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DOF
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PostPosted: Wed Sep 10, 2008 12:34 pm    Post subject: DOF Reply with quote

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.


PostPosted: Wed Sep 10, 2008 1:25 pm    Post subject: Reply with quote

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.)


PostPosted: Wed Sep 10, 2008 2:45 pm    Post subject: Reply with quote

Nesster wrote:
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.)


Sounds about right - switching to full frame from aps-c has reduced dof but increased the maximum useable aperture before diffraction starts softening...


PostPosted: Wed Sep 10, 2008 3:01 pm    Post subject: Reply with quote

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 ?


PostPosted: Wed Sep 10, 2008 4:35 pm    Post subject: Reply with quote

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


PostPosted: Wed Sep 10, 2008 7:55 pm    Post subject: Reply with quote

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.


PostPosted: Thu Sep 11, 2008 4:00 pm    Post subject: Reply with quote

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.


PostPosted: Thu Sep 11, 2008 4:09 pm    Post subject: Reply with quote

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 Wink


PostPosted: Thu Sep 11, 2008 4:46 pm    Post subject: Reply with quote

peterqd wrote:
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.


It's very very simple Peter. Toss away all the theory, get a large image with some progressive out of focus areas, resize it down and you'll see that some of the once out of focus areas will now seem in focus to your eyes. That's the effect of the circle of confusion depending on the size of the image (film or sensor).


PostPosted: Thu Sep 11, 2008 5:05 pm    Post subject: Reply with quote

Nesster wrote:
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.

You're right, but that's a different situation to what Cosmin asked. Smile