View Full Version : Bellow compensation.

Louie Devincentis

31-Mar-2012, 09:40

Hi Gang.

I have to premiss this question by saying that my knowledge of a LF camera and it's workings is extremly limited( never used one) and so this question might be in left field so bear with me.

When you are taking an exposure reading should you compensate for the distance from the lens to the film or is the light loss minimal?

Thanks.

Louie

Yes you must compensate for close up work, how much depends on the lens and the distance, there are tables published.

Ian

E. von Hoegh

31-Mar-2012, 10:29

Try the homepage. Wonderful source of info. http://www.largeformatphotography.info/

Light Guru

31-Mar-2012, 12:15

If you have a iPhone, iPod Touch or iPad there is a great app called Reciprocity Timer that helps calculate bellows compensation and several other things effecting exposure.

Hi Louie,

Bellows compensation is required any time a lens moves significantly forward from its infinity focus position,

regardless of the format in use or the lens focal length.

For example, if your image is life size (a 1:1 ratio between subject and image) you increase exposure 2 stops.

Here's the actual page that explains it: http://www.largeformatphotography.info/bellows-factor.html

The explanation is as follows:

The lens projects a cone of light on the film plane.

When focused at infinity, the height of the cone equals the lens focal length,

and the light fills a circle at the film plane that has a particular diameter.

As the lens moves away from the film, to focus on closer subjects, the diameter of that circle increases.

Since the total amount of light does not change, the illumination per unit area decreases.

When focused for 1:1, the distance from tilm to lens has increased to twice the focal length,

so the area of the illuminated circle is four times its area at infinity, which is a 2-stop difference.

- Leigh

Ken Sinclair

31-Mar-2012, 12:59

Louie..

I have, on numerous occasions, recommended Bob Wheeler's freeware for those photographers who own what may (by some) be considered the outmoded hand-held Palm Pilot. It will provide, to a tength of a stop, the compensation for bellows extension with all formats of cameras... as well as a lot more for those who need tha accuracy. I doubt there are 'Apps' for the Android or the iPhone that will provide all or (similar) information that can be provided from this still available freeware.

I only wish that I had been aware of this particular freeware when I was a "working" Technical/Scientific/Biological Photographer. It would have saved a LOT of time working out from the formulae, with the stubby pencil and a scrap of paper. Nowadays I never leave the house without making sure my Palm Pilot has 'good' batteries.

Ken

Heroique

31-Mar-2012, 13:16

Sheesh – how about a rule of thumb to ease our theoretical musings.

I don’t worry too much about “correction” unless the distance to my subject is 10x my focal length, or closer.

For example, w/ a 150mm lens, this threshold would be 10 x 150mm = 1500mm (1.5 meters).

That’s pretty close.

If I’m this close or closer – which is rare – my next rule of thumb is to add ½ stop for every 25% increase in the bellows extension (beyond infinity).

If you are doing close-up still life, macro etc then yes, you do need compensation. Have a look at Quick disc http://www.salzgeber.at/disc/ I've found that quite helpful.

my next rule of thumb is to add ½ stop for every 25% increase in the bellows extension (beyond infinity).

That's correct in all situations.

- Leigh

Lynn Jones

3-Apr-2012, 12:15

Focal length divided by bellow (similar measure) squared equals factor. Multiply ASA/ISO by the factor and that becomes your new film or sensor speed.

Lynn

Drew Bedo

3-Apr-2012, 14:04

You can get into extension compensation pretty deeply if you need to.

I try to make it as simple as my simple mind can deal with.

If the lens is extended to twice the focal length, two stops more light is needed.

If the total extension is one and a half focal lengths, only one more stop is needed.

For my 150mm lense, each additional inch of extension requires one third of a stop more light.

For my 20mm, the correstion is (about) one quarter stop for each inch.

This keeps it easy enough to do in my head while working on location. For critical close-up shots, I may get out the tape and calculator.

Louie Devincentis

3-Apr-2012, 14:22

Thanks everybody for the advice but lets see if I understand this not so easy subject. I only have to worry about light compensation when I am at very close distances and if I have to, compensate up to 2 f/stops.

Thanks again for your replies

Cheers.

