On a Canonet QL17, should auto exposure open wider than about f/2 at maximum aperture?
Asked 3/28/2021
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On my Canonet QL17, with the shutter cocked, the aperture opens fully in manual mode at f/1.7. But in AE mode, when I half-press the shutter to engage the meter/exposure linkage, the iris seems to stop at roughly the same opening as manual f/2 and never reaches the full f/1.7 opening.
I can see the mechanical coupling between the meter needle, the trapping/intercept mechanism, and the EE lever that sets the iris. There is also an adjustment cam/screw that appears to affect this limit. I could adjust it so AE reaches the same opening as manual f/1.7, but before changing anything I want to know whether this behavior is normal on a QL17.
Is AE on the QL17 expected to stop around f/2 in some cases, or should it always be able to reach the full f/1.7 opening?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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I've seen this occur as a limitation on meter range in my own QL17.
The way it works is this: depending on the ASA speed setting for the meter, the camera may not be capable of shooting auto-aperture at "wide open" before the meter sensitivity bottoms out. Use slower film and the meter can still "see" when correct exposure needs that addition one third stop. In other words, for a given shutter speed, f/2 for ISO 400 may be too dim for the meter, and the needle doesn't drop the rest of the way (hence can't be trapped at f/1.7), but if you set the ASA dial to 200, the meter won't run out of range before getting to maximum aperture.
Adjusting the movement to open wider may result in 1/3 stop overexposure throughout the entire range -- which isn't a terrible thing; even slide films like Velvia and Ektachrome (famous for narrow exposure latitude) can handle a third of a stop more or less light than perfect exposure.
Originally by user89902. Source · Licensed CC BY-SA 4.0
user89902
3y ago
0
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This can be normal. On the QL17, AE depends on the meter’s range as well as the mechanical linkage. If the meter bottoms out before reaching the last fraction of a stop, the auto system may stop around f/2 instead of opening fully to f/1.7. That can vary with the ASA/ISO setting: with faster film settings, the meter may run out of range sooner; with slower settings, it may still allow the lens to open all the way.
So AE not reaching full f/1.7 does not automatically mean the camera is faulty. Re-adjusting the cam so it always opens wider could shift exposure accuracy and potentially cause about 1/3 stop overexposure across the range.
A less likely possibility is that the mechanism was built from or adjusted with parts/calibration closer to a QL19/full-stop design, especially on an older serviced camera. But based on the answers, meter-range limitation is the most plausible explanation.
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