We often read that longer lenses give less depth of field.
When considering large format lenses, we also need to consider the fact that enlargement is correspondingly less.
So, even though a 300mm lens gives less depth of field than a 50mm lens, what if we compare an 8x10 contact print to an 8x10 print enlarged from a 35mm image taken with a 50mm lens.. Would the 8x10 enlargement still appear to have more depth of field than the 8x10 contact print ?
Sharpness or DOF? It really depends on the subject/picture being taken - how the plane of focus corresponds to the elements in the shot. And what does it matter? Who's shooting 35mm?
Content, content, content!
RJ
Depth of field is a brain bender for me, so whenever I am trying to figure out which format/lens combination to use when I want to maintain or maximize depth of field, I run calculations with online depth of field calculators (or DOF Master on my PDA) to see what hyperfocal distance I will be dealing with. Both:
http://dfleming.ameranet.com/dofjs.html and http://www.silverlight.co.uk/resources/dof_calc.html
allow you to select your film size to work out near/far hyperfocal distances for a given distance of focus. For example if you plug in: 35mm format, 50mm lens, f22 aperture, 25ft focus distance and compare those numbers to numbers generated by using: 8x10 format, 300mm lens, f22 aperture, 25ft focus distance; you will see that DOF shrinks considerably. Cheers,
Ken, while your question is a valid one, you've left some important variable out of the equation, maybe intentionally. lenses for 35mm formats become diffraction limited beyond f22, while a 300mm lens for 8x10 format can stop down to f 90 or beyond. More important are view camera movements which allow manipulation of the plane of focus. These factors combined with the enlargement factor cited in your question combine to eliminate most of the dof advantages enjoyed by smaller formats, and that says nothing of the other, equally important advantages enjoyed by larger formats.
Even if you allow for different size circle of confusion, the 35mm will have greater DOF.
For a subject 20 feet away (focused at that distance
35mm format, 50mm lens, f/16, CoC=0.025mm: DOF extends from about 10 feet out to 620 feet.
8x10 format, 300mm lens, f/64, CoC 0.188mm: DOF about 11 feet out to 88 feet.
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Trouble is, this is all apples to oranges. With the 8x10 camera, you may be able to use a judicious tilt or two to make the apparent DOF seem infinite. And don't forget the subjective impact of a contact print vs an 8X enlargement...
Bob Wheeler wrote an interesting article dealing with the differences in DOF in small formats vs large, it can be seen at:
http://www.bobwheeler.com/photo/DOFExpo2.pdf
Let's try to rule out any extraneous considerations, like view camera movements, apertures, etc.
I am not trying to determine what is better or easier, but just understand if the additional depth of field derived from shorter lenses, is compensated for, in larger formats, by an equivalently lower degree of enlargement. Forget about grain, lens quality, etc. I'm just talking about the optical principles themselves... I think. ;-)
Perhaps Professor Evens can throw some light on this one.
Okay, if you want to make a straight comparison with the only variables being enlargement factor and focal lengths, ie same aperture for both lenses, both focussed at infinity, which Circle Of least Confusion figure would you use? The one consistent with contact printing, or the one required for enlarging? You see, there's really no way to make the comparison you want to make without biasing the results one way or the other, which is why all of the applicable factors should be considered.
Let's say we set up both cameras along a picket fence, focus on one spot, and shoot both images at the same f-stop. Comparing the 8x10 contact print with the 8x10 enlargement, will there be an obvious difference that an average viewer would notice ? The difference between a 50mm and a 30mm lens is quite a bit - but is that difference ameliorated by the 8x enlargement ?
Sorry - I meant to say that the difference between a 50 mm and 300 mm lens is quite a bit.
Ken, the shallow depth of field of the 300mm lens on 8x10 would make most of the picket fence out of focus, which on a contact print would look far far worse than the loss of sharpness by enlarging a 35mm frame 8x, (assuming that the fence is mostly in focus due to the greater depth of field from a 50mm lens in the smaller format). This is assuming that both lenses are stopped down to say, f16. Aperture will always play a part. If both lenses are wide open, the difference between formats will be less.
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