Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-21-2010, 12:21 PM   #16
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by netrex Quote
I'm a bit confused about which metering problem you're talking about. So, what metering problem are you talking about?
there are 2 different topics in the thread,

the first is not so much a "metering problem" but a perceived functional limitation.

Specifically that with K mount lenses (as opposed to KA mount or later which have an A position) that you are limited at present to manual mode and pressing the green button to set an appropriate shutter speed. SOme people find this inconvenient and limiting, and wish pentax would offer some form of Av mode for these lenses. At present in Av mode the camera meters and shoots with the lens wide open.

There have been several threads on how this could be addressed in firmware

the second, which is a real metering issue, is most specifically aimed at the K10D and K20D, where they do not meter as accurately as other pentax cameras with manual aperture lenses. That is what the graph I attached shows, and the test description is in the link I attached, along with an earlier version of the chart, only comparing the K10D and *istD, and not showing K7 or K10D with split image performance.

For me, the first is a nice to have, but the second is more of a need to have. Accurate metering is all important, if you are going to shoot without checking each shot to make sure you had the exposure correct.

01-21-2010, 01:56 PM   #17
Veteran Member




Join Date: Jul 2008
Location: Var, South of France
Photos: Albums
Posts: 1,071
Original Poster
Well, someone gave me an idea over at DPReview : just enable the shutter while in DOF optical preview...

By the way, I've just found out that my K7 simply does not stop down my K lens while in Av mode!!! So they removed the only possibility for a workable Av mode with M lenses that consisted of using the Ev comp in conjonction of what aperture you selected on the lens!!!

So, for the above idea to work, they would have to put the aperture actuation back into the Av mode first...
01-21-2010, 01:58 PM   #18
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by dlacouture Quote
Well, someone gave me an idea over at DPReview : just enable the shutter while in DOF optical preview...

By the way, I've just found out that my K7 simply does not stop down my K lens while in Av mode!!! So they removed the only possibility for a workable Av mode with M lenses that consisted of using the Ev comp in conjonction of what aperture you selected on the lens!!!

So, for the above idea to work, they would have to put the aperture actuation back into the Av mode first...
Just for your information, the aperture NEVER stopped down a K lens in Av mode. this dates back all the way to the *istD. As i noted above, a K lens will work in AV mode only at maximum aperture but the camera does meter and adjust the shutter accordingly to this setting.

it always shot wide open.
01-21-2010, 03:22 PM   #19
Veteran Member
titrisol's Avatar

Join Date: Dec 2007
Location: In the most populated state... state of denial
Photos: Gallery | Albums
Posts: 1,098
Well, pentax could bite the bullet and make a body similar to the MZ5/6/7 that can take the older lenses as well as the new ones.
I do not see any other option

01-21-2010, 05:27 PM   #20
Site Supporter
GeneV's Avatar

Join Date: Oct 2008
Location: Albuquerque NM
Photos: Albums
Posts: 9,777
QuoteOriginally posted by Lowell Goudge Quote
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.
Maybe I'm missing something, but I don't understand how the problem could be Pentax programming the characteristics of the various lenses into the digital cameras. How would aperture affect the metering system? The meter would seem to only see the intensity of the light. One aperture setting could result in a large number of different light intensities actually hitting the meter.

If the metering system depending on the camera being programmed with an intimate knowledge of the lens capabilities, then no lenses introduced after the firmware was written would work.
01-21-2010, 07:14 PM   #21
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by GeneV Quote
Maybe I'm missing something, but I don't understand how the problem could be Pentax programming the characteristics of the various lenses into the digital cameras. How would aperture affect the metering system? The meter would seem to only see the intensity of the light. One aperture setting could result in a large number of different light intensities actually hitting the meter.

If the metering system depending on the camera being programmed with an intimate knowledge of the lens capabilities, then no lenses introduced after the firmware was written would work.
I believe the problem is not as complex as you make it out to be.

