Members: 77,756   Posts: 1,717,658   Online: 791

# Thread: How to Figure Friggin Focal length and f Stop Thingies.

1. ## How to Figure Friggin Focal length and f Stop Thingies.

2. yeh Jason, i kinda unnerstand

you've worked out the relative f stops

i'm a liddle confused, is that relative to pasta divided cheetohs or is it relative to your mum's door?

Ray

3. interesting...but the f-stop calculation doesn't seem to work on a known lens I tested -- is this only for a certain type of lens?

I'm in Maryland...we use crab claws, not pasta

fortunately, I prefer pasta to crabs

4. Originally Posted by DarkroomExperimente
interesting...but the f-stop calculation doesn't seem to work on a known lens I tested -- is this only for a certain type of lens?

I'm in Maryland...we use crab claws, not pasta

fortunately, I prefer pasta to crabs
It's a fundamental equation. Variables would be the actual location of the nodal point of the lens, the actual diameter used, and infinity focus. If you miss the nodal point, your focal length value will be off by that amount. If you focus on something too close, the focal length measured will be greater than the actual focal length of the lens. The longer the lens is, the more likely this is to be the case. If you measure larger than the diameter of the effective objective, like the barrel diameter, this would also skew the result.

5. "actual diameter used"....perhaps that's the issue

I used my Konica lens...so the only measurement I didn't read off the lens was the diameter of the element

6. Did you get the coal for Christmas?

Grin.

I sent this link to all my photo students. Thanks.

7. That is brilliant, John, I mean Jay. You really are John Belushi reincarnated, aren't you?

8. While there's a madness to his method - there's a method to his madness!

Meanwhile, I'd like to figure out his fetish with cheese. Cheetoz and (presumably) cheese tortelini figure prominently in this latest installment.

Quite odd.

"Paging Dr. Freud, paging Dr. Freud - attend STAT - misplaced Wisconsin cheesehead is found in Utah!"

9. Nice job! You're a funny guy.

10. Originally Posted by DarkroomExperimente
"actual diameter used"....perhaps that's the issue

I used my Konica lens...so the only measurement I didn't read off the lens was the diameter of the element
Its only a very approximate measure because it ignores pupil magnification. The actual aperture is the size of the entrance pupil which you can't see as it's hanging in space inside the lens. Its not the physical size of the iris.

Page 1 of 4 1234 Last

 APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY: