View Full Version : Exposure compensation for telephoto lenses?

Bill Glickman

10-Jun-2000, 01:15

I am using the Nikor 800 and 1200 mm tele lenses. These are the first teles I e ver owned and therefore am not aware of how exposure compensation is calculated. I don't beleive the normal exposure compensation formula works for teles? Doe s anyone have an exposure compensation formula that works for telephotos? I pla n to work at far distances, so exposure discs will not work. Thank you in adva nce.

Bruce M. Herman

10-Jun-2000, 03:18

Bill,

I have a Fuji 600 mm tele, and do not do any exposure compensation for distant subjects. Are you refering to compositions in which the bellows extension increases more than several cm beyond the extension required for focus at infinity?

Bruce

Bill Glickman

10-Jun-2000, 03:53

Yes Bruce, when focussing on something a bit closer than infintiy I was wondering how to calc. the amount of exp. compensation.

Robert A. Zeichner

10-Jun-2000, 08:16

Bill, What you are concerned about is something called exit pupil factor. You can actually see some difference in the apparent size of a given aperture by sighting through the lens first from the front, then the rear. Every tele has a different EPF, but you'll be relieved to know that it only comes into play in any appreciable way, when the object size on the gg is greater than 1/10th the actual object size. What this means is that in landscape work, where, with lenses that long you are usually quite some distance from the subject, it won't typically be a problem.

Bill ; I am sure that Robert is correct, but your question interested me in case you were going to use these lenses for macro work.

The basic principle with any lense used for macro work is that to get 1:1 magnification, both the subject and the groundglass need to be at twice the focal length of the lense being used. Easy to apply with a normally designed lense, but less obvious with a telephoto design where the front nodal point is in front of the lense, thus permitting a smaller flange focal length / bellows draw.

Looking at my Nikkor LF lense catalogue - the flange focal length of the Nikkor 1200mm f/18 is 755.7mm. Hence, if you want to get 1:1, the bellows draw would be 1511.4mm (2 x 755.7mm), and the subject would be 3288.6mm from the lense (2 x (1200-755.7) + 2400).

I think that the exposure compensation would then be 2 stops, following a normal calculation. Anyway, I hope this isn't a digression from the intent of your original question.

Bruce Gavin

10-Jun-2000, 13:16

Finnegan,

Would it be inconvenient for you to either post or email me the specs on the Nikkor-T lenses? I keep an extensive spreadsheet on all lenses, and am missing most of the vitals for the 500 thru 1200 lenses. I am looking for: image circle wide open, flange focus distance in mm, weight in gm, and length in mm. I have the other info.

Bill Glickman

10-Jun-2000, 16:01

Finnegan, are you saying that all lenses that have bellows draw at 2x lenght vs infinity are producing 1:1 on the gg? IF so that is very interesting.

So I guess the same basic principle applies here, double bellows draw vs. infintiy - yields 2 stops exp. compensation, just like any non tele lens, right? Thank you all for your input.

Bill ; 1:1 is produced when the distance from the lense of both<i/> the subject <i>and the groundglass are at a distance that is twice the focal length of the lense. This won't apply when the subject is at some distance from the lense, not least because your subject would not then be in focus. For most distance work, I think that Mr Zeichner's comments will apply.

I forget to add to my earlier post - the calculation example is similar to what I use when I use my Fuji 400T for close up photographs. I think that the calculation is correct, but if it is technically wrong, I would like someone to point out to me where any error may be. Cheers

Bill ; I have a question for you. For each exposure, are you then measuring the bellows draw, and if the draw > lense focal length, are you then calculating an exposure factor? So far I have only applied such exposure compensation when doing close up photographs.

Bill Glickman

10-Jun-2000, 19:45

FW.... I have not attempted it yet, that is why I made the post, I got stumped :-) My goal was to know either how to calc. the following two items... first, if a subject is 150 ft. away, how much exposure compensation do I need. Or, if the bellows is racked out to 1000mm, how much exposure compensation would I need. Either of these would solve the exposure compensation dilimna.

On my non tele lenses, I have listings for each lens for focus distance / exp. comp., and bellows draw / exp. comp. This way I know for each lens, what is the closest subject I can focus on with the amount of bellows or rails I have with me... I would like to do the same with these teles... I find it much easier to compensate for bellows extension by knowing the subject distance vs. measuring the bellows draw. I just grab my laser range finder, and in 1 second I know the submect distance in feet, then I look at my trusty charts and apply the proper exp. compensation. Now, how do I do that for the teles? I never saw formulas in view camera books for this?

