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01-21-2010, 01:51 AM   #1
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Easy solution to the M lenses metering problem

Having thought a lot on this problem through my K10, k20 and k7, I finally came to the following conclusion : things would be really easier if only Pentax made a tiny change in their firmware in the Av mode handling of non-A lenses to fully stop down the lens and then meter the scene on the shutter half-press... Think "optical preview on half-press", and you have it...

This would still work with m42 lenses, and would solve the wide-open metering problem of M lenses...

You could still focus wide open, and it would not slow the process...

Only drawbacks would be metering accuracy due to the focusing screen not being linear (but you already have this problem!), and a darkened viewfinder for those who like to half-press right to the last moment, but that would nonetheless ease things tremendously!

What do you think about this proposal?
Should Pentax add the following custom option : "optical preview on half-press with M lenses"?

01-21-2010, 05:48 AM   #2
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I think this proposal is not workable because it would itroduce a huge delay in the shutter release.

If you consider the noticible time for green button metering, add that to the shutter tripping time and the delay would be unacceptable.

the better solution would be to put in the min and max aperture and for non A lenses, control the aperture electronically. The problem as I have posted previously is the diameter changes linearly with movement of the activation lever as opposed to the area, therefore the precision would fall off at smaller apertures.

for me, and I am sure many others, the issue is not having to work in manual mode

It would be much more preferable to be able to enter even only maximum aperture, so when you press the green button, the camera meters wide open, then stopped down, and calculates the correct exposure knowing the maximum aperture metering to compensate for the errors.

Accuracy is , for me much more important than Av mode.,
01-21-2010, 07:32 AM   #3
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QuoteOriginally posted by dlacouture Quote

Only drawbacks would be metering accuracy due to the focusing screen not being linear
Could someone point to a link where this has been reputably proven?
I've read this here before and it smacks of urban legendism.
01-21-2010, 09:33 AM   #4
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QuoteOriginally posted by Wheatfield Quote
Could someone point to a link where this has been reputably proven?
I've read this here before and it smacks of urban legendism.
I don't know about the focusing screen part of it, but just about everyone who has tried to meter using the green button (myself included) has noticed that the metering is inaccurate at certain apertures.

I have wondered about the focusing screen as the culprit, though. It seems like if it were the screen, the inaccurate apertures would be the same at the same EV on all lenses. That is not the case in my experience.

01-21-2010, 09:41 AM   #5
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QuoteOriginally posted by Wheatfield Quote
Could someone point to a link where this has been reputably proven?
I've read this here before and it smacks of urban legendism.
You should know the link well.

https://www.pentaxforums.com/forums/241716-post69.html

the bottom line is that using the same lens, and the same test method, only separated by a year or two in time, I can, simply by replacing the focusing screen from the K10D with that of an *istD, reproduce perfectly the metering performance of the *istD with the K10D.

I don't know what else needs to be done. It is no ledgend, and it has been reported by other formum members as well, that metering improved when putting the *istD screen into a K10D or K20D.
01-21-2010, 09:49 AM   #6
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QuoteOriginally posted by GeneV Quote
I don't know about the focusing screen part of it, but just about everyone who has tried to meter using the green button (myself included) has noticed that the metering is inaccurate at certain apertures.

I have wondered about the focusing screen as the culprit, though. It seems like if it were the screen, the inaccurate apertures would be the same at the same EV on all lenses. That is not the case in my experience.
I believe the issue is that when you look at the response at differing apertures, it is clear that the performance of the metering is really dependant on the actual aperture of the lens.

My suspicion, and it can only be suspicion because I don't have the pentax source code, is that the camera uses the known by pentax performance of the metering system at different apertures, to adjust the metering with a correction factor applied to the wide open aperture of the lens.

THis point becomes evident when you do a simple comparison, and I challenge anyone to do it who has both teleconverters.

Take a teleconverter that simply feeds the aperture contact status through to the camera, (any sigma APO XX EX (DG) teleconverter, or a takumar A teleconverter) and use it on a fast lens with an A setting. You will have exposure errors, because the TC does not modify the aperture value to report the true aperture with the TC in place.

Then put the SMC-F 1.7x AF TC on the camera with the same lens, and you will find that there are no exposure errors.

