Working out the camera's Depth of field

kristelt said:

Hello there, Hope everyones keeping well. Not sure where the best place to add this post, so feel free to move to a more relevant section.
Im stuck on perhaps the basics and would be grateful for advice.

I'm trying to work out the depth of field/f numbers on my camera. Its a box camera with a paper negative and I'm using a lens with it.
When on f8 the depth of field is remarkably shallow-more like that of an f2 that im used to. A friend told me a calculation for the f number is
based on the focal length divided by the f stop...but it doesn't seem right when I try these calculations?

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BradS said:

f-stop is defined as focal length divided by the diameter of the entrance pupil.
Do you know the focal length of the lens?
Look into the from of the lens and measure the diameter of the aperture...it's difficult but an approximation is "good enough". Anyway, then divide focal length by this measured diameter....the result of this calculation is f-stop.

Depth of field is more complex. It involves focal length and aperture and human perception and all manner of fudge factors.

I suggest that you google "depth of field calculator"... there are specialized on line calculators...

also curious what camera you have.

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kristelt said:

Thanks for your reply. Yes Im aware of the focal ratio...just not sure how to calculate the depth field, as it seems it changes when the focal length does and its a separate calculation.
I'll check out the online calculators...
I've got an afghan inspired box camera made out of ply... I've attached a few pics-great fun and the kitten agrees!

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Donald Qualls said:

A couple useful generalities about DOF, however, are that the longer the focal length the shallower DOF will be at the same f stop (i.e. f/8 on a 200 mm will have less DOF, other factors equal, than f/8 on a 50 mm), and the smaller the negative the shallower the DOF (because you'll magnify the image more for viewing). These tend to offset each other to some extent, in that a normal lens for 35mm film is a much shorter focal length than a normal for 4x5.

Your box camera with paper negative is probably at least 4x5 (or 5x4, since you're in a British-derived area), so you need to be careful that you're not getting too close to the negative when examining sharpness -- that is, view from a "normal" distance for the image size. For 4x5 normal reading distance (around 300-400 mm) is about right, while for 8x10 longer than arm's length (perhaps a meter) is more common.

Another factor is the lens in use -- simple lenses often have significant spheric aberration and field curvature, meaning you may not be seeing actual lack of DOF so much as blur from other sources than defocus.

That said, simple cameras routinely have f/11 to f/13, and even as small as f/16 apertures, for good reason. See if stopping down another stop or so helps...

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BradS said:

Very cool!
It looks like you're using a lens in shutter so, presumably focal length and f-stop are well known.
The on line calculators will do the rest.

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RalphLambrecht said:

it's relatively simple math:

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Ian Grant said:

Rather than doing the calculation yourself there are web pages that will do the calculation for you, you just input the Focal Length of the lens and the distance you are focussing on assuming tit's not a fixed focus lens.

Ian

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its an 8x10 paper negative

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