*M lenses shoot wide open on my K100D

Hello from the States.

I bought this beautiful 40-80 mm Pentax-M lens that goes down to 2.8 so I have something to use for low light. The problem is that for anyone that uses the older lenses- you have to manually turn the f-dial for the aperture, and it doesnt send the f-stop to the camera.


What I've noticed is that no matter what aperture I turn to on the lens its always going to meter and shoot at f2.8 (max aperture). I can set it to f/11 or 22 but itll still meter at 2.8 when I press the AE-L button. Even though its in M mode. And I'm allowing the aperture ring. If you don't, then the cam just flashes Av and does nothing.

All the blades snap open and close very well with the lever. I'm glad to say that its a well-made and kept lens. I was reading up on this situation and I think what's going on is that one of the contacts on the camera is keeping the lens open. This is probably why I can open and close the aperture when the lens is off the camera but opens wide once I mount it. When I put the lens on my older K1000 the meter needle reacts as it should with the change in aperture. I guess it isnt too bad a problem since I use this lens in low-light situations only.

Also, I just tried my A lens with just using the aperture ring and the same thing happens. As soon as you mount the lens- it opens completely wide! Sigh.

Sorry about my introductory post being so lengthy but I'm at my wits end! I hate spending money on something and not be able to use it to the fullest. I may just have to get used to the fact that, technically, its a 40-800 mm 2.8 fixed aperture lens when put on a digital camera.

Thanks.

Hi Jeff,

WELCOME!

I don't know this particular lens, but I'm only able to use the older M lenses which have a Man / Auto switch. Once set to Man (Manual), I'm able to set an f stop on the lens, and set the camera to AV mode. The camera will then will select the appropriate shutter speed and ISO.

With M lenses without the switch, I too have to shoot wide open.

This also applies to all my screw mount lenses which I use with an adapter on DSLR - must have the switch set to Manual.

I hope this makes sense, and good luck.

Hi. You don't mention which camera you are using. I think you have already done this but with the K10D you need to find the appropriate menu in Custom Settings to change "Using Aperture Ring" to "Permitted". Once you have done that you can set your lens to whichever aperture you want and when you take the photo the lens will stop down. It works fine for me so far.

The problem lies with the metering. I just tried an experiment with my SMC-M 40-80mm, SMC-M 200 F4 and SMC-A 50mm F1.4 (using the aperture ring). On all these lenses you can use manual mode and meter by pressing the green button. As you stop down one can observe an appropriate change in the shutter speed until f/5.6. From then on the shutter speed remains fixed and the picture becomes increasingly under-exposed until at f/22 it is virtually black. By increasing the exposure 4 stops from the f/5.6 reading a reasonably good exposure can be obtained once more.

No wonder A series lenses command a higher price on fleabay.

Best wishes, Kris.

AFAIK You can use macs technique of using M mode, but then simply pressing the handy green button to make the camera select exposure rather than switching back to Av

For Pentax M and K bayonet lenses.......

As long as "Using Aperture Ring" is "Permitted", in Manual mode, the lens should stop down as the shutter is fired. If it doesn't, I'd say there was some physical problem with the lens or camera - I don't know what that could be.

A meter reading (albeit an unreliable one) is taken by pressing the AE-L button (or Green button on K10/20D).

To be sure it's working, with a smaller than max aperture selected on the lens, you should actually hear and see something happening when you press the AE-L button, and you can compare depth of field with shots taken with various apertures.

Sorry I didnt specify. Its the K100d.

It looks like ANY lens mounted on the camera will automatically open up wide. Even my A lens. At least the other lenses will stop down when I shoot but not the M. If there's any defect it may be in the camera body. The mount my be leaving the iris wide open. At least the lenses that can autoexpose can stop down.

On Pentax DSLR's, pre-A series lenses (M and K) will stop down only when Manual (M) exposure mode is selected (and maybe B). In any other exposure mode (Tv, Av etc) they will remain wide open irrespective of the aperture set on the lens.

