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05-05-2011, 02:19 AM   #1
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Will Pentax-A work on old K Mount cameras?

Hi, I have an K2 and MX along with several manual focus lenses: Pentax-M and Pentax K. I also have a new Pentax K-X with the kit lens.

When using the old lenses on the K-X, having to manually stop down (with the green button) every time I want to take a shot is kind of a pain (but doable). I also heard it is not that accurate when the lens is wide open. (UPDATE: Correction, it isn't as accurate as the F stop goes up.. I took a pic with it wide open and it exposed fine, but as I closed up the lens more (pressing green button each time) it got darker and darker..)

I read in the K-X manual that it will work with the Pentax-A lenses in all exposure modes. I am wondering if the meter readings are accurate at all f stops with these lenses. Also can I use these Pentax-A lenses on my K2 and MX?

What are the optics like in the Pentax-A lenses versus the Pentax-M and Pentax K lenses? How do the Pentax-A lenses compare to the K-X kit lens I got?

I am wondering if I should be investing in Pentax-A lenses instead of the older ones. I could sell the ones I have now and get the A ones. Or maybe I should be going with the Pentax-F lenses? Will these work on my K2 and MX?

Sorry for all the questions.


Last edited by geekette; 05-05-2011 at 02:45 AM.
05-05-2011, 03:01 AM   #2
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I can't figure this out.. with the flash up in manual mode with old lens, it gets darker as the f stop goes up (metering each time with green button). With it down it gets lighter the higher the f stop.

Sigh, I wonder if the Pentax-A lens would make all of these easier!?
05-05-2011, 04:29 AM   #3
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Let's start at the beginning

A lenses will work just fine on K and M bodies. These bodies use the aperture coupling only not the contacts

On a DSLR K lenses can't communicate the aperture setting to the body so they only work in manual mode with the green button, but there may be some metering errors of up to 1 1/2 stops

In flash mode on a DSLR the camera will only shoot with full power on the pop up flash with K lenses which is why the image gets progressively darker as you stop down. You need to calculate flash manually using the guide number, iso , distance and aperture

On a DSLR A lenses are clearly better (more useful) but because of this command a higher price

If you want to do a lot of flash with K lenses consider a flash that can operate in AUTO mode or get an *istD which supports the older TTL flash standard as well as the newer P-TTL
05-05-2011, 10:01 AM   #4
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With your older cameras, you need a lens with an aperture ring, or a lens with no aperture at all, like a mirror lens. Otherwise, you can't set the aperture. For Pentax lenses, that means avoiding the FA-J and DA lenses. All other K-mount lenses are usable.

With your DSLR, the camera is much happier not using an aperture ring at all. All of the A, F, FA and DA lenses can work this way.

05-05-2011, 01:55 PM   #5
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Some Pentax-A lenses are optically identical to their Pentax-M predecessors. Others aren't, and some are distinctly inferior. For instance, the Pentax-A 135/2.8 is a dog compared to the Pentax-M 135/3.5.

There is more plastic in the structure of many of the Pentax-A lenses compared to the earlier lenses. It's not in places that affects optical quality, but it can affect the "feel" of the lens.

Of course, the Pentax-A lenses often cost a lot more used than the Pentax-M lenses, since you're not the only person finding the green button metering inconvenient.
05-05-2011, 02:07 PM   #6
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I'm sure by now you have your answer to the original question. So entice us with which lenses you have.. I for one love the older manual focus lenses.

05-05-2011, 02:36 PM   #7
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A versions would work just fine on those cameras. the advantage of the A lenses however would be on automatic cameras, where stop down meter is not needed. aside from that, you can adjust at 1/3 stops on aperture where a K/M lenses are unable to do.
05-05-2011, 03:28 PM   #8
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QuoteOriginally posted by geekette Quote
When using the old lenses on the K-X, having to manually stop down (with the green button) every time I want to take a shot is kind of a pain
Unless the light has changed from one shot to the next, there shouldbe no need to keep hitting the green button. Just let it ride until yo see the light change. After a while it becomes second nature, and is arguably easier (less thinking, less work, better results) than using automatic modes and having to constantly worry about which scenes might need exposure compensation and in which direction.

05-05-2011, 03:49 PM   #9
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QuoteOriginally posted by geekette Quote
(UPDATE: Correction, it isn't as accurate as the F stop goes up.. I took a pic with it wide open and it exposed fine, but as I closed up the lens more (pressing green button each time) it got darker and darker..)
My experience is slightly different: the metering tends to be correct at fast apertures (e.g. F/2.8, ..., F/1.4); the slower the exposure, the more pronounced the error in metering. The error can be either way: over or under, but I've seen more over (washed out) than under.

QuoteOriginally posted by Lowell Goudge Quote
If you want to do a lot of flash with K lenses consider a flash that can operate in AUTO mode or get an *istD which supports the older TTL flash standard as well as the newer P-TTL
This is one of the reasons that I'd rather have a flash with auto and no p-TTL than one with p-TTL and no auto.
05-05-2011, 05:20 PM   #10
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QuoteOriginally posted by SOldBear Quote

This is one of the reasons that I'd rather have a flash with auto and no p-TTL than one with p-TTL and no auto.
or you can get an AF540FGZ and have it all
05-05-2011, 06:01 PM   #11
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QuoteOriginally posted by geekette Quote
I wonder if the Pentax-A lens would make all of these easier!?
Definitely!
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