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09-05-2012, 02:33 PM   #1
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a few k-mount questions....

First of all, I cannot find much on how kmount really works mechanically and electronically. Basically I'm trying to figure out how older film bodies read aperture off of K and M lenses with no pins. Does it simply measure the resistance or position of the aperture lever? Even my adaptall glass seems to pick up aperture from the ring, and I know that the adapter has no electronic couplings. So how does that work?

Also why did they cripple kmount with the ist? Was there a technical reason? Lack of the aperture couplings to read the position of the ring? Also many lenses have different numbers of aperture clicks, how does it know where aperture starts (ok it meters wide open and compensates?) and more difficultly where aperture ends. Does it assume most lenses peak @ f22? I think it sees my adaptall glass to f32......

So this is a big mystery. I don't know why aperture works on film bodies and why it is crippled on digital. I'm actually ok with stopping down and meter reading, but after using a film body for a while, I can see why it frustrates people that it doesn't just work like before. At least you get the advantage of focusing and composing wide open. Also am I correct to assume that the af film bodies all used center weighted for M and K glass? Why couldn't they multi-segment meter if it could interpet aperture? Seems like a avery pointless limitation. A glass has multimetering even on digital. I thought the only barrier to that was not knowing aperture, which the film bodies seem to somehow....

Sorry...really confused....I tried to look for specifics but came up empty handed with no explanations. The most I could find was that something changed in the aperture coupling on the move to digital. I can only assume they mean the physical coupling.

Anyone know kmount well enough to help me out here?

09-05-2012, 02:44 PM   #2
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On an uncrippled mount there is an aperture coupling (this is in addition to the diaphragm actuator) to tell the camera how many stops down from maximum aperture the lens is set. Correct, the camera meters off the wide open lens and compensates. The crippled mount was apparently to save expense on both camera and lens manufacture. I can't tell you the whys and wherefores of matrix metering but perhaps Lowell or one of the other experts will chime in.
09-05-2012, 02:48 PM   #3
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K and M lenses have no electronics in them. There's one lever that tells the camera how far the aperture ring is stopped down, and another lever that lets the camera stop down the lens to what's set on the aperture ring when it's time to shoot. All this is relative to the lens being wide open. This means that after performing open-aperture metering, the analog light meter in the camera simply multiplies the measured EV value by a constant corresponding to the position of the aperture ring, and then shows the reading to the user

The reason they "crippled" the mount was to make the new lenses cheaper to manufacture, and since the aperture ring was no longer to be used on future lenses, this was only a bad thing for manual lens users as it meant modern camera wouldn't be able to read the aperture ring setting.

"A" series lenses got contacts which solely tell the camera what the maximum aperture is, which was the only unknown on M/K lenses. Thus, this enabled P mode and aperture automation, whereas shutter automation did not require this extra information.

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09-05-2012, 02:52 PM   #4
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I never could figure out why K/M lenses are center weighted only. Maybe the metering electronics doesn't know enough so falls into a dumb mode and just uses the center? Just odd really. I mean it can work out exposure with one or a few of the cells but not all?

09-05-2012, 02:55 PM   #5
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I just pulled off my m28 from my film body. I moved the aperture. The lever raises. lightbulbs turn on. I know the lever gets toggled down to close aperture. Never noticed that it also moved up with the ring. Genius! Now I know how a k1000 works too. Always wondered that....
09-05-2012, 05:13 PM   #6
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Adams explanation was spot on. The aperture lever actually came in on very late sper tak's and SMC tak's for the Spotmatic F and ES II. The bayonet was introduced to provide an easier means of achieving the coupling, with positive rotational registration with the lens locking pin

Also as Adam suggests there is not a need for new cameras to read this because the aperture control is done elsewhere, via the body. Since the FA-J lenses that came with the *istD had no aperture ring, there is no point in having a body that can read that lever. After all it was made obsolete with the program plus in 1982, carrying a useless function forward for 20 years is mo than accommodating to the users even though I can understanding deleting it from new lenses why take it off the body
09-05-2012, 06:05 PM   #7
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So that the value of old lenses can be deprecated in favor of newer lenses. So that Pentax can make more money selling more new lenses.

Edit: This is why I think Pentax will never support an uncrippled mount in the future.
09-05-2012, 06:58 PM   #8
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QuoteOriginally posted by Tanzer Quote
So that the value of old lenses can be deprecated in favor of newer lenses. So that Pentax can make more money selling more new lenses.
If that was the main reason, then why not cripple it further? I'm sure they could find a way to keep the A lenses from working too, maybe no metering at all unless the lens reports a focal length, that way only auto focus lenses would meter. Why give us the option of stop down metering if they want to make it hard to use the M and K lenses?

They did it to keep the cost of the body down, since it isn't a feature that 90% of the buyers are going to care about.

09-05-2012, 08:54 PM   #9
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QuoteOriginally posted by elliott Quote
If that was the main reason, then why not cripple it further? I'm sure they could find a way to keep the A lenses from working too, maybe no metering at all unless the lens reports a focal length, that way only auto focus lenses would meter. Why give us the option of stop down metering if they want to make it hard to use the M and K lenses?

