Exposure correction with extension tubes

Rob Archer · Jun 28, 2003

I've just bought a 28mm extension tube for my Bronica ETRSi. I know I need to give extra exposure with a tube but how do I calculate it? I know the 'inverse sqare rule' comes into it but how do I apply it - I'm not paticularly mathematically minded! Also, how can I calculate depth of field when using a tube as it's obviosly so shallow?

Rob

glbeas · Jun 28, 2003

What you are seeking is a "bellows correction" of which you can find calclators and formula to find the extra aperture or time to add.
The amount varies with the focal length of the lens mounted on the tube.
This is quoted from a posting I made in Exposure Discussions- "Whats your coolest trick"

**Only real "trick" I can think of is figuring the approximate actual aperture with my calculator. Fer'instance 100mm len at f/8. Divide the focal length by aperture to get the aperture factor or 12.5 (which should be the real size of the aperture in mm). Measure the total extension, say its 150 mm from focal plane to lensboard. Divide that by the aperture factor to get 12. That tells me I'm really working with an aperture of f/12 for that magnification.
The math can be swapped around to figure what to set at a particular extension to get the working aperuter desired. Say the 150 mm total extension and the 100 mm lens, you want f/8. Divide 150 by 8 to get the factor of 18.75. Divide 100 by the factor 18.75 and you get 5.33. So you would set the aperture to a smidge wider than f/5.6 to get a working aperture of f/8.
It's worked every time I've used it. Only hard part is finding the damn calculator when I want it, it tends to turn invisible on me at times. **

In your case you will simply add the 28mm to the lenses focal length, and possibly add any amount of extension introduced when using the lenses focusing closer with its helicals and do the math.

Lemastre · Jun 29, 2003

One operator of large-format cameras says just regard the focal length of your lens as an f stop and add any extension to it to get the next f stop. E.g., if you're putting a 28-mm extension tube behind a 50-mm lens, you're going from f/50 to f/78. These numbers look more like f stops if they're divided by ten: f/5 and f/7.8. So your 28-mm extension tube would call for about a one-stop increase in exposure time or maybe a tad more. This means that f/5.6 on your lens would be about equivalent to f/8. I haven't tried this, but it looks pretty good.

David A. Goldfarb · Jun 29, 2003

Somewhere around here, if it's survived the migration to the new software, I've posted a magnification/exposure table as an MS Word doc file (but such tables can be found in any book on macro photography).

What I do with any format camera is put a ruler in the scene at the subject position or estimate the width of the subject field and compare it to the width of the film, so if I'm shooting a macro scene that's about 4.5" wide with a 2-1/4" square (6x6cm) camera, then my magnification ratio is 1:2. Then I look up the exposure factor on my table and make the correction--1-1/3 stops in this example. No need to think about focal length, lens extension or subject distance with this method--just magnification ratio.

For DOF--you'll usually need all you can get. Just stop down all the way and use multiple flash pops or very long exposures if necessary. There are various DOF calculators on the net. One that I like is called "f/calc" which is a shareware program that can do several photographic calculations.

Lemastre · Jun 30, 2003

David A. Goldfarb said:
. . . put a ruler in the scene at the subject position or estimate the width of the subject field and compare it to the width of the film, so if I'm shooting a macro scene that's about 4.5" wide with a 2-1/4" square (6x6cm) camera, then my magnification ratio is 1:2.

Isn't the width of the image on the film the significant value rather than the width of the film itself?[/i]

David A. Goldfarb · Jun 30, 2003

For the purposes of calculation of magnification ratio, it doesn't really matter whether you compare the size of the image on film to the size of the object in the world being photographed, or the width of the image field (the width of the film frame) to the width of the subject field (the whole area that the lens takes in at the subject distance). These ratios should be identical.

Methods like the "QuikDisk" available for free online and Calumet's device use the former approach, where an object is put into the scene, and a ruler marked in stops is used to measure the image of the object (the QuikDisk or Calumet's square card) on the groundglass, but this is only really practical with large format, where you have easy access to the groundglass. With a 35mm or medium format camera, I find it easier just to use the width of the frame as a known quantity for this purpose and to measure the subject field with a ruler or tape measure.

Lemastre · Jun 30, 2003

I misinterpreted "scene" to mean the width of the subject being shot, when in fact it includes the subject plus any space on either side that appears on the ground glass along with it.

Lemastre · Jun 30, 2003

I misinterpreted "scene" to mean the width of the subject being shot, when in fact it includes the subject plus any space on either side that appears on the ground glass along with it.

inthedark · Jul 3, 2003

Okay now you all have confused the crud out of me. . .if you put a tube on a lens how can the focal length possibly get larger??? My 760mm lens is wide and shallow whereas my 150mm lens with appropriate cone mount is decided similar to mounting a lens on a tube and the focal length is less. Doesn't focal length mean the greatest possible diagonal which the lens can handle?

