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Abel · **Depth of Field Variations Based on Sensor Size -** 05-27-2009, 02:54 PM

Depth of Field Variations Based on Sensor Size

I was looking up what the Four Thirds System actually was on Wikipedia and I came across a bit of info that I've seen argued and debated upon for quite some time now.

Depth of field variations based on sensor size...

This is what Wiki stated on the 4/3rd's "advantages list:

Greater depth of field at any given aperture and focal length due to the smaller format. This is an advantage in many applications (macro photography in particular), but a disadvantage in others (portraiture) where a shallow depth of field is desired.

Example:

Full Frame Sensor & 100mm lens shot at f2.8 will have a shallower depth of field than the exact same image taken with a Smaller (Crop Body) Sensor & 100mm lens shot at f2.8. Make sense?

I've seen this argued back and forth amongst photographers but have never seen either side mentioned in a resource online til this one.

Interesting...

Daniel Bates · 05-27-2009, 02:56 PM

Right. This is why P&S cameras don't have shallow DOF, and most DV camcorders can't replicate the film look without expensive 35mm adapters. Viva Red!

PhotoDude79 · 05-27-2009, 02:56 PM

Bingo.... That's what I was looking for.

kenw · 05-27-2009, 03:02 PM

I wouldn't use Wiki as a source for ANYTHING, Abel. Some of the links in the article, maybe. But any base article in wiki is way too open to be of any real value. (most academics and professional organizations now disallow any wiki source).

The problem with this argument is that small sensors in dSLRs are simply cropped. The focusing plane is no different than a full sized: it is in the exact same plane. Just cropped. There is no optical difference. FoV, yes, enlarge-ability, yes.....DoF, no way.

Move the focusing plane (EF-S), now you've got something to talk about. Same plane? No dif.

Abel · 05-27-2009, 03:23 PM

ken actually makes sense for a change.. heheee jk!

kenw · 05-27-2009, 03:32 PM

an infinite number of monkees on an infinite number of keyboards eventually can type Hamlet.....

jeffkohn · 05-27-2009, 03:40 PM

The article is wrong when it says

Quote:

Greater depth of field at any given aperture and focal length due to the smaller format.

What it should actually say is

Quote:

Greater depth of field at any given aperture and field of view due to shorter focal lengths used.

Smaller formats do have more DOF, but it's because they use shorter focal lengths for a given field of view. EG if Abel takes a shot with his 5Dmk2 at 75mm, and I'm standing next to him with my D300 I'll be using a 50mm lens to get the same image as him, therefore my image will have more DOF.

Quote:

The problem with this argument is that small sensors in dSLRs are simply cropped. The focusing plane is no different than a full sized: it is in the exact same plane. Just cropped. There is no optical difference. FoV, yes, enlarge-ability, yes.....DoF, no way.

The problem with your argument is that DOF is not purely a matter of optics or focal length. It's an approximation of what will be reasonably sharp in a final print. In fact, if you hold the focal length constant (eg Abel and I both shoot at 75mm), the larger format will actually have a bit more DOF, because the image doesn't need to be enlarged as much for a given print size (things get a little more complicated with digital though since you also have to factor in pixel density).

kenw · 04:04 PM

but what you're alking about is the old "circle of confusion" silliness. As an engineer i have a hard time with getting too technical with things so highly subjective.

You've complicated the comparison by enlarging different amounts. Print both at 100% and you'll see there is no difference. Granted, on print will be smaller than the other, but if you want to compare DoF directly, don't mux up the matter by introducing yet another variable (% enlargement).

jeffkohn · 05-27-2009, 04:50 PM

Quote:

but what you're alking about is the old "circle of confusion" silliness. As an engineer i have a hard time with getting too technical with things so highly subjective.

Every formula for DOF that I've seen has a CoC in it, so silly or not it's part of the equation. All the CoC does is define the criteria for how small a circle has to be in the original capture (sensor or film) to appear as an in-focus point in the final print. What many people don't seem to realize is that the value you use for CoC is subjective, and depends not only on the print size you want but also how picky you are about what is reasonably sharp. The 'standard' CoC values used in most DOF calculators (and the DOF scales on older lenses) are not of much use unless you're making smallish prints and aren't too picky about sharpness.

Quote:

You've complicated the comparison by enlarging different amounts. Print both at 100% and you'll see there is no difference. Granted, on print will be smaller than the other, but if you want to compare DoF directly, don't mux up the matter by introducing yet another variable (% enlargement).

And what is 100% print size? 24x36mm? I would argue that comparing different-sized prints is not very useful, since DOF by its definition is only relevent when viewing prints. DOF is not an optical property of lenses or sensors. It's all about prints (even if your 'print' is an on-screen web image).

Sonny · 05-27-2009, 05:16 PM

everyone just hush up and

kenw · 05-27-2009, 07:59 PM

you'll never get it and I'll never be able to explain it to you so that you can.

srwatters · 05-27-2009, 08:29 PM

I agree with Jeff!

The DOF really is a function of the focal length difference creating the delta (or more obvious is the same focal length, but closer distance to the subject creating shallower DOF on the larger sensor).

Tom · 05-28-2009, 09:45 AM

If I put my 150mm 2.8 macro on a tripod, put the 1Ds2 on it, adjust and write down the settings, take a picture, then put my 50D on that lens, without adjusting a thing, keep the same settings, the pic from the 50D will have the same DOF. The 1Ds2 will just have a 30% more FIELD OF VIEW. It is the same pic, only the 50D is cropped in more.

jeffkohn · 05-28-2009, 10:09 AM

Quote:

Originally Posted by kenw

you'll never get it and I'll never be able to explain it to you so that you can.

I understand just fine. You've been a bit vague about what exactly you disagree with me on though. If your only point is that for a given focal length, aperture, and subject distance both formats will have the same DOF when enlargement factor is held constant (eg same CoC, different print sizes), then that is true; I never said otherwise. I just happen to think that fact is only of interest to engineers, not photographers, because it's pretty much meaningless when actually taking pictures.

Jake · 05-28-2009, 10:14 AM

All I know is if I put a 85mm lens on my 5d and compose the shot how I like, lets say the persons eyes are 1/3 from the top of the frame.
And then take the same shot with a 85mm lens on the 20d and compose the shot the same way, eyes 1/3 from the top of the frame. Then the 5d will have shallower DOF.
Of course I had to move away from my subject with the 20d, but the FoV is what determines how you compose your photos. And the sensor size has a large part in the FoV.
So far all intents and purpose , larger sensors affect your DOF.