to largely mitigate the large sensor's high ISO performance benefit, and some will argue that it does. Well, it does.... (ignoring sensor design differences i.e. MP's/fill factors/backlit/etc).

My problem with this idea is that in order to use an equivalent aperture you must have the capability to open up the aperture. That means not only are you comparing cameras, you are comparing available system lenses. Quite often in low light scenarios you cannot open the aperture, either due to being at the limit or IQ considerations. Generally, when higher ISO's are used and are problematic it is in low light situations and the aperture is already maxed out in those regards.

It is true that if you are using higher ISO's in brighter conditions in order to achieve even greater DOF/SS's that you would likely have the ability to open up the aperture without limit/IQ concerns... but in those cases, using a higher ISO is less problematic to start with, and "higher" probably isn't "high." And in those cases where you are looking for more DOF, would you be inclined to open up the aperture for "equivalent" or would you probably want "even more?"

So the question is:
Should equivalent aperture be a consideration when comparing camera capabilities (ISO performance), or should it be left as simply a DOF consideration?
 
Err... Yes, I think it makes sense to assess image quality at equivalent aperture settings, a lot of the time :D

When shooting with a ff camera I'd normally use smaller apertures than I would when shooting with MFT unless very shallow DoF is my aim. For example f8 and be there with ff is f4 and be there with MFT so it makes sense to me to compare image quality as I'd use the cameras, at different apertures.

As you say, in low light you'd possibly use the widest apertures available and in this scenario ff may well show a clear image quality lead but if the smaller format gets a less ultimately good but still acceptable result it could well do so with more acceptable DoF. For example with ff and everything maxed out you may end up with razor thin DoF which is perhaps thinner than you'd ideally want but with MFT and everything maxed out you're probably going to get a bit more of your subject in the DoF for the same exposure and that could be a good thing despite any hit to ultimate image quality. You could stop the ff aperture down to match MFT DoF but then your shutter speed may drop too low so this is a scenario in which MFT could have an overall advantage despite some hit to ultimate image quality. Thankfully there are some nice wide aperture MFT lenses which perform well at wide apertures and that's not always the case with ff lenses so this could be another advantage in favour of MFT or some other smaller system.

I think that the best way to approach this is to think about your own personal needs and take a more real world view than a theoretical or test shot one. I've almost stopped printing and often don't crop much so MFT usually provides images that are good enough for me even at higher ISO's. It's obviously up to each of us to decide what image quality is acceptable and bigger usually does mean better but smaller could be acceptable and could be more likely to get a useable shot.
 
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I agree that one needs to consider their own requirements. For example, I prefer small sensors for macro work (although I have a hard time meeting the diffraction requirements).

But I think "equivalent aperture" is best left as a DOF consideration. I.e. "when using a smaller sensor you may be able to use a wider aperture and retain the same (or even more) DOF compared to using a larger sensor."
When otherwise combined with exposure/high ISO capabilities it confuses things greatly, it can make everything you thought you new "wrong," and it doesn't match most "real world" experience.
 
I'm not sure that concentrating too much on equivalence is really a good idea. I think it's best to shoot and process for the shot and result you want and then compare the end results. If you can then see that system A gets better results for you than system B you'll have your answer. I think that equivalence can be pretty much a red herring.
 
My Canon 17-55mm f2.8 on my 7D2 is not a 27-88mm f2.8, but a 27-88mm f4 using the equivalent aperture and focal length approach - you need to consider both aspects for comparison. Sure it lets in the same amount of light as any other f2.8 lens, but the visual effect on the background blur in the image is of an f4 lens.

The same thing applies to "equivalent ISO", which explains why ff cameras have better noise performance than APS-C. I think the difference is about 1.5 stops for an APS-C, so that at ISO 3200 on ff, the equivalent ISO on APS-C is around 1250, meaning that at these levels noise is generally similar.
 
woof woof said:
I'm not sure that concentrating too much on equivalence is really a good idea. I think it's best to shoot and process for the shot and result you want and then compare the end results. If you can then see that system A gets better results for you than system B you'll have your answer. I think that equivalence can be pretty much a red herring.
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Ah yes, but that requires actually purchasing/renting/or maybe borrowing... most just read the web and lust for better stuff to buy.
 
Brachytron said:
My Canon 17-55mm f2.8 on my 7D2 is not a 27-88mm f2.8, but a 27-88mm f4 using the equivalent aperture and focal length approach
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Nothing about the lens has changed.... cropping the FOV doesn't make it "equivalent" to a longer lens. There is a significant difference between actually having more magnification (longer FL) and cropping the field of view. The resulting FOV is similar, but lens resolution is (probably) not.
The *only* reason to consider the equivalent DOF (F4 equivalence) is if you want shallow DOF... would you be using a 27-88 at f/2.8 on FF, or would you more likely be using it stopped down for DOF? Or if you could use f/2.8 on APS instead of f/4 on FF for DOF then its DOF equivalent, but at a lower ISO so the ISO difference is negligible... or at a higher SS if ISO is already tolerable... And in the 27-55mm "overlap" it still has f/2.8 FF DOF equivalence...

Like I said, IMO it should be left as a DOF sensor size consideration... we should probably leave the lens consideration out of it entirely.
 
All ISO's are not equal though, the cells on a larger sensor are obviously larger and so capture more light. This means the signal has to be amplified less and so is less noisy so a FF sensor at a high ISO can still produce a better photo. My MFT is quite noisy at iso800 but there is no noticeable noise on my FF at iso6400.
 
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