[petez]

Lowell:
Thanks for the post and keeping it on track. Resolving the issue with Pentax seems to be the best approach rather that talking about all the work arounds.

All these same lenses are within +/- 1/2 stop over the entire range of f-stops on my *istD.

Clearly, it's not the lenses. Also, it appears not to be shutter speed related. You sound sure it's related to the focusing screen. How can you tell?
But more importantly, the problem occurs not just in metering. If I use a handheld lightmeter and manually set the camera, then I should get the correct exposure.
The screen shouldnt be an issue here.

It is curious to note that my Tamron 28-75 when using the manual apature ring as opposed to the camera controling the apature exhibited the same problem, but is perfect when using the apature controlled by the camera.

Exactly what I see. That made me think it's a software issue where the shutter is opening too soon in Manual Mode when the camera doesnt know the f-stop. So, I did the test at low light (.5 secs to 8 secs) thinking that if it was a timing issue then the impact would be less at longer exposures. Unfortunately, same results.

I'll speak with Pentax and see if he can conduct some more tests based on your excellent records.


Pete

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Last edited by petez; 04-03-2008 at 07:45 PM.

[petez]

I went through this with my K10D, and sent pentax a spread sheet with all the lenses I had tested up to that time.

Did you speak with someone? If so, can you email me their name so I can get them intouch w/ my contact.

[jeffkrol]

Did you speak with someone? If so, can you email me their name so I can get them in touch w/ my contact.

Good luck. You do realize this problem has been known since the k10 was released. And the k20/200(?) still have the same issue.
Personally I think it's a subtle way for Pentax to wean people away from OLD lenses...
Historically, weather it was an oversight or on purpose, the so called compatibility of old lenses even w/ the first D was crippled until a backlash suddenly created a firmware update to allow the "green button" metering. Was it an accident??? hmmm..........
As a side note, the K20 "chatter" regarding this issue is nowhere near that of the k10.
By the next camera release nobody (majority) will really care anymore.

[jeffkrol]

I went through this with my K10D, and sent pentax a spread sheet with all the lenses I had tested up to that time..............
If you plot grey scale vs f-stop (Fstop on a log scale) you will get an s shaped curve streight in the middle, showing the fariation of grey scale with F stops.

sorry if I missed it but did you do extensive testing w a ds screen???????

[Canada_Rockies]

sorry if I missed it but did you do extensive testing w a ds screen???????

I had read on this forum that the stop down actuators were lineal/not lineal, and tested this premise with the combination of M400 on K10D. In the range I used, 5 whole stops from wide open 5.6, 8, 11, 16 and 22, I got the same exposure using reciprocal shutter speed changes. This leads me to believe that for some reason the newer lenses "expect" a brighter screen where the meter looks than the older ones.

My conclusion is that the metering is somehow thrown off, and for the life of me cannot figure why. With the 400, if I meter at f/8, the exposure is correct, but at f/5.6 it is not. The pragmatic solution, of course, is to meter at f/8 and interpolate the settings from there, and that is what I do.

I don't have any more cash to spend on glass for a while, and I like to visit bird refuges, and that means the 400 will see more use. I have tested to figure out how to use it to give me acceptable images. PITA? Sure. Cheap? That, too.

[Lowell Goudge]

sorry if I missed it but did you do extensive testing w a ds screen???????

no, both cameras have their origonal screnes, I have not yet decided what I am going to do, Iunderstand the K10 meters well with the DS screen on manual lenses, but that this upsets automatic lenses and metering. (because of the fudge factor that Pentax has to sort out the K10 Metering) I have also heard that many split image screnes interfere with spot metering.

I use spot metering extensively because I shoot wild life and want what I am focused on properly exposed.

This has left me with a problem that I have not yet figured out fully.

I could put, for example a split image in my *istD, because it meters well, but I use it for flash with K mounts (because it is TTL) and don;t want to loose the spot (again flash for wildlife)

The K10 I use with most AF lenses, so if I change screens to suit Manual apature lenses, I get into trouble with my newer lenses.

As I said, I am kind of stuck for the moment.

[petez]

We really have two issues here - metering and exposure.
The metering is off and may be due to the screen as some evidence suggests.

However, the exposure should be reciprocal. That is, in manual mode, if I set the camera to f8 @1/125th I should get the same exposure at f16 @ 1/60 or f5.6 @1/250th. Exposure errors should not have anything to do w/ the screen. Or am I missing something in digital. WIth film, in manual mode, I got the exposure I asked for - F8 @1/125th was I got. Even on the LX which did meter off the film in other modes, manual mode gives what the camera is set for.

Pete

[Canada_Rockies]

We really have two issues here - metering and exposure.
The metering is off and may be due to the screen as some evidence suggests.

However, the exposure should be reciprocal. That is, in manual mode, if I set the camera to f8 @1/125th I should get the same exposure at f16 @ 1/60 or f5.6 @1/250th. Exposure errors should not have anything to do w/ the screen. Or am I missing something in digital. WIth film, in manual mode, I got the exposure I asked for - F8 @1/125th was I got. Even on the LX which did meter off the film in other modes, manual mode gives what the camera is set for.

