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# Thread: Bellows factor conversion method????

1. ## Bellows factor conversion method????

I just found this Bellows Factor Conversion method.

lens focal length of 210mm and a bellows draw of 450mm. Pretend these numbers are f-stops...drop the zeroes and you have 21 and 45.

How many stops between 21 (f22) and 45 (f45)... Right, the bellows factor is 2 stops. Increase your exposure by 2 stops.

If this is a valid method it is really simple. What do you experienced folks think?

2. I would say it is not only valid but simple. A no brainer for someone with basic math skills and a knowledge of a standard-ish aperture series. Cool beans.

3. This method is is amost identical to Paul_c5x4 method. He converts to inchs and the length becomes the f/stops and your bellows factor is the difference in f/stops.
I am trying to do a real close up shot right now with my bellows fully extended 16" (409mm) and a 180 mm lens. So 410 become f/41 and 180 becomes f/18.. The difference is 2 1/3 stops????
Is that correct?
I also have made a copy of the "Quickdisc" but in this particular example the disc is slightly larger than the 2 stop measuring scale.

4. My method:

"The bellows are way out there...I'd better double the exposure! And maybe I'll take another shot and double it again."

5. Not my method - I found it mentioned over on one of the large format forums. I don't think it makes any difference if you work in inches or millimetres, just stick with what ever you are comfortable with.

I am trying to do a real close up shot right now with my bellows fully extended 16" (409mm) and a 180 mm lens. So 410 become f/41 and 180 becomes f/18.. The difference is 2 1/3 stops????
Is that correct?
2-1/3 stops is correct. If you're using a filter also, then you need to ADD the filter's correction (in stops) to that. If your resulting exposure time exceeds one second, you may need to also consider reciprocity.

7. I sat down with a calculator and marked off the factors for each of my lenses on a ruler. Now it's a complete no-brainer...just hold one end of the ruler at the film plane and apply the factor that's adjacent to middle of the lens. For a morning of mathematical pain I now have a lifetime of convenience.

Murray

8. You should end up with a bellows factor of about 4.6 for that situation, and using the method, you end up with a bellows factor of 4. A fraction (about 1/6, or 1/2 of 1/3) of a stop of underexposure might not be noticed, but if you are shooting transparencies, I would just do the math, which is very simple...and since it is very simple, why not just do it for all types of film? Just carry a pocket calculator. Divide the smaller number into the bigger number. Then square the product. There is your factor. Multiply that by your exposure time to get the new exposure time at the same f stop. Since you are on a tripod and likely (hopefully, if you are actually thinking about what you are doing) have a set amount of depth of field in mind for the shot, shutter speed is the more sensible and easily fluid exposure parameter to change in most situations. However, if you would rather change f stops, you can just use it as you would use a filter factor. You can easily enough remember the commonly used filter factors, or carry a chart with you.

If this is something you do a lot, definitely do what Murray does. I use a tailor's tape, not a ruler, but that is not important.

9. Rob,the small number is focal length of lens focused on infinity,and the large number is a focus closer than infinity?

Mike

10. Hi, Mike.

That's right. The smaller number is the FL of the lens at infinity. The large number will be the extended distance measured from the film plane to the same point on the lens that corresponded to the FL when the lens was focused at infinity. This will usually be at the plane of the aperture.

(450/210)² = easy

With telephoto lenses, you are better off using magnification as a guide by using an object of a known size (such as a ruler) in the shot and measuring it on the ground glass, then using those two numbers to do the math.

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