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02-28-2014, 11:36 AM   #1
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Pentax 645N apperture mismatch and metering inaccuracy

Hi,

I have just bought used Pentax 645N with 45-85 and 80-160 and 120 macro lenses.

First, I have noticed that no matter what lens aperture number on ring doesn't match what is on display inside viewfinder. My question is what should I trust? Ring on lens or electronics inside the camera?

Second, I have noticed some metering inaccuracy. In shadows, it seems it is off 1EV. Since I am testing/learning how this camera behave. I always bracket shots and all +1.0EV looks normal too me. 0EV is underexposed. BUT, it was scene where was too many similar colours so I guess metering was fooled despite Ken Rockwell says it is spot on only film emulsion compensation may be needed.I have also taken a landscape where is diverse brightness and colours and there is is metering spot on. So I guess it is common problem with automatic metering if one color prevails in scene for instance.

Could you provide further information who has this camera, please?

02-28-2014, 01:35 PM   #2
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Hi,

Congratulations on your new purchases!

A few things that you might want to check since it seems the 645N might be new to you.

1. Setting the film's ISO. You do this by moving a lever under the exposure compensation dial then look to the small LCD window above the on/off switch to see what it is set at. Could be an incorrect ISO setting?

2. The metering pattern is selected by a lever under the shutter speed dial. Green is a dual six zone matrix pattern, middle option is a center weighted pattern and the one all the way to the left is a spot pattern. Might want to set your 645N on a tripod, point it at a variety of scenes and see what the readings are with the various metering patterns. In my experience, the Green (dual six zone matrix) is quite good for my purposes - mostly colour negative film.

Best of luck.

Yuri
03-02-2014, 12:24 AM   #3
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Thanks Yuri, all checked and manual double read in case notorious Chair<>Keyboard syndrome :-D

Looks like Pentax have long history of metering inconsistency and in case of DSLR also AWB. My former Pentax K-7 was most time off exposure metering and AWB (Auto White Balance) always off
03-02-2014, 06:05 PM   #4
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Frankly, I don't have the aperture mismatch at all with my 645N and 6 lenses, or any other Pentax camera, so the rather sweeping statement that Pentax does have this inconsistency does not apply to my cameras at least. In shooting slides, metering has been spot on. That said, I did read about someone who had this issue somewhere in these forums with the 645N I think.

As for the AWB, yes, my wife's K-x does have this problem, but it is hardly related to the aperture issue.

03-02-2014, 07:23 PM   #5
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Load the camera with e.g. Velvia 50 or 100 transparency film. With this, you have a very narrow working range to get exposure spot-on, and this is the ideal way to test a camera, assuming you have a good working knowledge of the idiosyncracies of transparency film and the values with which it returns the best results. Go out in conditions other than bright point light (ideally hazy to diffuse) and shoot. Expose the film in both shadow and highlight situations. Take notes. Process the film. If there are glaring inconsistencies with all of the exposures, then a problem may be indicated, assuming the ISO has been set correctly and there is no compensation active on the camera. THEN and only then draw assertions as to the accuracy of the camera's metering and/or exposure system. Never draw any sort of value from print film because the latitude will not show up minor problems.

The other thing to consider is the age of the camera. This model first appeared in 1984. So how many years ago is that?? Reliability of cameras can and does diminish over time, more so if they have seen constant hard professional service. If basic self-tests like the above return consistently abnormal results, the best thing to do is have the camera fully serviced.
03-03-2014, 11:24 AM   #6
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QuoteOriginally posted by Silent Street Quote
Load the camera with e.g. Velvia 50 or 100 transparency film. With this, you have a very narrow working range to get exposure spot-on, and this is the ideal way to test a camera, assuming you have a good working knowledge of the idiosyncracies of transparency film and the values with which it returns the best results. Go out in conditions other than bright point light (ideally hazy to diffuse) and shoot. Expose the film in both shadow and highlight situations. Take notes. Process the film. If there are glaring inconsistencies with all of the exposures, then a problem may be indicated, assuming the ISO has been set correctly and there is no compensation active on the camera. THEN and only then draw assertions as to the accuracy of the camera's metering and/or exposure system. Never draw any sort of value from print film because the latitude will not show up minor problems.

The other thing to consider is the age of the camera. This model first appeared in 1984. So how many years ago is that?? Reliability of cameras can and does diminish over time, more so if they have seen constant hard professional service. If basic self-tests like the above return consistently abnormal results, the best thing to do is have the camera fully serviced.
I usually shoot velvia 50 or provia 100f.

