bellows extension factor - getting the hang of it - any suggestions : )!!! ??

[Mark Sawyer]

But here's the real question... if I use a 200 mm lens focused at a bellows extension of 140mm, do I gain 1 stop? ;-) (ignore that whatever I photograph won't be in focus)

Yes. You've concentrated the same amount of light in a smaller area, regardless of focus.

[Kiwi7475]

Yes. You've concentrated the same amount of light in a smaller area, regardless of focus.

So Tele lenses, which require less extension than the focal length to focus at infinity, get a light advantage (“are faster”) than the theoretical equivalent non tele design (meaning for a constant diameter of the entrance pupil)?

[Mark Sawyer]

So Tele lenses, which require less extension than the focal length to focus at infinity, get a light advantage (“are faster”) than the theoretical equivalent non tele design (meaning for a constant diameter of the entrance pupil)?

Remember that calculating the f/stop means measuring from the rear nodal point of the lens to the focal point. On telephoto lenses, the rear nodal point is in front of the lens, while on conventional designs it's within the lens, so it's a wash. No advantage or disadvantage.

[Kiwi7475]

Remember that calculating the f/stop means measuring from the rear nodal point of the lens to the focal point. On telephoto lenses, the rear nodal point is in front of the lens, while on conventional designs it's within the lens, so it's a wash. No advantage or disadvantage.

Makes sense — there’s no free lunch I guess. Thanks!

[rodney@theloughroad.com]

Hope this is ok to ask here but…

Does anyone remember this thing?
The Quick Stick - For Large Format Bellows Camera Exposure Calculations