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

I'm still enjoying it

Me, too! Your turn...

[Jon Ruyle]

Here, I think you're wrong.

Yes, of course I was I misread your post, and I apologize. I thought we were comparing the 50mm f/1.2 to the 85mm f/1.8. But if the dude is using the same lens with two different sized sensors, then of course he has to move to get the same full body picture, and of course the pictures will be different pictures (even if they're both full body)

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.

True.

But yes, it would be even thinner if he shot the head/torso shot with FF

Yes, if he shot it with the same actual f/ number.


But none of this bears on the point I am trying to make (and not doing a very good job). What I am saying is, first: 50mm @ f/1.2 on 1.6 crop is equivelant to 85mm @ f/1.8 on full frame (or very close) in pretty much all ways except image quality (at non-macroish distances). So if you have a 50mm f/1.2 on your 7D and I have an 85mm f/1.8 on my 5DII, we can take the same pictures with the same working distance. I think we agree on this part.


The second part of what I'm saying is: because I'm getting the same picture at 85mm f/1.8 @ f/1.8 as you are with the 7D + 50 f/1.2 @ f/1.2, then to compare IQ, it makes sense to compare 50 @ f/1.2 with 85 @ f/1.8. This is where you disagree with me, if I understand correctly. But... why would you want to compare the two lenses at the same f/number? At the same f/number, we're getting different DOF's, different noise, etc. Shouldn't we compare IQ in a situation where the *picture* is otherwise the same?


The third part of my point is that, while we would like to just take a picture of a chart with the 7D + 50 f/1.2 @ f/1.2 and compare to 5DII + 85 f/1.8 @ f/1.8, we don't have a picture of the chart taken with the 7D. So instead of 7D + 50 @ 1.2 we use 5DII + 50 @ 1.2, and throw away the outer 60% of the image. This is an approximation. The 7D has higher pixel density, and there are other differences, but I believe it is a pretty good approximation.


Finally, it seems to me that the 85 @ f/1.8 picture would be far better than the cropped 50 f/1.2, but if you disagree on this last, I won't argue.

[neuroanatomist]

first: So if you have a 50mm f/1.2 on your 7D and I have an 85mm f/1.8 on my 5DII, we can take the same pictures with the same working distance. I think we agree on this part.

Absitively posolutely. [8-|]

because I'm getting the same picture at 85mm f/1.8 @ f/1.8 as you are with the 7D + 50 f/1.2 @ f/1.2, then to compare IQ, it makes sense to compare 50 @ f/1.2 with 85 @ f/1.8. This is where you disagree with me, if I understand correctly. But... why would you want to compare the two lenses at the same f/number? At the same f/number, we're getting different DOF's, different noise, etc.

Correct - because we cannot compare50 @ f/1.2 with 85 @ f/1.8 with the same lens. I'd want to compare images of a test chart using the same lens, because different lenses have different optical characteristics and performance. I'd want to use the same aperture, because the selected aperture affects optical performance of the lens, and for a test chart, I don't care about DoF since the subject is a flat chart parallel to the sensor - if it's well-focused, DoF is irrelevant (when you see sharpness on Bryan's charts change with aperture, that's not DoF, that's the effect of the changing aperture on the optical properties of the lens - at the wide end, stopping down helps most lenses; the sensor only comes into play when diffraction starts setting in). You could use a zoom lens on the charts, so you can vary focal length and aperture in the same lens. Even doing this simulation with most zoom lenses isn't ideal, since their optical performance changes through the zoom range. But, trying it with the 70-200 II (a very good zoom lens, as we know), there's really not much difference going from 70mm f/3.2 to 135mm f/5.6, or from 135mm f/3.2 to 200mm f/5.6 (the closest I could get to a 1.6x factor).


So, I think there's no real way to "compare the IQ in asituation where the picture is otherwise the same."


