# Thread: How to Calculate Bellows Extension correction for a true Telephoto lens?

1. ## Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

Hello There - I am new to the forum. To answer your question on how to address the bellow extension as it affects exposure... this is what I came up with. The F stop number is a calculation of the diameter of the len's design. F means fraction in this case. So F8 means 1/8 of one. One being the actual size of the lens opening which would equal the focal length of the lens. The focal length being the distance between the lens and the film plane when it is focused on infinity. That means F8 is only F8 when the lens is focused on infinity. So a 380mm lens will produce a 380mm length of the bellows when focused at infinity. Thus whatever you focus on just measure the length of the lens to film plane to calculate what your "real" f stop is. That is, even if you set the lens on F8, the actual f stop is less if you focus at any distance less than infinity. Hope this makes sense. When focusing at 10 or 15 feet away the difference is negligible. But when focusing up close it is not. I posted a video on this subject some time ago. Here's a link.... https://www.youtube.com/watch?v=mpJWccGXe5E&t=1s

2. ## Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

Thank you very much TimHGuitar. Unfortunately we are talking about special design lens here.
Thanks.

3. ## Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

Iga, a lens' focal length is the distance from the lens' rear node to the film plane when the lens is focused at infinity. Telephoto lenses' rear nodes are in front of the diaphragm, not behind it, and are sometimes in front of the front of the lens.

When your 360/5.5 Tele-Xenar is focused at infinity its rear node is 360 mm from the film plane.

Extension is measured from the infinity position. The relevant magic formulas for magnification given extension and focal length and for exposure correction given magnification and pupillary magnification are in post #3 above.

Focus on the subject, measure extension, calculate magnification. Then calculate exposure correction.

4. ## Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

Focus on the subject, measure extension, calculate magnification. Then calculate exposure correction.

OK, will do. Thanks Dan !
Best wishes to all,
Igor

5. ## Re: How to Calculate Bellows Extension correction for a true Telephoto lens?

Two good things to learn this morning.
First - 360 Xenar illuminates 5x7 with a lot of movements ( rise and fall at least ),
At 3 meters it illuminates Whole Plate with almost 0 movements, but thats good news.
And as for bellows factor, may be I was close enough with my initial idea ?

https://www.shutterbug.com/content/t...g-and-short-it

Best,
Igor.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•