Pentax 645N 45mm - aperture ring not matching viewfinder display?

tomfrh · May 18, 2015

Hi,

I have a pentax 645N and am trying out a 45mm lens.

With the 45mm lens the viewfinder aperture will show half a stop less than the aperture ring setting. E.g. f/22 on the aperture ring appears as f/19 in the viewfinder. Below f/5.6 they match. This has occurred with both the 45mm lenses I've tried. My 75mm lens works correctly.

Can someone explain why this might be occuring? How does the lens tell the camera what setting it is using? How is the aperture information passed to the lens? Electronically via the contacts or mechanically via a lever?

If I use aperture priority mode am I correct in assuming the camera will be metering wrongly by half a stop?

Tom

tomfrh · May 18, 2015

...I've found in an old thread that this is a well known problem with this system and you have to live with it. So my question is -when will this issue affect exposure?

Presumably the camera will be exposing according to the aperture setting in the viewfinder?, and any time this occurs with manually set apertures (be it manual mode or aperture priority) the metering/exposure will off by half a stop?

Also, when aperture ring is set to "A", it stops down correctly according to what's in the viewfinder. E.g. if program mode says "f/22" in the finder, then the aperture will stop down to f/22, i.e. the same size aperture as if you'd set f/22 on the aperture ring....

pronvit · May 18, 2015

Don't know much details, but there's a mention of this effect in the official P645N manual, so I suppose it's ok.

tomfrh · May 19, 2015

pronvit said:
Don't know much details, but there's a mention of this effect in the official P645N manual, so I suppose it's ok.

How about that! I just read the manual and there it is! Thanks for the information. The manual is a bit vague though. It just says the finder display will "approximately" match the aperture ring.

I think you're right that it's ok. I just did some metering tests on my monitor to check.

Aperture set to f/16 (viewfinder shows f/13), shutter speed set to 4 seconds, meter is dead centre.
Aperture set to f/16 (viewfinder shows f/13), shutter set to A, viewfinder says 4 seconds
Aperture set to A, shutter speed 4 seconds - aperture in viewfinder says F16

So it seems that f/16 is the correct aperture in this instance, and the "f/13" viewfinder value when you set the aperture ring is simply a quirk.

pentaxuser · May 19, 2015

Ken Rockwell mentions it, I think, in his very good review of the P645N.

pentaxuser

tomfrh · May 19, 2015

pentaxuser said:
Ken Rockwell mentions it, I think, in his very good review of the P645N.

pentaxuser

I couldn't find where he mentions it?

pentaxuser · May 19, 2015

tomfrh said:
I couldn't find where he mentions it?

That's because he doesn't, having checked his review again. My apologies, I was sure it was his site. The only other site I occasionally visit is Pentaxforums so I can only think it was there. Worth a look but just bear in mind that 80% of the posters only talk about the latest Pentax digi MF. However the posts about lenses and the lenses reviews are covered.

pentaxuser

tomfrh · May 27, 2015

pentaxuser said:
That's because he doesn't, having checked his review again. My apologies, I was sure it was his site. The only other site I occasionally visit is Pentaxforums so I can only think it was there. Worth a look but just bear in mind that 80% of the posters only talk about the latest Pentax digi MF. However the posts about lenses and the lenses reviews are covered.

pentaxuser

You were right orginally, it is there - except he mentions it his 645 lens reviews, not the 645N review. I just stumbled upon it....

pentaxuser · May 27, 2015

tomfrh said:
You were right orginally, it is there - except he mentions it his 645 lens reviews, not the 645N review. I just stumbled upon it....

Thanks for that. I can now cancel my memory-loss doctor's appointment Seriously not many would bother to let me know. Nice of you to do so.

pentaxuser

Pentax 645N 45mm - aperture ring not matching viewfinder display?

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