Originally Posted by

**Michael Mutmansky**
In a word, No.

In an idealized lens (no real lens is ideal), the aperture at f11 for a 90mm lens will be 8.1mm. It will actually be a little different on a real lens, due to the location of the aperture relative to the center of the lens. However, if you can determine what the diameter is at f11, then you can figure out an adjustment factor to apply until you get home.

Focal length divided by the 'stated' aperture will be close to equalling the diameter of the aperture if the scale is correct.

If not, take the focal length of the lens and divide by the actual diameter to get the effective f-stop. Compare that to the 'stated' aperture and determine the difference (in stops). Apply that on the aperture scale to all shots, and you'll be pretty close. If you calculate a larger aperture, then you need to stop the lens down further (higher f-stop value). If you calculate a smaller aperture (not likely, as that means the scale was from a shorter focal length), then you open up the lens (lower f-stop value).

---Michael

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