From testing all of my lenses with my *istD, and my K10D, there is a distinct pattern that appears.

If you look at the graph I produced using te SMC 50mm F1.4 lens, this appears to be consistent over a very large range of lenses, and apertures.

As a result, I believe pentax programs a "correction" curve to make linear measurements based on a "generic" curve similar to what I have attached for the maxum aperture of the lens. from there on the error on each lens is constant, for an A series lens, and probably within an acceptable +/- 1/3 stop.

the biggest part of the problem appears to be the new focusing screen technology used to get brighter screens as demonstrated by the change in performance when I put an older technology screen in the camera.

as a result, there is not a real issue with keeping the firmware upto date with lenses
01-22-2010, 01:53 AM   #22
Site Supporter
GeneV's Avatar

Join Date: Oct 2008
Location: Albuquerque NM
Photos: Albums
Posts: 9,777
QuoteOriginally posted by Lowell Goudge Quote
I believe the problem is not as complex as you make it out to be.

From testing all of my lenses with my *istD, and my K10D, there is a distinct pattern that appears.

If you look at the graph I produced using te SMC 50mm F1.4 lens, this appears to be consistent over a very large range of lenses, and apertures.

As a result, I believe pentax programs a "correction" curve to make linear measurements based on a "generic" curve similar to what I have attached for the maxum aperture of the lens. from there on the error on each lens is constant, for an A series lens, and probably within an acceptable +/- 1/3 stop.

the biggest part of the problem appears to be the new focusing screen technology used to get brighter screens as demonstrated by the change in performance when I put an older technology screen in the camera.

as a result, there is not a real issue with keeping the firmware upto date with lenses
It is not my intention to make the problem out to be more complex, but just to accurately describe the problem or understand why the description you have offered is accurate. From my use of various m and k lenses, I see definite differences between the lenses. Yes, I would agree that in general, the curves are similar, but one lens may hit accurate readings at F4, and another at F5.6 or F8. And it remains accurate at that particular stop over various EVs. It is not always exactly the same stop or the same number of stops from wide open on each lens that is the accurate one. If it were, then I could see that a generic curve would work. (I would also think that it would be a relatively simple matter for Pentax to alter that curve when the camera detected a lens without aperture information)

My question is, if we are talking about a curve programed to compensate for the response of the metering sensor, why is it that aperture that determines accuracy, rather than the intensity of light? If the meter is not linear, then it should behave the same if the same intensity of light hits it, whether that EV is the result of a large aperture or stronger light through a smaller aperture, unless aperture affects the metering in some way other than changing the EV.
01-22-2010, 03:22 AM   #23
Pentaxian




Join Date: Apr 2007
Location: Sweden
Posts: 1,932
QuoteOriginally posted by Lowell Goudge Quote

As a result, I believe pentax programs a "correction" curve to make linear measurements based on a "generic" curve similar to what I have attached for the maxum aperture of the lens.
But to use the correction curve the camera needs to know which aperture you are using, thus it will not work with M-lenses anyway.

01-22-2010, 03:32 AM   #24
Veteran Member




Join Date: Jul 2008
Location: Var, South of France
Photos: Albums
Posts: 1,071
Original Poster
GeneV : Aperture has a big impact because the screens are in fact some sort of fresnel lenses, instead of the good old ground glass of our film days... They are optimized to increase perceived brightness above f/5.6, and that's why we have these s-shaped response curve with underexposure for anything wider than f/5.6...
01-22-2010, 05:16 AM   #25
Veteran Member
blende8's Avatar

