DOF with DSLR??

I'm still discovering the basics of my digital Rebel. Today I'm

wondering why I am not achieving a shallow depth of field, with wide

apertures, since that was one of the reasons I bought the dSLR

(rather than the point and shoot).

Others in my local camera club moved from film Nikons to the D70 and

they are also puzzled by the absence of subject isolation.

So the question is - is shallow depth of field less available with

dSLR than we anticipated?

It's less than with equivalent angles of coverage on full-frame slrs--or DSLR's. The EOS 1Ds and the Kodak 14MP full-framers give the same DOF as a 35mm SLR. On your DReb to get the same coverage of a 50mm lens on a full-frame you're using bewteen a 28mm and 35mm lens with the same DOF as those lenses have on 35mm.

A lens has the same depth of field regardless of whether you use it on a DSLR or a film SLR. Of course when you use a DSLR the magnification factor means that for any given situation you would select a shorter focal length lens. So where you're used to using, say, a 50mm lens with film, you probably use a 28mm lens with digital, with correspondingly greater depth of field. So, yes, if you fill the frame with the same subject with digital and with film, from the same viewpoint, you will have greater depth of field (less subject isolation) with digital - because you will be using a shorter focal length. It's only a problem if you can't change your viewpoint, though.

Read all about it right here on photonet at:

 

http://www.photo.net/learn/optics/dofdigital/

Jay,

 

Take Mark's advice and study Bob Atkins's article in the link. Lots of people say lots of things about depth of field, and most of it is wrong. The link is one of the few which gets it right.

 

Steve,

 

Same advice.

Leonard: Steve and I both said the same thing in different ways and we're both right so if Bob says the same thing in a third way or else says something contradictory, either way neither Steve nor I need to waste our time.

Jay,

 

Steve says that the same focal length lens will have the same DOF independent of the format, full frame SLR or DSLR. As Bob Atkins explains in detail, that is in fact not true. DOF also depends on the degree of enlargement necessary to make the final print. The format of the D70 is about 1/1.5 = 2/3 the size of the full frame SLR, so it has to be enlarged about 3/2 as much for the same size final print. Paradoxically, for the same focal length and same f-stop, you actually get more DOF with the larger format. You can see this by looking at the formula for hyperfocal distance, which is f^2/Nc where f is the focal length, N is the f-number, and c is the diameter of the maximum allowable circle of confusion. c depends on the degree of enlargement to make the final print. It is larger for the larger format, which means the hyperfocal distance is smaller, which means more DOF. But, as I said, Bob Atkins does a better job of explaining it, so look at what he has to say.

how wide are you apertures? what lens are you using?

this is shot with 50mm 1.8F wide open:

I am really confused by all these.

 

Lets do not look at sensor size but just use 35mm film as the basis. Does it means that the DOF changes with the final print size I make from it? Say a 8X10 has more DOF than a 16X20 which in term is more than a 20X30 print? Should we factor in viewing distant also? Viewing at close distant will make image appears to be less sharp and may induce an out of focus impression for shallower DOF.

Yes. I had failed to take into account the effect of the different format and the correspondingly greater enlargement of digital.

 

On the other hand, the conclusion I reached:

 

"...if you fill the frame with the same subject with digital and with film, from the same viewpoint, you will have greater depth of field (less subject isolation) with digital"

 

I think is correct, and is borne out by Bob Atkins' article.

You can use the coef (x1.6 for Canon D Rebel) to get equivalence on 35mm Full Frame (& film) by multiplying focal & f number:<br>

A 50mm at f/1.8 used on D Rebel will give you the same image (Field of View and Depth of Field) as a 80mm at f/2.88 that would be used on film.<p>

 

Olivier

David,

 

You are confused because it is confusing. Some things are that way and there aren't any shortcuts. Read Bob Atkins's article and then read it again. It may require several tries, but you should eventually get it right. I am a professional mathematician, but I had to work through it all myself before I got it right. The math is not complicated, but the urge to oversimplify gets in your way. If you are still confused about some points after studying Atkins's article, I will be happy to help you understand it.

Thanks to all respondants-

 

The theoretical understanding of the phenomenon, though interesting, was not as important as simply knowing that I could not expect to achieve the isolation of a subject as I had anticipated.

Marvin,

 

You can obtain subject isolation by appropriate manipulation in a photoeditor. Books about Photoshop and websites devoted to photoediting usually have extensive discussions of how to do that. In essence, you have to select the subject carefully and gradually apply more and more blurring outside the selected area. You can also isolate two different subjects at different distances, if you want, which would not be possible just using optics in a camera.

Leonard, shallow DOF can be had without any PS work.

 

Martin, it sounds like the folks in your camera club aren't controlling their apertures.

Shoot in Av mode. Maybe you folks are shooting in the green circle or P modes!

 

Anyway, to suggest that there's a DOF problem with DSLRs is absurd. Are you using a

slow zoom lens from several yards away from your subject at f/8 or so? The DOF is very

deep in that sort of situation.

 

Here's a photo I made the other day with my 10D. DOF is very shallow.<div></div>

Andrew,

 

Your shot is like what I hope to achieve. I have the same lens that you used so I guess that I'll need some more tries and experience. Thanks.