Louie

goamules

3-Apr-2012, 17:44

I learned from Mark Sawyer to simply measure the focal length wherever the bellows is extended, then divide by the apparent aperture looking in the front of the lens. If my camera with an 8" F4 Petzval (with a 2" diameter circle of light seen looking into it's front) is stretched out to 16", it just became an F8 exposure. Then I meter the scene and calculate based on the f-stop I just determined.

Mark Sawyer

3-Apr-2012, 20:58

What Garrett said! :) There are all sorts of formulas for figuring bellows extension factor, but in the end, the very definition of an f/stop is just the focal length divided by the aperture. And figuring it from scratch helps keep you in touch with what it is.

If you have a iPhone, iPod Touch or iPad there is a great app called Reciprocity Timer that helps calculate bellows compensation and several other things effecting exposure.

Thanks for the link to that great App, I was surprised to see reciprocity kick in as fast as 2 sec on FP4 of 1 stop!

Ian Gordon Bilson

5-Apr-2012, 22:39

I'm with Drew Bedo on this subject - and probably less enabled than he.

This,from a Kodak Photo Guide. "Bellows Factor for View Cameras" : " Place a white card, 2 inches long,at the subject position.

Measure the width of the card on the ground glass. Apply the following ( +F stops) correction. Note the 1/3 f stops.

7 millimeters : + 1/3

14 : + 2/3

22 +1

30 +1 1/3

41 +1 2/3

52 +2F

64 fill in the gaps..

76.5

93 +3

110

Sorry about the Imperial/Metric combinations, but metrics are more precise for this application.

I use a cheap pair of dividers at the glass, and read off the mm to a scale on my Horseman LF.

Brian Ellis

6-Apr-2012, 07:55

It's easy to get way too involved and worried about bellows compensation, especially when using negative film which generally has a good bit of exposure latitude. I used a rule of thumb under which no compensation was made as long as the thing on which I was focusing was farther than 8 times the focal length of the lens from infinity (some people use 10 times which probably works fine too) and if it was just a little more than 8 times I added a half stop of exposure, if it was somewhat more I added a stop, and if the bellows was fully extended or thereabouts I added a stop and a half or two. I just eyeballed the distance, I didn't carry a ruler or other measuring device around with me. I had four lenses and I converted the focal lengths into feet (rounded inches up or down) and typed a little table so that I didn't have to do the "8 times focal length" math in my head but after a while you remember them and don't need a table. Always worked fine for me, this isn't nuclear physics.

Drew Bedo

6-Apr-2012, 11:49

Does anyone lower the blast visor and access THE FORCE?

the thing on which I was focusing was farther than 8 times the focal length of the lens from infinity

This is where English screws me up. Lets assume a 12 inch lens; 8X12 inches is 8 feet.

Does this mean the thing you focus on is 8 feet closer than infinity? or 8 feet father than infinity away?

Or it is farther than 8 feet away from the camera?

Or is the bellows 8 feet from its infinity position?

I used this the other day, was easy, and worked great for me.

If you are doing close-up still life, macro etc then yes, you do need compensation. Have a look at Quick disc http://www.salzgeber.at/disc/ I've found that quite helpful.

Mark Sawyer

6-Apr-2012, 16:53

Just divide the bellows extension by the aperture diameter and you have your f/stop value... :p

Tim Meisburger

6-Apr-2012, 17:56

Thanks Mark, and Garrett. This is the simplest and easiest to remember explanation I've heard. I have, and have used, the Quick Disk, and it is easy and effective, but when I am wandering around I never have it with me (I could stick it in my bag, I guess).

goamules

6-Apr-2012, 19:23

Simple is, as simple does.

WoodyBoinker

9-Apr-2012, 07:47

Would the plots in this article be usefull for bellows compensation in the field?

Certainly would be an easy quick reference.

http://people.umass.edu/~rbhome/bellows15.pdf

E. von Hoegh

9-Apr-2012, 07:57

The chart in fig. 3 and a tapemeasure could be carried in the field, but it's better just to learn and remember the rule. It's really quite simple, and you're not as likely to leave your brain at home.

Drew Bedo

9-Apr-2012, 13:38

I think goamules has it right: Keep it simple. Find a simplified way to make the inverse square law work for you in the field and work with it till its automatic for you.

While I do not use this method, one way to simplify the correction would to pre figure the correction in thirds or quarters of a stop for your commonly used lenses and lay out a scale on the bed or rail of your camera. A different scale for each focal length of course.