The scene I noticed inaccurate metering is this waterfall Photo of the month – January | David "HAJES" Hajek Fine Art Photography (BETA) (this picture is from DSLR)

I bracketed this scene. EV0 underexposed. EV+1.0 spot on at least what I have seen on light table. There is overblown highlights which may fool metering of camera.

An other scene was landscape reflection in water with Provia 100F and EV0 was spot on.

Otherwise, as someone said...I already take notes and will adjust accordingly.

Camera came from respectable German Dealer. But I guess they just check if camera works mechanically and that's it.

thanks for suggestion guys

---------- Post added 03-03-14 at 11:27 ----------

QuoteOriginally posted by Smolk Quote
Frankly, I don't have the aperture mismatch at all with my 645N and 6 lenses, or any other Pentax camera, so the rather sweeping statement that Pentax does have this inconsistency does not apply to my cameras at least. In shooting slides, metering has been spot on. That said, I did read about someone who had this issue somewhere in these forums with the 645N I think.

As for the AWB, yes, my wife's K-x does have this problem, but it is hardly related to the aperture issue.
I am not sure whether you have understood my question or just my english is weak.

I have asked for two unrelated issues.

Aperture ring and viewfinder F number doesn't match.

And metering inconsistency. Unsure, why you refer aperture is cause of metering inconsistency.
03-03-2014, 03:07 PM   #7
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The image of the waterfall is correctly exposed to my eyes, but DSLRs are notorious for burning out highlights at the expense of everything else. Where water is thick and tumbling, the longer the exposure in seconds, the greater the likelihood of even minor spectrals burning out. Essentially, you have lost detail in that foreground thick, white tumble. Elsewhere, there is no evidence of adverse exposure in this image, with excellent striated detail retained in the major flow of the water as would be expected. A major error I see with photographers is using a shutter speed of around 30 seconds to photograph a simple stream or a waterfall. The result of this is that light striking spectrals will burn out very early in the piece and the water itself, where detail would otherwise be retained and highly visible, is completely lost. With MF and e.g. RVP 50, 1/4 to 1/2 second exposures in diffuse (not hazy, and definitely not bright light) are a good starting point.

This, however, is odd and strikes me as incorrigible:

EV0 underexposed. EV+1.0 spot on at least what I have seen on light table.

There is a significant difference there and that difference is too much when using e.g. RVP 50 it introduces something like Russian Roulette in anything other than benign uncomplicated lighting. I am not sure if you have had the camera professionally examined as would be a good idea.
03-04-2014, 01:18 AM   #8
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QuoteOriginally posted by Silent Street Quote
The image of the waterfall is correctly exposed to my eyes, but DSLRs are notorious for burning out highlights at the expense of everything else. Where water is thick and tumbling, the longer the exposure in seconds, the greater the likelihood of even minor spectrals burning out. Essentially, you have lost detail in that foreground thick, white tumble. Elsewhere, there is no evidence of adverse exposure in this image, with excellent striated detail retained in the major flow of the water as would be expected. A major error I see with photographers is using a shutter speed of around 30 seconds to photograph a simple stream or a waterfall. The result of this is that light striking spectrals will burn out very early in the piece and the water itself, where detail would otherwise be retained and highly visible, is completely lost. With MF and e.g. RVP 50, 1/4 to 1/2 second exposures in diffuse (not hazy, and definitely not bright light) are a good starting point.

This, however, is odd and strikes me as incorrigible:

EV0 underexposed. EV+1.0 spot on at least what I have seen on light table.

There is a significant difference there and that difference is too much when using e.g. RVP 50 — it introduces something like Russian Roulette in anything other than benign uncomplicated lighting. I am not sure if you have had the camera professionally examined as would be a good idea.

Image from DSLR and transparency were compared and my Canon 5D.2 is always consistently spot on with exposure metering and AWB. Also my good old "75MPx" Canon 3D was always spot on so I wonder whether is that common Pentax feature or just my camera is broken. I have seen this phenomen by my ex-DSLR Pentax K-7

Looks like, I will have to send camera somewhere for inspection once I find professional service in Europe.

Velvia is known for its sensitivity of correct exposure.
Thanks for suggestions

03-04-2014, 01:46 AM   #9
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QuoteOriginally posted by sniper29a Quote
Hi,

I have just bought used Pentax 645N with 45-85 and 80-160 and 120 macro lenses.

