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08-31-2010, 01:59 PM   #16
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Lowell,

Is there any aperture at which manual lenses meter the same way as automatic lenses?

I've been pondering through why the camera behaves like it does. With an automatic lens, the camera consistently meters a very wide range of light; with a manual lens, and when metering a scene that is well within the limits of the meter's ability, exposure is calculated differently at different apertures. So it isn't about the amount of light. Which makes me wonder whether the angle of incidence matters for the light sensors. With a large aperture, there's a wide range of angles of incidence; with small aperture, a low range. If that matters, and the camera makes adjustments depending on the aperture... then perhaps with a manual lens, when the camera doesn't know the aperture, it assumes a given aperture for its calculations. If an aperture is assumed, then any manual lens should meter "properly" when set to that aperture.

I think I'll do some tests with my A lens. But if there's somewhere that automatic and manual lenses reliably agree on exposure, then one could measure at that aperture and simply reciprocate shutter/aperture adjustments from that baseline. Just a wild guess, and I'm sure someone else has tried it, but, heck, today's my Friday.

08-31-2010, 02:12 PM   #17
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No luck. With one scene, f/9.5 metered the same automatically as manual + green button. With another scene, f/9.5 was different by a full stop. Hm.
08-31-2010, 02:50 PM   #18
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QuoteOriginally posted by JonPB Quote
Lowell,

Is there any aperture at which manual lenses meter the same way as automatic lenses?

I've been pondering through why the camera behaves like it does. With an automatic lens, the camera consistently meters a very wide range of light; with a manual lens, and when metering a scene that is well within the limits of the meter's ability, exposure is calculated differently at different apertures. So it isn't about the amount of light. Which makes me wonder whether the angle of incidence matters for the light sensors. With a large aperture, there's a wide range of angles of incidence; with small aperture, a low range. If that matters, and the camera makes adjustments depending on the aperture... then perhaps with a manual lens, when the camera doesn't know the aperture, it assumes a given aperture for its calculations. If an aperture is assumed, then any manual lens should meter "properly" when set to that aperture.

I think I'll do some tests with my A lens. But if there's somewhere that automatic and manual lenses reliably agree on exposure, then one could measure at that aperture and simply reciprocate shutter/aperture adjustments from that baseline. Just a wild guess, and I'm sure someone else has tried it, but, heck, today's my Friday.
I would bet between F4-5.6 since that is what most kit lenses are. This would be consistent with my graph as well. Your proposal is also sound, meter at that point and manually change afterwards, but oh what a pain in the but compared to just pushing a button like my *istD

as to your understanding of the issue, I think you hit it right on, the wide aperture lenses hae light at a large number of incident angles hit the focusing screen, and they reflect back to the meter sensor, causing much more reflection than expected from the screen, more reflections = more perceived light at large apertures, and as a result the camera over-compensates and under exposes. I think at very small apertures there is something else at play that causes the exposure to come back down towards correct, perhaps scatter from defraction or something but that is only a guess.
09-02-2010, 09:01 PM   #19
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What I find interesting, after some brief testing, is that my manual lenses seem to meter for the same exposure at a given aperture--even if I consider that exposure to be under- or over- exposed. I.e., my Auto Tak 55/1.8 and ST 35/3.5 meter the same at a given aperture, which gives me hope for an easy (if slow) way to adjust. Up until now, I had assumed that if the meter readings were different, they'd be unique for each lens. Now, I'm thinking that I'll have to remember something like "f/4 requires +1 EV from the meter," regardless of lens, which seems quite workable.

No, my manual lenses + GX10 don't lend themselves to snapshops. But the optics are wonderful, at least compared to my kit lens. Well worth the effort when I have a scene that's worth taking the time to capture thoughtfully. Ultimately, it is about the image for me. Sometimes, like for my animated nephew, the time allowed for composition is minimal, which of course requires more automation.

I'm curious to see how well I like an automatic lens' focusing. I don't like, but may still find it preferable, focusing and re-framing. One thing I love about manual focus is that the focal point can be anywhere in the visible frame.

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