Bellows Length Relative to the Adjustment of f-stops

[bobwysiwyg]

I wonder if someone could explain something that, thus far, is eluding me. I've recently acquired my first view camera (Cambo 4x5) the complement of lenses consists of 135mm, 150mm and a 210mm.

I've been reading about anything I can get my hands on to learn the basics and I "think" I understand most of it. One thing that I'm still having trouble getting my arms around is the fact that I may meter a scene, but under certain circumstances (bellows length/extension) have to alter the indicated f-stop. I understand the concept of extended bellows resulting in less light to the film plane, but can't quite grasp under what circumstances this would be necessary.

I suspect most shooting will be various scenes at some distance, but I may try a portrait, say at 10 ft. or so. If someone could explain, in relation to the lens focal lengths I have, I would be very appreciative. Thanks.

[JBrunner]

You don't have to worry about bellows factor until the length of the bellows exceeds the focal length of the lens (ie with a 150mm lens, the distance between the lens and the film plane exceeds 150mm), which isn't going happen when focusing at ten feet with any of your lenses.

It will happen most often when shooting still life objects. Set up something small and fill the frame with it, and see how far your bellows has to extend to get focus, and you'll see first hand.

This may also help:

http://www.youtube.com/watch?v=vwfRA615Mx8

[rob champagne]

don't you mean 150mm from infinity focus position?

[Ed Sukach]

You don't have to worry about bellows factor until the length of the bellows exceeds the focal length of the lens (ie with a 150mm lens, the distance between the lens and the film plane exceeds 150mm), which isn't going happen when focusing at ten feet with any of your lenses.

The "f/stop" is defined as the aperture diameter (actually it has to do with aperture AREA) divided by the distance to the film plane. The optics - the lens itself, and its focal length, really has nothing to do with the f/stop. "T/stops are different; the effects of the optical system, the glasses, etc., are considered.

Most of us use "f/stops". If a 150mm lens is focused at infinity ... Whoops ... if a "simple" 150mm lens is focused at infinity (and the diaphragm is placed at the optical center of the lens), the f/stop will be as indicated. As soon as we extend the lens (diaphragm) to a position farther than 150mm, the effect is to reduce the amount of light reaching the film plane - the equation:

Distance of aperture to film plane / aperture diameter = f/stop.

Holds, in any event.

I've just checked out:

http://www.youtube.com/watch?v=vwfRA615Mx8

... And he explains it very well, actually. I haven't downloaded his "scale and target", but I've seen similar, and they work very well - accurately enough for any sane ... (???). or at least, practical purposes.

[Pinholemaster]

Simply: (bellows draw / lens focal length) squared = exposure factor.

Bellows draw is measured from the film plane to the nodal point of the lens. The nodal point is usually around where the aperture is. This is not rocket science.

So (150mm bellows draw divided by a lens of 150mm) squared equals 1. An exposure factor of 1 means no adjustment of the aperture is needed.

[johnnywalker]

So an exposure factor of 8 would be 3 f-stops?

[JBrunner]

don't you mean 150mm from infinity focus position?

Yes, of course, which would be the focal length of the lens, 150mm. The long hand version was completed by Pinholemaster, and as noted it's not rocket science, unless you want to make it so.

[Mick Fagan]

When out in the field or studio/kitchen/lounge and you wish to know your effective f/stop number I use a simple equation to do so. I require only one piece of extra equipment, but in fact, use two to do so.

The first piece of equipment is a cloth tape measure available at any sewing or haberdashery shop. The other is a small credit card sized calculator powered by a solar cell inbuilt, that I acquired at a trade show as a freebie.

Assuming here, you are using your 150mm lens, and your light meter is telling you that at your chosen shutter speed, you require f11.

Focus first, and then measure the distance from the film plane to the centre of the lens.

In this instance we assume you have decided to get a same size image on your negative of the object being focused, therefore your extension from the film plane is 300mm.

Your light meter measured f/stop was f11, therefore your effective f-number = f/11 x 300/150 = f/22

In this instance you doubled the distance, so you lost exactly two stops of light.

Doing it again at ½ life size on the negative it goes like this:- your measured f/stop number was f11, your effective f-number = f11 x 225/150 = f/16.5

In this instance you only added 50% distance, so you lost exactly one stop of light.

This method works well enough for normal lenses, like those that you have. I also find it extremely easy to use as I use my light meter to give me shutter speed - f/stop combinations, which I suspect is what most of us do.

Mick.

[Steve Sherman]

Another method to determine the amount of increased exposure you'll need.

Take the length of the lens your using and turn it into F stops, i.e. 120mm = 4" 150mm = 6", 210mm = 8 1/4", 300mm = 12"

If you have a 4" lens and when closed focused on subject matter the lens nodal to the film plane measures 5.6" then you add one stop of exposure, if it measures 8" then you would allow 2 stops of exposure, this is also known as "life size". In other words the size of the actual subject is recreated on the film plane as the exact size that it is in real life.

A 300mm lens measures 16" when close focused you would correlate to F11 to F 16, therefore add one stop of exposure, 19" when focused add 1.5 stops of exposure.

To those who suggest that a 12" lens does not exactly correlate to F 11. For years I used this system in a studio setting shooting nearly life size most of the time using Ektachrome film which has little tolerance for exposure error.

The beauty is you will not have to search for anything, F stop scale should be in your head to draw reference from.

Cheers

[eddym]

Simply: (bellows draw / lens focal length) squared = exposure factor.

That's the method I use. It's very easy if you carry a pocket calculator in your camera bag. Actually, it's even easier with a slide rule!