Bill Glickman

10-Jun-2000, 20:36

Just checked some gear to be more specific. I can get bellows extension out to 1,900 mm. So my goal is to know, at each 1/3 stop of bellows compensation, i.e. 1/3, 2/3, 1, etc. what the focus distance is and the respective bellows draw for each 1/3 stop increment. I am using the Nikor 1200 mm and 800 mm TED. These proper formulas, if we ever get to them should be then posted on this forums home page, since this data is so very difficult to find. And the forumula should apply (I think) to all teles, regardless of their make or fl. I am confident someone out there has done this calc. before?

Sorry about the</i>italics. Bill, I've been poring through the references I have available, and the only one of any help is on p.68 of Ansel Adams' "The Camera", where he writes ;

"For close subjects, the f-stop must be corrected to account for the fall-off in illumination caused by the lense extension. This effect becomes significant whenever the distance from lense to subject is about 8 times the focal length of the lense, or less."

Hence I would contend that you would probably not need to calculate an exposure factor, unless your subject is 6.4 metres or closer with the 800mm lense, and 9.6 metres or closer with the 1200mm lense.

In addition, in the appendix on p.194, after going through the various formulae associated with focal length, subject distance and magnification, he writes ;

"You should have no difficulty in using them (the formulae), except where the lense is of retrofocus or telephoto design, in which case the nodal plane must be located and used in measuring all distances."

I gave you the flange focal length of each lense earlier, so you can fairly easily estimate where the nodal plane will lie, i.e. at a distance in front of the lense of focal length less flange focal length. Can someone please correct me if I am wrong?

Struan Gray

12-Jun-2000, 08:18

There are two nodal points, which are usually situtated where the two principle planes cross the optical axis. If you measure the image distance (i) from the secondary nodal point to the film, and if you measure the object distance (o) from the primary nodal point to whatever it is you're photographing, then you can use the familiar formula:

1/f = 1/o + 1/i

So what? Well, if you don't change the relative positions or strengths of the lens elements the principle planes and points also stay fixed relative to each other. So in the absence of floating elements, extending a tele by a given distance beyond its infinity position has exactly the same effect as extending a normal lens by the same amount past its infinity position.

If you have a press-type camera with infinity stops you can easily measure the actual extension beyond the stop and just plug it into one of the bellows-factor formulae which gives the exposure compensation in terms of the focal length and the extension.

With cameras which have moving backs it is easier to measure the total bellows length. Adding the difference between the flange focal length and the true focal length will give you the distance to the rear nodal point, which is the bellows draw you would have had with a non-tele lens. Put that into your favourite formula which uses the focal length and the total bellows draw.

An example: with the 1200 mm tele at 1:1 you will have 1200 mm of extension because 1:1 always moves the lens one focal length beyond the infinity focus. The shutter speed needs to be lengthend by a factor of ((f+e)/f)**2, which in this case equals four - i.e. two stops.

Alternately, the total bellows length will be 1955 mm (1200 + the flange focal distance at infinity focus). The difference between the flange focal distance and the focal length is 445 mm, so the total equivalent bellows draw is 2400 mm, twice the focal length, which needs two stops more exposure.

The object distance will depend on the position of the front nodal point, and that's easier for you to measure than for me to guess. Once you know it for one setup, you can use the modified thin lens formula above to find the position of the front nodal point and then calculate or plot object distances for all other magnifications and bellows extensions. Alternately, just move the camera until it looks right :-)

Stpephen Willlard

12-Jun-2000, 14:11

Bill, Here is my two cents worth on this subject. Two years ago I talked with a Nikon technician and he told me that their telephone lenses for large format cameras were optimized for use at infinity only. Anything closer than infintity was not recommended because of the deterioration in resolution and accutance (sp).

I have not done any close up work, but if I were then I would get a meter that is designed to read off the ground glass. This would eliminate managing the bellows factor, and I believe would provide you with very accurate readings.

Well that is all I have to say. Hope this helps.

By the way, I would really be interested on what you are using to stablize your camera when shooting with those big lenses.

Bill Glickman

12-Jun-2000, 14:45

Struan, this was brilliant! Struan, or anyone, please bless my simplificaiton of the post.

1. I focus to infintiy, mark the position of the front standard. 2. Focus closer, then note the additional bellows extension. 3. Add this additional extension to the lens fl, in this case 1200mm. 4. Use this value in the normal exp. compensation formula.

I think thats right... pretty simple, of course after someone else explained it so eloquently!

Stephen, I appreciate the input from Nikor. I will try some test shots at 50 ft. and see what happens. I wonder if this issue is true for all Teles, or possibly just Nikor teles?

rails, then focus on the closer subject and mark the rails, subtract the two

Struan Gray

13-Jun-2000, 04:17

<<1. I focus to infintiy, mark the position of the front standard. 2. Focus closer, then note the additional bellows extension. 3. Add this additional extension to the lens fl, in this case 1200mm. 4. Use this value in the normal exp. compensation formula.>>

That'll do it. I was just being longwinded about it :-)

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