DOF

[montecarlo]

Hello all.

Maybe this is a well debated issue but I think I didnt' understand how DOF works. I thought DOF is related to the focal lenght and aperture only and not sensor/film format. I would say that using a 80mm lens on a 35mm camera, a APS-C camera and a 6x6 camera would give the same DOF . Only the view angle would be different.
How come then when using this calculator http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
the resulted values for a 80mm lens at f:4 and a subject at 5 meters are:
-for APS-C 1.5 crop sensor: 0.68m
-for 35mm FF: 1.025m
-for 6x6 : 1.91m
Shouldn't be the same value ?
Thanks everyone.

[Nesster]

Yeah, I have some problems with this stuff also.

I've settled on this image: start with a large view camera, everything is large: the lens, the film, the 'lines per mm'. If you need to photograph a comb, the whole thing covers, say, 5 cm on the film. Doesn't need a lot of lpm to get the whole thing.

Then start miniaturizing: everything gets smaller: lens, film, the requirement for lpm. That same comb now has to fit onto, say, 1 cm on the film. You have to resolve much finer detail.

And so on, every time you get smaller, everything gets smaller.

Including the circle of confusion, which gives us the sensation of DOF. (Also, I believe, the onset of diffraction becomes earlier, as the physical stuff miniaturizes but light itself does not.)

[Richard_D]

[montecarlo]

But, but, but... is you use CZJ Biometar 80m f:2.8 at set at f:4 and focus on a objec placed at 5 meters, mounted on Pentaxon Six, Canon 5D and Canon 400D the DOF won't be the same ?
Or only if the calculations are made for a lens desing for a specific format only than the resulted DOF valued will be different ?

[A G Photography]

As Nesster said it is related to the circle of confusion size.

Actually for each lens, whatever the focal and the aperture, there's just ONE point of exact focus. All the others are approsimations which seems in focus or not depending on the circle of confusion size for that film or sensor size.

Here there are the circle fo confusion size for every film and sensor formats:

http://en.wikipedia.org/wiki/Circle_of_confusion

[Rob Leslie]

DOF is all about subject to lens distance. In theory all lenses have the same DOF if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field. But we shoot at different distances with wide angle, telephoto and different DSLR crop factors.
Here is link to an article explaining it.

http://luminous-landscape.com/tutorials/dof2.shtml
Once you understand all lenses do have the same DOF and forget the myths about wide angle having more than telephoto things become very easy.

[peterqd]

I read the articles but I'm still confused. In Cosmin's example I can see there will be a different circle of confusion with the P6 compared to the Canons because of the different flange to film distance. But the two Canons have the same distance. If they both use the same lens and aperture setting and focus on the same object, how can the image be any different? The APS-C sensor image is just a crop of the FF.

[Nesster]

I think you're right IF you take the view that the same distances apply, only the crop is different.

OTH, if you keep the subject with the same coverage in the image area, which is reasonable, as with 35mm you'd want to fill the frame and with crop cam's you'd also not want to cut parts of the subject off, then all the dof / resolution matters come into play.

Of course, I could be entirely wrong about this, I can't wrap my head around the whole theory of it, so I just shoot

[A G Photography]

[peterqd]