I find that metering doesn't work on my K20d in Av mode but works fine in M. But as the others have said above, make sure "using aperture ring" is set to "permitted" in the custom menu.

On the K20d the "green button" enables the stop down metering to work but on the K100d you need to use the AE-L button.

The first 4 posts of this forum thread explain in detail: GOOGLE IS YOUR FRIEND

It is normal for the lens to open wide when mounted on the camera. This is to provide a bright viewfinder.

Can you cofirm that you are using Manual exposure mode and "Using aperture ring" is permitted?

Have you compared depth of field with different aperture settings? Test with a subject where you can get a shallow DOF with f2.8, then compare with an attempt at f11. That will prove definitely whether the lens is stopping down when the shutter is fired.

If not working, I would suspect the lens before the camera. If the camera can stop down an A lens, it can stop down an M lens. First of all, though, you must be sure that you are using the appropriate camera settings, hence me asking you to confirm.

As for taking a light reading with M and K lenses before firing the shutter: it's well known that this can give unreliable results - worst with the K10D, but can be unreliable with other models as well. With smaller apertures, I never found it much use with my *ist DS as well as my K10D. So you may need to do some trial and error or calculation before achieving the exposure you want.

Hi Pete.
Yes, I can definitely say that I have "aperture ring permitted" and I have the knob set to M. But as far as the camera's concerned, its still in Av mode. When I set the f-stop on the ring and then press the Ae-L button I can the same shutter speed no matter what aperture I pick. So I can honestly say the lens isn't stopping down at all, and that goes for ANY lens that has an aperture ring, even my more modern A lenses.
It does make that "whoosh" sound when I press the Ae-L button when I meter so it does sound like its stopping down. I don't think my camera will allow me to adjust the aperture at all thru the ring. It'll meter thru any lens at whatever max aperture that lens has.

Sorry, its just frustrating.

Sounds to me there is something wrong with your camera body. When I press the green button on my K10D I can visibly see the aperture closing through the viewfinder. Is there anybody out there with a K100D that can test this?

I hope you sort it.

Best wishes, Kris.

the only other thing I can suggest is that the aperture stop-down lever on the lens can sometimes become ever so slightly bent. If that is the case then it doesn't contact properly with the stop down coupler on the camera.

TBH though, that should only effect one lens, and I think it should stop the lens opening up rather than stopping down. I'm afraid it sounds like theres a fault with your K100D which is preventing it from going into M mode with an old lens attached. It is behaving as you would expect if it were in any mode other than M, but M is not engaging properly.

As a last ditch, have you tried resetting your camera to factory defaults then just changing the Aperture ring one to "Permitted"?

As Kris says, you should see the resulting loss of light in the viewfinder whilst it stops down to meter.

You could try the DOF preview as this essentially stops the lens down whilst you hold it on. Try it at f22 so the change in light will be plainly obvious. If this works on your A lenses & not M ones then it's the lenses. If it doesn't even work on your A lenses then your camera is at fault. - Take the lens off completely, turn your camera to Manual mode & turn it on. Then try the DOF preview button to see if the aperture lever on the camera is moving.

i have used Manual lenses on a k100d and i have the camera set to Manual mode... the aperture WON'T display in the LCD screen, only the shutter speed... you adjust the aperture via the ring on the lens.

i noticed that the k100d does only seem to meter as if the lens is wide, so i tend to take a reading in Av mode and work out what it will be if i close the lens down. as i use the M lenses for macro i can get away with playing. if in doubt, use the preview button instead of taking a shot.

i did find that as i used the lens(es) more i could judge roughly what the shutter speed should be if using the aperture closed down.

alas, i don't think there is a way to meter accurately on a 100d at anything other than wide open - unless my body has the same problem as yours! i must admit, it makes no sense as the aperture is physically being closed.

It may be a physical "defect" of the K100 "non-super" body. At least the lens does physically work. I'm going to upgrade to the K200 pretty soon and I'm hoping that this won't be an issue with it. For now, I'll just use it as my "low-light" lens.

Thank you for all of your help. Maybe now I can contribute information rather than asking for it!