Same thing with bellows extensions adjustments. The longer the bellows are extended the larger the aperature needs to be wherein f8 would become f5.6, not the other way around.

inthedark · Jul 3, 2003

Oops! I think I misread, particularly Lemaster's remark. Okay. Sorry, jumped in a bit too fast. Analytical dislexia I guess

Donald Miller · Jul 3, 2003

A quick way to get the corrected f stop for bellows extension for all formats is to convert the focal length in mm to inches by dividing by 25.4.

If you then make up a list of F stop numbers as follows:
2.8* 3.0 3.5 4* 4.5 5 5.6* 6.3 7.1 8* 9 10 11* 13 14 16* 18 20 22* 25 28 32*
The numbers with asteriks are whole stops. These conversions of lens mm to inches can be rounded to nearest corresponding stop.

If we were to use a 75 mm (2.95 inch) lens on the Bronica and add the 28 mm extension tube to come to an actual 4.05 inch lens to film plane distance the exposure increase would be 2/3 stop increase in exposure.

With a 150 (6 inch) mm lens on the Bronica adding the 28 mm extension tube would give the lens an effective 178mm (7 inch) lens to film plane distance. The exposure compensation would be an additional 1/3 stop.

In the case of a 210 (8.25 inches) Symmar lens with a lens to film plane distance measurement of 300 (12 inches) would require a 1 and 1/3 stop increase in exposure.

In another case of a 90 mm Super Angulon (3.54 inches) with a actual lens to film plane distance of 120 (4.72 inches) would require slightly more then 2/3 stop of increase in exposure.

inthedark · Jul 3, 2003

Don, I see you too are suggesting that adding a tube enlarges focal length. What is your definition of focal length, please. Mine must be different.

Thomassauerwein · Jul 3, 2003

Add some exposure and shoot a polaroid. Do a polroid test keep track. then do a film test. Make sure your chemistry is balanced how you like it. as your test subject use a subject your comfortable with then slide a grey scale in the set. Eventually you'll know by instinct,

Donald Miller · Jul 3, 2003

The focal length of the lens is not modified in and of itself through the use of extension tubes with fixed focal length lenses. In the case of view cameras the same effect is realized by increased bellows extension beyond infinity focus.

Through these alterations (extension tubes and bellows extension beyond infinity focus) lens to film plane distance is increased and the lens to subject distance is decreased which affects (increases) magnification. As magnification is increased exposure must be increased to compensate.

To answer your question about focal length. The focal length of an asymetrical lens would be the distance from the lens nodal point to the film plane when the focus is at infinity.

inthedark · Jul 4, 2003

Really? Huh. When I was taught/told about focal length by the builder of my cameras, he said the focal length represents the diameter of flat of field for the lens. For example a 780mm lens divided by 25.4 gives just shy of 30". So I can safely use this lens to receive on film or PMT an image up to 21"sq with zero edge distortion, or print from an image 21"sq with zero distortion. I wonder if that explanation is because my cameras can't see to infinity. The frame work limits the lenses from getting close enough to the film plane. For those of you who just went, HUH!?. I am not a photographer, my cameras are reproduction cameras in a darkroom.

inthedark · Jul 4, 2003

By the way, I am in no way arguing with your definition. It actually makes more sense than the one I mention. And I now understand also that the point to the bellows/tube addition is a way to a end; not an actual indication of increased focal length. thank you for clearing that up.

Donald Miller · Jul 4, 2003

Well, that is an interesting point. Since I am not well versed on process lenses I can not speak to the definition that you propose. I do have a 600 mm Apo Nikkor process lens. This came from a graphics application and is barrel mounted without shutter. This lens on my 8X10 camera requires a bellows extension of approximately 24 inches when focused at infinity. I will say though that this lens will project a very large image circle. It will certainly cover a 16X20 negative and possibly larger when stopped down. However it would not cover that large a projection for a graphics application since the requirements for resolution would be much higher in my understanding.

glbeas · Jul 4, 2003

Your builder was just giving a simplistic explanation of the coverage of the lens, which can vary with the construction of the lens. If you were using the same lens in a view camera shooting landscapes you would most likely have about half the coverage, shooting 1:1 increases your coverage ability with any given lens. If you want a really good technical book on the subject of view cameras try Leslie Stroebels book View Camera Technique. That will get into the subject of coverage of various lens designs as well. The length of the lens barrel of any focal length lens has a large effect on the covering power as well as stopping down the aperture. J.L. Woodens Large Format Optical Reference Manual gives very specific details on lens specifications.

inthedark · Jul 4, 2003

Thanks, glbeas, I will go in search of one or both asap. The better I understand, the better I can make best choices.

Exposure correction with extension tubes

Rob Archer

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Lemastre

David A. Goldfarb

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Donald Miller

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Exposure correction with extension tubes

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