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07-28-2009, 11:34 PM   #1
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Exposure meter with M lenses.

On my K10D, if I am using my M-50 f/2 there is no exposure meter, is there a fix for this?

Typically I have the DOF preview switch set to take a preview photo and would adjust the exposure from there.

I found this thread which I don't follow 100%.

I didn't realise you could push the green button and it would set the exposure, that's handy, but is there still no exposure meter?

I don't understand this statement:
Another method is to engage the optical preview (stop down) button in manual exposure mode. This turns on the lightmeter and the exposure bar. While keeping the optical preview engaged, adjust aperture or shutter speed until the exposure bar shows the correct exposure or shows over/under exposure to you liking.
Can anyone elaborate on that?

Cheers.

07-29-2009, 01:49 AM   #2
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What happens here is that with the crippled KAF mount, which all Pentax DSLRs have, there is no mechanical connection between manual lenses and the camera to transmit aperture information. This means until you manually stop down the lens (which happens with the mechanical stop-down lever, which is still there) the camera has no idea what aperture is set at the lens and will meter as if it was wide-open.

To use M lenses, the only mode you can use is M. Select your desired aperture with the aperture ring, then press the green button and the camera will momentarily stop down the lens to obtain a meter reading and set a shutter speed. If aperture or light conditions change, you will have to press the green button again to correct the shutter speed. It's a little bit of a nuisance, but that's how it works: YES, there is metering with M lenses, but not continous, immediate metering. You need to press a button to take each reading.

An alternate method is to set the DOF preview function to optical, instead of the way you have it now. This means when you engage the DOF preview in M mode, the lens will stop down to your desired aperture and the viewfinder will display how many stops under- or over-exposed you are, so if it says "+3" you need to dial down the shutter speed three stops from wherever you are at the moment. To me, this method seems clumsy and completely unnecessary, seeing as the green button method is there. Just forget about it.
07-29-2009, 10:02 AM   #3
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QuoteOriginally posted by blwnhr Quote
Typically I have the DOF preview switch set to take a preview photo and would adjust the exposure from there.
You'll need to change that - you need to set the DOF preview to actually do an optical DOF preview. You can live without the "taking a preview photo" function - you can always take an actual test photo to serve the same purpose. But you can't live with the optical DOF preview (well, obviously, you'll live, but you know what I mean - there is no substitute for optical DOF preview). The optical DOF preview is the only way to get the meter reading.

QuoteQuote:
I didn't realise you could push the green button and it would set the exposure
Right, that's an important feature of Pentax DSLR's. Without it, with manual lenses you'd be doing a lot of guessing. No wonder you were relying on the digital preview so much! With the Green button, you can get a very good exposure right away, meaning you don't really need to do so many of those test exposures.

QuoteQuote:
I don't understand this statement:
Another method is to engage the optical preview (stop down) button in manual exposure mode. This turns on the lightmeter and the exposure bar. While keeping the optical preview engaged, adjust aperture or shutter speed until the exposure bar shows the correct exposure or shows over/under exposure to you liking.
Set your DOF preview lever to actually do a DOF preview instead of taking a picture, then see if the above doesn't make sense. It works just as described - as long as you hold the DOF preview lever in position, a meter reading will be displayed, just as it is normally for "regular" lenses.
07-29-2009, 10:08 AM   #4
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Is it too early to mention metering errors?

Not to confound / confuse the OP a little, but he needs to be very aware of checking his histogram when using manual lenses especially with a K10D/K20D due to the problems associated with metering on those bodies.

Specifically, below F4 the camera will tend to under expose, with up to 1.5 stops at F1.4, and above F5.6 the camera will tend to over expose by as much as 1-2 stops around F11-16 before settling down to +1 stop by F22-32.

07-29-2009, 08:26 PM   #5
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Original Poster
Thanks Erik and Marc, that clears it up.

QuoteOriginally posted by Lowell Goudge Quote
Not to confound / confuse the OP a little, but he needs to be very aware of checking his histogram.
Lowell, I'm pretty aware of the metering system in the K10D, I've owned it for well over 2 years and have 20k+ photos on it. I have loaned the camera to someone to use with their Pentax M lenses and I didn't know how to get the metering right as I rarely used an M lens.
07-29-2009, 10:06 PM   #6
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What Llowell is trying to tell you is that M lenses do not meter correctly on the K10D. The camera's meter does not determine even exposure for all combinations of f-stop.

To use an M lens on a K10D, you need to check your results as you shoot with the histogram.

This is only the case with M lenses and aperture preset lenses. "A" lenses do not have this problem.
07-29-2009, 10:56 PM   #7
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QuoteOriginally posted by KungPOW Quote
...This is only the case with M lenses and aperture preset lenses. "A" lenses do not have this problem.
Put another way...your metering mileage may vary if:
  • The lens is screw mount
  • The lens is K-mount but lacks the electrical contacts on the mount face
Steve

P.S. You did remember to enable the aperture ring in the settings menu?
07-29-2009, 11:08 PM   #8
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Guys, is it possible to make poll for owners of M lenses to get statistics about metering errors on M lenses?
It is also interesting to understand source of this stupid errors.
In theory, stop-down metering must be more accurate then calculated one for A lenses.
But in reality we see quite opposite.

07-30-2009, 12:04 AM   #9
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QuoteOriginally posted by tr13 Quote
Guys, is it possible to make poll for owners of M lenses to get statistics about metering errors on M lenses?
There is no point for the poll now. It's known that the errors vary from lens to lens. Each lens has its own characteristics.

