Help -- Exposure Compensation for Bellows

[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!

[Jim Becia]

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

[diversey]

What do you think you should do in this case?

[mediopollito]

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!!

[mediopollito]

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)

[William Whitaker]

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

[Joe Smigiel]

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.

[mediopollito]

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!

[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

[Tracy Storer]

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.