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08-27-2016, 01:42 AM   #1
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Manual lens seems to work ok without use of green button stopping down?

Hi, I've been exploring my new K3 and started down the fun road of manual lens acquisitions. I already had an SMC 135mm f3.5 prime knocking around from an experiment on a samsung nx camera, so this has been a test subject after getting the K3. I followed, mostly, the instructions I found on how to use a manual lens and have been getting some lovely results. But .... I've not used the green button stopping down and don't understand why my exposure seems to be ok notwithstanding.

When I have tried the green button, I'm not sure it does anything in particular. I can manually adjust the aperture from wide open to minimum and the shutter speed and iso, if on auto, remain unchanged between the two extremes yet the exposure is ok. I've seen reference to the aperture remaining wide open unless stopped down, but doesn't the manual adjustment of aperture on the lens do the physical adjustment of aperture and the camera can't do anything automatically to aperture? I noticed in some photos that even if the aperture is stopped down, there can be a narrow depth of field. So, I'm guessing I'm wrong about my assumption. Could someone shed some light. Is changing the aperture manually with the ring not altering the aperture as I thought. If it is changing as I thought, how come shutter speed and ISO remain the same even though I radically change the aperture with the ring?

As I say, had some nice results so far, but I'm keen to iron out this misunderstanding. I'm uploading a shot which clearly shows the narrowness of the DoF. If shot wide open, I wouldn't have expected it to be so narrow, yet I'm pretty sure I chose a narrower aperture to makes sure I got more in focus.

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08-27-2016, 01:59 AM - 1 Like   #2
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According to the EXIF data you are shooting in Av mode. With an M or K type lens the aperture will remain wide open regardless of what you set the f-stop to. Thus the camera takes the meter reading and exposure while wide open, setting the appropriate shutter speed. Some users get around this by pressing the lens release button and twisting the lens enough that the auto stop-down lever disengages with the lens unlocked in the mount. Others will remove the lever - either destructively (not recommended) or non-destructively.

With the camera set to Manual mode (M) and the ISO not on Auto the iris will close down to the setting on the aperture ring when you take the exposure. Pressing the Green button first will stop-down the lens to the set aperture, turn on the meter and set the appropriate shutter speed. Otherwise the last set shutter speed will be used. This of course is assuming that the appropriate menu settings for aperture ring and green button use are chosen.

Pentax Forums threads on Manual Metering and MF lenses:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/110657-how-use-me...k-x-k-7-a.html
https://www.pentaxforums.com/forums/53-pentax-dslr-camera-articles/100260-man...ic-lenses.html

Note: with both the Samsung nx and K3 you should be able to set the DOF preview to optical and in manual mode operating the DOF preview will stop-down the lens and turn on the meter with the exposure value bars active. Using the EV bars as an exposure guide you can then set the EV by setting the shutter speed with the eDial or the f-stop with the aperture ring.

Last edited by Not a Number; 08-27-2016 at 02:11 AM.
08-27-2016, 04:13 AM   #3
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ok, the mist is clearing now. My assumption of physically changing the aperture by use of the aperture ring, in effect, despite the camera, was wrong. Is there a quick explanation of how this is done?

Will play with settings etc to get the hang of it properly. One thing occurs - with the k3 is it not possible to choose an alternative shutter setting (eg a faster speed to freeze action) which isn't reset when I press the green button? I can't seem to set the camera to use auto iso in this situation. Am I left with a hit and miss process of manually selecting a higher iso till I hit the shutter speed that I want to use, given the metered exposure of the camera?

Thanks for your help
08-27-2016, 04:33 AM   #4
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QuoteOriginally posted by c_j_m Quote
ok, the mist is clearing now. My assumption of physically changing the aperture by use of the aperture ring, in effect, despite the camera, was wrong. Is there a quick explanation of how this is done?

Will play with settings etc to get the hang of it properly. One thing occurs - with the k3 is it not possible to choose an alternative shutter setting (eg a faster speed to freeze action) which isn't reset when I press the green button? I can't seem to set the camera to use auto iso in this situation. Am I left with a hit and miss process of manually selecting a higher iso till I hit the shutter speed that I want to use, given the metered exposure of the camera?

Thanks for your help
At the time the image is captured or the green button is pressed, the camera pulls the aperture level as far down as it will go, which will stop the lens down to the setting selected on the aperture ring.

You can certainly do that with the ISO, and I'm not sure it would be hit or miss- just turn it up the number of stops that you want the shutter speed to increase

To get the exact behavior you want you'd have to use an A series lens or newer in TAv mode, or M mode with the green button set to Av priority.


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08-27-2016, 04:39 AM   #5
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just to add to my last post, I noted and have partially understood your comment about twisting the lens to disengage the auto stop down lever.

