[Ira]

That is simply a mix-up. Depending on the camera, the old K-mount lenses (any lens of the pre A-generations, including the K and M series) will stop down for metering (Green Button) and also stop down to the chosen aperture during exposure, if you use the camera in M-mode. Only in the automatic exposure modes, the aperture is locked fully open.

Ben

That's not according to what I read according to one of the external tutorials on using manual lenses on the KX.

Did I screw up because the camera model makes a difference?

[smcook99]

Ok, so to recap. Pentax Made A-Series lens will auto-meter. Any other lens will work in manual mode I just have to meter myself, this is an K-mount or M42 mount. Am I correct on all that now?

[Entropy]

All K lenses will work on all K bodies, from the old days of film to today. But they will only shoot wide open.

Um, no. That is WRONG on the K20D.

It will only meter wide open unless you use the green button in M mode, but it WILL stop down to the selected aperture when you shoot.

In aperture priority mode, Bad Things will result if you don't shoot wide open, since the camera meters wide open (with no idea what the real aperture is) and then will stop down to take the shot.

In manual mode, hitting the green button will cause the camera to stop down to the aperture selected on the ring, meter, then return to wide open.

(Not sure how different this may be on the K-x. A coworker is getting one soon so I might be able to play with one of my M lenses on her K-x.)

[RioRico]

Ira IS losing it. OK, here's the skinny: Many lenses made by Pentax and others use a bayonet mount introduced on K-series film cameras, called the K-mount or PK. The prior standard used my Pentax (and many other brands) was a 42mm screwmount called M42. Many many good M42 lenses exist. There are also (mostly Russian) lenses with a 39mm screwmount called M39 that can be used on Pentax cameras, but other 39mm lenses CAN'T, so forget them for now.

PK lenses (those with the K-mount) come in various series, some with identifiers on the lens, some without. Those with an A on them have an aperture ring with an A setting (except some cheaper digitals) and contacts that communicate aperture information with the camera. *ANY* A or FA or DA lens can automatically stop down when you meter and shoot.

PK lenses WITHOUT an A on them, including the M series and the unmarked so-called K-series, will not stop down automatically. BUT you can use M(anual) mode and the Green button to stop-down when you meter and shoot. This just makes using them a little slower. In any of the auto modes, they only shoot wide-open; in M(anual) mode, they'll stop down.

Screwmount (M42 or M39) lenses require adapters to mount on your K20D. There is much debate here about the quality of various adapters. The Official Pentax sort are safe and expensive; third-party clones such as Bower are cheap and MUST BE MODIFIED OR YOU WILL GO APESHIT; neither of these affects the focus of the lens. A third sort, with a flange, is safe and cheap but you lose infinity focus, which doesn't matter if you are shooting closeups or macros.

NO screwmount lenses will stop-down when you meter and shoot. All must be manually set. Many were made to stop-down on M42 bodies; those that ONLY stop-down automatically on M42 bodies will ONLY shoot wide-open on your K20D unless they're modified. Don't hesitate to ask here about this if you're considering buying any screwmount lens. There are many sweet cheap deals out there, if you just know how to use them.

I think that's a pretty accurate overview.

[smcook99]

""
It will only meter wide open unless you use the green button in M mode, but it WILL stop down to the selected aperture when you shoot.

In aperture priority mode, Bad Things will result if you don't shoot wide open, since the camera meters wide open (with no idea what the real aperture is) and then will stop down to take the shot.

In manual mode, hitting the green button will cause the camera to stop down to the aperture selected on the ring, meter, then return to wide open.
""


I usually shoot in Aperture mode because I want the lens wide open, so in that case I would be fine to assume the camera will meter and adjust shutter and iso fine? If for some reason I want to stop down, then I put it in Manual mode, use the green button to meter, adjust my iso and shutter myself and than take the shot.

[smcook99]

There is someone within driving distance selling a Tamron 70-200 for $30 that is screw on. I figure for $30 + adapter I can't go wrong just to have it to feel out the zoom range for a later decision on a final walk around range I'd like. So, in shooting with that lens, I will most like want it to be wide-open. I'm going to assume it's f4 or f5.6. In that case I can leave my k20d on auto, which will force the lens to f5.6, meter and adjust for me?

[reeftool]

My M 135/3.5 must be broke because it stops down to the aperture I set when I push the green button and when I shoot. I shoot in M when I'm using a manual lens. The aperture data doesn't show up in the exif because the camera can't read it. A K mount lens from another camera brand or aftermarket lens mfgr. may not work the same but so far, I have had no issues of a lens that won't stop down in M mode to the setting on the aperture ring.

[smcook99]

Clearly I'm not the expert here, but I thought everyone just got done explaining that in M mode, the lens will stop down for the green button and the shot.

[boriscleto]

This should have been the first response. Read this thread

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

[Marc Sabatella]

You're right - that should have bee the first response!

I think Ira's confusion comes from the fact that in all modes *except* M, a manual aperture K-mount lens (eg, one in which the aperture ring is the only way to control aperture) will indeed only shoot wide open. But in M mode, all K-mount lenses - manual or otherwise - stop down correctly when shooting.

Bottom line: any K-mount lens can used just fine, no adapter. But if it's manual aperture - meaning it has an aperture ring but lacks an "A" position - then you have to use it in M mode if you don't want to shoot wide open. Any Pentax lens with the letter "A" or "F" in the main part of the name (eg, A50/1.7, F50/1.7, FA50/1.4, etc) works in all exposure modes.

[smcook99]

The pentax store does not have the screw mount adapter in stock and I couldn't find it in any of the big stores. bhphotovideo.com has a "General Brand" M42 adapter. I know someone said the universals are problems. Has anyone used this "General Brand" product from bh?