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04-26-2011, 12:34 PM   #1
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Using K, M, and A lenses

Hi,

I've been shooting Pentax for a while and I still don't understand the K and M lenses.

I've used screwmount "auto" lenses and A lenses. I know how those work. The screwmount requires stopping down by hand before shooting (at least when used on a modern digital body). That means I'll either take longer to get a shot, or I'll focus at the stopped-down aperture and make the viewfinder dim and make the point of focus less obvious. I don't really like doing this.

My question is really, "Do M lenses stop down and meter automatically?" I know I can do DOF Preview and then green button and then shoot. But I'd like to select an aperture, focus, and then just fire.

Next, I'd like to know what the difference is between K and M lenses. Obviously there are mechanical/construction differences. I know that the K Takumars are generally lower quality. It seems to me that K and M lenses are mostly the same. Is that correct?

Thanks for the help.

04-26-2011, 01:10 PM - 1 Like   #2
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QuoteOriginally posted by Aegon Quote
My question is really, "Do M lenses stop down and meter automatically?" I know I can do DOF Preview and then green button and then shoot. But I'd like to select an aperture, focus, and then just fire.
Short answer, NO. Long answer, you have to use stop down metering. The "crippled" K-mount does not have the necessary mechanical linkage to transmit aperture data to the camera from the lens. A lenses use an electrical connection to transmit aperture data.

QuoteQuote:
Next, I'd like to know what the difference is between K and M lenses. Obviously there are mechanical/construction differences. I know that the K Takumars are generally lower quality. It seems to me that K and M lenses are mostly the same. Is that correct?
The K series is generally identical to the SMC Takumars. Many of the M series have a different optical formula, but not all. Some lenses didn't change until the A series.
04-26-2011, 01:29 PM   #3
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Interesting. So that's what all the fuss is over the crippled K mount.

My Super Program and ZX-7 film cameras aren't crippled. Does that mean that they could use M lenses without stop down metering? That might still be useful.
04-26-2011, 01:31 PM   #4
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1. In M mode, with a K or M lens, DOF preview will stop down the lens to the selected aperture and activate the meter so you can See it. It will Not set the shutter speed. You must still do that yourself which you can do while DOF is activated (I have my shutter speed set for the rear dial). The Green button momentarily stops the lens down (just as it would with DOF preview) but sets the shutter speed for you. That is the case on the 'flagship' cameras anyway. See This for a quick tutorial on how to use stop down metering.

2. Functionally, there is no difference between an M lens and a K lens. The Takumars you refer to are Bayonet Takumars which from my understanding is a budget class of lenses that lack the SMC coatings. Actual K lenses in most cases, are in a completely different class. Witness the Takumar (Bayonet) 135 f2.5 compared to the K135 f2.5.

3. Yes.



04-26-2011, 01:52 PM   #5
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QuoteOriginally posted by Aegon Quote
My Super Program and ZX-7 film cameras aren't crippled. Does that mean that they could use M lenses without stop down metering? That might still be useful.
Yes, K & M lenses work with the Super Program and ZX-7 in AV mode & M mode.
04-26-2011, 02:11 PM   #6
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Great info, thanks to both of you.
04-26-2011, 09:21 PM   #7
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M42 lenses with an Auto/Manual switch and 'A' lenses will work on dslrs in Av mode. The 'A' lenses meter correctly. The M42 lenses do not in general meter 100% correctly without a little compensation dialed in. This varies by lens and by the aperture used. They are quite usable in Av but you have to be aware that you will need to correct based on the lens you are using.

Takumar (Bayonet), 'K' and 'M' lenses will not allow shooting in Av mode. You will need to shoot in manual, set the aperture by hand using the lens ring and use the green button or AV+- button to set the shutter speed (depending on camera model).

Functionally I do not believe there is any difference between 'K' and 'M' lenses. It appears that the 'M' lenses are in general a little smaller and lighter than their 'K' counterparts, although not all lenses were changed.

There is a sticky on this subject here.
04-26-2011, 09:51 PM   #8
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QuoteOriginally posted by Aegon Quote
My Super Program and ZX-7 film cameras aren't crippled. Does that mean that they could use M lenses without stop down metering? That might still be useful.
Yes.

You are local to me. P.M. me if you want an in-person demo of the various lens types on various bodies. I have a full assortment!


Steve

04-27-2011, 06:06 PM   #9
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Thanks for the offer Steve. If the Portland/Vancouver Area Meetup ever happens then I'd love a demo.

I bought an M 50/1.7 to try on my film bodies. I'll give them a run on the digital bodies too, but I'm not expecting miracles.
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