QuoteOriginally posted by tr13 Quote
It is also interesting to understand source of this stupid errors.
Pentax never admits this problem. But I've heard that it has been fixed, at least to a certain degree, in the K-7.

The problem seems to be related to the focusing screen. Replacing the stock screen (L?-80) in the K10D/K20D with the ones for the ist*D series (L?-60) seems to fix the problem.

QuoteOriginally posted by tr13 Quote
In theory, stop-down metering must be more accurate then calculated one for A lenses.
But in reality we see quite opposite.
Yup. This is similar to the problem in exposure inconsistency with P-TTL. With the pre-flash strobe for light metering and the camera's/flash's computing power, one would think P-TTL is more accurate and consistent. But in reality it is worse than technologies used decades ago: TTL and flash-auto.
07-30-2009, 01:40 AM   #10
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I am sorry, but being slightly related to image processing I don't agree with your points.
I understand that errors could vary, but they must not vary from lens to lens.
Replacing focusing screen could only result in constant exposure correction, it is just pure physics.
As I could see sources of this errors:
1) Firmware still uses same algorithm as with A lenses with recalculations.
2) Errors could also arise from wrong lever position during step-down metering, but I don't
strongly believe this.
3) Most probably exposure is not calculated propertly because stopdown metering integrate some light from intermediate diaphragm positions. From programmers point of view, metering is faster, but delay to stop down is not sufficient. In other words - correct algorithm requires stopdown-resetting exposure sensor-metering-opening. And it is not so today.
07-30-2009, 05:56 AM   #11
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QuoteOriginally posted by tr13 Quote
Guys, is it possible to make poll for owners of M lenses to get statistics about metering errors on M lenses?
It is also interesting to understand source of this stupid errors.
In theory, stop-down metering must be more accurate then calculated one for A lenses.
But in reality we see quite opposite.
I don't think there is need for a poll.

I have checked all my manual lenses on both K10D and *istD, and the post below was done substituting the *istD screen into the K10D (they are the same size)

You will need to accept in advance my appology for the title as this was a hotly disputed argument at the time.

https://www.pentaxforums.com/forums/241716-post69.html
07-30-2009, 06:00 AM   #12
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QuoteOriginally posted by blwnhr Quote
Lowell, I'm pretty aware of the metering system in the K10D, I've owned it for well over 2 years and have 20k+ photos on it. I have loaned the camera to someone to use with their Pentax M lenses and I didn't know how to get the metering right as I rarely used an M lens.
Adam, that was the specific point of the posting, the K10D metering with M42, K and M series lenses is rarely "right". It was not meant to suggest you were not familiar with the K10D, but not all people use non "A" lenses on a regular basis.

I have posted a link to my analysis of the root cause of the problem in response to another post here, but the bottom line is that you have to continually check your histogram when using Non "A" lenses on the K10D and K20D.
07-30-2009, 10:38 AM   #13
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Interesting measuring, Lowell.
But I think that poll is much more useful, anyway.
As for focusing screen non-linearity is could be possible in case of special materials with concentrating lenses (not just glass).
But I strongly belive in my previous points.
As for how we could make M lense to behave like A lens - it is quite easy. All that we need to change is lever moving distance. As M lenses have different distance to diaphragm relation.
I'll try to make comparisons between M42, M and A lens in same situations. As M42 don't have any stop-down during measure and A don't have it either (only calculations are made).
07-30-2009, 11:34 AM   #14
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QuoteOriginally posted by tr13 Quote
Interesting measuring, Lowell.
But I think that poll is much more useful, anyway.
except what is the poll going to ask, you are dealing with a characteristic that changes potentially with every lens at every aperture. about the best you could do is have a poll where you enter error for ewach f-stop. i am not sure it can be done
QuoteQuote:
As for focusing screen non-linearity is could be possible in case of special materials with concentrating lenses (not just glass).
note the errors exist even if yoou take an A lens out of A and meter, and although there is a variation lens-to lens, most lenses have this characteristic
QuoteQuote:
But I strongly belive in my previous points.
you are entitled to your opinion, but note. The brighter focusing screen of the K10/K20 may also have a property to scatter light differently, causing more than just the light that the metering should look at hitting the meter. This scatter may be a function of how parallel the light is hitting the focusing screen. the more you stop down a lens, the more parallel the light is hitting the focusing screen at each point, therefore the screen could cause the problem entirely by itself. If you want to confirm this
QuoteQuote:
As for how we could make M lense to behave like A lens - it is quite easy. All that we need to change is lever moving distance. As M lenses have different distance to diaphragm relation.
not as easy as you think, you also need to know the maximum aperture of the lens mounted on the camera.
QuoteQuote:
I'll try to make comparisons between M42, M and A lens in same situations. As M42 don't have any stop-down during measure and A don't have it either (only calculations are made).
I have the graph for all 19 of my lenses, both A and non A lenses, but have not checked 100% of the A lenses in Non A mode. I also have not checked these lenses on my K7 Yet, but plan to. Note each lens takes about 15-30 frames per camera body to test for each metering mode,

Last edited by Lowell Goudge; 07-30-2009 at 11:54 AM.
07-30-2009, 11:59 AM   #15
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Very useful post, Lowen. Thanks.
As for light scatter this was my point - lenses in focusing screen could cause such behaviour (as K10D/D20D screen is similar to rear projection TV screen). But I do not belive it as my primary suspect is software implementation.
Anyway, my proposed solution is to make special configuation text file. And you will be able to scroll throught lenses defined in this file, and each will have SR distance, exposure compensation for each diaphragm value, etc.

The more I look at DSLR metering the more I notice how dated this thing is.
My panny TZ5 meters much more accurately then GX20 in matrix mode :-)
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