Calculation question bellows

[/dev/null]

Hi,

Since I am having troubles with 4x5" (underexposed negatives) I was checking at apug.org and found the 'bellows sticky'.

Just to make sure I understand, this is my situation:

- Linhof Kardan ST-E 4x5"
- 210mm (21cm)
- Portrait (see attached image, I place the model quite near the lens)
- shoot at f11.



Do I measure the bellows from start bellows to film back or do I include the lens too?
And usually I tip the front lens panel downwards a bit, do I need to correct this too?

So according to the calculations this gives me following if I pull out the bellows about 60cm:

(60cmx60cm) / 210mm (=21cmx21cm) = 8.16 (that is 3 stops)

This seems to be correct, as I usually just added 2 stops and my negatives were underexposed a bit (the attached image was 3 stops added, but still had to correct a little bit).

Appreciate the help.

 

[polyglot]

It's much easier to work the computation from the magnification, because that doesn't require you to know where the centre of the lens is. In this case, the subject is about 12" high and therefore the magnification is 5/12=0.42. The correction factor is E=(M+1)^2 or 2x, which is basically exactly one stop.

I struggle to believe that your bellows was at 60cm, you must be measuring something wrong. A 210mm lens will hit 1x magnification at a total bellows extension of 42cm. At 60cm, you're going to be at about 2x and therefore seeing only 2x2.5" of subject!

If you added 2 stops for bellows factor and that wasn't enough in this case, then you have some other problem causing the underexposure. Maybe a bad aperture scale, damaged/inaccurate shutter or bad light meter?

Tilting the lens does reduce the quantity of light slightly (e.g. cos^4 falloff), but not enough that you'll really have a problem on B&W negatives as long as you remain within the lens' coverage circle.

If you insist on measuring bellows, start by focusing at infinity and measure the bellows then. That length is zero-extension, i.e. the lens is one focal length from the film plane. If you then focus closer, you can re-measure the bellows and the additional length that you measure will be the extension.

PS: that photo doesn't look underexposed to me!

 

[/dev/null]

Thanks, polyglot.

I guess you are right, 60cm seems a bit much (I recalculated back), I was planning to do a test the weekend using the calculation I read here and stick with that first. If I still have underexposure issues my problem lies probably somewhere else.

If you insist on measuring bellows, start by focusing at infinity and measure the bellows then. That length is zero-extension, i.e. the lens is one focal length from the film plane

Let me have a look at that sticky again and see how to calculate this way and see what works best for me.

 

[adelorenzo]

I just use a Quick Disc.

Jason Brunner had an excellent (and entertaining) video about it but he appears to have disappeared from the Internet.

 

[shutterfinger]

Measure from the shutter/aperture blades, any lens, barrel or shuttered, to the film plane.

Infinity focus will be close to the focal length of the lens from the film plane. Any extension 1/3 stop or more needs exposure compensation. Divide the lens focal length by 3, add that number to the infinity distance to find the add 1/3 stop point. Divide the focal length by 2 and add to the infinity distance the get the 1/2 stop point. 2X focal length from the film plane add 1 stop. Not as precise as using the formula but will be within 1/3 stop of correct.

 

[Doremus Scudder]

Measure from the center of the lens to the ground glass.
Forget the calculations. Make yourself a table that you can quickly consult. If memory serves correctly, I posted my tables in the "bellows sticky" that you mention. You can just download them and have done with it.

Yes, magnification works too, but I find it a lot easier to measure and check the table.

Best,

Doremus

 

[BradS]

it is really not complicated. Simply focus on your subject and measure from the lens board to the ground glass. When focused on a subject far away (at infinity), this distance will be approximately the same as the focal length engraved on the lens. So

bellows factor = ((distance from lens board to GG) / (focal length))2

that factor multiplies the shutter speed...like a filter factor. If you want it in stops:

stops = log(factor) / log(2)

Make sense?


example:
focal length: 210mm
bellows length when focused on close subject: 260mm

factor = (260.0 / 210.0)2 = 1.533288
stops = log(1.53288) / log(2) = 0.6 stops


polyglot's answer is also correct...and a little algebra shows it is entirely equivalent.

