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Old Dec 9, 2003, 5:31 PM   #1
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Default Focal lenght multiplier and deep of field

When I use a lens with an APS format DSRL (Canon DRebel, D10, ...) I have to apply a multiplier to the focal lenght because the size of the sensor is smaller than the size of the film. With a 100 mm foclal lens, for example, I get an imagine like a 150 mm. But the deep of field that I get is the same of that I get with the lens on the full frame or is smaller, like that I get with a 150 mm? I think that with an APS frame I get a small deep of field (like a 150 mm) because I need to enlarge the imagine, to print maybe an 8x10, more than a full frame imagine. I am right?
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Old Dec 9, 2003, 7:06 PM   #2
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Sorry, you got to the wrong conclusion. Depth of field depends on the size of the circle of confussion and on the focal length of the lens - the real focal length, not the 35mm equiv.

You are correct in saying that when the size of the film/sensor is smaller, the circle of confusion must also be smaller in the same ratio to get the same resolution in the same size print - if the photo is taken with the same focal length lens. However the focal lenght of the lens is much smaller and the depth of field is reduced (roughly) by the square of the ratio of focal length.

So if you reduce the size of the sensor by half, the focal length will be reduced by half to keep the same field of view. That will roughly double the depth of field. In many consumer digicams, the ratio is often more like a factor of five.

You can find details at many sites: search for "circle of confusion" (with quotes) and you will find all kinds of explainations. On is at http://www.outsight.com/hyperfocal.html
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Old Dec 9, 2003, 7:14 PM   #3
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Here is a handy online Depth of Field Calculator I found a while back. It supports popular film and digital camera models, and calculates DOF using true (versus equivalent) focal length, aperture and distance to subject.

http://dfleming.ameranet.com/dofjs.html
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Old Dec 9, 2003, 8:00 PM   #4
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IMO Bill is right, the DOF is based on the true focal lenght since the lens always projects the same image over the entire area of the 35mm frame. The APS sized sensor only crops this area down and should be called as so: ie the 1.6x "multiplier" just muddle up things... The DOF is the same as on a full-frame dSLR (or film) :?

BTW the 1/focal lenght rule of thumb applies here for shutter speed as well, ie it's just a crop (do not mutiply the focal lenght by 1.6x)!

Pretty cool hey? 8)
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Old Dec 10, 2003, 5:05 PM   #5
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Thanks for your explanations.
I found very useful the Jimc's calculator.
However the conclusion is that because the circle of confusion (that is a conventional size dependent from human eye resolution) is smaller the depth of field for an APS DSLR with a given focal lenght is smaller than a 35 mm frame DSLR or film SLR at the same aperture. You can get this from the Depth of Field calulator.
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Old Dec 10, 2003, 6:17 PM   #6
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Very true, however you are not comparing the same picture. The one taken with the APS will have a smaller field of view. If you crop the center of the 35mm photo, and recalculate the smaller Circle of Confusion needed to enlarge it to the same size, you will find the depth of field to be identical.

Alternately, put a lens on the APS camera that gives the same field of view as the 35mm lens - a shorter lens. Do the calculations and you will find that the APS shorter lens has a greater depth of field even given a smaller Circle of Confusion to account for the greater enlargment needed.
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Old Dec 10, 2003, 8:15 PM   #7
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BillDrew is correct again!

... Think about it this way: load an APS film inside a 35mm SLR if you can. What do you get? The projected image from the lens onto the film box of the 35mm is still the same, but the APS film only records the cropped portion. -> ditto with APS sized CCD/sensor (it just look like the focal lenght have been "magnified", but not really. Same with DOF, the lens/camera hasn't changed: you're just capturing less of the image with the same DOF! :idea:
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Old Dec 11, 2003, 1:00 PM   #8
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Well this new lenses release will clear it up (if you can read french!)
http://www.photim.com/Infos/UneInfo.asp?N=936

They talked about the E1 with the smaller sensor and smaller lens designed for digital... and where FrancescoC can now stop pondering, since this 4/3 format actually has a smaller circle of confusion and illustrate his point quite well!
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Old Dec 11, 2003, 4:50 PM   #9
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Quote:
Originally Posted by BillDrew
Very true, however you are not comparing the same picture. The one taken with the APS will have a smaller field of view. If you crop the center of the 35mm photo, and recalculate the smaller Circle of Confusion needed to enlarge it to the same size, you will find the depth of field to be identical.

Alternately, put a lens on the APS camera that gives the same field of view as the 35mm lens - a shorter lens. Do the calculations and you will find that the APS shorter lens has a greater depth of field even given a smaller Circle of Confusion to account for the greater enlargment needed.
BillDrew maybe that I explained badly my thought, but that is what I think.
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Old Dec 11, 2003, 4:57 PM   #10
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Quote:
Originally Posted by NHL
BillDrew is correct again!

... Think about it this way: load an APS film inside a 35mm SLR if you can. What do you get? The projected image from the lens onto the film box of the 35mm is still the same, but the APS film only records the cropped portion. -> ditto with APS sized CCD/sensor (it just look like the focal lenght have been "magnified", but not really. Same with DOF, the lens/camera hasn't changed: you're just capturing less of the image with the same DOF! :idea:
NHL:
DOF is a relative concept and depends from the size of the confusion circle. This size is conventional and depends from the resolution of human eyes. If I want print at the same size a image, got with the same focal length, from a 35 mm and a APS, I MUST enlarge the APS image more than the 35 mm. Obviously the 35 mm image has a larger field of view than the APS. This means that, in the print the subject is smaller and the DOF is larger. If I want that the field of view is the same I need to enlarge the 355 more, like the APS, and I get the same DOF.
Thanks for the link: I can read French.
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