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06-16-2011, 06:36 AM   #1
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K-r and Asahi 50 1.4

Please help me understand what is happening or what I'm not understanding.

By the way, I have researched for two weeks and can't seem to make sense of it. Probably just something stupid I'm over looking but it's iritating me not understaning it.

I recently picked up an Asahi 50mm 1.4 kmount for my K-R.

When I have the camera in "M" mode I can set the aperture, focus, hit the green button for metering, take my pics and everything happens like it should. I can see the aperture close to the selected stop when looking into the lens, the depth of field and metering change. Everything functions and I get control and great shots.

When I put the camera in P mode, it will sdjust the shutter for proper exposure and i can get good shots automatically under most conditions. Changing the aperture on the lens has no effect, I understand how this is not supposed to work. But when I look in the front of the lens I can still see the aperture doing the exact same thing as when in M mode. It will close to the correct stop for wherever I have the aperture ring set.

Why is it I can see the aperture closing to the selected stop in P mode but no change in image exposure, DOF, etc..? like nothing is changing.

I probably got over 100 shots of me looking like an idiot peering into the lens watching the aperture playing with the camera.


Last edited by beesmyer; 06-16-2011 at 06:37 AM. Reason: adding detail
06-16-2011, 06:52 AM   #2
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QuoteOriginally posted by beesmyer Quote
Please help me understand what is happening or what I'm not understanding.

By the way, I have researched for two weeks and can't seem to make sense of it. Probably just something stupid I'm over looking but it's iritating me not understaning it.

I recently picked up an Asahi 50mm 1.4 kmount for my K-R.

When I have the camera in "M" mode I can set the aperture, focus, hit the green button for metering, take my pics and everything happens like it should. I can see the aperture close to the selected stop when looking into the lens, the depth of field and metering change. Everything functions and I get control and great shots.

When I put the camera in P mode, it will sdjust the shutter for proper exposure and i can get good shots automatically under most conditions. Changing the aperture on the lens has no effect, I understand how this is not supposed to work. But when I look in the front of the lens I can still see the aperture doing the exact same thing as when in M mode. It will close to the correct stop for wherever I have the aperture ring set.

Why is it I can see the aperture closing to the selected stop in P mode but no change in image exposure, DOF, etc..? like nothing is changing.

I probably got over 100 shots of me looking like an idiot peering into the lens watching the aperture playing with the camera.
There is no communication between what you set the aperture ring to and the camera so it doesn't know how to set for a correct exposure. I'm not familiar with the lens. Does it have an A setting? If it does then use that when in P mode. If it is solely a manual lens then you'd need to use M.
Check here on how to use stop-down metering with manual lenses: https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html
06-16-2011, 06:58 AM   #3
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If I have this straight, in K mount non auto focus we have K, M, and A models and only the A models will allow your K-r to set the app for you. Can you either upload a pic of the front of the lens or tell us what it says around the front element looking straight into the lens? That should tell us a lot about which lens you have and what it will do.
06-16-2011, 06:59 AM   #4
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QuoteOriginally posted by Bill_R Quote
There is no communication between what you set the aperture ring to and the camera so it doesn't know how to set for a correct exposure. I'm not familiar with the lens. Does it have an A setting? If it does then use that when in P mode. If it is solely a manual lens then you'd need to use M.
Check here on how to use stop-down metering with manual lenses: https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html

Hes right. You can use the lens wide open only (1.4) in Av mode (regardless of the aperture ring setting), in M mode you use it as you have been with the aperture ring, green button and manual settings. The other modes wont work


Last edited by Deimos; 06-16-2011 at 08:27 AM.
06-16-2011, 08:17 AM   #5
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Thanks for the the quick replies and willingness to help.
Lens is SMC Pentax-M 1.4 50mm Asahi Kmount.

I follow everyone so far in that their is no communication between the lens and camera and thus camera must be in M mode to function. Took me a while but I got it working well.

What I'm wondering is what is the difference in the workings of the aperture because I thought what I can see closing when looking through the front of the lens and hitting the shutter release button was the aperture that controlled the shot. When the camera is in P mode, I can set the aperture ring, look in the lens, take a shot, and it works just like it does in M mode. But it actually has no efect. Is there something else happening in the camera that I don't see that controls the aperture result? Not sure if that makes sense... I don't see any difference in the camera and I'm wondering what is actually the aperture if it's not what I'm seeing through the lens since that is working the same way no matter whether it's in p or M mode.
06-16-2011, 08:36 AM   #6
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Modern lenses, any K mount and even M42 mount on some bodies, allow you to focus with the lens wide open and then "stop down" or close the aperture to actually take the picture. They do this so you can see what you are shooting in the view finder. Hard to understand unless you have a truly manual lens but with a lens that does not automatically stop down, if you set the aperture to f/22 you cannot see anything in the view finder to focus on, it will be dark.

With a K, M or A lens (not in A position) the camera has no way to tell what the aperture is set at. There is no communication with the lens.

In M mode you use the green button to "stop down" the lens to the aperture set by the aperture ring. The camera then takes an exposure reading and sets the shutter speed to match. When you take the picture the camera again stops down but uses the speed setting determined when it took the reading with the aperture stopped down.

If you set it in P mode, when you take a picture the camera will still stop down the lens to the aperture set by the aperture ring but it is not taking an exposure reading, it assumes the lens is wide open. So the aperture will move, as you discovered, but the shutter speed will be set incorrectly so it does not work.

These lenses only work in M mode where you physically set the speed and aperture. Pentax has the green button which allows you to take a reading and set the shutter speed without having to look at the indicator and scroll the speed up or down like you used to on manual cameras.
06-16-2011, 08:46 AM   #7
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QuoteOriginally posted by jatrax Quote
These lenses only work in M mode where you physically set the speed and aperture. Pentax has the green button which allows you to take a reading and set the shutter speed without having to look at the indicator and scroll the speed up or down like you used to on manual cameras.
Or if you only want to shoot wide-open at f1.4 you can use Av mode and the camera will meter accordingly...

