Bellows factor

[christopher walrath]

So, I have heard that there are like, oh I don't know, a googillion ways to figure bellows factors. How do you do it?

 

[Sw1tchFX]

For bellows comp, each 10-inches (i think, might be a foot) is a stop loss in light.

for example, if your light meter says the exposure is 2 seconds, f/32 at ISO 100, if your bellows is extended 10" (or a foot) it becomes 4 seconds, f/32 at ISO 100.

 

[christopher walrath]

But what if it's a 6 inch or a 12 inch lens?

 

[Battou]

How are you metering?


I TTL and don't have any issues about factoring. I have only had two shots fail due to underesxpoure, that was due to not reading it properly and falling short in Max long exposure time, I wanted a shot that exceeded the ability of the camera on it's own and that was my mistake.

 

[christopher walrath]

I'll be honest, I'm not doing it. Look below at my gear. Just wanting to stir up some good information in a forum that is near and dear. SO if you have a way of doing it then by all means . . .


But I will one day . . .

 

[Battou]

I need to pay a little more attention to where I am on the forum, I have gotten a little rusty on forums capible of subforums, then again my bellows can't be all that different can it?

 

[Judge Sharpe]

There is a formula- can't think of it off the top of my head some thing like the bellows length squared divided by the apeture of the lens (what the heck is a 6 inch len anyway?) to give you a factor. once you have figured it and writen it on a card for a particular lens/ camera, you can forget how to do it. Small changes in bellows length for focus would not matter.

The used to be a folk band in Birmingham, Alabama named Three on a String- They played at one of my watering holes from a past life. Any relation?

 

[Helen B]

Apart from TTL metering and marked bellows, the easiest way I know of is to use a Quick Disk.

As Christopher says, there are many ways of calculating it. I find the easiest way of remembering the general formula is to think of the effective f-number as being the distance from the rear node to the film plane divided by the pupil diameter. That may sound difficult, but it is very easy once you have the physical concept in your head. It's not correct in all cases, but it is often near enough. Retrofocus and telephoto (true telephoto, not just long focus) lenses mess the formula up because the location of the exit pupil (or the pupil magnification) is a key factor.

For a one-stop reduction the rear node will be 1.4 times the focal length from the film (ie a bellows extension of 0.4 times the focal length beyond the infinity focus position), for a two-stop reduction the the rear node will be 2 times the focal length from the film (ie a bellows extension of one focal length beyond the infinity focus position). You can use the scale on a light meter as a calculator. It's a lot harder to describe than to do it. You just need to have a clear understanding of why you are doing it

Best,
Helen

 

[Hertz van Rental]

In simple terms, f-numbers only work as marked until an object approaches to within about 10 focal lengths of the lens.
Various calculations can be used depending upon what values are known.
The simplest is based on the calculation for f-stops:
f-number = N
focal length of lens = f
diameter of iris opening (aperture) = d

N = f/d

This works when the bellows extension is about the same as the focal length of the lens.
At greater extensions:

N1 = v/d

Where N1 is the effective aperture and v = the bellows extension.
The practical rule of thumb derived from this:
(Bellows extension relative to the focal length / approx. increase exposure by)

1.5 x f = x2.25
1.75 x f = x3
2 x f = x4
2.5 x f = x6
3 x f = x9
4 x f = x16
5 x f = x25

Then you just use a ruler, measure the bellows and relate it to the focal length of the lens in use.
The table is only approximate so bracket your exposure.
TTL metering, or using a meter that works off the ground glass screen on LF, will automatically adjust for bellows.

 

[Helen B]

f-number = N
focal length of lens = f
diameter of iris opening (aperture) = d

N = f/d
...

Just a minor point: the f-number is the focal length divided by the diameter of the entrance pupil, not by the diameter of the iris opening (unless the two are equal, of course). This is a fairly important concept to understand, particularly when you are trying to comprehend zoom lens specifications.

Best,
Helen

 

[christopher walrath]

You can also get a cheat way to do this from a fantastic photographer's site. Jason Brunner of Utah (an e-friend of mine). Check it out.

http://www.jasonbrunner.com/coolstuff.html

 

[Helen B]

That looks like a less well thought out version of the Quick Disc. There's a reason for the QD to be circular rather than square.

Best,
Helen

 

[Hertz van Rental]

Just a minor point: the f-number is the focal length divided by the diameter of the entrance pupil, not by the diameter of the iris opening (unless the two are equal, of course). This is a fairly important concept to understand, particularly when you are trying to comprehend zoom lens specifications.

On a lens that is stopped down the two are basically the same when shooting LF.
What you are doing with Bellows Factor is working out what the effective aperture is as it will differ from what is indicated on the aperture ring*.
For example a marked aperture of f-8 would behave as if it were f-11 when the bellows are extended to twice the focal length of the lens, and as f-16 at an extension of 4x the focal length.
It worked well enough for me in the studio


*Note: f-numbers are a convenient way to simplify and standardise exposure calculations across lenses of different focal lengths.
f8 on a 85mm lens gives the same level of illumination on the film as f8 on a 300mm lens.
The aperture on the 300mm is wider at f8 than that aperture on the 85mm to compensate for light loss caused by the greater distance of the film from the lens. That is to say, f-numbers are constants (calculated by the formula above) that take into account the effects of the Inverse Square Law.
But f-numbers assume the focal length of the lens remains constant (for primes).
Extending the bellows more than the focal length of the lens (or the use of extension tubes) brings the Inverse Square Law back into play so that the f-numbers are no longer accurate indicators of illumination levels on the film.
Bellows Factor is nothing more than an adjustment to compensate for this effect.

 

[Helen B]

On a lens that is stopped down the two are basically the same when shooting LF.

No, they often aren't. Check it out with your LF lenses. Even if they were, I think that it is poor guidance to give a formula for f-number that is only true when the iris opening diameter equals the entrance pupil diameter. The f-number is calculated from the diameter of the entrance pupil, not from the diameter of the iris opening.

Best,
Helen

PS The reason that I gave 'bellows extension of x times the focal length beyond the infinity focus position', for example, instead of just 'bellows extension' in my discussion of effective aperture is that that accounts for an unknown location of the rear nodal point. Simply using 'bellows extension' does not. It's not a big deal for most LF lenses (except true telephotos, which a re rarely used for LF macro work), so long as you measure from the iris to the film plane; but it can be important for many MF lenses used for macro work.

 

[JC1220]

It is not so complicated for LF work, once you extend the belows past the focal length of your lens you will need to make some adjustment. In the field, typically 1 to 2 stops additional exposure is all that is needed. No need to break out the commodor64.