WHat's the difference, the SMC-F 1.7x AF TC modifies the reported aperture to the camera to report the trrue aperture. The camera uses this and applies the proper correction,
01-21-2010, 10:58 AM   #7
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One question

I apologise in advance for the minor thread-jack, but I was under the impression that the K-7 meters correctly with "K" and "M" lenses, and the metering anomalies only occur on the KxD bodies. Is this a false assumption?

NaCl(enquiring minds want to know)H2O
01-21-2010, 11:03 AM   #8
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Well, Lowell, I can see how this would add a delay, but it would be so small to the point of being nonexistent that it would not bother me at all for my kind of shooting... It would still be way faster than fiddling with green button in my opinion...

Anayway, the added time is only the stop-down action, and I think it's near instantaneous (except with sluggish apertures, but you can always wait a little before pressing for the metering to stabilise)...

01-21-2010, 11:08 AM   #9
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QuoteOriginally posted by dlacouture Quote
Well, Lowell, I can see how this would add a delay, but it would be so small to the point of being nonexistent that it would not bother me at all for my kind of shooting... It would still be way faster than fiddling with green button in my opinion...

Anayway, the added time is only the stop-down action, and I think it's near instantaneous (except with sluggish apertures, but you can always wait a little before pressing for the metering to stabilise)...
the stop down time is not instantaeneous.

when you press the green button, there is about a 1/2 second total time of measurement.

at the present time, the stop down is happening at the same time as the meter rising, but these would need to be two separate events in series for your scheme to work, as opposed to parallel

At the end of the day, unless you are constantly changing the camera settings ald lighting conditions, the green button is easy. you set your exposure by focusing on an approximately 18% grey surface, press the button, and the metering is set until lighting changes.

I don't reset exposure on each shot, I set it, and then shoot, essentually in the same process as using an incident light meter, and then let the shadows go where they want to,
01-21-2010, 11:13 AM   #10
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QuoteOriginally posted by NaClH2O Quote
I apologise in advance for the minor thread-jack, but I was under the impression that the K-7 meters correctly with "K" and "M" lenses, and the metering anomalies only occur on the KxD bodies. Is this a false assumption?

NaCl(enquiring minds want to know)H2O
thread-jack accepted.

the K7D definitely meters better than the K10D, but it is not perfect;

It is close to the *istD for fast lenses, but still worse for slow lenses, or significantly stopped down, but if you consider the average value of the K7 Metering in my curve as about 130, then the deviation either side is in worst case within +/-1 stop.

with the K10D it is more in the range of -1.5 to +2
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01-21-2010, 11:20 AM   #11
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Well, I mainly shoot my kids, and "light meter" is a word they don't know or have time to ponder ;-)
I mean, I swear these little devils follow some Shrödinger wave/particule duality law, as they can be at two location at the same time!

I know that the current Green button is satisfying for a lot of people... I don't say "replace it by what I propose", but merely "give us the choice"...
Anyway, Av mode is useless as it is with M lenses, so what is the harm in giving the choice? Some will still prefer the old way, some the new...
01-21-2010, 11:38 AM   #12
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QuoteOriginally posted by Wheatfield Quote
Could someone point to a link where this has been reputably proven?
I've read this here before and it smacks of urban legendism.
Huh? Which part is urban legend? The non-linear behavior or the fault being the screen? The non-linear behavior is well established based on the camera manual as well as tons of posts on this site by myself and others. Relationship to a particular screen as well as various other fixes are another matter.

As Lowell pointed out, substitution of the *ist screen has been claimed by a number of users as a fix. That being said, IIRC changing screens screws up performance with A-contact glass. So the fix may only be useful if with non-A-contact glass. When I switched to the Katz Eye product, I was hopeful to have some relief for the metering issue. Unfortunately, exposure studies both with and without Optibrite showed no difference in metering from the stock screen. I would have liked to have tested the *ist screen except that I am too cheap to buy one just for testing.

