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11-30-2007, 11:36 AM   #1
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terminology question

When a K mount lens is described as having a manual diaphragm, what exactly are they refering to?

NaCl(enquiring minds want to know)H2O

11-30-2007, 12:34 PM   #2
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I would guess it's an older lense with aperture-ring so you manually set the aperture on the lens (not in-camera with the eDial).

but what do I know
11-30-2007, 02:11 PM   #3
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Manual diaphragm means that the lens needs to be stopped down manually to take a picture.
11-30-2007, 02:16 PM   #4
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There are several series of "K mount lenses"

true "K" mount lenses I refer to as K mount, they have mechanical only apature coupling. Lens series with this mount are SMC Pentax and SMC-M lenses

KA mount are lenses with automatic apature with an A setting beyond the smallest apature on the lens, and up to 6 electrical contacts on the lens base, all on the flange that the lens mates to the camera body.

KAF lenses are the first series of auto focus mounts, And lenses are marked SMC Pentax-F They have an additional lens contact that gives focus distance information, focal lenght etc to the camera.

KAF2 mounts are the autozoom lenses that came out with the PZ series cameras, and have an additional 2 contacts for power that are in the same position as the new HSM lenses. They are on lenses marked SMC Pentax FA, these lenses can still be switched into manual apature.

KAFJ are what many people call the latest lenses where there is no apature ring what so ever . they cannot be used on the oldest pentax cameras due the fact they have no mechanical apature coupling to tell the camera the apature setting.

11-30-2007, 02:19 PM   #5
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Manual diaphragm lens are those that require you to focus and compose while the lens is wide open. Then with the aperture ring set it to the f stop that gives you the DOF you want. Then on a film camera like a K1000 the meter would give you a reading to adjust the shutter. On a K10D you'd hit the green button to get the exposure reading. Then adjust the shutter and fire the shutter.
The reason for focus and composing wide open is the camera doesn't control the aperture blades so as you close the lens down the image in the viewfinder gets dimmer and harder to work with. Examples would be M42's with an adapter.
12-01-2007, 08:28 AM   #6
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Thanks Peter, I thought it was something like that. Every once in a while I'll run across that description in refering to long teles. I always wondered what they were talking about, as I had thought that all of my "K" lenses were "manual diaphragm". But they really aren't as they are wide open during focusing and automatically stop down to the f stop you've selected on your aperture ring during the shot. So a "manual diaphragm" lens refers to one that doesn't automatically stop down to your dialed aperture.
If I understand you correctly, there would be an extra step or so in shooting
1) figure out your final aperture
2) go wide open (so as to get enough light) to focus and frame your shot
3) stop the lens down
4) hit the "green button" (or the AE-L button on the DS) to meter the shot
5) push the shutter
Is this the story?

NaCl(a light meter would help here)H2O
12-01-2007, 08:49 AM   #7
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That's the story. Of course you need to do some research on the lens in question that you're buying or looking at. Many older K mounts are referred to this way, when it's not actually the case in the strict sense of the term. My K 300mm for example will stop down when the shutter is fired and stays wide open to compose and focus no matter what the aperture setting is. I'm no guru but I think the only true manual diaphram lenses are M42's and they need the extra step of manual step down/exposure setting after focus and composition just before hitting the shutter.

These MD lenses (as I understand it) have no aperture activation lever (or whatever you call it) so the camera can not move the aperture blades. So for example you're DOF preview lever on the camera can't work.

A light meter here could help as in my experience the green button doesn't always give an accurate reading since the K series DSLRs use the crippled mount and were never really built with the old glass in mind. I've done a fair amout of shooting with the K 300 f4 to learn how to use it on the K10D. it needs -1.5 to -2 Ev from the meter readings almost all the time.
Big problem with a hand held light meter is if the light conditions are changing quickly or the subject is fast moving. You're just not going to get the shot right. If you buy a lens like this ( I have a couple of M42's) practice with it and learn how it reacts to the camera's metering. Once you are used to it, you can get good results fairly quickly.
12-01-2007, 09:06 AM   #8
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A manual diaphragm will stay closed down to the value set as has been concluded above. It will not open up to full aperture after the shot.

I found a lens brochure from 1983 - it lists these manual diaphragm K-lenses:

500mm f/4.5
1000mm f/8
Zoom 135-600mm f/6.7
28mm f/3.5 shift

In the M42 world there were a few more, but Pentax managed to make those "automatic" in the K mount version. Of course, all M42 lenses operate as manual diaphragm when mounted on a K mount body.

12-01-2007, 11:16 AM   #9
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Thanks Peter and Ole, the lens I'm contemplating is the 500mm F4.5. Peter, I have the 300 F4, and have found it to be a very nice lens. I've not had to dial any EV on my copy. It's only real problem is the lack of a tripod mount.

NaCl(but I think I'll pass on the 500, just got the 35-105 anyway)H2O
12-01-2007, 01:02 PM   #10
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Well maybe i can solve that problem for you Salty, I got one of these for it and like it a lot. Yeah a bit pricey but not too bad and well worth the $$ and cheaper/better than the Manfrotto option I thought. Seems to be made from a thick plastic like PVC and the only metal is the knob on the tightening screw. Has made using this longer lens a pleasure now on a tripod. I add my AF1.7 TC to the lens and have the length (but not speed) I need for a fixed length.

The 35-105 is a smokin' lens. Sharp as a prime with decent close focus ability. I had one for years and stupidly slimmed down the bag from over 25 lenses to the current 14. I don't know if I miss it for it's superb quality or just this LBA collecting thing.

I don't mind dialing in a little EV comp. fast and easy to get what I want out of the lens. I think it's a camera issue as this lens was fine on my LX.
I have to agree though. This is a very nice lens, it has a sharp image, nice colour and built like the preverbial....

Support Collar for SMC Pentax 300 mm 1:4 ,FA 300 1:4.5 - (eBay item 230198110762 end time Dec-06-07 23:59:26 PST)

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