# Thread: Help -- Exposure Compensation for Bellows

1. ## Help -- Exposure Compensation for Bellows

Hello,

I did a quick calculation of the bellows extension factor for my 210mm lens in a specific situation. I got:

210mm = approx. 8"
Length of bellows when focused on subject = 9"

(9/8)2 = 1.265

Log2 of 1.265 is about 0.37, which would be the difference in stops.

I know that's not a big difference and probably wouldn't matter in this case, but my question is, once the extension factor is calculated and the difference in stops is determined, do I increase or decrease the stops?

An easier example: if I'm currently at f/8 and I calculated the extension factor to be 2, which would mean a 1 stop difference, do I correct it to an f/11 or an f/5.6?

Thank you!

2. ## Re: Help -- Exposure Compensation for Bellows

Originally Posted by mediopollito
Hello,

I did a quick calculation of the bellows extension factor for my 210mm lens in a specific situation. I got:

210mm = approx. 8"
Length of bellows when focused on subject = 9"

(9/8)2 = 1.265

Log2 of 1.265 is about 0.37, which would be the difference in stops.

I know that's not a big difference and probably wouldn't matter in this case, but my question is, once the extension factor is calculated and the difference in stops is determined, do I increase or decrease the stops?

An easier example: if I'm currently at f/8 and I calculated the extension factor to be 2, which would mean a 1 stop difference, do I correct it to an f/11 or an f/5.6?

Thank you!
You would increase your exposure. In your example you would have to add approximately 1/3 stop, either by increasing the time, or opening the f stop up by a third (for example) from f11 to about f8.7

3. ## Re: Help -- Exposure Compensation for Bellows

What do you think you should do in this case?

4. ## Re: Help -- Exposure Compensation for Bellows

Originally Posted by Jim Becia
You would increase your exposure. In your example you would have to add approximately 1/3 stop, either by increasing the time, or opening the f stop up by a third (for example) from f11 to about f8.7
Thank you, Jim!!

5. ## Re: Help -- Exposure Compensation for Bellows

Originally Posted by diversey
What do you think you should do in this case?
I know I'm supposed to "increase" the stop, but that is confusing because I don't know if that means to increase the number (which would make the f-stop smaller, say from f/8 to f/11) or open the f-stop more, which would decrease the number (for example, from f/8 to f/5.6)

6. ## Re: Help -- Exposure Compensation for Bellows

Ya don't get nuthin' fer free. Gotta buy another stop's worth of light...

7. ## Re: Help -- Exposure Compensation for Bellows

Originally Posted by mediopollito
I know I'm supposed to "increase" the stop, but that is confusing because I don't know if that means to increase the number (which would make the f-stop smaller, say from f/8 to f/11) or open the f-stop more, which would decrease the number (for example, from f/8 to f/5.6)
You need to increase the exposure as bellows extension increases. That means more light is needed so the increase would be towards the larger opening in the f/stop sequence, not the larger number. In your comment above, f/5.6 would represent the proper choice.

8. ## Re: Help -- Exposure Compensation for Bellows

Originally Posted by Joe Smigiel
You need to increase the exposure as bellows extension increases. That means more light is needed so the increase would be towards the larger opening in the f/stop sequence, not the larger number. In your comment above, f/5.6 would represent the proper choice.
Got it. Thank you, Joe!

9. ## Re: Help -- Exposure Compensation for Bellows

Just to maybe simplify things for later:

F-stops are fractions. f/22 (notice the slash...) is an opening 1/22nd of the focal length of the lens; f/16 = 1/16th of the focal length, etc. 1/16th is obviously larger than 1/22nd... Remember this, and you'll never have to ask which way to go to add more exposure.

Best,

Doremus

10. ## Re: Help -- Exposure Compensation for Bellows

Originally Posted by Doremus Scudder
Just to maybe simplify things for later:

F-stops are fractions. f/22 (notice the slash...) is an opening 1/22nd of the focal length of the lens; f/16 = 1/16th of the focal length, etc. 1/16th is obviously larger than 1/22nd... Remember this, and you'll never have to ask which way to go to add more exposure.

Best,

Doremus
To expand on Doremus excellent esplanation.....a 50mm f/1.0 lens(superfast) has infinity focus of 50mm and an entrance pupil of 50 mm, so 50/50 = 1. And THAT'S what f-stops are all about Charlie Brown.