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02-18-2015, 01:27 AM   #91
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QuoteOriginally posted by thibs Quote
Don't forget the aperture lever functioning is NOT linear with K/M lenses.
It became linear with A and further lenses.
Correct but that does not affect the problem discussed here.
The metering is done stopped down to the point set on the lens.

What one has to do is telling the camera the aperture set on the lens. The camera then does an exposure measurement and notes the value. Then the next aperture is set. In the end one gets a curve. This must me linearized by the camera. It is enough to do this roughly.

The non linearity of the aperture lever is only is a problem, if you set the aperture on the camera.
This makes sense only if you have a transformed M to A lens (by adding the appropriate contacts).

02-18-2015, 06:51 AM   #92
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QuoteOriginally posted by blende8 Quote
Correct but that does not affect the problem discussed here.
The metering is done stopped down to the point set on the lens.

What one has to do is telling the camera the aperture set on the lens. The camera then does an exposure measurement and notes the value. Then the next aperture is set. In the end one gets a curve. This must me linearized by the camera. It is enough to do this roughly.

The non linearity of the aperture lever is only is a problem, if you set the aperture on the camera.
This makes sense only if you have a transformed M to A lens (by adding the appropriate contacts).
Indeed you're right. I stand corrected.
02-18-2015, 07:50 AM   #93
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QuoteOriginally posted by Fogel70 Quote
2. If using Av-mode the camera do a stop down metering before the image is captured, and compares it to open aperture metering to know how much the lens has been stopped down to be able to set correct shutter speed.
I wouldn't think any comparison to wide open would be needed; but is it possible to do this fast enough not to increase shutter lag? How much time does the camera need in order to meter accurately in this situation?
02-18-2015, 08:57 AM   #94
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QuoteOriginally posted by blende8 Quote
This would be a work-around.
Either Pentax stores the exposure characteristics of every Pentax lens in a list, or it allows users to create such a characteristic on their own.
For this you have to measure the exposure for every aperture set on the lens.

The main reason why the behaviour is not linear is probably that the focussing screens transmission is not linear, but depends on the lens opening.
IMO it would be better if the users set this them self, as I doubt Pentax would like to spend time on it.
It would probably only take a few weeks before the community has updated the lens database on PF with this info on most lenses.


QuoteOriginally posted by Scintilla Quote
I wouldn't think any comparison to wide open would be needed; but is it possible to do this fast enough not to increase shutter lag? How much time does the camera need in order to meter accurately in this situation?
It depends on how long it takes to stop down the aperture. Old manual lenses can be quite slow to stop down. But this would be a problem in using the coupler too.
Maybe this is a major reason for Pentax not doing anything about handling of manual lenses. If they try to automatize it, it might lead to a lot of problem when the camera try to capture the image before the aperture has stopped down completely.
The metering itself does not take much time. It can probably be done in a few hundreds of a second.

It would need to compare to wide open metering to know how much it has been stopped down, so it can calculate how much it should compensate for inlinearity of the metering.

02-18-2015, 12:33 PM   #95
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I don't understand why this is still such an issue when Pentax has given two separate ways to deal with older lenses.

First the green button.

Then there is also the ability to stop down the lens with the on/off lever. This is the way I do it. The lens doesn't just stop down you also are able to see exposure doing it this way. You can move the aperture ring on the lens until you get the exposure you want.

I don't understand why this is not sufficient enough.
02-18-2015, 02:23 PM   #96
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QuoteOriginally posted by Rico Quote
I don't understand why this is still such an issue when Pentax has given two separate ways to deal with older lenses.
Because it does not work accurately. Please read the thread above.
02-18-2015, 03:14 PM   #97
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QuoteOriginally posted by Rico Quote
Then there is also the ability to stop down the lens with the on/off lever. This is the way I do it. The lens doesn't just stop down you also are able to see exposure doing it this way. You can move the aperture ring on the lens until you get the exposure you want.
The aperture coupler complaints are specific to old K-mount lenses. I didn't know there were any old K-mount lenses with a switch that would let you do what you describe?
02-18-2015, 04:05 PM   #98
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QuoteOriginally posted by dcshooter Quote
He's talking about the DOF preview on Pentax digital bodies that is activated by pushing the on-of switch further toward "on." If you hold it down, you can turn the aperture ring on a K/M lens and it will continuously meter.
Well darn, how did I just now find out about this feature? Would be quite handy for my M 28/3.5, but the K-30 manual didn't say anything about it. Have a like, good sir (or madam?)!

02-18-2015, 04:29 PM   #99
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QuoteOriginally posted by Scintilla Quote
Well darn, how did I just now find out about this feature? Would be quite handy for my M 28/3.5, but the K-30 manual didn't say anything about it. Have a like, good sir (or madam?)!
Optical DoF preview on Pentax dslrs is a feature on all older bodies and newer high end models located on the camera's "on" switch. Beginner and mid rage bodies have the option but it has to be set to the "RAW/fx" button.
02-18-2015, 06:37 PM   #100
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QuoteOriginally posted by blende8 Quote
Because it does not work accurately. Please read the thread above.
If you don't think it meters accurately whether it is metering a half stop to much or a half stop to little then just compensate accordingly.

Personally I don't have a problem how it meters. I know how the K-5 meters. It meters consistently the same way every time then go from there. It is not like it meters all over the place.

It is like the little white alignment knobs on the older lenses. It might not align with the release button on the newer bodies but it still allows you to know the orientation of the lens when you can not see the red dot in dim light. So you have a good idea how to attach the a lens in the dark regardless if it aligns exactly with the placement of the lease button on newer bodies. You just compensate for the difference in the placement of the release button.
02-18-2015, 07:08 PM   #101
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QuoteOriginally posted by Rico Quote
If you don't think it meters accurately whether it is metering a half stop to much or a half stop to little then just compensate accordingly.
..and so you go to compensate and the shot is gone...
02-18-2015, 07:16 PM   #102
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QuoteOriginally posted by krazykat Quote
..and so you go to compensate and the shot is gone...
I went through that process for eleven years with my Pentax Super Program, because I learned on my first roll of film (I shot almost exclusively slide film back then) that the camera tended to under-expose.
02-18-2015, 07:20 PM   #103
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QuoteOriginally posted by p38arover Quote
Even more reason to keep my *ist D - it also works with my AF080C TTL ring flash and AF400FTZ TTL flash.
TTL yes, but the istD* had a crippled mount. I doubt that we get an uncrippled mount. It would be cheaper to get TTL back for A lenses.
02-18-2015, 07:33 PM   #104
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QuoteOriginally posted by reh321 Quote
I went through that process for eleven years with my Pentax Super Program, because I learned on my first roll of film (I shot almost exclusively slide film back then) that the camera tended to under-expose.
Good for you! Maybe we should not have autofocus on any lenses either then
02-18-2015, 07:46 PM   #105
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QuoteOriginally posted by krazykat Quote
..and so you go to compensate and the shot is gone...
Maybe for you but I know before hand to compensate so I don't miss the shot. I don't see what is so hard to understand. I know I need to be +1 or +1.5 over 0. No matter what the meter reading might be.

Besides if you are relying on metering with old lenses at the point of doing action shots then you are going to be missing a lot of shots regardless. If I am out on the street I am already prepared with a proper aperture and shutter speed and ISO before I attempt to capture any image. How do you think they did it before there was metering available? They had a full understanding of the relationship of aperture shutter speed and ISO and knew how to adjust accordingly to varying conditions.
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