What is Bellows Factor about.

[Grandpa Ron]

I have restored a 4x5 view camera. If I base the lens on a diagonal measure of 6.4 inches I would need a 162.5 mm lens.

I have a 128 mm lens that would be a focal length of 5 inches, a bit of a wider angle lens.

If my light meter indicates 1/100 sec. at f 16, do I need to compensate for the shorter bellow length?

I am not sure what bellows length factor means.

 

[Ysarex]

I have restored a 4x5 view camera. If I base the lens on a diagonal measure of 6.4 inches I would need a 162.5 mm lens.

I have a 128 mm lens that would be a focal length of 5 inches, a bit of a wider angle lens.

If my light meter indicates 1/100 sec. at f 16, do I need to compensate for the shorter bellow length?

I am not sure what bellows length factor means.

No compensation needed in that case.

Bellows factor is required when you start doing close-up work and you extend the bellows. Think of it this way: You have a 150mm lens on your camera and you set the lens to f/8. That f/stop value which effects exposure is in fact focal length divided by the diameter of the aperture which in this case would be 18.75mm. As you focus on subjects closer and closer to the camera you will extend the bellows moving the lens away from the film. You are in effect increasing the lens focal length. If you extend the bellows so that the lens is 200mm from the film and the lens is still set to f/8 does the aperture change to compensate? No it doesn't. So 200/18.75 = 10.6. You have the lens set for f/8 but because of the extension of the bellows your exposure is occurring at an effective f/10.6. You need to make a compensation.

Bellows factor will be directly correlated to magnification. At a magnification of 1/2 life size you'll need to open the lens 1 stop more than the metered exposure. At full life size you'll need to open the lens 2 stops more than the metered exposure, etc. So the easiest way to measure bellows factor is to measure magnification which you can do on the ground glass when you focus.

Joe

P.S. This holds for all lenses on all cameras. The reason you don't hear too many small hand camera users talking about it is because the camera's internal light meter compensates. Back in the day when not all cameras had internal light meters they actually built the compensation into the lens. If you ever get your hand on an old 55mm f/3.5 micro Nikor lens set the f/stop to a mid value like 5.6 or 8 and then watch through the lens as you turn the focus ring -- the aperture will begin to open up.

 

[compur]

You only need to worry about bellows factor when doing close-up photography which means when the bellows is extended to a length that is longer than the focal length of the lens.

 

[Tim Tucker 2]

As @Ysarex says, bellows extension is linked to magnification and not focal length. Though I use a different calculation it is essentially the same.

The f-stop system already compensates for focal length so you only need to compensate when you *extend* the bellows and make a *significant* change to the focal length, the distance between the lens and the film. The f-stops marked on the lens relate to the focal length of 128mm

It becomes significant when the distance between the lens and the subject is *less* than 8 x the focal length. In the case of a 128mm lens then that is when your subject in focus is less than 1m from the lens simply because you have to extend the bellows by such an amount to achieve focus that you essentially alter the focal length enough to noticeably effect exposure, and therefore have to re-calculate the aperture diameter/focal length to achieve accurate exposure.

I use the formula:

Extension factor = Bellows extension squared/focal length squared

If a lens is focussed at infinity then with the 128mm lens it will be 128mm from the film.

So 128mm squared/128mm squared = 1

Indicated exposure x1

At 1:1 magnification you will be doubling the focal length, the lens will be 256mm from the film.

So 256 squared/128mm squared = 4

Indicated exposure x4 (two stops, 1/100 second becomes 4/100 or 1/25 second).

As indicated it is the same calculation as above but it can be done simply by measuring the distance between the lens board and the film plane.

BTW, the compensating Nikkor 55/3.5s are the ones with the old style scalloped focus rings, in case any of you were rushing to check...