Pete

FWIW, I took five shots with my M 400 using reciprocal shutter speeds, based on "sunny 16" and they are all close enough to the same exposure. I put them up on my Flickr site so folks could check them out.

[petez]

FWIW, I took five shots with my M 400 using reciprocal.

Glad to hear it's working.

Is the M400 a Pentax lens?

Pete

[jeffkrol]

We really have two issues here - metering and exposure.
The metering is off and may be due to the screen as some evidence suggests.

However, the exposure should be reciprocal. That is, in manual mode, if I set the camera to f8 @1/125th I should get the same exposure at f16 @ 1/60 or f5.6 @1/250th. Exposure errors should not have anything to do w/ the screen. Or am I missing something in digital. WIth film, in manual mode, I got the exposure I asked for - F8 @1/125th was I got. Even on the LX which did meter off the film in other modes, manual mode gives what the camera is set for.

Pete

OK, the light bulb went on a bit... Yes, in very controlled conditions (controlled light) any reciprocal change in ap/shutter should create all equally exposed photos. One thing to touch on though: This assumes complete linearity of the aperture. No micro changes in lighting, ie a passing cloud/ voltage surge/no flourescent lights or CRT's where you could hit a "pulse", accurate control of shutter speeds during the exposures ect. No green button metering w/ each ap/shutter change. Just meter on one and interpolate from the rest. ........
If you are experiencing more than a 1/2 stop difference in full manual mode when adjusting from a good exposure then there is something wrong w/ your camera/lens.
One factor that may be noted is that this may not apply w/ iso changes due to some of the digital voodoo that occurs in the camera/processing software.
Personally I think sensors are more apt to look more visually lighter/darker w/ subtle changes in light during exposure than one would expect. Film does seem more ..err... forgiving in this aspect. Weather this is real or not I can't say...

[petez]

If you are experiencing more than a 1/2 stop difference in full manual mode when adjusting from a good exposure then there is something wrong w/ your camera/lens.

Thanks for the comment.
I've been eye-balling the exposure differences and believe that there is a lot more than 1/2 stop. I may look at quantifying the differences using the techinque Lowell outlined. But, one thing for sure is that I'm going back to the camera store and trying the test on a different camera w/ my lenses. That should answer this question.

I'm not sure how many other folks have done this reciprocal exposure tests with out green button metering - perhaps it's just me. Hopefully some others will give it a try and let us know.

[Canada_Rockies]

Glad to hear it's working.

Is the M400 a Pentax lens?

Pete

Sorry, it's the 400 in my sigature. SMC-P M 400mm f/5.6
FWIW, I only use Pentax glass. Every time I have tried other brands I have been disappointed. I know this is just my luck, but that's the way it is.

[Lowell Goudge]

OK, the light bulb went on a bit... Yes, in very controlled conditions (controlled light) any reciprocal change in ap/shutter should create all equally exposed photos. One thing to touch on though: This assumes complete linearity of the aperture. No micro changes in lighting, ie a passing cloud/ voltage surge/no flourescent lights or CRT's where you could hit a "pulse", accurate control of shutter speeds during the exposures ect. No green button metering w/ each ap/shutter change. Just meter on one and interpolate from the rest. ........
If you are experiencing more than a 1/2 stop difference in full manual mode when adjusting from a good exposure then there is something wrong w/ your camera/lens.
One factor that may be noted is that this may not apply w/ iso changes due to some of the digital voodoo that occurs in the camera/processing software.
Personally I think sensors are more apt to look more visually lighter/darker w/ subtle changes in light during exposure than one would expect. Film does seem more ..err... forgiving in this aspect. Weather this is real or not I can't say...

I agree with this given one additional point that I made in my way to test. Make sure that the camera is set to 1/2 stop increments, in order to match the lens detents. otherwise, you can sometimes get a surprise and an apparent jump or inconsistency in exposure because you were very close to a threshold at either extreme of the 1/3 stop, and by moving the apature 1/2 stop, you don't have that half stop available, you only have 1/3 and 2/3 stop with the shutter.

[petez]

I1/2 stop increments..

I have a hard enough time w/ 1/2 stop shutter speeds like 1/180th and 1/2 fstop like f13. I cant image dealing with 1/3rds!

[stevebrot]

There is yet another consideration, though it is probably not as much a problem for auto-diaphragm lenses.

At narrow apertures, light entering the prism through the eyepiece becomes significant. I have been taking quite a few pictures lately with my newly-acquired Jupiter-9 on my K10D. That lens has a manual diaphragm and meters with the lens stopped down. I was very surprised when in AV mode at f/5.6 to do all the settings with my eye to the camera and then see the shutter speed increase 4-fold when I moved my face back from the camera to take the picture. Eyepiece exposed...1/250 second, eyepiece shaded...1/60th second. The weird part is that this happened in the shade on a hazy day!

Think about it the next time you are working on a tripod. It might be time to figure out what happened to that little eyepiece shade gizmo that came with the camera!