*sigh*

The third part of my point is that, while we would like to just take a picture of a chart with the 7D + 50 f/1.2 @ f/1.2 and compare to 5DII + 85 f/1.8 @ f/1.8

Exactly. But, we've established that those two pictures would have (essentially) the same framing and DoF. It's also apparent that for these specific lenses (which we can compare, since they are both tested with a 1DsIII), the85 f/1.8 @ f/1.8 is sharper than the50 f/1.2 @ f/1.2 (not surprising, I suppose, sinceoptically-speakingthe 85/1.8 is really a great value, and the 50L is optimized for bokeh over sharpness). Added to the reduced noise of a FF sensor vs. an 18 megapixel crop, it's obvious that overall IQ will be better with the85 f/1.8 on a 5DII than the50 f/1.2 on the 7D. So, do I agree with your final point, too.


All of this really just reinforces my sentiment that I'll eventually want a crop body with the shooting performance of the 7D for wildlife and action, and a FF body with the IQ of the 5DII for portraits and landscapes.


Your point that I may be better off getting a 5DII (instead of an ultrafast prime for use on a crop body) is valid, especially for one of the reasons that's been only a minor part of these discussions - significantly better noise performance.

Consider the 5DII, dude. It would be like getting a whole new set of fast lenses.

I am definitely considering it.


The only problem I see with that is exemplified by a story I read (for the umteenth time) to my daughter last night - "If You Give a Mouse a Cookie," ... as in, if you give John a 5DII, he's going to want an 85L to go with it...

[ddt0725]

The only problem I see with that is exemplified by a story I read (for the umteenth time) to my daughter last night - "If You Give a Mouse a Cookie," ... as in, if you give John a 5DII, he's going to want an 85L to go with it...

**sigh** After all that, we're back to what I said to begin with ...you should get the 85L! [I] [:P]
I guess sometimes it just takes a non-analytical mind to spot the obvious! []

Denise

[Keith B]

The only problem I see with that is exemplified by a story I read (for the umteenth time) to my daughter last night - "If You Give a Mouse a Cookie," ... as in, if you give Keith a 5DII, he's going to want an 85L to go with it...

Huh? What?


The best way for me to describe the 85 1.2, is like being addicted to heroine without ever trying it. You don't know why, your body just needs it.

[Jon Ruyle]

Take the following, both using the same
FF camera - 85mm @ f/4 with the outer 60% cropped away(to simulate the
crop sensor), should be compared to uncropped 135mm @ f/6.4,
right?

Yes! (Though we're now in a much higher IQ zone- closer to being pixel limited. In this case, throwing away all those pixels is unfair to the 7D. I still think the 135 will do better, though.)

By that logic, you could compare an
image taken with an 85mm f/1.2L II @ f/4 on FF with the outer 60% of the
image thrown away with a complete FF image taken with an EF 28-200mm
f/3.5-5.6 @ 135mm f/6.3. Here's the comparison. Wow - that 'crop-simulating' 85L
is making the crop sensor look way better than the FF.

Of
course I don't mean any lens in the world will do better on FF than any
effectively equivalent lens on a crop body. Even here, though,
after cropping, the difference might not be as great as you think. (I
dunno). Anyhow, making a non-stellar 7x zoom improve beyond a legendary
prime might be a little much to ask, but I think that often, cheap primes on FF will
far outperform expensive primes on crop.


Here's the thing. If you want a faster shorter lens, you can buy it. It will be expensive and the IQ will be worse than your longer slower lens. Or you can go FF and *all* your lenses will act like more expensive faster shorter lenses, only the IQ will get *better* rather than worse. (In some situations you'll like the longer slower version of the lens better, so I suggest you don't throw your 7D away [])

So, I think there's no real way to "compare the IQ in asituation where the picture is otherwise the same."

True. But what I've been doing is comparing a cheap lens on the 5DII to an expensive lens on the 7D, and trying to convince you that the cheap lens comes out way ahead. I gave the example of the 85 f/1.8 compared to the 50 f/1.2 and the 135 f/2 compared to the 85 f/1.2, but in general, lenses get faster and IQ gets better when you go to FF.

The only problem I see with that is exemplified by a story I read (for the umteenth time) to my daughter last night - "If You Give a Mouse a Cookie," ... as in, if you give John a 5DII, he's going to want an 85L to go with it...

Oooh, my daughter loves that one, too.


I say if John wants an 85L he should get one. The only problem I see is, if you give John an 85L, he's going to want a 5DII to go with it...