computing exposure

[Kat_4663]

I have a few questions on computing exposure times?

If I know the correct exposure time at one f-stop (e.g. 1/50 at f/11), how do I compute the correct exposure time at a different f-stop.

If I am using a 4 in. focal length lens but the object that I am photographing is further away than that, how do I compute the correction exposure time dependent on how far away I place the object?

Any response will be helpful as I am lost.

[Mark Windom]

1. The exposure time is doubled with each one stop reduction of aperture size and halved with each one stop increase in aperture size. In other words, if you get a reading of 1/30 sec. at f11 comparable exposures would be 1/15 at f22 or 1/60 at f8; 1/8 at f32 or 1/125 at f 5.6, etc. The smaller the aperture size, the larger the f stop number (11,16,22,etc) and the longer the exposure time required. At f11 the size of the aperture opening is twice as large as it is at f16 and 1/2 as large as it is at f8. Hope this isn't too confusing.

2. I'm not sure I understand your second question but if you are asking about exposure compensation for bellows extension beyond that required for focusing at infinity I use the Quik Stick.

[Ralph Barker]

Full f-stops either double or halve the amount of light passing through, depending on whether you're using a larger opening or a smaller one. Shutter speeds do the same with the duration of the exposure. So, if you use a smaller f-stop, such as f/16 instead of f/11, compensate by using a correspondingly longer exposure time, like 1/25 instead of 1/50.

For shots where the object is close, exposure compensation is also required for the additional bellows extension. There is a good article on how to calculate the bellows extension on the home page of this site at http://www.largeformatphotography.info/bellows-factor.html.

Calumet also offers a small device that is placed in the scene, and then measured on the ground glass with the small "ruler" supplied. Search for bellows extension calculator and you'll find it on www.calumet.com.

[Ernest Purdum]

Kat, I think your decond question has us a little confused, so if I seem to be giving you a silly or patronizing answer, please forgive me.



The focal length of a lens is the distance of the rear nodal point from the film when the lens is focused at infinity, something a really long way off. The rear nodal point is sort of an optical pivot point. Tilting the lens about the rear nodal point won't move the image around. Usually, it is somewhere in the back piece of glass. The f stop is the ratio of the focal length divided by the effective opening of the diaphragm.



Although the f number is only precisely correct when you are focused at infinity, you don't need to worry about correction until you are quite close to your subject. Remember what Mark and Ralph told you about each change of f number doubling or halving the amount of light the film gets. You can see that a small change of effective aperture isn't going to matter.



When to start using compensation depends a little on the kind of film you are using (color transparency film is the most finicky) but about eight focal lengths is pretty safe, so you wouldn't need to compensate with your four inch lens unless you were less than about three feet from your subject.



If you can find an old "Kodak Master Photoguide" there is a handy little calculator in there for figuring exposure compenstion. (Mine is almost forty years old, but the principle doesn't change.)

[Brian Ellis]

Others have answered your first question. I take you second question to mean how do you adjust the metered exposure for bellows extension when you're focusing closer than infinity. If that's what you're asking, the short answer is that when doing general photography (as opposed to macro) you won't usually need to make any adjustment. There's a rule of thumb to the effect that if the object on which you're focusing is farther away than 8 times the focal length of the lens, bellows compensaiton is unnecessary (some people use 10 times to be conservative). So if, for example, you're using a 210mm lens you don't need to adjust for bellows compensation unless the object on which you're focusing is closer than 96 inches or 8 feet from the lens. It's possible that people who use slide film can't follow this rule since slide film is less forgiving of even minor exposure "errors" than negative film, I don't know because I've never used slide film, but I know it works fine with negative film.

Even with things closer than the 8 (or ten if you choose) distance, you don't need to get too anal about it. I usually just increase the exposure by half a stop and let it go at that except for the rare occasions when I'm doing a real close up, then I add a full stop. There are a lot of things going on when you make an exposure that will create minor "errors" besides bellows extension, e.g. your shutter speeds are probably slightly off, the film speed you're using may not be exactly right, your focus may not be perfect, your film isn't perfectly flat in the holder, your ground glass may not be in the absolutely perfect plane, filter factors are pretty much a guesstimate, etc.