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poco
19-Sep-2007, 13:00
The shutter on my 90mm broke and I'm thinking of temporarily swapping it for the shutter on my 72mm, which I don't use much. I hang my head in shame, but could someone tell me how much to adjust exposure by if I keep the 72mm aperture scale as is?
Bob Gentile
19-Sep-2007, 14:16
All other things being equal, the 90mm should need "about" 1.5 times as much light in the 72mm lens's shutter.

f/8 on a 72mm lens corresponds to a 9mm diameter aperture. The area of that opening is 63.6mm. f/8 for the 90mm corresponds to an 11.25mm diameter aperture with an area of 99.4mm. 99.4/63.6 is "around" 1.5.

f/4 on the 72mm is an 18mm diameter and a 254mm area. f/4 on the 90mm requires a 22.5mm diameter with an area of 397mm. And 397/254 is "around" 1.5.

f/16 on the 72mm is a 4.5mm diameter aperture with a 15.9mm area. The 90mm lens needs a 5.6mm diameter with an area of 24.8mm. 24.8/15.9 is "around" 1.5.
erie patsellis
19-Sep-2007, 14:39
the above disregards the "apparent size" issue. Easiest way is to stick on a piece of label material, calculate the required apertures, close the iris til you get the size, measured held at about arms length (with a caliper) and make a mark on the label, once you have all the marks, label them and go shooting. Sounds harder than it is, really, takes me about 10 mins to label one temporarily.


erie
John Kasaian
19-Sep-2007, 15:15
Why not swap scales while you're at it? Assuming that the shutters are identical models of course.
Jim Galli
19-Sep-2007, 15:36
open 1/2 stop. Fix the other shutter.
poco
19-Sep-2007, 15:46
Thanks, guys. I'd guessed 1/3 stop difference so I was close.

"Why not swap scales while you're at it?"

Because I've already managed to break one shutter and don't need to break two :D
Michael Graves
20-Sep-2007, 05:23
I did that for a long while with an 203 7.7 Ektar mounted in a shutter marked f4.5. I took the easy way out. Just did a film test with that lens and used a different film speed so that the aperture scale "read" the way I was used to. Can't really predict DOF that way, but I never learned how to do that anyway. That's what the ground glass is for.