aperture scale

[allsystemsfail]

Hello, I have a caltar 240 F5.6 in a copal 3, and just got some nikon 450 F9. cells. My question is if I put the 450 cells into the 5.6 copal 3 will the aperture scale be off? I know putting them in that shutter dose not make it a 5.6 lens, but when I stop down to F22 - F 45 will they be off? If so how do you figure out by how much? Thanks for any help.

Rich

 

[Nick Zentena]

You can use math to get a less then perfect but likely good enough result.

240/22= 10.9

450/Fstop=10.9

So 450/10.9=Fstop or

about F/44 if I didn't screw it up.

Or you could take out a ruler etc and measure the size the aperture looks like when you are examining the assembled lens. Then plug that size figure into the formula.

Neither is 100% perfect but both can be done at home.

 

[allsystemsfail]

Thanks Nick,
So if I want to shoot at 22 I would set the lens at 45ish. I did the math for 32 and 45 and it seems to be 2 stops. I guess due to the almost double the focal lenth. If that works it will be easy to remember while shooting. I'll try it and see what I get.
Rich

 

[jacobus]

For a 240mm lens the F5.6 mark means an entrance pupil (or aperture) of 42.82mm ( 240 : 5.6). For a 450mm lens the same entrance pupil produces a relative aperture of approx. F11 ( 450 : 42,85 = 10.5). Just use the scale and remember that it's 2 stops off because your Nikon lens is almost twice as long as your Caltar.

 

[Mark Sawyer]

Remember that the aperture size should be measured through the front element, so what may be a 5mm opening through one lens may not be a 5mm opening with another lens, even though the diaphragm may be set in the same place...

You could also measure the aperture and divide it into however much bellows extension you have at the time, eliminating the need to figure the bellows extension factor. (But remember the reciprocity failure...)