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05-05-2010, 08:36 AM   #1
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K-x and SMC-M Lenses

Hello all,

Just wondering if you could help a new Pentaxian understand SMC-M lenses and metering on the K-x -

I'll be getting a K-x in a few days, and while I would love to get the DA pancakes right now (which are the main reason I shifted to Pentax from Nikon) unfortunately budget constraints means I have to wait for a few more months

Anyways, I'm planning on instead going for a combo of the SMC-M 28mm f/2.8 and the SMC-M 135mm f/3.5 to tide me over until then.

Now, from what I understand of the lens compatibility chart on this page -

The Pentax Camera Lens Compatibility Chart

M lenses on the K-x would work on Av and M mode. Specifically, in Av mode all I need to do is set the aperture via the aperture ring on the lens, and the camera meters and sets the shutter speed? Did I get that right?

So if I use SMC-M lenses on Av mode, it's basically like using a modern lens on the K-x, except that I don't get AF and I have to set the aperture via the lens?

Or am I missing something else?

Thanks in advance.

05-05-2010, 09:47 AM   #2
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QuoteOriginally posted by umbalito Quote
Hello all,

Just wondering if you could help a new Pentaxian understand SMC-M lenses and metering on the K-x -

I'll be getting a K-x in a few days, and while I would love to get the DA pancakes right now (which are the main reason I shifted to Pentax from Nikon) unfortunately budget constraints means I have to wait for a few more months

Anyways, I'm planning on instead going for a combo of the SMC-M 28mm f/2.8 and the SMC-M 135mm f/3.5 to tide me over until then.

Now, from what I understand of the lens compatibility chart on this page -

The Pentax Camera Lens Compatibility Chart

M lenses on the K-x would work on Av and M mode. Specifically, in Av mode all I need to do is set the aperture via the aperture ring on the lens, and the camera meters and sets the shutter speed? Did I get that right?

So if I use SMC-M lenses on Av mode, it's basically like using a modern lens on the K-x, except that I don't get AF and I have to set the aperture via the lens?

Or am I missing something else?

Thanks in advance.
M lens can only be used wide open in Av mode. You can turn the aperture ring but the camera doesn't know that and will always meter and shoot wide open.

M mode works fine. You can use the aperture ring to set the aperture and use the "Av +/-" button or the green button to do stop-down metering (so the shutter speed is set properly).
05-05-2010, 09:48 AM   #3
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Take a look at this thread.

https://www.pentaxforums.com/forums/pentax-beginners-corner-q/59272-shooting-...x-dslrs-f.html

On the K-x I suggest setting the Green Button to Optical Preview as the AV +/- button will set the exposure. With Optical Preview you can see the DOF and then you use the e-dial to set the EV.

Last edited by boriscleto; 05-05-2010 at 09:53 AM.
05-05-2010, 10:26 AM   #4
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QuoteOriginally posted by boriscleto Quote
Take a look at this thread.


On the K-x I suggest setting the Green Button to Optical Preview as the AV +/- button will set the exposure. With Optical Preview you can see the DOF and then you use the e-dial to set the EV.
That's a biggie--that the AV button on a KX in M will default to setting the shutter speed, so you can use your green button for something else.

I don't use optical preview much (I guess the DOF in my head), but maybe I'll start. I heard that you don't get the greatest preview anyway, like you do on a film SLR. So since I've been shooting JPEGs, I have my green button set to being me into RAW.

05-05-2010, 10:40 AM   #5
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Is there a particular reason you're not considering the M 50mm f1.7? That would be my first choice for an excellent and cheap manual focus lens.
05-05-2010, 11:22 AM   #6
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QuoteOriginally posted by EricT Quote
Is there a particular reason you're not considering the M 50mm f1.7? That would be my first choice for an excellent and cheap manual focus lens.
I would agree, it's a fantastic lens. I shoot with this lens ALL the time when it starts getting dark and i don't want to use the flash, you've just got to be careful you don't get too close to your subject if you're shooting on Av though or the shallow depth of field can really mess things up.
05-05-2010, 12:58 PM   #7
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QuoteOriginally posted by xjx Quote
M lens can only be used wide open in Av mode. You can turn the aperture ring but the camera doesn't know that and will always meter and shoot wide open.

