# Thread: Minimum focusing distance with Fotoman 45PS and Nikon Nikkor 90 SW f8 ...

1. ## Minimum focusing distance with Fotoman 45PS and Nikon Nikkor 90 SW f8 ...

Hello! I am trying to figure out what would be the minimum focusing distance with this combination of camera, helical focusing system and the Nikkor 90 sw f.8 lens.
I appreciate any sort of help!!!
best regards and thanks to anyone who answers! pietro

2. ## Re: Minimum focusing distance with Fotoman 45PS and Nikon Nikkor 90 SW f8 ...

If you already have the kit—why not just screw it all the way out and see?

What is your application. What are you trying to accomplish?

3. ## Re: Minimum focusing distance with Fotoman 45PS and Nikon Nikkor 90 SW f8 ...

Here you go:

1/F = 1/S + 1/B

F is focal length. S is subject distance. B is bellows draw. Since you want to know minimum focus distance, B is the maximum bellows draw of your camera.

4. ## Re: Minimum focusing distance with Fotoman 45PS and Nikon Nikkor 90 SW f8 ...

I really wanted to do the algebra on this but just couldn't. Guess my mind isn't so agile. So I took the inverse.

1/90mm = .01111111111111111111
1/135mm = .0074074074074074074 Assume a bellows draw of 5.3 inches or halfway to 1:1.
1/S = .0037037037037037
S = 270mm or about 10.5 inches

Does this sound right? Then 1:1 subject distance is about 7 inches.

Bear with me and lets try another camera. Say a 360mm lens. Bellows draw of 500mm (about 19".)

1/360 = 1/S + 1/500
.0028 = 1/S + .002
.0008 = 1/S
S = 1250mm or 49 inches

I guess that sounds about right. Thanks for the formula, Christopher.

6. ## Re: Minimum focusing distance with Fotoman 45PS and Nikon Nikkor 90 SW f8 ...

Originally Posted by Mark MacKenzie
S = 270mm or about 10.5 inches

Does this sound right? Then 1:1 subject distance is about 7 inches.

I guess that sounds about right. Thanks for the formula, Christopher.
Yup that sounds right. Solved for S, that equation is S=BF/B-F

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