FF vs Crop DOF (+5d2 focus points)
So here's the question: does the DOF/bokeh of f4.0+full frame sensor approximately match the DOF/bokeh of f2.8+crop sensor? Just curious.
There's no doubt that FF is in my future, and if that means I can keep my never-expected-to-love-it-as-much-as-I-do 70-200F4 and get some creamier backgrounds out of it, that would be a lovely bonus (I of course have a 135L to crush backgrounds as needed, but the convenience of the zoom means I reach for it a lot more than I ever expected I would!)
Sub-question: are the outer points on the 5dII really THAT bad? How would they compare, for instance, to the outer points on the xsi? Not shooting sports, although I do shoot shallow DOF a lot and need accuracy. Again, just wondering. The 5dIII is obviously the camera of my dreams (it would be hard to imagine something that's closer to my wishlist), but it's well out of my $ reach right now. With all the good deals on 5dII's, it's hard not to wonder what one could do for me....
Thanks in advance!!
There's no doubt that FF is in my future, and if that means I can keep my never-expected-to-love-it-as-much-as-I-do 70-200F4 and get some creamier backgrounds out of it, that would be a lovely bonus (I of course have a 135L to crush backgrounds as needed, but the convenience of the zoom means I reach for it a lot more than I ever expected I would!)
Sub-question: are the outer points on the 5dII really THAT bad? How would they compare, for instance, to the outer points on the xsi? Not shooting sports, although I do shoot shallow DOF a lot and need accuracy. Again, just wondering. The 5dIII is obviously the camera of my dreams (it would be hard to imagine something that's closer to my wishlist), but it's well out of my $ reach right now. With all the good deals on 5dII's, it's hard not to wonder what one could do for me....
Thanks in advance!!
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As I understand it (and dofmaster calculates - numbers below), the answer is approximately yes, once you account for the crop factor by changing subject distance or focal length.
5D: 200mm, f/4, 20 ft from subject, dof = 0.71 ft
7D: 125mm, f/2.8, 20 ft from subject, dof = 0.82 ft
or
7D: 200mm, f/2.8, 32 ft from subject, dof = 0.82 ft
So you're an inch or so different in dof (in the good direction for shallowness), and of course your angle of view/background changes with the different perspective. Given how much you love your 135, though, seems like once you get that 5D, you'll never put anything else on it.
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Assuming the same subject matter, there are 2 scenarios that you need to consider, in order to understand your question:
2) ... wanting to use the same distance to subject, so changing the focal length.
The premise is that a 70-200mm lens presents different FOV combinations, depending on whether it's used on a crop or FF body. Using the same subject matter and filling the frame to the same degree will force using one scenario or the other, or some combination of the two.
The short is that "yes", a FF body does tend to force you to use one of the two scenarios above, and that will cause a similarity in bokeh between subject and background, using f2.8 on crop and f4 on FF.
Will they be identical? No, but scenario 2 should be most similar.
To put it another way, f4 on a FF body should be similar to f2.8 on a crop 1.5x/1.6x body, at the same FOV and using the same subject matter and composition. (The focal length changes in this instance.)
As a method of some level of proof, go to the following site:
http://www.dofmaster.com/dofjs.html
... and plug in the following values:
Camera, film format, or circle of confusion: Canon 7D
Focal length (mm): 50mm
Selected f-stop: f2.8
Subject distance: 10ft.
You should generate:
Depth of field
Near limit 9.39 ft
Far limit 10.7 ft
Total 1.29 ft
In front of subject 0.61 ft (47%)
Behind subject 0.69 ft (53%)
Hyperfocal distance 152.8 ft
Circle of confusion 0.019 mm
Duplicate the test using:
Camera, film format, or circle of confusion: Canon 5D (II/III)
Focal length (mm): 80mm
Selected f-stop: 4
Subject distance: 10ft.
You should generate:
Depth of field
Near limit 9.47 ft
Far limit 10.6 ft
Total 1.12 ft
In front of subject 0.53 ft (47%)
Behind subject 0.59 ft (53%)
Hyperfocal distance 175.2 ft
Circle of confusion 0.03 mm
Note that the calculated DOF are similar to within around 15% of each other's values.
Cab beat me to it.
Moderator of the Cameras and Accessories forums
Then again, since you are effectively magnifying the blur, what looked in focus in the non-cropped photo might not in the cropped photo. I think I am getting a headache!
Very nice explanation indeed! Regarding Divamum's original question, I think this answers it: If you use the same lens on a small-sensor camera and a full-frame camera and crop the full-frame image to give the same view as the digital image, the depth of field is IDENTICAL.
It depends on what you use the outer points for, and how contrasty the subject is. For still subjects that have decent vertical and horizontal contrasts, the outside points are fine. For moving subjects, they're almost worthless. In dim light and/or low contrast, you're much better off focusing manually or setting focus with the center point and moving the camera.
That said, I love my 5D2! I get my best images from it; better than my 1D4 bodies when I nail the shot. It's just that it is harder to do this.
That's also my experience. I use the 5D MKII mostly for posed formals.
If I want to use that body for any sort of candids, I use center dot to focus, frame loosely, then crop in post to compose the shot.
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Ok, so here's the peripheral question: why is it that FF is always recommended for "more DOF control", and shots typically have shallower depth of field? I'm not arguing with the numbers, just trying to reconcile "at the same aperture which encompasses the same field of vision, regardless of whether that requires moving or changing the focal length, the resulting DOF will be about the same" with the numerous user reports and comments that you get shallower depths of field on a FF camera than a crop.
