DOF Scale use?

[Josh Z.]

I'm trying to use the DOF card that is described on this site (http://www.largeformatphotography.info/dofknob/). My question is how to use the DOF card (not knob). I'm not sure how to use it. The table listed there is for N=45, what does this mean? Is it for f/45, or for a 45mm lens?

Thanks.

[Josh Z.]

Ah... I think I have figured it out. The delta described for the card is from reading from f/0 to f/45 on the printed paper, should messure 8.1mm for the 4x5 example. I confused my self in thinking that I would need a card per lens.

From what I've been trying out in the house (dark out), I think the card may be a little small to actually use in the field, but from learning how this card works, I have a much better understanding on how to obtain optimum DOF.

[Leonard Evens]

You are right. The card tells you which f-number to choose depending on the focus spread in mm. It is completely independent of focal length. Focal length does enter indirectly because focus spread will depend generally on focal length, so if you focus at the same subject distance but use a shorter focal length lens, the focus spread will be less. That will lead you to use a smaller f-number, i.e. a larger apreture.

For close-ups, however, you should correct by dividing the recommendation by one plus the magnification. Close-up range is usually defined to be within ten times the focal length. One may also obtain the factor one plus the magnification by measuring the bellows extension and dividing that by the focal length. If you don't apply this factor, it usually won't hurt, but you will end up using a smaller aperture than need be. This will only be an issue if diffraction is beginning to affect the image.