Could a Lens Adversely Affect Meter Readings on an SLR?

bvy · Jun 12, 2017

This is an offshoot from (there was a url link here which no longer exists), but a question in general. Could a lens be responsible for an SLR giving bad meter readings? In brief, here’s what’s going on: I acquired three Sigma SLR’s, and all three (!) are giving faulty meter readings with the lens I have -- two to three stops under. I only have one SA-mount lens -- the 28-80 AF kit lens that came with it. I’m trying to determine if somehow the lens could be the culprit, or if (unbelievably to me) I got triply unlucky and ended up with three bodies with bad meters. It was part of an auction lot, but I want to perform due diligence before approaching the seller.

Yes to fresh batteries, and checking all the settings (ISO, exposure compensation, etc.).

Furthermore, is there a way to test the meter with no lens attached? Or by jerry-rigging some other lens over the mount?

Thanks.

AgX · Jun 12, 2017

To my understanding, no.
However there are cases (in the Canon range) where with certain combos between lenses and extenders the metering will be off, as explained in the manuals.
This applies for properly working lenses (see the comments on metering-linkages below).

A camera needs no lens for (some) metering. At least for comparing bodies a test without a lens can be set up. With bodies put in exactly the same position and indirect lighting as a white screen. Alternatively a opal glas, again in exactly repeatable lighting set-up, put onto the mount.

thuggins · Jun 12, 2017

It would depend on the make of camera and how it "reads" the lens setting for metering. Back when cameras used stopped-down metering this was never a problem as you metered at the aperture you were going to shoot at. But all modern cameras need some sort of linkage or connection to compensate the meter for the number of stops between a fully open lens and the setting of the aperture ring. The basic problem is that different lenses have a different "zero" setting (fully opened aperture).

You can clearly see this in the Pen FT's metering system, where there is no linkage between the lens and camera. If you don't know which stop equates to zero, the meter reading does you no good. Another example are "non-auto" extension tubes, that don't have a meter linkage in them. These have to be used with stopped down metering.

Paul Howell · Jun 12, 2017

As I thought about this, as all 3 bodies are giving the same reading and all 3 likely have different levels of discoloring in the viewfinder, my best guess is that the lens is sending a faulting reading to the meter, telling the meter that the lens is set say F22. Set the camera to manual, open the film door set the shutter to 1 second, the F stop to wide open and see if it wide open, then stop it down to f16 and see if the stops down. but even if the lens is stopping down does not mean the lens chip is working.

Ian Grant · Jun 12, 2017

Choice of Focal length affects how a TTL meter reads a scene but this is impacted more by the placement of highlights, shadows etc in a scene.

Ian

Chan Tran · Jun 12, 2017

A lens can certainly effect meter reading for example a lens with the T stop significantly different from the f stop would. (although it still gives you good exposure) but the reading compared with a handheld meter is wrong. However I don't think it's 2-3 stops off.
Do all 3 of your cameras give the same readings? And they are higher or lower than the correct readings? And by how much?

bvy · Jun 12, 2017

Chan Tran said:
A lens can certainly effect meter reading for example a lens with the T stop significantly different from the f stop would. (although it still gives you good exposure) but the reading compared with a handheld meter is wrong. However I don't think it's 2-3 stops off.
Do all 3 of your cameras give the same readings? And they are higher or lower than the correct readings? And by how much?

I compared against my Olympus E-520 (DSLR) and Nikon F100 (SLR) pointing each to a grassy patch under constant light. Both agreed giving 1/8 at f/16 (ISO 100). The Sigma bodies gave 1/2, 1/3 and 1.

bvy · Jun 12, 2017

Paul Howell said:
As I thought about this, as all 3 bodies are giving the same reading and all 3 likely have different levels of discoloring in the viewfinder, my best guess is that the lens is sending a faulting reading to the meter, telling the meter that the lens is set say F22. Set the camera to manual, open the film door set the shutter to 1 second, the F stop to wide open and see if it wide open, then stop it down to f16 and see if the stops down. but even if the lens is stopping down does not mean the lens chip is working.

You've been a great help, Paul. I really thought (still kind of think) that it has something to do with those discolored mirrors. They all have a magenta cast -- not yellow, as seems more common. Anyway, in all my reading and research online, there's a lot of talk about the mirrors discoloring, but no one describes bad metering as a side effect.

AgX · Jun 12, 2017

Many SLRs have got their meter cell located at an extension of the prism block. Thus a fault at that prism indeed can affect metering.

Kawaiithulhu · Jun 12, 2017

AgX said:
meter cell located at an extension of the prism block

Note that this is why SLRs sometimes offer covers to slide over the eyepiece, or like some Nikons a little manual shutter to block out the eyepiece... so that light from the back won't distract your exposures when shooting away from the eyepiece and using a shutter release.

Paul Howell · Jun 12, 2017

My SA 7 and 9 were yellow not magenta, both appeared to be metering ok when I chucked them. I guess you can use them in manual mode using a hand held meter.

RalphLambrecht · Jun 12, 2017

bvy said:
This is an offshoot from (there was a url link here which no longer exists), but a question in general. Could a lens be responsible for an SLR giving bad meter readings? In brief, here’s what’s going on: I acquired three Sigma SLR’s, and all three (!) are giving faulty meter readings with the lens I have -- two to three stops under. I only have one SA-mount lens -- the 28-80 AF kit lens that came with it. I’m trying to determine if somehow the lens could be the culprit, or if (unbelievably to me) I got triply unlucky and ended up with three bodies with bad meters. It was part of an auction lot, but I want to perform due diligence before approaching the seller.

Yes to fresh batteries, and checking all the settings (ISO, exposure compensation, etc.).

Furthermore, is there a way to test the meter with no lens attached? Or by jerry-rigging some other lens over the mount?

Thanks.

Murphy works in strange ways.

Could a Lens Adversely Affect Meter Readings on an SLR?

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