Regarding the outer points, that's a major bummer - I was hoping they'd have the useability of my old xsi (not brilliant, but definitely useable). Focus+recompose was a pretty epic fail for the way I shoot, and moving to the 7d's multiple points was a big bonus for me. Hmmm... have to think about this.....
I always follow these DOF discussions in hopes that I can develop some intuition about it - and so far the only result is that my brain also hurts.
The 'conventional wisdom' is probably born of incomplete knowledge in this area by many/most photographers (at many experience levels).
What gives me fits, is that I hear (and believe) that there are only 3 factors that should affect DOF:
Distance to Subject
Focal Length
Aperture
So why does DOFMaster ask what sensor size is in use? It gives different results for a crop sensor than for a FF sensor. Why and how could that be the case, since crop sensor is giving you the same result as cropping the image in PP?
Is it because a different acceptable circle of confusion is thought to apply? If that is the reason, is it a valid one?
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This is the one that confuses me... as you said below (from the earlier post), I have a hard time understanding why the DOF changes by cropping an image that has already been taken...
Is that the answer? I'd think that at 100%, it wouldn't matter what the crop was, and zooming to 100% doesn't change what is or isn't sharp, it's just the impression of it being sharp when viewed at normal size? This gives me a headache, too...
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For example:
Let's choose two bodies, like the Canon 7D (crop 1.6x) and Canon 5D MKII, and both using the same Canon 135mm, f2L lens.
Assuming the same subject matter criteria and photographic task, let's say a full-length portrait, and keeping the same background and the same distance from subject to background, what are the ramifications of the shoot?
The 7D, in order to fill the frame with the full-length subject, will be further from the subject than will be the 5D MKII. At the same aperture of f2 that means the background will be more blurred on the 5D MKII image because we are closer to the subject, shifting focus further from the background.
Conversely, the 7D allows an aperture of around f6.9 before diffraction starts to affect the image, versus f10.2 for the Canon 5D MKII.* (This is a valid comparison partly because the 7D and the 5D MKII share a fairly similar gross pixel count of 18 versus 21 MPixels.) Basically this means that we can start with a softer background on the 5D MKII and largest aperture, and wind up with a small aperture too (although in this scenario f6.9 on the 7D at the further distance to subject will not be the same background blur as the 5D MKII at f10.2 and closer to the subject.)
Since we start at f2 and finish at the diffraction limited apertures (and others will remind you that there are still usable apertures beyond the diffraction limits, but the comparison still holds), that means more total stops that are usable, and that's how I define the FF imager as having, "more control" over DOF.
Additionally, more usable stops (and fractional stops) means finer "granularity" of DOF control.
To summarize, the 5D MKII will have more background blur wide open, while still allowing deep DOF at small apertures, and more total stops between, compared to the 7D and using the same lens, subject, and distance from subject to background.
*(Diffraction Limited Aperture, DLA, information from "the-digital-picture.com" site.)
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To answer my own question, I have read the rest of that link, and it makes much more sense to me now. He reports that DoF makes assumptions that include the size of the print (usually 8x10 or 8x12), the viewing distance (~15"), and being viewed by a person with normal eyesight. So viewing it at 100% is completely irrelevant. Therefore, the magnification of a crop sensor image up to 8x10 is larger than the magnification of a FF image to 8x10. This means that what would be small enough to be rendered as a point in the FF print is magnified more and is now a disc, so it appears to be less acceptably sharp.
Maybe I didn't restate that well, but at least it's making sense to me in my own head. I am much less confused about the circle of confusion than I was earlier...
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I think that is correct. If you take two shots of a subject from the same distance and at the same aperture, one with 1.6 crop and one FF, then crop the FF shot so that it has the same FOV as the 1.6 crop, both will have the same DOF. They have too, because it shouldn't matter whether the crop is done by the camera or in the post.
The complexity comes in when you change the distance from the camera to subject so that the FF has the same FOV as the crop because of altered position. Since DOF is a function of distance from camera to subject (assuming you're below the hyperfocal distance), you'll see a difference.
Are you telling me you've not shot with a FX? Rent! Esp. Rent that 5Dmk2 first before buying>!<
The AF system is certainly the weak point on the 5D2, but I will say that the center point works just fine. I do occasionally use my 5D2 for sports shooting--especially in low light, such as where the Stanford women's soccer team have their home games--and it's almost never a problem. I also do a fair bit of candid portraiture, and it works fine for that as well. Overall it is a great camera, with lovely IQ. It just takes some practice to use most effectively.
sure, and that's why people would pick full fram if all other things were equal (which of course they are not, including price !
Ziggy, you are a teaching superstar for distilling all the empirical optical facts into a simple example I can easily understand. I now get it. THANK YOU!!!!
Now to continue debating whether to hold out for the 5dIII (which is what I really would LIKE to do - the only impediment there is the cost, and it will require selling both of my current bodies even to consider spending that much), pick up a rapidly-getting-cheaper-by-the-day 5dII, or wait and see what this new "entry-level" ff camera that is rumored is going to offer.....
For a hundred plus shipping you can get your 5Dmk2 answer: http://www.lensrentals.com/rent/canon/cameras
Based on my following and interpretation of your shooting, I'd say you'd not be a happy Camper. But for a hundred bucks it is worth finding out. At least I obviously think so.:D