Aside from that sort for approach, I take it there is simply no way with the K3 then to select and stick with a chosen aperture (ie anything other than wide open) from one shot to another without the stop down process? Sometimes, would be good to just know it's always shooting a particular f stop till I choose otherwise. Before you put me right on the fact I was shooting wide open despite believing I was manually selecting aperture, I liked the ease with which I could adjust shutter speed and use auto iso.
08-27-2016, 05:21 AM   #6
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If you're shooting in AV mode, the lens always shoots wide open, and will properly meter. If you wish to choose say F8, then you need to use the green button and manual mode. It is however nice to have both in my opinion, one give you a slower approach, the other is great for quick snaps.
08-27-2016, 05:43 AM   #7
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Not entirely correct - the camera meters as though the lens is wide open. The aperture control lever will still stop the lens down during actuation of the shutter - resulting in an under exposed image if in fact your aperture has been set to a value less than wide open.
The whole problem with pre 'A' lenses is there is no communication of aperture to the camera body for the auto exposure to calculate exposures with. There was a petition on the forum requesting the communication coupling be restored on K mount bodies - this would enable all shooting modes and TTL flash metering. That being said: shooting in manual has been infinitely helpful to understanding exposure for me.
08-27-2016, 05:48 AM   #8
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QuoteOriginally posted by mattt Quote
Not entirely correct - the camera meters as though the lens is wide open. The aperture control lever will still stop the lens down during actuation of the shutter - resulting in an under exposed image if in fact your aperture has been set to a value less than wide open.
That's not true. If you are in AV mode, the lens will not stop down at all. Try it for yourself!
The lens will only do what you describe in M mode.

08-27-2016, 05:49 AM   #9
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The lens stops down in every mode. Unless you are talking about Green button metering - - in which case you are correct M is the only mode.

V V What he said down there V V

Last edited by mattt; 08-27-2016 at 12:09 PM.
08-27-2016, 05:58 AM - 1 Like   #10
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QuoteOriginally posted by mattt Quote
The lens stops down in every mode. Unless you are talking about Green button metering - - in which case you are correct M is the only mode.
It clearly does not. It only stops down in M. It does not stop down at all in AV. Seriously, try it out for yourself. Mount an M or K lens, stick the camera in AV, take a photo wide open. Change the aperture to something mental (eg F22), and take another shot. Both of the shots will be wide open with loads of bokeh. Repeat the experiment in M, and you will see it stopping down. I noticed this behaviour on the K-3 initially, so it's perhaps different with older bodies, but this is how it works on both the k-1 and the k-3.
08-27-2016, 06:10 AM   #11
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I rarely use the green button. With eg my SMC 28mm f3.5 I'll "chimp" a shot or two in Av to give me wide open exposure, switch to M mode and then just mentally calculate the new shutter speed for one stop down, two stops down... and spin the e-dial accordingly. It's easy to do, it's just half, half again.... One reason I do this is because I find I have to chimp when using the green button too and adjust the exposure, usually + a half to one stop. My way is quicker and eliminates green button inconsistencies.
08-27-2016, 06:16 AM   #12
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big thanks for all the help. Left unsure about whether the lens will stop down with my manual lens in any mode other than manual and green button. When I was unknowingly shooting with TAV selected, I didn't get under exposure even though I selected a smaller aperture. With this in mind and based on the helpful explanations of Not a Number and Adam, I thought this indicated that the camera wasn't stopping down to the selected aperture but merely metering wide open. Have I not understood correctly?
08-27-2016, 07:28 AM   #13
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Correct. With non-"A" type lenses the camera defaults to Av in all modes except manual B or X. There is no linkage from the lens to the camera to tell the camera what F-stop is selected. It meters wide open and the lens remains wide open when you take the exposure.

When a K-mount auto lens is not mounted on the camera the diaphragm will close down as you operated the aperture ring. If you remove the auto aperture lever or you only mount the lens by twisting it in the mount part way so the lever does not engage you can close the lens down with the ring. In Av mode the camera will meter through the stopped down lens and set the appropriate shutter speed. Usually past f8 metering may not be accurate as there may not be enough light.

Try this: as you mount the lens slowly twist the lens into the mount as you look through the front element. When you start off the iris should be fully stopped down. At some point the iris will start to open. Back off until it stops down fully again. You can shoot with the lens in this position and the iris will respond to the aperture ring. Just take care as the lens is not secure and can twist in the mount.
08-27-2016, 03:26 PM   #14
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I have a rather stupid question. How does the camera stop down a M series lens?
08-27-2016, 04:07 PM   #15
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QuoteOriginally posted by macman24054 Quote
I have a rather stupid question. How does the camera stop down a M series lens?
With the M and K series, when you mount the lens, and look through the viewfinder, the lens will be wide open. So whilst you may have f8 selected on the aperture ring, it will actually be at f1.7 (or whatever the max aperture is). When you take the shot, the camera will release the aperture lever on the lens, which will cause the aperture to close to the f-stop you have selected (e.g. F8). The slight problem with this, is that the camera (well, a modern digital body) has no idea what F-stop you have selected (older non-crippled k-mount bodies could communicate this information mechanically). So in a mode such as AV (which chooses the exposure automatically), it cannot meter the exposure for a stopped down lens, so it simply doesn't bother, and always takes the shot wide open.

If however you are in manual mode, then you are in control of the metering, so the lens will close to the f stop you have selected. If you want to get an accurate metering from the camera, then you'd set the green button to activate stop down metering - which simply releases the aperture lever, and determines the exposure with the aperture closed. If you think about this for a moment, it makes sense why AV mode can't stop down the lens. If the camera doesn't know what aperture you have selected, and must always keep adjusting the exposure depending on where you point the camera, even if you were to press the green button, that exposure would be overwritten by a new value immediately.
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