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04-04-2008, 08:44 PM   #31
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QuoteOriginally posted by stevebrot Quote
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!
that is a good point except that light through the view finder would lead to under exposure not the over exposure seen with manual lenses, and I doubt that the under exposure on my 50mm F1.4 is due to a little bit of light in the finder, a whole lot more goes in the big ol' hole in the front.

04-04-2008, 08:45 PM   #32
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QuoteOriginally posted by stevebrot Quote
T It might be time to figure out what happened to that little eyepiece shade gizmo that came with the camera!
The camera strap that came w/ my MZ-S had a slot for holding that eye piece. I was sure dissapointed that the K20D didnt have the same slot.
04-04-2008, 08:52 PM   #33
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QuoteOriginally posted by petez Quote
The camera strap that came w/ my MZ-S had a slot for holding that eye piece. I was sure dissapointed that the K20D didnt have the same slot.
My ricoh XR-2s had an eyepeice shutter built in. nothing to forget, except close the shutter
04-04-2008, 09:06 PM   #34
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Introduction:
QuoteOriginally posted by jeffkrol Quote
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.
Methods:
So, I took this to heart and did some controlled tests (constant light source) and some controlled analysis. (I use Lightroom and did a test to see what it thinks a 1/2 stop is compared to 2 images that were take 1/2 stop apart. The Lr exposure slider looked right on.)

Results:
With the camera in manual mode and the lenses in A mode, all the exposures were very close-within a +/- 1/2 stop easily. Yeah for PKA Adaptall mounts.

Now, when I switch the lens out of A mode, I immeadiately see all the images ~2/3stop underexposed across all f-stops - except for my 90mm lens. At f16 and f22 I loose 1.66 stops!

I've repeated the test and those 2 fstops on that one lens are just wrong.

Conclusions:
Certainly, on the digital preview window, a 1/2 stop looks like a lot more than what I think of a 1/2 stop is on a light table while looking at slides.

So, I figure if I just add 1/2 stop when using the PK adaptall mounts, I'll be close enough. (Besides, i still cant think in 1/3 stop increments). Then w/ the 90mm, I'll have to remember to add a lot of light at the smaller fstops. I think I can live w/ it.

Discussion:
So, anyone want to venture a guess why the PK mounts are underexposing and why 2 f-stops on my 90mm are really underexposing?

04-05-2008, 07:25 AM   #35
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QuoteOriginally posted by petez Quote
Introduction:


Methods:
So, I took this to heart and did some controlled tests (constant light source) and some controlled analysis. (I use Lightroom and did a test to see what it thinks a 1/2 stop is compared to 2 images that were take 1/2 stop apart. The Lr exposure slider looked right on.)

Results:
With the camera in manual mode and the lenses in A mode, all the exposures were very close-within a +/- 1/2 stop easily. Yeah for PKA Adaptall mounts.

Now, when I switch the lens out of A mode, I immeadiately see all the images ~2/3stop underexposed across all f-stops - except for my 90mm lens. At f16 and f22 I loose 1.66 stops!

I've repeated the test and those 2 fstops on that one lens are just wrong.

Conclusions:
Certainly, on the digital preview window, a 1/2 stop looks like a lot more than what I think of a 1/2 stop is on a light table while looking at slides.

So, I figure if I just add 1/2 stop when using the PK adaptall mounts, I'll be close enough. (Besides, i still cant think in 1/3 stop increments). Then w/ the 90mm, I'll have to remember to add a lot of light at the smaller fstops. I think I can live w/ it.

Discussion:
So, anyone want to venture a guess why the PK mounts are underexposing and why 2 f-stops on my 90mm are really underexposing?
How did you set exposure and what were the camera settings?

this result, while encouraging for you (constant exposure error) disagrees with all other reported performance.

What metering mode also spot, or center weighted average for the not in A mode shots?
04-05-2008, 10:54 AM   #36
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QuoteOriginally posted by stevebrot Quote
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!
Or just hold your hand very close to the eyepiece. Anything that prevents most light from entering the eyepiece will do the job.
04-05-2008, 03:04 PM   #37
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QuoteOriginally posted by Lowell Goudge Quote
that is a good point except that light through the view finder would lead to under exposure not the over exposure seen with manual lenses, and I doubt that the under exposure on my 50mm F1.4 is due to a little bit of light in the finder, a whole lot more goes in the big ol' hole in the front.
Yes, under-exposure!

