View Full Version : Compensating exposure for bellows extension
I have been re-reading Adams' book on the Negative which contains a chapter on t he 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 "a djusted exposure" calculation to account for filter factor(s) and bellows extens ion/focal length.
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 d rawn. 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 fin al exposure (assume no filter is used) as .07 + (.07 *.25) (or .09 seconds total )?
Are there instances where one never wants to consider adding in this kind of exp osure compensation?
Thanks for answering what may be an easy question for many of you out there in z one system land...
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'.
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
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!
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).
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.
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).
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.
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
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.
Also, save yourself some money and get the QuickDisc from here (http://www.salzgeber.at/disc/). 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
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.
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.
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.
Thanks again, Robert
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.
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..
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