They did it to keep the cost of the body down, since it isn't a feature that 90% of the buyers are going to care about.
Spot on. Pentax has moved forward, while allowing us old Pentaxians to use old lenses with much less hassle than any other brand. Haul out the old M lens, set your favourite aperture for the subject, push the green button and shoot away.
09-06-2012, 06:57 AM   #10
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Of course they wanted to make it harder to use old M and K lenses. Ever wonder why you have to hit two buttons? They could release a firmware update today, supporting a new menu option to reprogram the shutter release, so that when you hit the shutter release button, the camera would actuate the stop-down metering, then fire the shutter. That would not cost anything in additional mechanical parts.

Pentax is playing a delicate and brilliant balancing act, crippling the mount just enough to encourage new lenses, while not greatly alienating users who happen to have old lenses.

Sure there are some secondary benefits of being able to say, from a marketing perspective, that the K-5, K-30, etc. works with millions of lenses. But they don't actually care if we actually use those old lenses, in fact they would much prefer we don't use the old lenses at all. Pentax makes no direct profits from the PF Marketplace or from Craigslist.
09-06-2012, 07:23 AM   #11
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QuoteOriginally posted by Tanzer Quote
Ever wonder why you have to hit two buttons? They could release a firmware update today, supporting a new menu option to reprogram the shutter release, so that when you hit the shutter release button, the camera would actuate the stop-down metering, then fire the shutter.
Interesting thought, it would be terrible for so many reasons though. Most of all it would introduce annoying shutter lag. Also, with stop down metering you get center weighted metering, so if you want to meter off something that isn't in the center, you're out of luck. How would I disable this feature easily so I'm not metering before every single shot when light conditions aren't changing, yet still be able to quickly meter when the light does change?

If you think using the green button is unnecessarily hard, then maybe you should not bother with manual lenses to begin with. It is by far the simplest solution without adding hardware cost, it isn't like the button is inconveniently placed either, you can easily press it without taking your eye away from the viewfinder and it is so close to the rear dial that I can hit it without even taking my thumb away from that either. It becomes a reflex, you don't even think about it after a while.

Last edited by elliott; 09-06-2012 at 07:32 AM.
09-06-2012, 08:08 AM   #12
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Tanzer
Elliott
You are both correct. Pentax is brilliant by keeping my loyalty while encouraging me to upgrade my lenses. I would dearly love to be able to use my M400 other than wide open in an automatic metering mode, but I cannot, and my old lenses have "the dreaded purple fringe" which, although easy to fix in post processing, is still annoying. I will have to start buying lottery tickets I guess - or put my lottery ticket money in my lens account
09-06-2012, 08:19 AM   #13
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While I agree with most of the technical posts, I have to disagree with the posts that suggest pentax is deliberately making it a PITA to use older lenses. The fact is, that they deleted support for open aperture metering on K mount lenses something like 20 years after they made the last camera that needed it. They retained full backward compatibility for years after any other camera maker would have.

just look at canon, they abandoned their lens mount and backward compatibility not once but twice, the first was the breach mount, then a failed bayonet before they came out with the present mount. We are not talking here about being able to shoot, but meter manually, or not meter at all, but rather, we are talking about not even being able to mount the damn lenses!

Nikon is not much better, although you can mount most lenses, the cameras don't meter at all and you simply guess exposure, look at the histogram and move forward. It is only a select few bodies that can do auto exposure with old lenses, and for those you enter maximum aperture into the body.

Now you might ask, why can't pentax do that, and I suspect the reason is that the K mount does not have a strictly regulated aperture movement with activation lever, like the KA mount. some have suggested diameter is linear not area, and while that may be true, it is not clear as to whether the body can control the lever accurately enough, and consistently lens model to lens model, to make it worth while. So they came up with the best they could, reliably. set the exposure relitively accurately once, and just shoot with it until lighting changes.

and by the way, you are not restricted to center weighted metering, you do have spot, so you can selectively meter off anything you want.
09-06-2012, 08:30 AM   #14
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QuoteOriginally posted by Lowell Goudge Quote
and by the way, you are not restricted to center weighted metering, you do have spot, so you can selectively meter off anything you want.
Quite true. Also you are absolutely correct that Pentax has maintained backwards compatibility infinitely better than other brands. I had a friend with US $6,000 invested in Minolta when it suddenly became Maxxum. He was somewhere beyond livid.
09-06-2012, 08:41 AM   #15
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QuoteOriginally posted by Canada_Rockies Quote
Quite true. Also you are absolutely correct that Pentax has maintained backwards compatibility infinitely better than other brands. I had a friend with US $6,000 invested in Minolta when it suddenly became Maxxum. He was somewhere beyond livid.
My brother in law lived through the canon changes, first from the FD mount, to bayonet, but then went to nikon when they obsoleted the first bayonet. By the time he was with nikon everything was AF already, but he does use some older MF glass from time to time. Since he usually purchases the higher end bodies, they allow for manual aperture control. Seems they force entry level users to purchase new glass, but those with the money to buy an expensive camera can save and use older glass. (something along the lines of pay me now or pay me later!)
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