Manual lens - green button - stop down meassure


Link Posted 24/07/2009 - 11:57
I have a few fully manual lenses like Petri 135mm F2.8 and the Kenlock 28mm F2.8.
Aperture is manual - without the A setting on the aperture ring.

Focus is manual as well. Not relevant in this post though.

My problem is that I find the meassurement of my K20D camera, with the Stop Down function on the Green Button or with Optical Preview, to be underexposing a lot in low light.
It is as much as four to six exposure stops wrong.
I.E.: F22. Stop Down and Optical Preview show the same meassurement: 3''
Correct Exposure is 30''

This goes for both lenses.

Is this normal?
Is there a way out?

15mm - 450mm equivalent, and still can't reach...


Link Posted 24/07/2009 - 12:11
I think I see what you mean having just tried it... it seems 3" is the min. shutter speed it wants to set... even with my hand over the lens my camera sets 3"

Not sure why this should be the case though!

If there's a solution which doesn't involve using a lighmeter or fitting an A lens to meter, I'd also be interested to know.
Last Edited by Pentaxophile on 24/07/2009 - 12:23


Link Posted 24/07/2009 - 12:17
This is one off the known issues when using older manual diaphragm lenses on Pentax DSLR's. Some people here have improved the problem by changing the focussing screen.

If you search the forum, you should be able to find plenty of info.
Peter E Smith

My flickr Photostream
Last Edited by Mannesty on 24/07/2009 - 12:18


Link Posted 24/07/2009 - 12:24
Firstly have you enabled the arpeture ring in the cameras advanced settings.
If not it wont work!
Secondly i found the best method of using manual lenses on k20d is to have the camera set to fully manual and when holding the optical preview switch in place turn the dials to set your exposure and speed until the green hexagon lights up and exposure meter is in centre.

Hope thats some help.
Lil Andy

K20D, PENTAX DA 18-55, TAMRON DI 70-300, PENTAX SMC-K 135 f2.5


Link Posted 24/07/2009 - 12:38
Andy, if you set your 'green button in manual' to 'TV Shift' (in custom settings), the green button will select the correct shutter speed in manual! Give it a go, it's almost like having AV mode!

But I think Niklas has found a limitation of the metering system when using non-A lenses... 3" is the limit of the shutter speed it displays / selects.


Link Posted 24/07/2009 - 14:21
All My manual lenses are Non A.

Thats the method I use with them so hopefully will work for you guys to
Lil Andy

K20D, PENTAX DA 18-55, TAMRON DI 70-300, PENTAX SMC-K 135 f2.5


Link Posted 24/07/2009 - 16:32
I appreciate all the answers, some of which gave me new approaches for the use of fully manual lenses on the K20D.
Thank you very much.

However, it did not yet solve the ~4-stop underexposure I experience in low light conditions.

So if anyone has an input on this issue, bid in.
Elsewise I will hold on to Mannesty's explanation that it's a known and described issue.

15mm - 450mm equivalent, and still can't reach...
Last Edited by niklas on 24/07/2009 - 16:33


Link Posted 24/07/2009 - 16:56
If you look at the instruction manual, Pentax tell us that exposure may not be accurate. There is an even worse issue with using OM lenses on Olympus 4/3 cameras, which leads me to think that in a general sense making sure that old lenses were 100% useable would compromise the function with modern lenses.

It may well be that this is technically solveable, but if that means the cameras became uncompetitive compared to other marques, then it would not happen.
Best regards, John


Link Posted 24/07/2009 - 17:19
johnriley wrote:
If you look at the instruction manual, Pentax tell us that exposure may not be accurate.

I have found the metering with K lenses to be reasonable.. occasionally a half to a stop out, but very usable.

However shutter speeds longer than 3" are simply not available, which explains Niklas's problem of gross underexposure.
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