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08-23-2011, 01:17 PM   #1
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A question from the terminally dense

OK, after lugging a Pentax PZ-1 around for years, then giving up on film and walking around with first a Nikon, then a Canon single-digit megapixel point-and-shoot, I've bit the bullet and bought a Pentax K20D (I'm not wealthy -- in fact, I'm retired and broke)

I'm trying to get my head around the "older K.x.x mount film camera lenses on a Pentax DSLR." I pretty much understand the 35mm film exposure area vs. the smaller CMOS exposure area, and the resulting Lens Focal Length and Field of View factor (from Bojidar Dimitrov's website) and the possible vignetting/distortion problems -- what I haven't quite figured out yet is what sort of behavior I can expect from the older "film' lenses, and what kind of in-camera settings I need to use for the NON-DA lenses I have.

First, to the mount descriptions (Wikipedia, and who knows how accurate THAT is?):

<< KAF2 mount:

The KAF2 mount is the same as the KAF mount except that it adds two extra contacts to the inside of the mounting ring for power zooming, and transmits modulation transfer function (MTF) data through the digital seventh contact. On the K10D digital SLR model (and later models like K100D Super, K20D, Pentax K-7, Pentax K-5 and K200D) the two power contacts can also power Silent Drive Motor lenses.

"Crippled" KAF2 mount:

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 Silent 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. >>

I'm not sure if my K20D has a KAF2 mount or a "Crippled" KAF2 mount, but can someone please give me an idea what the K20D settings should be for a couple of lenses I have -- from the primitive, to the not-so-primitive -- and maybe I can figure out mister in-between:

SMC Pentax-M 50mm F2.0

SMC Pentax-FA 1:3.5 - 4.7 28-80mm Zoom

I don't mind losing auto focus -- I prefer manual -- but the powered zoom function would be nice, as would looking through a NON-manually stepped-down aperture.

Thanks for your help!

Bart

08-23-2011, 01:35 PM   #2
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Your FA 28-80 will work without problems, autofocus will work and exposure will be automated.
You 'll maybe be disappointed by its image quality, but it will work.
I personally find such zooms quite handy for portraits, as it starts as a true normal and end up as a small telephoto, which is the ideal range for portraits. If you can spend the money, the tamron 28-75/2.8 is quite good as a worthy replacement, BTW.

Now, your M lens is another beast. It will expose "automatically" (as in "dynamically") only at full aperture, or you'll have to use the M mode and the Green button in order for the camera to stop down the lens to the selected aperture on the aperture ring and meter the scene. Quite time-consuming in fast-changing conditions.

You can use some tricks to ease its use. For instance, you can drill a new locking groove into your lens so it will lock halfway turned into the mount. The aperture lever will not be engaged, and so the aperture ring will have a direct effect on the aperture. This way, using Av mode, you can have a dynamic metering at any aperture, without the extra hassle of the green button. Downside is that the viewfinder will darken, and focusing will be harder if you use small apertures.

Bending the lens' aperture lever has the same effect, but then your lens cannot be used like a M lens anymore (whereas the 2nd locking groove still allows you to lock the lens in its usual position and use it like a M lens).

Please also note that the K20 focusing screen does not have a linear response to stop down metering, meaning that stopping down the lens by one stop won't necessarily be metered as one stop darker (do a search on this forum for posts by Lowell and its famous "K20 vs *istDS screens" thread).
It means that using your M lens will be frustrating, as using a different aperture on the same scene will result in a wildly variating metering. You have a -1.5/+3 Ev metering variation in the worst cases...

Fortunately, you can change the focusing screen for a better one, either a *istDS screen, like the LL-60, or better still a Canon ee-S screen if you are not adverse to some DIY wizardry (quite easy to do, and those screens are cheap!).
The Canon ee-S screen has the exceptional capability of being absolutely linear in exposure (meaning that stopping down the lens by one stop will result in a one stop difference in the camera metering), which is the only screen I know capable of this. It also has the added benefit of being able to faithfully render DoF below f/2 (usual screens are not able to render a f/1.4 DoF)...

Voilą!

Last edited by dlacouture; 08-24-2011 at 12:05 AM.
08-23-2011, 01:50 PM   #3
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I can report that 50mm F/2 lens takes very nice images on my K-5. I have to remember to let the shake reduction do its work, but focusing usually allows enough time. I've been mostly shooting with catch-in-focus (CIF) with that M-50. As long as it's calibrated and you shoot at a reasonable f-stop - say 8-11 you'll get your subject in focus.

Look in your manual to find the CIF option - I'm pretty sure the K-10 has it. That's one less thing to worry about when shooting.
08-23-2011, 02:55 PM   #4
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Hi Bart,
AFAIK, all Pentax dSLR mounts have been crippled, meaning there needs to be added user steps in order to be able to stop down an M lens during exposures. This shouldn't bother you if you don't have/like to shoot with M lenses.

There are articles in this forum that help explain this better if you're interested.
Otherwise, your K20D is a fine choice in camera, and will allow you to use all M42, A, F, FA, DA and DFA lenses (and their 3rd party equivalents) as you would have previously on a film body.

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