Exposure, DOF, and Equivalence

Started Apr 13, 2013 | Discussions thread
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Great Bustard
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Exposure, DOF, and Equivalence
Apr 13, 2013

It seems that there is some confusion on these points, which was made painfully clear here:

http://www.dpreview.com/forums/post/51283797

You figure its all about DoF, fine, but not everyone agrees including the makers of your equipment who include a speed priority mode, its there for a reason, and 'primary' to that is exposure.

I think the lack of understanding expressed in the above statement may be representative of the lack of understanding of Equivalence in general, and why there is so much conflict over an otherwise simple concept.

First of all, Equivalent photos are defined as photos that have the same:

  • Perspective
  • Framing
  • DOF
  • Shutter Speed
  • Display Size

That's it.  The concept of Equivalence is not a mandate that if one is using two systems that they much shoot Equivalent photos with each system.  Indeed, the exact opposite is often the case, that is, that one uses one system over the other for photos it can do better than the other system, which depends entirely on what one considers to be "better".

That said, let's begin with the comment, "You think it's all about DoF".  No, I really don't think that.  However, if the sharpness of a photo is central, especially the corners, then it's absolutely ridiculous to compare the photos from two different systems when the systems do not have the same portion of the scene within the DOF.

In other words, it's silly to talk about corner sharpness of f/2.8 on mFT vs f/2.8 on FF, as the DOF on FF is half as deep.  The exception would be if the whole of the scene is within the DOF at f/2.8 on FF, and f/2.8 is as wide as the mFT lens can open.

So, let's talk about scenes where the whole of the scene is within the DOF, like astrophotography.  For example, 12mm f/2.8 on mFT vs 24mm f/2.8 on FF.  These settings will result in the same exposure for both systems for a given shutter speed, but 4x as much light will fall on the FF sensor during that exposure than on the mFT sensor:

Total Light = Exposure x Effective Sensor Area

which means the FF photo will have half the noise for equally efficient sensors (same QE and read noise / area).

OK, fine -- we all understand this.  So what?  Well, the "so what?" is what happens if the photo from the mFT camera is sharper in the corners than the photo from the FF camera, and this sharpness matters more than the noise?  Well, the FF photographer could simply stop down to f/5.6, which not only results in the same DOF (albeit irrelevant for this particular example), but also the same total amount of light being projected on the sensor, and thus the same noise (again, for equally efficient sensors).

So, if the mFT camera still produces a sharper photo at f/2.8 than the FF camera at f/5.6, we would say it is the better system for the task (in terms of IQ), but we cannot make that statement on the basis of comparing at the same exposure.

Of course, the ideal situation for FF would be if it resolved at least as well at 24mm f/2.8 as the mFT camera at 12mm f/2.8, because then it would be able to deliver a less noisy photo that was at least as sharp.  But, obviously, this would only matter for scenes where DOF is not a factor, otherwise the FF photographer would have to stop down to f/5.6 regardless, just to get "enough" DOF.  Alternatively, the photographer might prefer the more shallow DOF of f/2.8 on FF, and consider the lesser noise that comes with it a bonus, but then things like corner sharpness would not be a concern.

In other words, Equivalence is not about how you use a camera, but a meaningful framework by which to compare two formats in terms of the visual properties of the final photo.

That is, comparing systems on the basis of exposure leads does not tell us anything useful about how a competent photographer would use the equipment, whereas comparing two systems on the basis of DOF and, consequently, total light, will.

In short, if 12mm f/2.8 1/100 ISO 400 resulted in the "best" photo on mFT, then 24mm f/5.6 1/100 ISO 1600 will result in the "best" photo on FF, unless, of course, we prefer a more shallow DOF and the mFT system cannot open up wider, or, alternatively, the lesser noise that comes with a wider aperture on FF makes for an overall "better" photo, despite a more shallow DOF that is undesirable.

Thus, I'd be please to answer:

I gave you specific terms which I had asked you to respond to, you failed, now you're just bouncing. You so screwed the beginning that you would sooner do anything but relax and revisit how you got trashed in the first place. That isnt my problem, just sayin...

'cause I'm afraid I missed those "specific terms which I had asked you to respond to".  It's not like me to duck questions, after all, but sometimes, I confess, I do miss something someone said.

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