The exposure issues with manual lenses is not as clear-cut as some of the original reports. Behavior varies according to firmware version , the lens in question, and the metering mode. My original experience with my adaptall-2 lenses was over-exposure at apertures smaller than f/16 and under-exposure above f/5.6 (using manual mode and the green button on my K10D). The over-exposure issues resolved (for the most part) with a firmware update (currently at v1.20).

I recently purchased a Pentax-M 200 f/4 and a MC Jupiter-9 f/2. Both show significant underexposure wide open regardless of metering method. Apertures f/5.6 and smaller meter accurately except for 1/2 stop under-exposure on the 200mm at f/22 and f/32.

(The J-9 was evaluated using AV, M with the green button, and M with the stop-down lever. The 200mm was tested using the green button and the stop-down lever. Diaphragm linearity for both lenses had been previously confirmed.)

As for how much light goes in the front vs. the back...It all depends on the aperture when the meter is set. With stop-down metering, potential light entry through the eyepiece is an issue that I remember reading about way back from the days of the Spotamatic. (I could not afford the Pentax, but the same applies to the Ricoh TLS that I could afford...and still have...)

As Rocky pointed out, it is usually enough to shade the eyepiece with a hand, but is important to remember when shooting with slow glass or when metering in stop-down mode.

Back to the original thread topic...The real frustration in all of this is that it "in the old days", stop-down readings through the lens were the gold standard for reflected light metering. It is really frustrating to not be able to override the software "splines" and just meter the light "straight" when in manual mode. For sure, some of this is the result of excess image circle and/or archaic lens coatings and/or reflective surfaces on the rear of the lens and/or angle of incidence to the sensor, but the behavior is too sketchy to place all the blame on the lens.

Last edited by stevebrot; 04-05-2008 at 03:10 PM.
04-05-2008, 04:17 PM   #38
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Folks:
Sorry if I'm adding confusion here, but there are two issues being discussed.
The stop down metering, which is the original title of the thread, is a problem that I do have. The results are in the original post.

But I was also concerned about a recripocal exposure. That is, if I pick an exposure, any one, then go thru the f-stops and change the shutter speed accordingly, I should see the same exposure. That is what is in the 2nd set of results.

QuoteOriginally posted by Lowell Goudge Quote
How did you set exposure and what were the camera settings? What metering mode also spot, or center weighted average for the not in A mode shots?
Camera in Manual Mode, matrix metering (which switches to cw). I had a single light bulb for a light source and a grey card about 6" in front of the lens. None of this really is a factor for this test. I did get a base exposure of f2.8 @1/20, via a light meter which happened match the camera meter. I then progressed thru all the f-stops to f22@2secs. I did not meter between each image.

QuoteOriginally posted by Lowell Goudge Quote
this result, while encouraging for you (constant exposure error) disagrees with all other reported performance.
No, this test has only been reported by Canada_Rockies who saw consistent exposure thru all f-stops. The 2/3rds f-stop when exposing w/ a lens not in A mode (manual lens mode?) is a problem for the lens I tested and hope others will verify. I additionally have another problem with my 90mm at f16 and f22 underexposing by 1.66 stops. I'm doubtfull that this is a common problem but most likely a characteristic of this lens.


One thing I found just this AM. With the camera in manual mode, manual metering, and manual lens mode, we've all noted that there is one f-stop that meters properly. It turns out that if you set the lens to that f-stop, then press the green button, the press the AE Lock button you can change either f-stop or shutter speed and the other will change automatically. Pretty cool. It's even in the manual-pg 95. Should have used that to do these tests!


Last edited by petez; 04-05-2008 at 06:01 PM.
04-05-2008, 07:55 PM   #39
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QuoteOriginally posted by petez Quote
One thing I found just this AM. With the camera in manual mode, manual metering, and manual lens mode, we've all noted that there is one f-stop that meters properly. It turns out that if you set the lens to that f-stop, then press the green button, the press the AE Lock button you can change either f-stop or shutter speed and the other will change automatically. Pretty cool. It's even in the manual-pg 95. Should have used that to do these tests!
I think this only works with A and later lenses, though. The aperture is set on the mechanical ring. BUT I'll have to try this out tomorrow and see for myself. If this works you will not have to wait for the posting - you'll hear the "YES" clear over to the East Coast.
04-05-2008, 08:53 PM   #40
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QuoteOriginally posted by Canada_Rockies Quote
I think this only works with A and later lenses, though. The aperture is set on the mechanical ring. BUT I'll have to try this out tomorrow and see for myself. If this works you will not have to wait for the posting - you'll hear the "YES" clear over to the East Coast.
Oh, of course! I was using the A setting this AM, changing f-stops to get better depth of field. Sorry to get your hopes up.
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