View Full Version : Kowa 240 f9 and Computar 270 f9 in same Copal 3s

David Kashuba
13-Feb-2004, 15:24
"I have a 210 G Claron in a Copal 1 shutter. I also have a 240 G Claron which I would like to mount in the same shutter. If I do this can I use the same aperture scale? i.e. do the apertures correspond in size between the two lenses? Or, do I have to get a scale specifically for the 240? Many thanks as always........ --Stephen Vaughan, 2004-01-14 01:09 AM

You can do what you wish. There is a correction factor of 1/3 of a stop. The 240 len is 1/3 of a stop more closed down than indicated. ie, When using the 240 in this instance, if the iris says F11, the actual stop is F12.5. Having even more fun, the equal focal length 1/3 stop increments are.... 210mm, 240, 270, 305. Sound familiar? --Frank Filippone, 2004-01-14 04:35:36 "

Can I assume that my Kowa 240 will be 1/3 more open than the Computar 270 scale reads? Thanks - David Kashuba.

CP Goerz
13-Feb-2004, 20:32
I would just give the iris lever a wee nudge more to the open end when you make your final setting. I'm sure there is a formula somewhere for the exposure difference for the additional 30mm its not going to make that big a deal though, just a nudge L do it.

CP Goerz.

Jim Galli
14-Feb-2004, 12:07
You're on the right track. Easy to get confused and nudge the wrong way though. It may be helpful to think of f10. If your scale is for a 240, f10 in theory is 24mm. For your 270 you need 27mm. See where I'm going. 1/3 stop is correct. This isn't to try to actually measure things it's just a helpful hint for me because I'm always thinking "which way do I nudge?" You could make up a whole "casket set" for that 3S because 240, 270, 305, and 360 would all fit and they're all 1/3 stop apart from eact other. After you've ruined enough film and scratched enough elements you'll get shutters with scales for each and this discussion will become moot.