View Full Version : What F stop with one element removed?

Jorge Gasteazoro
9-Aug-2001, 18:50
Hi all, I need some help since I have never done this, if I remove the front ele ment 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 for mula or method out there? I would greatly appreciate it if you guide me on this! Cheers and good shooting to all!

N Dhananjay
9-Aug-2001, 18:58
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.

Jorge Gasteazoro
9-Aug-2001, 19:02
N thank you very much, I should have thought of that! It was a great help your response.

david clark
9-Aug-2001, 20:35
Hi Jorge,

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

10-Aug-2001, 17:38
Dagors are usually NOT convertible. Cheers,

N Dhananjay
10-Aug-2001, 17:45
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.

david clark
11-Aug-2001, 01:15
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

11-Aug-2001, 01:34
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