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Bosaiya
31-Jul-2009, 16:49
Would someone take pity on me and point me in the direction of a formula for determining the f/stop of an unmarked lens? I've been searching for calculations and formula but keep getting the wrong kind of f/stop calculator. I know it's been posted but I can't seem to find it anywhere.
Blueberrydesk
31-Jul-2009, 17:06
Aperture diameter divided by focal length. A 25mm aperture opening on a 50mm lens would be f2. (really f 1/2, but the numerator is usually dropped)

Or is the lens you have a barrel lens with no aperture? The math would be the same, but I'll let someone else comment on how to determine aperture size. I don't know if it would just be the diameter of the front (or rear) element.
Bosaiya
31-Jul-2009, 17:10
Or is the lens you have a barrel lens with no aperture? The math would be the same, but I'll let someone else comment on how to determine aperture size. I don't know if it would just be the diameter of the front (or rear) element.

Thanks! Yes, a barrel lens. I'm not sure what to measure.
Greg Lockrey
31-Jul-2009, 17:11
Aperture diameter divided by focal length. A 25mm aperture opening on a 50mm lens would be f2. (really f 1/2, but the numerator is usually dropped)

Or is the lens you have a barrel lens with no aperture? The math would be the same, but I'll let someone else comment on how to determine aperture size I don't know if it would just be the diameter of the front (or rear) element.

At the focal length....
Donald Miller
31-Jul-2009, 17:15
Paul, I think you have the formula backwards. Should be focal length divided by entrance pupil diameter.

Example: 360 mm focal length divided by 40 mm entrance pupil diameter equals F9. As lens is stopped down to 16.36 mm it becomes F22.

Your barrel lens focal length divided by the front element diameter will get you near enough for usage.
Blueberrydesk
31-Jul-2009, 17:26
You're right Donald. As usual I'm thinking upside down. :)
Bosaiya
31-Jul-2009, 17:43
Paul, I think you have the formula backwards. Should be focal length divided by entrance pupil diameter.

Example: 360 mm focal length divided by 40 mm entrance pupil diameter equals F9. As lens is stopped down to 16.36 mm it becomes F22.

Your barrel lens focal length divided by the front element diameter will get you near enough for usage.

Now I'm finding that determining the focal length is no easy task. Nodal points and all that.
Blueberrydesk
31-Jul-2009, 17:52
Bosaiya, if you can post a pic of the lens there's a good chance someone here will recognize it and tell you what it is. Not me, of course...but someone. :)
Bosaiya
31-Jul-2009, 18:07
Bosaiya, if you can post a pic of the lens there's a good chance someone here will recognize it and tell you what it is. Not me, of course...but someone. :)

If only it were that easy - It's a cobbled together job.

I'm pretty sure it's around an f/3.x or something. I have a meter with a ground glass attachment and between that and a few other methods that won't be mentioned it seems a little faster than a known f/4.7 that I have in the same mount. I was just hoping there was an easy formula I might be able to apply. I think it's good enough to do some tests with.
Andrew
31-Jul-2009, 18:40
I was going to suggest putting the lens in front of an slr body to get a reading off the inbuilt light meter and then working back from that but you've already done something very similar...

given the exposure latitude of most films if you're correct to half an f-stop [or even a bit more] you'll never notice the error
Bosaiya
31-Jul-2009, 20:05
I was going to suggest putting the lens in front of an slr body to get a reading off the inbuilt light meter and then working back from that but you've already done something very similar...

given the exposure latitude of most films if you're correct to half an f-stop [or even a bit more] you'll never notice the error

I tried that towards the end as confirmation, but I blew it using a dSLR that had the lens coupled to the body (it freaked out without the lens attached). I was able to use my noggin and kept the lens on, focusing on the back of the LF lens. I'll run some sheets of film through tomorrow to see what I end up at.

It's sort of like an adventure.
Andrew
1-Aug-2009, 01:39
I blew it using a dSLR that had the lens coupled to the body (it freaked out without the lens attached).
there's something to be said for old technology...
if you'd used an older film slr it'd have just given you a shutter speed without tossing a hissy fit
Bosaiya
1-Aug-2009, 08:31
there's something to be said for old technology...
if you'd used an older film slr it'd have just given you a shutter speed without tossing a hissy fit

But then I'd miss out on the paradox of mixing ultra-modern with antique.
jnantz
1-Aug-2009, 09:35
the whole nodal point throws me for a loop too ...
i usually just measure from the rear element,
or s if someone suggests the iris ...

i have been not too far off ... and i never worried much about it
seeing i usually use these things with b/w film
(and i'm not too worried about exactness ... )


But then I'd miss out on the paradox of mixing ultra-modern with antique.

i LOVE that!
Bosaiya
1-Aug-2009, 09:56
I'm a file-to-fit kind of guy. I never stop learning. For example I keep learning that if I want to make one of something I need to buy enough for at least two - inevitably I will screw things up along the way.
Ernest Purdum
2-Aug-2009, 16:32
For focal length, if you ave enough bellows you can first focus at infinity, mark your rail or bed, then focus on a scale to get 1:1 on the groundglass. The distance moved is the focal length.

For aperture, it sounds silly, but it is the size the diaphragm seems to be when seen from the front that matters. There are several ways of getting a fairly accurate measurement of this and I'm sure you'll find them here by using the search.

The math is easy enough even for me. Just focal length divided by effective aperture.