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03-20-2015, 03:12 AM - 1 Like   #1
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16-85 lens: Uneven Exposure?!

From the In-depth review of the 16-85 lens:
Exposure is somewhat uneven across the aperture range. Wide-open the lens underexposes. Stopping down half a stop increases exposure to being correctly exposed. Exposure then drops back into underexposure again until F16, where exposure increases some.

Read more at: HD Pentax-DA 16-85mm F3.5-5.6 Review - General Image Quality | PentaxForums.com Reviews

What can be the cause of this uneven exposure?

Do the aperture blades not close and open properly?
Is the lens a bad copy?
Does the lens get another aperture value than the one given by the metering?
Is there another explanation? I find it odd.

Regards
khenna




03-20-2015, 08:47 AM   #2
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An electronic reading (exposure) has to be translated to a mechanical setting (aperture) there is going to be small variations. In the industry I work in we have a term for small things like that 'mice nuts'
03-21-2015, 03:38 AM   #3
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QuoteOriginally posted by Painter Quote
An electronic reading (exposure) has to be translated to a mechanical setting (aperture) there is going to be small variations. In the industry I work in we have a term for small things like that 'mice nuts'
I wouldn't call stopping down half a stop 'mice nuts'. Also, normally exposure values are in- or decreased by one-third of a stop, half a stop seems unreal and cannot be explained by 'mice nuts' only. Variations within a third of a stop are normal and don't need to be mentioned.
The extra one-sixth of a stop wide-open is strange and puzzling. So, what can it be?
03-21-2015, 04:03 AM   #4
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QuoteOriginally posted by khenna Quote
From the In-depth review of the 16-85 lens:
Exposure is somewhat uneven across the aperture range. Wide-open the lens underexposes. Stopping down half a stop increases exposure to being correctly exposed. Exposure then drops back into underexposure again until F16, where exposure increases some.

Read more at: HD Pentax-DA 16-85mm F3.5-5.6 Review - General Image Quality | PentaxForums.com Reviews

What can be the cause of this uneven exposure?

Do the aperture blades not close and open properly?
Is the lens a bad copy?
Does the lens get another aperture value than the one given by the metering?
Is there another explanation? I find it odd.

Regards
khenna


Overall it seems like under exposed. But I mean really who should expect an electronic sensor to expose exactly the way our brain would interpret the original scene on side or even more complex how it should be exposed from an artistic point of view?

The photographer should not accept the default exposure all the time and should instead choose the one that is really a match from an artistic point of view. With modern sensors the key thing is to avoid over exposure while a bit of under exposure is nothing to wory about. A bit of post well done post processing create the mood, style and help subtain the point/subject of the photo.

Technically, It look like the 16-85 would just benefit of +0.3/0.5 exposure by default on camera but this might be a lot linked to the metering mode and exact scene.

But really while the test has value, I would say to really state there an issue, the testing should be done on a few different scenes with different lighting/colors/tones and with different exposure mode. To really see what's going on. To see if the exposure bias doesn't come from camera, one should also try with another lense (like the 18-135) to see if that's not just the camera fault here.

03-21-2015, 05:48 AM   #5
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Unless I see a series of shots against a uniformly lit surface, and the measure of grey scale at each aperture I will take nothing for granted.

What mode of metering did you use, is the metering making assumptions based upon aperture, etc, for what should be the correct exposure?

Let's go back to basics.

I am not saying your observations are wrong, just that the method used to make the observation is not controlled properly.

I Have also seen lenses with inaccurate apertures,monotony the tamron 28-75/2.8 which starts out correct wide open but which drifts gradually to over expose by one stop at F32. So I believe it can happen. I just want to see a better test
03-21-2015, 06:09 AM   #6
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QuoteOriginally posted by khenna Quote
I wouldn't call stopping down half a stop 'mice nuts'. Also, normally exposure values are in- or decreased by one-third of a stop, half a stop seems unreal and cannot be explained by 'mice nuts' only. Variations within a third of a stop are normal and don't need to be mentioned.
The extra one-sixth of a stop wide-open is strange and puzzling. So, what can it be?
The difference between your acceptable 1/3 stop and your unacceptable 1/2 stop is most certainly "mice nuts".
I make bigger adjustments in PP on just about every image I have ever taken.
03-21-2015, 06:29 AM   #7
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QuoteOriginally posted by khenna Quote
I wouldn't call stopping down half a stop 'mice nuts'. Also, normally exposure values are in- or decreased by one-third of a stop, half a stop seems unreal and cannot be explained by 'mice nuts' only. Variations within a third of a stop are normal and don't need to be mentioned.
The extra one-sixth of a stop wide-open is strange and puzzling. So, what can it be?
We don't know the metering mode, if the lense or camera was defective had maybe a cateyes or something know to mess up with exposure... if it would happen consistantly all the time or only on the specific scene used...

04-15-2015, 10:25 AM   #8
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Ok, so I finally got around to doing a more controlled test of the 16-85 and exposure.

For those who don't know me, my favourite test area is a concrete block wall on the south side of my house.

For the test I select spot metering and meter on the same block all the time, set the ISO to a single value, and run through the apertures letting the camera pick shutter speed,

The measurement is done using the approximately central 10-15% of the total frame (the same block I focus on) and wiht my photo editor, recording the mean greyscale value for the selection area.

So here are the results.

At16 mm the exposure Starts wide open at 120 greyscale, which from experience is where Pentax sets their nominal exposure. As the lens stops down to F5.6 the exposure drops about 17 greyscale (~1/3 of a stop), and from F5.6 through F22 it rises approximately linear to a greyscale value of 140 or about +1/2 stop, but over most of the range is within +/- 1/3 of a s top of nominal, and given the camera response in 1/3 stop increments there is always a risk of being +/-1/3 of a stop difference any way


At 85 mm however, the response is quite different and the exposure is a little irregular vs aperture. The best way to characterize it is wide open, exposure is spot on, but above F6 the lens will over expose consistently by 1 stop +/-1/3 of a. Stop.

As a result, unless shooting wide open, at maximum focal length, I would suggest setting EV comp at -1

I did not try any measurements in the middle of the focal length range.

I suspect the issue is with all of the mechanical adjustments in the lens as it zooms, the aperture does not always stay at the nodal point of the lens
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