15" and 10" TeleOptar

[jeffrey_abelson]

I have 10 inch and 15 inch TeleOptars - one made by Graflex and one

made by Wollensak for Graflex - why would the 15" inch lens and

Shutter be so much larger? The 15" is a real beast while the 10" is

quite small - both are 5.6. I know there's a difference between 250mm

and 380mm, but this difference seems considerable. BTW - the newer

15" is coated and Alphax shutter while the 10inch is uncoated in a

graphlex shutter.

[pvp]

Simple: they are the same maximum aperture of f/5.6. The f/stop is a ratio of the focal length over the diameter of the aperture. Longer focal length means larger diameter to achieve the same stop.