 

[Mainecoonmaniac]

I not smart enough to use formulas. I use the amount of magnification on my ground glass. Ralph Lambrecht's book Way Beyond Monochrome has a template for such a tool. All you have to do is place the target in your shot, then measure the target with the ruler on the ground glass to see how much compensation you need for the correct exposure. I often shoot 8x10 that requires 1 stop compensation. I that's the case, I'll half the ASA on my meter so I don't have to do any brain gymnastics.

He has excerpts from his book with the template here.
http://www.waybeyondmonochrome.com/WBM2/Library_files/TemplatesEd2.pdf

I suggest buying the book because it's a wealth of information.

Cheers!

 

[BradS]

Measure from the shutter/aperture blades, any lens, barrel or shuttered, to the film plane.

Infinity focus will be close to the focal length of the lens from the film plane. Any extension 1/3 stop or more needs exposure compensation. Divide the lens focal length by 3, add that number to the infinity distance to find the add 1/3 stop point. Divide the focal length by 2 and add to the infinity distance the get the 1/2 stop point. 2X focal length from the film plane add 1 stop. Not as precise as using the formula but will be within 1/3 stop of correct.

This is totally wrong. For example...
When bellows is 2*f, bellows factor = (2)2 = 4 ==> 2 stops (not one)
when bellows extension is f + 0.5*f = 1.5*f, then bellows factor = (1.5)2 = 2.25 ==> 1.17 stops (not 1/2 stops)
When bellow extension = f + (1/3)*f = (4/3)f , then bellows factor = (4/3)2 = 1.78 ==> 0.83 stops (not 1/3 stop)

 

[jeffreyg]

A simple solution that I have found quick and useful .... take some time to do the math and work backwards. Start with the exposure factor (in f stops) for each lens and find the bellows extension. Mark the f stop changes (1/2, 1,1 1/2 and so on) on a small retractable ruler with a permanent marker at that position. Use different color markers for different lenses. Focus the camera and measure from the shutter to the film plane and look on the ruler for the number of stops. Take a meter reading and then make the change needed. This way you can do the math once and forget the formulas and concentrate on the composition.

http://www.jeffreyglasser.com/

 

[wildbill]

Measure from the shutter/aperture blades, any lens, barrel or shuttered, to the film plane.

Infinity focus will be close to the focal length of the lens from the film plane. Any extension 1/3 stop or more needs exposure compensation. Divide the lens focal length by 3, add that number to the infinity distance to find the add 1/3 stop point. Divide the focal length by 2 and add to the infinity distance the get the 1/2 stop point. 2X focal length from the film plane add 1 stop. Not as precise as using the formula but will be within 1/3 stop of correct.

wth?

you guys with all your fancy alegebra & shit. Are you intentionally trying to confuse the op?

those little gadgets you must stick in the frame are fine until you have a shot of water that you can't get to or snow, sand, mud, etc that cannot be disturbed. I have one somewhere and it was the worst $12 I've ever spent.
1. get a tape measure
2. Convert your focal length to inches*, ie 210mm=8 inches and pretend that's f8
3. measure bellows draw for the shot you have set up. Lets say 11 inches and call that f11. the difference between f8 and f11 is?
4. give the film 1 additional stop of exposure

*no need to measure all of your lenses, it's just not that critical

 

[BradS]

I not smart enough to use formulas....

you guys with all your fancy alegebra & shit. Are you intentionally trying to confuse the op?

I'm kinda surprised and dismayed by this type of response....is it really so hard to divide two numbers and square the result? That's all there is to calculating the bellows factor. I'll never understand the hostility some folks have to anything the smells like math...even basic arithmetic.

 

[Mainecoonmaniac]

I don't mind math. But why do math when there's a clear relationship between magnification and exposure compensation?