Beyond 1.4 its M mode only and the green button

(you will never see the aperture listed on the LCD)
06-16-2011, 08:52 AM   #8
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QuoteOriginally posted by jatrax Quote
If you set it in P mode, when you take a picture the camera will still stop down the lens to the aperture set by the aperture ring but it is not taking an exposure reading, it assumes the lens is wide open. So the aperture will move, as you discovered, but the shutter speed will be set incorrectly so it does not work.
Just so I'm clear on what I'm seeing since our definition of "not work" might not be the same.

In P mode it does not work in the sense that the aperture setting has no affect on the image. But it does work in the sense that I can get a perfectly exposed photo no matter what the light conditions are as long as it's not so bright as to cause over exposure.

This is why it took me so long to figure out I can't use P mode. When I set it on P and the lens at f1.4 the photo looks exactly the same as if I set it at f22 and it looks almost exaclty the same if I have all the lights on or just a couple.

So it's taking pictures at wide open, its taking exposure readings at wide open, even though I can see the aperture closing to the selected setting. I thought since this is closing to f22 that my pictures would be well underexposed since no other settigns on the camera change.


In P mode...Shutter speed and ISO will be exactly the same for all aperture settings from f1.4-f22 and the picture will turn out exactly the same, good.

I change light conditions and the camera adjusts shutter and takes the same good image no matter what the light condtion of f setting is.

I just don't understand why I can change the fstop, see the aperture close, thus cutting down on the light, but still get the exact same picture.

I might not be making any sens and if no one can answer it I'll just wait but I appreciate your patience.

06-16-2011, 09:43 AM   #9
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QuoteOriginally posted by Deimos Quote
Or if you only want to shoot wide-open at f1.4 you can use Av mode and the camera will meter accordingly...

Beyond 1.4 its M mode only and the green button

(you will never see the aperture listed on the LCD)
What I did find doing this is I can set the aperture from one extreme to the next and the pics come out the same. I thought it should come out under exposed if I set the f ring down to f22 because I can see the aperture snapping down to a real small opening.
06-16-2011, 11:14 AM   #10
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Unless you have the camera in M mode, the camera will ALWAYS read the aperture as wide-open at 1.4 regardless of the aperture ring setting. If you want to use another aperture setting other than 1.4 you need to have the camera in M mode, select the aperture you want on the lens and push the green button to meter
06-16-2011, 11:17 AM   #11
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One other thing, is that P mode really does not exist when using this type of lens. When you turn the dial to P, watch the screen and it will show Av mode. Since the camera knows P will not work it really sets it to Av.
06-16-2011, 11:23 AM   #12
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Interesting thread. I don't know the answer and don't have my camera with me to check. I suspect that the mount and the mechanics of the K mount require the lens to momentarily stop down and open up. The camera is likely taking the picture after the aperture is wide open again. This would explain why you get the same exposure no matter the aperture setting. You could try setting it to manual and see if you get the same exposure wide open as you do with the P mode set at any other position than wide open. I bet they you end up with the same shutter speed assuming no change in ISO.

You may want to try P mode with live view and see what the aperture looks like. Again, I bet it is wide open regardless of the aperture setting. You may notice that the aperture closes and then opens back up. (again, I am just guessing as I am cameraless at the moment.)
06-16-2011, 11:38 AM   #13
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Guys this is not rocket science, it is simple:

Manual Pentax M lens

No electrical contacts

Can only be used wide open f1.4 in Av mode, but M mode allows other apertures

In Av mode the camera will expose properly for a wide open f1.4 shot only and will ONLY read the aperture setting as 1.4 regardless of the aperture ring setting (example: aperture ring set at f22 in Av mode, result auto-exposed f1.4 photo)

In M mode you can change the aperture using the aperture ring, press the Green button when looking at your target and then focus and shoot. You will be able to shoot at any aperture like this (example: aperture ring set to f22 in M mode, Green button or manual setting use to set exposure, result f22 photo)

Enjoy!

Last edited by Deimos; 06-16-2011 at 11:57 AM.
06-16-2011, 12:18 PM   #14
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QuoteOriginally posted by MSM Quote
Interesting thread. I don't know the answer and don't have my camera with me to check. I suspect that the mount and the mechanics of the K mount require the lens to momentarily stop down and open up. The camera is likely taking the picture after the aperture is wide open again. This would explain why you get the same exposure no matter the aperture setting. You could try setting it to manual and see if you get the same exposure wide open as you do with the P mode set at any other position than wide open. I bet they you end up with the same shutter speed assuming no change in ISO.

You may want to try P mode with live view and see what the aperture looks like. Again, I bet it is wide open regardless of the aperture setting. You may notice that the aperture closes and then opens back up. (again, I am just guessing as I am cameraless at the moment.)

I understand what everyone is saying that in AV it only reads 1.4 no matter what setting is selected. The question I have is that if the aperture is still closing down to 22 when f22 is selected, I'd expect underexposure. Since it looks fine, this post above is what makes the most sense to me... That it is actually taking the photo after it opens all the way again.
06-16-2011, 03:36 PM   #15
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Maybe we should go back to basics here. Exactly what lens are we talking about? What does it say on the front. Post a picture if you can.

I just did your test on my 50mm 1.4 in Av and the aperture does stop down but the resulting shot is under-exposed. It exposes as if the aperture were 1.4 not the 22 I have it set at. Shutter speed in Av is 1/200th but change to M and I need 1/13sec to make the exposure correct.
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