Wallet full of $100 bills

[neuroanatomist]

The first step is for us to agree on one fact.

Say this hypothetical dude shoots a full-body portrait shot at 85mm f/1.8 on FF, and takes the shot so the subject's body vertically fills the frame - we'll say he's 10 feet from the subject. There's a certain DoF associated with that aperture and subject distance. Now that dude sets down the FF camera and puts the 85mm f/1.8 lens on the 1.6x crop. To take 'the same shot' (i.e. so the subject'sverticallybody fills the frame), he has to move further from the subject, out to about 16 feet.

With respect, I *believe* you are mistaken here (and of course I admit that it could be me). The hypothetical dude would not move at all. He would stay 10 feet from the subject to maintain the same framing.

Here, I think you're wrong. 85mm f/1.8 on FF - put that 85mm on a 1.6x crop, I hope we can agree that you now have the equivalent of 136mm f/2.9. We changed the camera, not the lens. To maintain the same subject framing now that 'the dude' has the equivalent of a 136mm lens, he'd have to move back. Put another way, you've got a 5DII with a 70-200mm zoom. Frame a shot at 85mm. Now, zoom your lens to 135mm (which is what the 1.6x crop factor does to the angle of view). You have step backward (to 1.6x the original subject distance) to maintain the same framing, right? When you do move further from the subject to maintain framing, if you leave your aperture setting the same, your DoF will get deeper. Those are the effects of cropping we're talking about - longer effective focal length, and narrower effective aperture because the distance changed to keep framing the same.


I remember a discussion with Daniel on the noise/compression/perspective issue, and I brought up subject framing - his response was that in the example photos he posted, he changed the focal length (using 70-200mm zoom lens) to 'simulate' the cropping effect. Here's a relevant quote, it's from this post:

If you wanted to see what happens when you change just subject distance and not focal length, here's what you would compare:


* 5D2 70mm f/6.4 at 10 ft
* 7D 70mm f/4 at 16 ft


That results in the same subject framing ("field of view"), but not the same angle of view and not the same perspective.

So, his example is essentially the same as mine - to get the same subject framing with the same lens on a crop body, compared to FF, you'd need to move further back, and that would result in the deeper DoF. You mentioned that, "The 200mm example is a bad one because we're getting different effective focal lengths when we use the same lens with different sized CCD's." When you compare different sensor sizes with the same lens, you're always going to get different effective focal lengths. Keep in mind, the sensor is not affecting the iris diaphragm diameter of the lens. A 100mm lens at f/2 is going to have a 50mm diameter iris diaphragm, regardless of what camera the lens is mounted on. The effects of decreasing sensor size on DoF are due to the fact that with the same focal length, you've got to move further away from the subject to get the same framing.


Here's a second excerpt from that same thread with Daniel:

In other words, as Bryan states, "...as a generalization, using a higher FOVCF DSLR will yield more DOF in your similarly cropped pictures because you will be farther from the subject."

That's correct. The same focal length and f-number combined with further distance results in deeper DOF for smaller sensors.

So, that would seem to be the first fact to agree on, yes?

However, if instead that dude didn't move, and used the 1.6x crop + 50mm f/1.2 to take a torso shot that filled the frame, the depth of field would be similar to the 85mm f/1.8 on FF.

If the dude does not move, he'll not have a head and torso shot. He'll have the same framing as with full frame, and (I'd be shocked if Daniel wouldn't agree) same compression and perspective. There's even the same amount of light striking the CCD, so assuming same sensor sensitivity, same photon noise. Lens aberrations aside, IQ aside, he has the same picture.

You're right, I'm wrong here (I actually edited the first example after I wrote it, but neglected to make this one match - that's what I get for posting before heading home from work). By changing both the lens (the actual focal length goes from 85mm to 50mm) and the body (1.6x crop brings the equivalent focal length back up to 80mm (close enough to 85mm for me), you'd get the same framing. To get the head/torso shot, our friend 'the dude' would have to move forward. The point I was trying to make is that if he took that head/shoulder shot (by moving forward), he'd get a thinner DoF than the body shot. But yes, it would be even thinner if he shot the head/torso shot with FF

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