Calculating Optimal F-stop for 8x10

[G Benaim]

Hi all,

When using 4x5, I used the defocus distance method for choosing optimal f-stop. Now that I'm using 8x10 for contacts, I've been using the same values, though I know I can be more liberal. Does anyone have a chart listing f stop values for 8x10 to use the defocus method? I'm often in need of more than the 10mm listed in the LFF article, and don't really know how much dof I'm getting beyond f64. Thanks,

GB

[Leonard Evens]

I believe you are using Hanma's method which tries to give you the best result, combining defocus and diffraction at the near and far points, If you use Hansma's method, then the result is independent of format. So you would use his tables unaltered. However, for 8 x 10, for the same angle of view, the focal length for 8 x 10 would be twice that for 4 x 5. As a result, the focus spread would be much larger for the same angle of view and same near and far points, so you would end up with a much larger focus spread. You have already noticed this.

Hansma's formula tells you to take the square root of the the product of 375 with the focus spread to get the recommended f-number. So for a focus spread of 10 mm, you would take the square root of 3750, which is about 61.24. You can extend Hansma's table with a scientific calculator and carry the extended table with you. Here are some additional values.

12 mm f/67.1

14 mm f/72.5

16 mm f/77.5

18 mm f/82.2

20 mm f/86.6



I don't do 8 x 10, but I suspect that Hansma's method is likely to suggest apertures much smaller than are actually needed. That is because since less enlargement is needed, you are less likely to get in the range where diffraction significantly affects the result.

Using only defocus, the rule should be the following. Divide the focus spread by twice the coc. A typical coc for 4 x 5 would be about 0.1 mm, and one for 8 x 10 would be about 0.2 mm, since generally prints only need half the enlargement. Thus, if you had a focus spread of 10 mm, you would divide that by twice 0.2 or 0.4 to get 10/0.4 = 25. That is about a third of a stop beyond f/22. But you might want to stop down to f/32 instead, just to be sure. (As noted above, the focus spread would be four times as large for the same near and far points in the scene, and the net result would actually be that you would end up using an f-number twice the one appropriate for 4 x 5.)

This rule applies for distant objects. For close-ups, you can also divide by the ratio of the bellows extenstion to the focal length, which would give you a larger aperture.

[G Benaim]

Thanks Leonard, that's exactly what I needed.