What F stop with one element removed?

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Hi all, I need some help since I have never done this, if I remove the front element or rear element of a lens to obtain a different focal lenght, how do I know what F stop it is? I suppose the f stop changes since thr ratio of the aperture to focal lenght has changed, but how do I calculate it? does anybody have a formula or method out there? I would greatly appreciate it if you guide me on this! Cheers and good shooting to all!

-- Jorge Gasteazoro (jorgegm@worldnet.att.net), August 09, 2001

Generally, lenses designed as convertibles owuld have these marked as two (or three) aperture scales. If this is missing, you can calculate them by calculating the actual physical size of the aperture. So if the combined lens was a 150mm at f/5.6 wide open, that means wide open, the physical size of the aperture is 150/5.6. No, if you know what your converted length is, you can calculate the f stop. For e.g., if it converts to a 240mm lens by removing the front element, then you know the physical size of the aperture wide open = 150/5.6 i.e., approximately 26.8mm. Divide your new focal length i.e., 240 by this number and you have your f stop i.e., 240/28.6 which make for approximately 8.4 i.e., the f stop is approximately f/8.4 (a little slower than f/8). Hope this hlps. Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), August 09, 2001.

N thank you very much, I should have thought of that! It was a great help your response.

-- Jorge Gasteazoro (jorgegm@worldnet.att.net), August 09, 2001.

Hi Jorge,

For Dagors, I think you multiply by scaled f stop by 1.8 . Best, David

-- david clark (doclark@yorku.ca), August 09, 2001.

Dagors are usually NOT convertible. Cheers,

-- Geoffrey Chen (DB45TEK@AOL.COM), August 10, 2001.

As the previous response states, Dagors are not really convertible. Although, they were sometimes advertised as such, the individual cells are not fully corrected for coma. As a result, the individual cells of a Dagor have to be stopped down considerably to produce a reasonable image. Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), August 10, 2001.

Hi, well if you find yourself in an unusal situation with a Dagor and find you'd like a longer focual length, remove the front cell, stop down, and multiply the f by 1.8. Cheers, David

-- david clark (doclark@yorku.ca), August 11, 2001.

I've always been quite happy with the image from the single element of my Dagors. I've always used a conversion factor of 1.7

-- Wilhelm (bmitch@home.com), August 11, 2001.