 

[goamules]

Easiest, lowest math method. Set up your shot, set your aperture as you want. (Say, wide open F4.5)

1. Measure your camera bellows from ground glass to center of lens. (perhaps 16 inches)
2. Measure the apparent aperture (the circle of light) looking into the front of the lens. (say 2 inches)
3. Divide the first by the second. That will be the actual F-stop you are shooting. (16/2 = F8) Expose for F8, not for F4.5.

 

[wildbill]

Easiest, lowest math method. Set up your shot, set your aperture as you want. (Say, wide open F4.5)

1. Measure your camera bellows from ground glass to center of lens. (perhaps 16 inches)
2. Measure the apparent aperture (the circle of light) looking into the front of the lens. (say 2 inches)
3. Divide the first by the second. That will be the actual F-stop you are shooting. (16/2 = F8) Expose for F8, not for F4.5.

easier than the method I described?
what do you use to accurately measure the apparent aperture, the same tape measure that you use to measure the bellows?

I'm only asking because I've never heard of this method and I'm trying to picture measuring the aperture without removing my compendium shade and unscrewing the front element.

 

[BradS]

Easiest, lowest math method. Set up your shot, set your aperture as you want. (Say, wide open F4.5)

1. Measure your camera bellows from ground glass to center of lens. (perhaps 16 inches)
2. Measure the apparent aperture (the circle of light) looking into the front of the lens. (say 2 inches)
3. Divide the first by the second. That will be the actual F-stop you are shooting. (16/2 = F8) Expose for F8, not for F4.5.

yes! This is, of course, the most direct method. It can be hard to do in a macro set up however.

vinny, you do not remove the front element.

 

[RalphLambrecht]

distances are measuredwithout tilt from the nodal plane of the lens. the precise location of the nodal plane is rarely known, but you can just assume the center of the lens and, in my experience, that is a fair assumption.to be most accurate use a focusing target and measure off the ground glass

 

[Rolfe Tessem]

Good grief this is complicated.

If you've used the little magnification tool you put in the scene once or twice, you can clearly remember that a head and shoulders shot with a 210mm lens on 4x5 requires one stop additional.

 

[adelorenzo]

Any perl hacks in the crowd? TIMTOWTDI!

 

[BradS]

Any perl hacks in the crowd? TIMTOWTDI!

except...for this problem, all these different ways to do it boil down to the same thing...
bellows factor = (bellows length / focal length)2

 

[Wayne]

wth?

you guys with all your fancy alegebra & shit. Are you intentionally trying to confuse the op?

those little gadgets you must stick in the frame are fine until you have a shot of water that you can't get to or snow, sand, mud, etc that cannot be disturbed. I have one somewhere and it was the worst $12 I've ever spent.
1. get a tape measure
2. Convert your focal length to inches*, ie 210mm=8 inches and pretend that's f8
3. measure bellows draw for the shot you have set up. Lets say 11 inches and call that f11. the difference between f8 and f11 is?
4. give the film 1 additional stop of exposure

*no need to measure all of your lenses, it's just not that critical

I like this method, and hate most of the rest. It's amazing how complicated people can make this. My own method is to do the calculations once for each lens, then write it on a tape measure. Then all I have to do is measure the extension when I'm set up, and the compensation is right there on the tape.

 

[polyglot]

Any perl hacks in the crowd? TIMTOWTDI!

Of course.

 

[Rafal Lukawiecki]

Any perl hacks in the crowd? TIMTOWTDI!

Python (there was a url link here which no longer exists) on standby.

 

[TimFox]

In my fanny pack with extra shutter release and light meter, I keep a seamstress' tape that I filched from my wife: english/metric, plasticized fabric, no weight, and folds to nothing. Then, I use the basic equation: (bellows/focal length)^2.

 

[Salem]

I use the milestone technique. For each lens, I memorize two numbers: the extension that require opening one stop and the extension that requires two stops, then I just interpolate roughly for any extension (or extrapolate).