First, I have noticed that no matter what lens aperture number on ring doesn't match what is on display inside viewfinder. My question is what should I trust? Ring on lens or electronics inside the camera?
From the 645N manual - page 41:
4. The f-stop that you have selected and appropriate aperture indication determined by the camera can be seen in the viewfinder when the shutter release button is depressed halfway.
The approximate aperture indication appears in the viewfinder. It may not be the same indication as you select on the lens aperture ring, especially when the A645 150 f/3.5 or A 645 45-85 Zoom f/4.5 is attached.

[Edit:]Also on Ken Rockwells site - his review of Pentax-A 645 150mm F3.5:
"The weirdest thing about it, as cautioned in the Pentax 645N manual, is that in aperture-priority mode it reads, and film-edge-imprints, as f/19 when set to f/22 and f/27 when set to f/32. Weird, but nothing that even a geek like me would worry about. Pentax mentioned this in the manual so we wouldn't freak out."

Last edited by LaHo; 03-04-2014 at 06:22 AM. Reason: addition
03-05-2014, 04:37 AM   #10
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There are certain lenses with which the inconsistency between aperture ring and camera aperture display are present. I don't remember exactly which lenses but I think there are several. But always trust the aperture ring, not what it says in the viewfinder.

As for underexposure, there have been times when I've seen some of this but here is a link to a couple rolls shot on color slide film which provides very little margin for error and I was pleased with the results.
link so if you are consistently getting underexposure you may have a mechanical problem with your camera.
03-05-2014, 09:25 AM   #11
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QuoteOriginally posted by LaHo Quote
From the 645N manual - page 41:
4. The f-stop that you have selected and appropriate aperture indication determined by the camera can be seen in the viewfinder when the shutter release button is depressed halfway.
The approximate aperture indication appears in the viewfinder. It may not be the same indication as you select on the lens aperture ring, especially when the A645 150 f/3.5 or A 645 45-85 Zoom f/4.5 is attached.

[Edit:]Also on Ken Rockwells site - his review of Pentax-A 645 150mm F3.5:
"The weirdest thing about it, as cautioned in the Pentax 645N manual, is that in aperture-priority mode it reads, and film-edge-imprints, as f/19 when set to f/22 and f/27 when set to f/32. Weird, but nothing that even a geek like me would worry about. Pentax mentioned this in the manual so we wouldn't freak out."
I have read Ken's thoughts about the camera. But missed info about lenses. I see it is quite common problem. I have got SMC-A 45-85, SMC-A 80-160, SMC-A 120 macro. All mentioned lenses show what was described earlier. I do not see focal lens imprinted on negative. I always use lens ring as I consider it as accurate.

---------- Post added 05-03-14 at 09:29 ----------

QuoteOriginally posted by revdocjim Quote
There are certain lenses with which the inconsistency between aperture ring and camera aperture display are present. I don't remember exactly which lenses but I think there are several. But always trust the aperture ring, not what it says in the viewfinder.

As for underexposure, there have been times when I've seen some of this but here is a link to a couple rolls shot on color slide film which provides very little margin for error and I was pleased with the results.
link so if you are consistently getting underexposure you may have a mechanical problem with your camera.
Thank you sir, that's what I wanted to hear. I trusted always ring since it is mechanical with lever by flange/mount of lens.

I will shoot a test roll with common scenes I shoot. If I see consistent results I will compensate according my notes. Galen Rowell has also written that every camera measures different. For instance his NIkon F4 with Kodachrome ASA64 gave best results at ASA80 EV+1/3

If I don't get consistent reading after test, I will seek an expert for reapair
03-05-2014, 11:44 PM   #12
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Lens focal length data only gets imprinted on the film when using FA series auto focus lenses. All six of my lenses are the older A series manual focus version so I never get focal length info either.
04-10-2014, 03:42 PM   #13
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QuoteOriginally posted by sniper29a Quote
I am not sure whether you have understood my question or just my english is weak.
sniper, I probably misunderstood your English, and my phrasing was not too accurate with hindsight.
On another note, perhaps that ‘long history of metering inconsistency’ is consistent. It is known, I believe, that Pentax underexposes rather than overexposes on purpose, I recall such observations in comparison with Canon/Nikon, but others will be better placed to comment on that. And of course it also depends on metering points, which in the 645N are far fewer than in a modern dslr.
Just FYI, my 645-A 120mm shows the same aperture on the lens and in the viewfinder (as well as the A 3.5/35mm, FA 2.8/75mm, FA 2.8/150mm and FA 4/200mm). My 67 4/55mm does not show the aperture in the finder. I do not remember it on the FA 4.5/35-85mm and A 2.8/75mm either but I no longer have these.
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