Join Date: Feb 2007
Location: Bremen, Germany
Photos: Albums
Posts: 1,484
QuoteOriginally posted by Gimbal Quote
But to use the correction curve the camera needs to know which aperture you are using, thus it will not work with M-lenses anyway.
I would say the lens must be mounted completely stopped down, like an A-lens.
Since the camera knows which lens it is, it behaves accordingly.
01-22-2010, 05:45 AM   #26
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by GeneV Quote
My question is, if we are talking about a curve programed to compensate for the response of the metering sensor, why is it that aperture that determines accuracy, rather than the intensity of light? If the meter is not linear, then it should behave the same if the same intensity of light hits it, whether that EV is the result of a large aperture or stronger light through a smaller aperture, unless aperture affects the metering in some way other than changing the EV.
as responded to by dlacouture it appears to be due to the type of viewing screen, and the impact that aperture has is that it will change the angles at which light strikes the viewing screen, although I cannot give youthe physics behind the issue, it appears to be an issue of scatter, one part caused by the micro lenses of the screen, the other caused by the F Stop of the lens and the angle light hits the viewing screen.

When I first heard about the issue, I didn't think it impacted my K10D because the images were generally close visually. It was not until I shot not a scene but a uniform lit surface which had a very narrow exposure band and measured the grey value of the peak that I could actually quantify the error,m and also test a solution.
QuoteOriginally posted by Gimbal Quote
But to use the correction curve the camera needs to know which aperture you are using, thus it will not work with M-lenses anyway.
Not totally, if it knows maximum aperture, and takes 2 metering readings, one at maximum and one stopped down, it can , with the correction curve calculate the used aperture. and set the shutter accordingly.

As a result, the green button function would change from stop down - measure - open to measure - stop down - measure - open
01-22-2010, 05:47 AM   #27
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by blende8 Quote
I would say the lens must be mounted completely stopped down, like an A-lens.
Since the camera knows which lens it is, it behaves accordingly.
if you entered both maximum and minimum aperture, and programmed the activation lever to match the K mount lens, i.e. linear movement results in linear change in aperture diameter not liner change in aperture area, then yes,
01-22-2010, 05:56 AM   #28
Veteran Member
blende8's Avatar

Join Date: Feb 2007
Location: Bremen, Germany
Photos: Albums
Posts: 1,484
QuoteOriginally posted by Lowell Goudge Quote
if you entered both maximum and minimum aperture, and programmed the activation lever to match the K mount lens, i.e. linear movement results in linear change in aperture diameter not liner change in aperture area, then yes,
These things could be stored in the camera.
You only have to enter the lens name.

But all this is fantasy.
01-22-2010, 06:11 AM   #29
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by blende8 Quote
These things could be stored in the camera.
You only have to enter the lens name.

But all this is fantasy.
yes they could but unlike the modern lenses where the camera knows what lens is on the camera, i think it is much simpler to prompt for focal length, then max and min aperture

but it would still be fantacy.
01-22-2010, 07:28 AM   #30
Site Supporter
GeneV's Avatar

Join Date: Oct 2008
Location: Albuquerque NM
Photos: Albums
Posts: 9,777
QuoteOriginally posted by dlacouture Quote
GeneV : Aperture has a big impact because the screens are in fact some sort of fresnel lenses, instead of the good old ground glass of our film days... They are optimized to increase perceived brightness above f/5.6, and that's why we have these s-shaped response curve with underexposure for anything wider than f/5.6...
That was exactly the discussion I was looking for. That makes some sense of it.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, dslr, half-press, lenses, pentax, photography, preview
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
43 mm 1.9 metering Problem stevemgap Troubleshooting and Beginner Help 6 07-09-2010 11:50 AM
KX blurring problem diagnosed - solution below jlaubza Pentax DSLR Discussion 37 05-15-2010 10:53 PM
Metering problem on K10D? hyyz Pentax DSLR Discussion 4 03-03-2010 04:17 PM
k100d stop-down metering problem stanjo Pentax DSLR Discussion 4 11-05-2007 12:37 PM
Off camera flash. Problem, solution and lesson learned Ed in GA Flashes, Lighting, and Studio 0 03-12-2007 07:56 AM



All times are GMT -7. The time now is 02:07 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top