M mode works fine. You can use the aperture ring to set the aperture and use the "Av +/-" button or the green button to do stop-down metering (so the shutter speed is set properly).
are you saying you are doing the same thing twice? you set it manually and you set the camera...f/4 on the physical lens and set the camera to f/4 also and your set? If that's the case then M is not as intimidating as I thought. The whole "wide open" thing was throwing me off (don't know about the OP). Doesn't that just mean the computer thinks things are wide open and cannot detect the aperture?
05-05-2010, 01:18 PM   #8
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QuoteOriginally posted by justtakingpics Quote
are you saying you are doing the same thing twice? you set it manually and you set the camera...f/4 on the physical lens and set the camera to f/4 also and your set? If that's the case then M is not as intimidating as I thought. The whole "wide open" thing was throwing me off (don't know about the OP). Doesn't that just mean the computer thinks things are wide open and cannot detect the aperture?
When you attach a K or M series lens the lens has no way of communicating the aperture to the camera (crippled KAF2 mount) so all you will see in the display is f---. There is no way to set the aperture from the camera, you have to use the aperture ring. In M mode, you then press the AV +/- button and the camera stops down the lens and meters. When shooting in AV mode there is no way to stop down the lens, so it only shoots wide open.

M42 lenses work differently. When set to manual the aperture stops down when you turn the ring.

QuoteQuote:
"Crippled" KAF2 mount


Pentax K10D "Crippled" KAF2 mount. The mechanical stop-down coupler/indicator found at 2 o'clock in the KAF2 is missing from the "crippled" version.
The "crippled" KAF2 mount is found on Pentax DSLRs released beginning with the K10D/K100D Super. It supports the extra features of the KAF2 mount including the two power contacts for the lens. Power zoom is supported (not full support and only in K10D), as well as lenses with focus motors. Such lenses support Supersonic Drive Motor (SDM), allowing for quieter autofocus. Like the "crippled" KAF mount, it does not have a mechanical stop-down coupler/indicator and thus can only use stop-down metering on pre-A lenses.


05-05-2010, 02:53 PM   #9
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QuoteOriginally posted by boriscleto Quote
When you attach a K or M series lens the lens has no way of communicating the aperture to the camera (crippled KAF2 mount) so all you will see in the display is f---. There is no way to set the aperture from the camera, you have to use the aperture ring. In M mode, you then press the AV +/- button and the camera stops down the lens and meters. When shooting in AV mode there is no way to stop down the lens, so it only shoots wide open.

M42 lenses work differently. When set to manual the aperture stops down when you turn the ring.
This seems like a good time to ask this... if you're shooting a manual M42 lens (i.e. one with an open/close ring) on Av mode, will the metering still work properly as you stop down or will the sensors just go haywire?

Cheers, roliver
05-05-2010, 03:08 PM   #10
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QuoteOriginally posted by roliver Quote
This seems like a good time to ask this... if you're shooting a manual M42 lens (i.e. one with an open/close ring) on Av mode, will the metering still work properly as you stop down or will the sensors just go haywire?

Cheers, roliver
When stopping down in AV mode you will have to use EV. I've found that you have to use between +1.3 and +2 EV. My Helios requires +3. I'm sure Ira will just tell you to use M mode and hit AV +/-
05-05-2010, 03:49 PM   #11
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I've got both the SMC-M 28mm f/2.8& the SMC-M 50mm f/.7.
And on my K-x the 50mm IMO, is far superior to the 28mm.
05-05-2010, 04:16 PM   #12
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QuoteOriginally posted by roliver Quote
This seems like a good time to ask this... if you're shooting a manual M42 lens (i.e. one with an open/close ring) on Av mode, will the metering still work properly as you stop down or will the sensors just go haywire?

Cheers, roliver
Yes, you can use M42 lenses in AV mode, just make sure you have the Auto/manual switch ON the lens set to manual. like boriscleto said, a lot of times you'll have to use a little exposure compensation. On my K10 I use several Taks, and love them....
05-05-2010, 04:31 PM   #13
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QuoteOriginally posted by boriscleto Quote
When stopping down in AV mode you will have to use EV. I've found that you have to use between +1.3 and +2 EV. My Helios requires +3. I'm sure Ira will just tell you to use M mode and hit AV +/-
Give that man a cigar!!!
05-05-2010, 04:32 PM   #14
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By the way:

My pictures don't stink based on my technical techniques. They stink from a lack of talent.
05-05-2010, 05:00 PM   #15
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I have a couple of M lenses, M135 3.5 and M200 used on my K20d.
The results are great. I always use them in M mode and if you do the same you will quickly get the hang of taking some really good quality shots.

Example of 135/3.5 on k20d.

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