StudioPhoto

[Martin_Johansen]

Hi, I am going to the studio later today shooting two persons, wide open, with a 150 mm. I want to go closer, so I need to make the bellow longer and therefore compensate with more light. I am shooting with a sinar, bw 400 iso. My question is, if I make the bellow 300 mm long, how much light do I need yo compensate? One f-stop, two? Thanks for all help, best regard Martin

[J D Clark]

The bellows factor is calculated as: 300 squared divided by 150 squared = factor.

90000 / 22500 = 4x

Thus, two stops.

John Clark
www.johndclark.com

[Martin_Johansen]

Hey, and thank you John, i have trouble understanding this. I did the pictures, with the bellow at 150 mm, but they still got underexposed.

I managed to fix them in photoshop/scanning though -

if anyone knows any drawings or something that could explain the calculations you are telling me about, pls let me know

http://martinjohansen.no/wordpress/2...ays/217-sinar/

[E. von Hoegh]

For the pictures in your example, no bellows extension factor is neccesary. You were nowhere near 300mm bellows extension.

If you extend the bellows 300mm, using a 150mm lens on 4x5, you will be focussed on an area that is the same size as the film, and the subject will be reproduced on the film at life size.

Your underexposure is due to some other cause.