Question about Aperture Scale Compatibility

[robphoto]

My only suggestion, if you want to experiment before you spend money, is to use the ratio of the focal lengths to make yourself a conversion scale.

Because the f:stop is the ratio of the focal length to the aperture diameter, if both lenses fit into the same shutter (an unlikely if), you can make a conversion scale.

For a simple example, if the scale is for a 90mm lens, and I mount a 180mm in the same shutter, the f:stops are proportional to the focal length, so:

90mm f:8 ---- for 180mm lens in same shutter, that would be f:16 (8 x 180/90)

note this is if both are mounted in the same shutter: you can't convert between different shutters

in your case, if you mounted the 400mm lens into the shutter that had held the 150:

150mm f:8 ----- 8 x 400/150 = 21.3, roughly f:22, or three stops different.

Another way to get a real rough scale would be to measure the maximum aperture opening, mark it on a tape scale on the shutter, figure out what that aperture number would be (focal length / diameter), stop down to half that diameter (two stops different) etc., and fill in the intermediate points by eye. This is very rough and ready, but would be fairly close. The advantage is you get to try out the lens before investing money in it.

(edit: I see the above poster saying that what you measure isn't the actual aperture circle, but the apparent size of the aperture looking through the front element, otherwise this should be correct)

[dahlen59]

Ok if you want to spend money. My way to do it was to unscrew the scale flip it around an write on the back. To get the right meusure you have to unscrew the front element and measure the iris. Easy in the beginning but as the f stop gets smaller it is more problematic. I calculated the diameter for ex f 11 and then adjusted the aparture to that diameter. Different tubes and pencils was handy.

[maltfalc]

Ok if you want to spend money. My way to do it was to unscrew the scale flip it around an write on the back. To get the right meusure you have to unscrew the front element and measure the iris. Easy in the beginning but as the f stop gets smaller it is more problematic. I calculated the diameter for ex f 11 and then adjusted the aparture to that diameter. Different tubes and pencils was handy.

no. read the previous replies.