[disord3r]

This question relates more to the mount than any specific DSLR, and relates to the aperture lever.

Is there a formula that I can use for approximating the f number of a given lens based on how far the aperture lever is moved? Different lenses have different minimum and maximum ratios, so that would obviously have to factor in the formula, but I'm attempting to the approximate the f number based on a percentage. For example, if I have an adapter which has aperture control, the movement of the adapter's aperture ring is considerably small. If I move it, say, 50% of the way, I want to be able to guestimate what the f number is for a lens that has a maximum of 1.4 and minimum of 22, a maximum of 2 and a minmum of 22, etc. It's not going to be the same for both but I know the number is not linear.

Does such a formula exist, or is there a better rule of thumb to use? I realize this is fairly inconsequential since the body will still meter properly.

[dms]

Lenses used to have a very large distance between the f-stops at the full open end (fmin), becoming successively smaller towards the most closed end (fmax). Now they are generally equally spaced. For one that is equally spaced, it likely is:

percent travel = L/Lmax *100% = (log f - log fmin)/(log fmax - log fmin) * 100%

BTW my basis is: travel distance L is proportional to log of f-stop, and then I used a linear interpolation formula for L going from L=0 to L=Lmax

---------- Post added 05-31-17 at 09:29 AM ----------

If you rather not use logarithms you could number the f-stops starting with 0. E.g., if it goes from f/2.8 to f/22:
f/2.8 --> N=0, f/4 --> N=1, f/5.6 --> N=2, f/8 --> N=3, f/11 --> N=4, f/16 --> N=5, f/22 --> N=6
and replace the log f/stop by N in the equation.

[Thagomizer]

Interesting question, and one I've been wondering about as well. Mounting a lens without an aperture ring on extension tubes without contacts results in wide open operation only. If the aperture lever is fixed to a given position by introducing something to keep it from moving, this would allow shooting at the aperture it's fixed at? Assuming that one is careful with the object used and its material, will doing this with the aperture lever result in damage to either the lens or the camera?

[disord3r]

I'll play around with that and see if that correlates with the metering. Thanks!

---------- Post added 05-31-17 at 01:48 PM ----------

Assuming that one is careful with the object used and its material, will doing this with the aperture lever result in damage to either the lens or the camera?

I would think you could get away with that if it's something minimal (tape?) because there's really nothing there to interfere with the aperture lever except the inside of the adapter. I'd be more concerned with the object leaving a residue on the lever.