Beginner's lightmeter question

hrothgar41 · Jun 26, 2025

I've shot B&W film on rangefinders with an external lightmeter for several years, but beginning again after a long pause, a really stupid question popped into my mind: how does the lightmeter recognise, or how can it adapt to, the lens power? I'm using a Leica IIIf which so far has had its Elmar 3.5 and has now acquired a Nikkor H 50/2. With an Ilford 400 film I set my Sekonic Twinmate meter for 400 ISO, and it reads aperture 4 at 1/60. But I'm using an 50/2 instead of a 50/3.5: how can I regulate the meter to take that into account?

Chan Tran · Jun 26, 2025

Change to 1/250 if you set the lens at f/2. Or you can simply set the lens to f/4. The dial shows a range of aperture and shutter speed combination you can use.

xkaes · Jun 26, 2025

You don't say what meter you are using. As Chan mentioned, most meters show a range of shutter speed & f-stop combinations that you can choose from -- even though your lens is an f2 lens, does not mean you have to use f2.

Other meters only show one f-stop. On these you need to read the manual to reset the shutter speed to get to the f-stop that you want to use.

Whatever meter you are using will work with any lens. No adjustment is needed. You can shoot the same scene with both lenses at f8. The only difference is that one lens opens up a little more than the other -- f2 vs f3.5 -- but from f3.5 down to f16 they are both the same.

snusmumriken · Jun 26, 2025

I hope I'm understanding your question correctly.

f/4 on a lens with maximum aperture of f/2 passes the same amount of light as f/4 on a lens that can only open to f/3.5. Apertures are expressed in terms of focal length. That's where the 'f/' comes from: the f/ number is the focal length divided by the effective diameter of the iris. The beauty of this is that the same f/ number passes the same amount of light regardless of lens design, maximum aperture or focal length. So relax and use the aperture your meter indicates .

Chan Tran · Jun 26, 2025

xkaes said:
You don't say what meter you are using. As Chan mentioned, most meters show a range of shutter speed & f-stop combinations that you can choose from -- even though your lens is an f2 lens, does not mean you have to use f2.

Other meters only show one f-stop. On these you need to read the manual to reset the shutter speed to get to the f-stop that you want to use.

Whatever meter you are using will work with any lens. No adjustment is needed. You can shoot the same scene with both lenses at f8. The only difference is that one lens opens up a little more than the other -- f2 vs f3.5 -- but from f3.5 down to f16 they are both the same.

He said he used a Sekonic Twinmate that is why I said the meter shows a range of aperture/shutter speed combination.

hrothgar41 · Jun 26, 2025

Thank you all greatly. You have reminded me of what I shouldn't have forgotten - perhaps old age really does soften the brain! Back into the world with a beloved old Leica and new courage......

BrianShaw · Jun 26, 2025

May I make a suggestion that might aid your re-education: search the WWW for a copy of Roger Hicks’ book “Perfect Exposure.” It’s really inexpensive and a great source of information on both basic and advanced light metering, as well as exposure techniques.

wiltw · Jun 26, 2025

Questions are 'dumb' ones only if one first understands how things work, otherwise questions are perfectly fine to ask!

To explain how things work...think of your situation, with a rangefinder cameras with lens, plus external meter as...

lens mounted on camera presents an image to film, lens and lens FL which effects area of view, and it has aperture which effects amount of light striking the film
meter 'sees' and area of view thru its own virtual lens (which has no FL nor aperture) so its area of view never changes and it is only effected by the amount of light striking the scene that it sees

...so changing the lens on the camera (#1) has ZERO EFFECT on the area which is seen by the meter! Changing the lens FL from 28mm to 135mm has no effect on what the external meter sees, and changing lens from f/3.5 to f/1.4 max aperture (or from f/4 to f/22 shooting aperture) has no effect on how much light the meter sees

If one were using an SLR with thru-the-lens meter, only then would changing #1 have any effect on #2, and changing the shooting aperture would change the suggested exposure by the TTL meter!

Beginner's lightmeter question

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