My research on the subject places the blame on both the screen technology and the metering firmware. In simplified terms:
  • The meter reads the light transmitted through the screen
  • The light from the screen does not reflect the actual value appropriate for the lens aperture (may be more/less) due to design features to increase screen brightness
  • The raw meter reading is "massaged" by camera's firmware based on maximum aperture information from the "A" contacts on the mount
  • If the maximum aperture information is unavailable (non-A-contact lens), the camera's firmware uses a default curve
  • The default curve is not accurate at wider apertures (and smaller by some reports)

It is important to note that the body's correction applied to the meter reading and how the system functions differs according to camera model. Some users of KxxxD models have reported improved metering by shorting the body contacts with foil or by removing paint from the lenses mount surfaces.

One small ray of hope...This last summer I did a quick and dirty test of the K-7 with my M 50/1.7. Exposure was essentially linear at all apertures. I would love to do a more extensive test, but that would require purchase of the K-7!

Steve

Last edited by stevebrot; 01-21-2010 at 11:45 AM.
01-21-2010, 11:43 AM   #13
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QuoteOriginally posted by NaClH2O Quote
I apologise in advance for the minor thread-jack, but I was under the impression that the K-7 meters correctly with "K" and "M" lenses, and the metering anomalies only occur on the KxD bodies. Is this a false assumption?

NaCl(enquiring minds want to know)H2O
Salty...this was my experience during a brief hands-on with the K-7 when it was on-tour here in Portland. I mounted my M 50/1.7 and shot a blank wall at all apertures using green button metering. The meter settings for the series were as expected and resultant images were essentially identical.

Steve
01-21-2010, 11:48 AM   #14
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QuoteOriginally posted by stevebrot Quote
Huh? Which part is urban legend? The non-linear behavior or the fault being the screen? The non-linear behavior is well established based on the camera manual as well as tons of posts on this site by myself and others. Relationship to a particular screen as well as various other fixes are another matter.

As Lowell pointed out, substitution of the *ist screen has been claimed by a number of users as a fix. That being said, IIRC changing screens screws up performance with A-contact glass. So the fix may only be useful if with non-A-contact glass.
while this is true, it is somewhat more managable with A lenses than non A lenses for the following reason. When you install the *istD screen, and you use an A lens, it always meters wide open, and then calculates the other exposures, even when yoou press the green button, it is a wide open metering. as a result, and also in the same manner as my sigma 70-200F2.8 with TC attached, you have a constant exposure error that you can apply with exposure compensation. (except in manual mode on the K10D where there is no exposure compensation in manual), a constant error is much easier to deal with than the banana curve the K10/20D produces with the stock screen on manual lenses
QuoteQuote:
When I switched to the Katz Eye product, I was hopeful to have some relief for the metering issue. Unfortunately, exposure studies both with and without Optibrite showed no difference in metering from the stock screen. I would have liked to have tested the *ist screen except that I am too cheap to buy one just for testing.
see my curve of the K10D with junfinance screen, better at big apertures no change at smaller ones
QuoteQuote:

My research on the subject places the blame on both the screen technology and the metering firmware. In simplified terms:
  • The meter reads the light transmitted through the screen
  • The light from the screen does not reflect the actual value appropriate for the lens aperture (may be more/less) due to design features to increase screen brightness
  • The raw meter reading is "massaged" by camera's firmware based on maximum aperture information from the "A" contacts on the mount
  • If the maximum aperture information is unavailable (non-A-contact lens), the camera's firmware uses a default curve
  • The default curve is not accurate at wider apertures (and smaller by some reports)

It is important to note that the correction applied to the meter and how the system functions differs according to camera model. Some users of KxxxD models have reported improved metering by shorting the body contacts with foil or by removing paint from the lenses mount surfaces.
I think this applies to M42 mode and using Av where I find as a minimum everything is offset to -1 stop
QuoteQuote:

One small ray of hope...This last summer I did a quick and dirty test of the K-7 with my M 50/1.7. Exposure was essentially linear at all apertures. I would love to do a more extensive test, but that would require purchase of the K-7!

Steve
See my chart in this thread, better than K10/20D but still not as good as my *istD.

I can't comment on the other models, i.e. single thumbwheel models, but if people would do the same test with a K50mmF1.4 I would be glad to add the data to the chart.

it would be great to have the full range of all pentax bodies mapped out.
01-21-2010, 11:57 AM   #15
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I'm a bit confused about which metering problem you're talking about. So, what metering problem are you talking about?
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