This way you only calculate the correction once for each lens.

Just divide the bellows extension by the aperture diameter and you have your f/stop value... :p

But first you have to do the math (using the focal length and f/stop number of the lens) to get the aperture diameter, correct?

Brian Ellis

9-Apr-2012, 14:02

This is where English screws me up. Lets assume a 12 inch lens; 8X12 inches is 8 feet.

Does this mean the thing you focus on is 8 feet closer than infinity? or 8 feet father than infinity away?

Or it is farther than 8 feet away from the camera?

Or is the bellows 8 feet from its infinity position?

Sorry, I really shouldn't have talked about infinity. I meant no bellows extension if the subject on which I was focusing was farther than 8 x focal length of lens from the camera (didn't worry about farther from lens or farther from film plane or farther than somewhere else on the camera, since I was just eyeballing it anyhow I didn't think I needed to get that precise).

Sorry, I really shouldn't have talked about infinity. I meant no bellows extension if the subject on which I was focusing was farther than 8 x focal length of lens from the camera (didn't worry about farther from lens or farther from film plane or farther than somewhere else on the camera, since I was just eyeballing it anyhow I didn't think I needed to get that precise).

I reckoned that and it is a good rule.

Regards

Drew Bedo

9-Apr-2012, 16:22

Vaughn: You are so right!

Mark Sawyer

9-Apr-2012, 18:25

Just divide the bellows extension by the aperture diameter and you have your f/stop value... :p

But first you have to do the math (using the focal length and f/stop number of the lens) to get the aperture diameter, correct?

Yes, there's one simple division problem to do. And you have to make two measurements with a tape measure.

But if you're reeeeally lazy, set your f/stop to one inch and measure the bellows extension in inches. Or set it to one centimeter and measure the bellows extension in centimeters. Then you're just dividing by one; you bellows extension is your f/stop.

See this lens currently on ebay, #330714904774:

http://i55.photobucket.com/albums/g139/Owen21k/KGrHqJnoE8YJtNFO0BPP5PfQwg60_58.jpg

Notice it's got a big white mark at about f/11? Let's check my old Velostigmat and see what that measures to be...

http://i55.photobucket.com/albums/g139/Owen21k/fstop.jpg

Hmmm... about an inch. (Remember, there can be a little slop in the iris accuracy, just like on shutter times.) There once was an old photographer who had the option of measuring his bellows extension, and that was his f/stop, period. 18 inches of extension? f/18. 14 inches? f/14. 26 inches? f/26.

I can't swear that this is what was going on, but I've seen a few shutters marked this way, and it's always one inch, sometimes also at half an inch (easy math, cut your extension in half!), or one centimeter. If you aren't fussy about your taking aperture, it can make life easy.

Today's photo-tip for lazy math-phobes! :rolleyes:

goamules

9-Apr-2012, 19:10

But what if with your shown lens, and 1 inch iris setting, you focus and your bellows measures 11 inches? You have F11, that easy?! Uncanny! Unheard of! What will they think of next?!

Mark Sawyer

9-Apr-2012, 19:25

Yup, although technically, at 11 inches extension with a twelve inch lens, you've pulled a Buzz Lightyear, focusing "at infinity, and beyond!!!" :)

And technically, a 12" lens should have its 1" aperture at f/12, but as I said, there's often a little slop...

BTW, I thought it was a neat little trick, and one I'd never heard talked about before, so I gave it it's own little thread under the Lenses & Lens Accessories forum, (though in retrospect, I guess it should have gone in the Style & Technique forum...)

Skip Roessel

10-Apr-2012, 08:38

Lynn's right. Divide the FL by the Bellows Draw first (always gives you a number less than one) and square that, getting an even smaller number. That's fraction or percentage of light the film is getting. The reciprocal, a number greater than one, is how much you multiply the old exposure time to get the new, longer time.

The same math, a little simpler is: new exposure equals ( BD / FL ) squared, then multiplied times old exposure in seconds. If Bellows Draw is divided by Focal Length, that will always be greater than one, and squared it is greater still.

In Excel I think that formula would be NEW EXPOSURE IN SECONDS =((BD/FL)^2)*OE (OE is Old Exposure in seconds, what the meter says. Bellows Draw and Focal Length are both in centimeters, or whatever.)

Skip

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