# Thread: Do these figures look right?

1. ## Do these figures look right?

I was working out some minimum focusing distance numbers tonight with the standard equations, but I'm not sure I did it correctly. Do these look OK, or am I missing something?

150mm lens w/270mm bellows draw = 13.25" min focus distance

150mm lens w/360mm bellows draw = 10.1" min focus distance

210mm lens w/270mm bellows draw = 37.2" min focus distance

210mm lens w/360mm bellows draw = 19.8" min focus distance

Sorry for the bother...thanks all!

2. ## Re: Do these figures look right?

Originally Posted by PViapiano
I was working out some minimum focusing distance numbers tonight with the standard equations, but I'm not sure I did it correctly. Do these look OK, or am I missing something?

150mm lens w/270mm bellows draw = 13.25" min focus distance

150mm lens w/360mm bellows draw = 10.1" min focus distance

210mm lens w/270mm bellows draw = 37.2" min focus distance

210mm lens w/360mm bellows draw = 19.8" min focus distance

Sorry for the bother...thanks all!
I only calculated the last one, which is correct.

3. ## Re: Do these figures look right?

Thanks, Ron...if the last one is right, they're all OK. Something just didn't look right to me at the time.

4. ## Re: Do these figures look right?

Just remember, before you start focusing closeup with a tape measure, that y'r results are slightly approximate because you don't know where to measure extension from and because y'r lenses actual focal lengths probably aren't exactly what's engraved on them.

5. ## Re: Do these figures look right?

"you don't know where to measure extension from"? - if he's got a 150mm lens he can measure - at infinity - these 150mm from the nose of her grandmother, if he likes so... 150 is 150. And the difference of a few tenths of mm or even 1-2 mm from the nominal focal length doesn't make any difference in the exposure correction. Common sense.

6. ## Re: Do these figures look right?

Piano, for some reason you use in the first calculation 2 decimal digits. Instead of 13.25 it should be 13.29 - useless precision anyway.

7. ## Re: Do these figures look right?

Thanks, guys...I just needed a ballpark figure...one decimal point ;-)

Also, if my bellows extension is fully deployed and I'm as close as I can focus, will I need to use compensation? When do I require compensation? Is it only when I'm at the 1:1 ratio?

I haven't fooled much with this stuff yet...and I know I need to read the extension article on the LF home page first.

8. ## Re: Do these figures look right?

Originally Posted by PViapiano