Exposure adjustment with Bellows

[JayRmichigan]

Hey, I picked up a bellows extension recently, and I'm wondering if anyone knows if there's a general rule for how much you should compensate for the decrease in light?

Thanks,

 

[payasam]

There is a rule all right. I think it's related to what is called the inverse square law. You should find it if you search through Google for "bellows" or "extension tubes". I can't help because I gave away all my photography books.

 

[benmacphoto]

I Know for 4x5 8" is 1 stop. Not sure about sure about smaller formats though.

 

[payasam]

Related to magnification, isn't it? For a 50mm lens, 50 mm extension is 1:1, or two stops. And so on. Damn all weak memories and bless all TTL meters.

 

[Kristopher]

Benmac photo is partially right but only if you use a 150mm 6 inches lens.
And payasam is right.
As a rule of thumb, mesure the extension in inches and compare it to the infinity extension in inches. (mesure from the lens aperture blades to the film plane except for some telephoto desings) then consider these as being f stops. Hence a 4 inches lens with a 8 inches extension is + 2 stops. (f4 -> f5.6 -> f8) 4 inches at 6 inches +1 stop. etc.

For any lens, 1:1 magnification gives you two stops to compensate. because your lens focal point will be exacly midway between your subject and the film plane.
You should not get more than 2 stops except if you go past real life, then the same rule applies.

There is plenty of different ways of calculating this, but I found this one to be the most easy to apply.

Hope it will help,

Kris

 

[Chris101]

The distance the lens is from the film squared divided by the square of the focal length is called the "extension factor". That is how many times more light (f-stop or time) must be let through. For example, if you have a 60mm lens, and extend it to 80mm from the film, then 80^2/60^2 = 1.78. That is about 3/4 of a stop increase. (1.0 is normal exposure.)