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06-12-2016, 05:50 AM   #1
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K50 Green button query with nice old manual lens


I've recently got a K50, which I love - a fantastic digital camera compared to many others I've had. I've recently started to try and use it with some nice old manual lenses , but have encountered a couple of problems. I've followed the instructions on two threads , and adjusted the green button and edial to optical preview and t-av , but perhaps am not understanding something fundamental.

At first sight it doesn't seem possible to have an extended preview of the stopped-down aperture unless I'm in liveview mode; and I've pressed the Green button, let it 'flash' , stop-down and adjust the setting ; and then I've waited a second or two. Then, shifting the edial adjusts the brightness of the image on the liveview screen for a while ( then it sometimes seems to stop doing this ? ), and I can see how I'd ideally like the image to be. If I use the viewfinder though, there's no "extended preview" - just the sudden stop-down and adjustment for a second or two. Is it just not possible to preview the changed aperture for any length of time through the viewfinder at a proper large size, as I flick the edial, or have I just not understood something basic ?

Thanks very much in advance for your help.

---------- Post added 06-12-16 at 05:54 AM ----------

Incidentally this is a Pentax-K compatible lens, with no "A" automatic aperture on its dial - thanks !

06-12-2016, 06:46 AM   #2
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With M lens should be in manual mode
06-12-2016, 07:00 AM   #3
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The camera has no idea what aperture the lens is set to so it has no idea what the exposure settings should be, so it needs to be in "M" (manual) mode.. The green button closes the aperture to whatever it's set to so that the camera can see what the shutter speed should be for a good exposure. Even then, this kind "stop-down metering" as it's called can be unreliable, especially at the extremes of the aperture settings (very big or very small).

I'm not sure what you mean by "extended preview" - the behaviour I've described applies equally to live-view and using the viewfinder.

After a while you get the feeling for what settings to use in a given level of light and no stop-down metering is required - just take a picture and use an educated guess to see if any changes are needed.

I'd also highly recommend shooting in RAW (+jpg) mode because RAW files can be edited later to account for exposure errors, while jpgs can't.
06-12-2016, 07:22 AM   #4
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Hi there, thanks for your replies.

Just to clarify - the camera is in M mode and the edial is set to TAv Tv.

I have used an old SLR a few years ago and wonder if I just haven't understood how DSLR's work.

QuoteOriginally posted by Jonathan Mac Quote
I'm not sure what you mean by "extended preview" - the behaviour I've described applies equally to live-view and using the viewfinder
Yes, this is exactly the area that I don't understand. I think on an old SLR I had, pressing the "stop down" button would keep the camera permanently on this setting as long as I had my finger on it, so I could see much more closely what the image would look like at the end product. When I press the green button in viewfinder mode pm here though, I get just the instantaneous change and the camera resetting ( the shutter speed I think ?) with a flash of a different image not long enough to properly to see it. In the screen mode though, after I press the green button and get the jolt-adjustment etc, the edial seems to adjust the exposure because the image on the screen seems to be getting darker and brighter. Is this just the computer on the camera simulating the change in exposure, not something I'll be able to see through the viewfinder ? When I'm in my viewfinder mode and do the same, nothing seems to change at all. Maybe what I want just isn't possible on a DSLR 's viewfinder ?


06-12-2016, 07:25 AM   #5
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"viewfinder pm mode" is a typo there, should just say viewfinder mode.
06-12-2016, 07:46 AM   #6
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QuoteOriginally posted by Image Enthusiast Quote
If I use the viewfinder though, there's no "extended preview" - just the sudden stop-down and adjustment for a second or two. Is it just not possible to preview the changed aperture for any length of time through the viewfinder
The optical viewfinder isn't actually giving you a preview of the exposure - since it doesn't take into account shutter speed - and so it isn't able to do what you think you want.

However, once you get accustomed to making exposure decisions you will find that this isn't what you want to do anyway.
06-12-2016, 08:11 AM   #7
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If you assign the RAW/fx button to Optical Preview this will stop-down the lens so you can visually check the DOF through the viewfinder. It does not turn on the meter or display any information in the viewfinder. It should remain stopped down as long as you press the button. On the flagship in manual mode and MF lenses this will stop down the lens and turn on the display and the EV bar will display the EV steps over or under EV 0. You can adjust the exposure by turning the aperture ring or eDial for shutter speed. The green button behaves the same - momentarily stop down the lens and set the shutter speed relative to the light measured through the stopped down lens.
06-12-2016, 09:37 AM   #8
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Also I'd say that you shouldn't expect or rely on the Live View display brightness to be your guide to correct exposure ... As you noted it can readjust itself anyway following metering. The Green Button Stop Down process is kind of similar to the Optical Preview function (mentioned earlier) ... This is the equivilent' of the old 'DOF Preview ' on film SLRs .... You hold the control to lock the aperture down to the set amount and try to see the DOF ...(not always very practical at small apertures). The difference is that Optical/DOF Preview does not carry out metering, it just closes the aperture down for you to see that effect.

But really none of this is useful to you for exposure evaluation, not via Live View or viewfinder .... To assess the exposure value you should use a histogram and/or brightness /darkness indications on the LCD screen). As said above, we can't expect metering to be so accurate with old lenses, so let the camera take a stab at it first (green button) and then look at your chosen indicator on the LCD and adjust the exposure as needed (usually with a shutter speed change). Try to choose realistic apertures and ISOs for the scene .. this should help the metering to perform as best it can.

Don't keep pressing the green button unless you have moved or the lighting has changed, otherwise you just keep on returning to the original metered shutter speed ... Do one stop down, assess that, then adjust as needed and leave it while you shoot the images.

06-12-2016, 12:41 PM   #9
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Cheers everyone - will get out there in the camera in the next decent bit of summer evening light, and put all your advice to work !
06-22-2016, 11:39 AM   #10
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If I leave the camera (k50) in AV it works ok, I usually need to adjust compensation based on aperture value, but after tuning that, the camera reacts fines and exposes accordingly, this even with a old prinzflex 80-200 that doesn't work with stop metering.

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