Compensating exposure for bellows extension

[robert_ruderman1]

Hi,

 

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I have been re-reading Adams' book on the Negative which contains a chapter on the Zone system. I am still quite a neophyte when it comes to the Zone system. In his chapter, there is an exposure record form (by Ted Orland) which has an "adjusted exposure" calculation to account for filter factor(s) and bellows extension/focal length.

 

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My question is, when does one apply the entension/focal length factor? I have a 150mm lens and (for example) I focus on my subject with about 75mm of bellows drawn. According to Adams' book I should compensate about .25 => (75^2)/(150^2). If my determined exposure is 1/15 (or .07) second, do I want to compute my final exposure (assume no filter is used) as .07 + (.07 *.25) (or .09 seconds total)?

 

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Are there instances where one never wants to consider adding in this kind of exposure compensation?

 

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Thanks for answering what may be an easy question for many of you out there in zone system land...

 

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Robert

[tom_meyer_at_home___1]

Yo Robert, The Anti-Tech here, responding to your overly analytical

quandry. Others may call me sloppy, but I use a neat little device

sold by Calumet to avoid this sort of painful figurin'.

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It's a 2 part tool, one is a Square (white on one side, the other

black) that you place in a scene that's close enough to make you

think about bellows extension. The 2nd piece is a ruler that has

fractions of F-stops printed on it instead of inches. Use this

"ruler" to measure the image of the square piece on the ground glass

and it tells you the bellows factor. Very fast. Just remember to take

the square out before taking the picture (voice of experience,

here)...t

[gary_frsot]

I'm with Tom on this. Nothing sloppy about the method. It's

accurate, simple and requires no battery or brains.

There is a site: http://www.salzgeber.at/disc/index.html

That will let you make one of these with your printer and

some scissors!

[fw1]

I don't think that your calculation above is correct. The bellows

factor only applies in practice when a lense is extended beyond its

normal focal length, in order to magnify the image. In your

calculation you produce an exposure factor of less than 1, which

implies that you would be reducing, not increasing, the exposure. I

have never come across this, for the simple reason that, by

definition a lense cannot be focussed at a bellows draw less than its

focal length - i.e. a 150mm lense could not be focussed at 75mm.

(Telephoto designs, by dint of their design where the lense nodal

point is in front of the lense, can however be focussed at a bellows

draw less than focal length, but I don't think that this applies to

the example you have given).

 

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Normally you get a 1:1 subject : image size by extending the bellows

to twice the focal length of the lense - in your example, 300mm. The

bellows factor is therefore (300^2)/(150^2) = 4, or two f-stops.

[alan_gibson]

By 'extension', Adams means 'film-to-lens distance', which in

Robert's case will be 150+75 = 225mm. So (225^2)/(150^2) = 2.25. If

the metered exposure was 1/15s (0.07s), the new exposure will be 1/7s

(0.15s).

 

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This isn't specific to the Zone System, it applies whatever exposure

method you use, with whatever format. When do you bother with it?

Whenever the exact exposure matters to you. If you are unconcerned

about 1/3 stop errors, then extensions of less than 1/6 of the focal

length need not bother you.

[c_matter1]

Forget the calculations...use the direct visual devices. The Calumet

one previously described costs 7 or 8 bucks (as I recall). Sinar

makes a clever combination grease pencil(for marking on the groung

glass) / bellows extension exposure comp device for $19. Probably the

best value they offer. The point is..bellows extension compensation

will come into play much sooner than you'd ever imagine. These

devices allow you to do a quick accurate check

[n_dhananjay3]

When does bellows extension become a factor? One answer is at

distances of 10 times the focal length or less. So, if you're using a

150mm lens, start worrying about compensating for bellows extension

when your subject is about 5 ft away. Though this little convention,

of course, is the result of someone's call on how many stops off

exposuree he's willing to put up with. If you go with notion that 1/3

stop is when you should compensate (assuming smaller amounts anyway

go out in the wash of normal random errors), you start worrying about

it at 1/8 magnification or so and larger.

 

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Also, save yourself some money and get the QuickDisc from <A

HREF="http://www.salzgeber.at/disc/">here</A>. It works in

essentially the same way as the other stuff. Do get onee of thesee

though. Frees your mind up in the field. Hope this helps.

DJ

[robert_ruderman1]

Hi,

 

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Thanks to all for helping me in trying to figure out this part of

exposure compensation. I guess my example (and math) was a little

flawed (as was my interpretation of what defines "bellows extension"

or at least how to measure it). I do greatly appreciate all the help.

 

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My example of 75mm extension with a 150mm lens was to imply that there

was no magnification of the subject, rather that the lens was focused

at (or near) infinity as I might do for a landscape scene in an open

field.

 

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I'll take a look at these devices/freebies for determining when/if I

need compensation and work them into my exposure scheme. I am, as

some suggested, not sure that I should worry about scenarios where

there are +/- 1/3 stop changes in exposure; but anything greater than

that might be of interest.

 

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Thanks again,

Robert

[bill_moore]

The basic thing to keep in mind: If the bellows is extended to twice

the focal length, you will need two f stops more exposure, In your

example, 75mm is half of the focal length, making your extension one

and a half times the focal length, you will need one stop more

exposure. A simple tape measure is all you need. Just base you

calculations on this.

[bill_glickman]

I do not do any table top or close up work where the Calumet item

above is the way to go... In the field, I have tried many methods,

and have settled on some tiny cheat cheats for each lens. I used to

measure the bellows, however, you need to take into consideration the

lens nodal point for each lens and this is very cumbersome with

movements. So I have very small charts for each lens, showing at

each focal distance, the compensation required in 1/3 stops. It is

easier to calc. the bellows extension mathematicaly then plug it in

the bellows extension formula and have this on a quick cheat sheet,

compated to all the work that would need to be done in the field...

you simply find the distance to your focus point and check the

chart... It is much easier for me to determine the distance to your

sujbect than the distance of bellows draw on your camera.. good luck..