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11-12-2010, 06:05 PM   #31
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QuoteOriginally posted by cyclone3d Quote
Let the discussion begin.

When I finish up getting everything worked out, I will post a full diagram.

I want to get your thoughts before I go and give it away.
I just use the old dodge of not rotating the lens until it clicks on the mount. By only part rotating it (while making sure it's secure enough not to fall off!) you can usually get at least two extra stops of control in addition the wide-open functionality that goes anyway with the AV setting. The less you rotate it the more stops you get.

11-12-2010, 06:19 PM   #32
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back to square 1

troglodyte

K and M lenses have no aperture contacts, and as a result the camera does not know maximum and minimum apertures,

As a result when put on a DSLR the camera will do nothing with them (assuming you set Use aperture ring) except shoot them wide open in every mode but manual. In manual the camera will let the lens stop down during exposure to the preset value on the aperture ring.

Also in manual mode, you can press the green button, and the camera will stop the lens down transiently and take a meter reading while stopped down, and store those settings. on single thumb wheel bodies it is the AE lock button.

Now for the conversion.

The idea behind the conversion is as follows.

Put the aperture contacts where they need to be to code the max and min aperture, AND install a spring loaded contact for the A pin so it contacts the recessed 3rd pin on the body, and then the camera will think you have an A lens. You then set the lens to its smallest aperture and the camera will control the aperture through the body, just like an A lens.

OK That's the theory behind the conversion, now the reality.

The conversion works, but, the aperture mechanism is nor mechanically the same between A llenses and M/K lenses. A lenses have the area of the aperture change linearly with movement of the aperture activation lever, K and M lenses have the area change as a function of the square of the movement of the lever. As a result, the camera does not select the aperture it thinks is required and you get the exposure error I plotted on the first page of this thread.

therefore although it is possible to do the conversion the exposure error that results makes the conversion useless.

Now, M42 lenses, in manual mode, will stop the lens down as you turn the aperture ring, because the camera has no aperture control at all for M42 lenses. These work in AV mode because the lens is actually closing down, so the camera can meter correctly.

The real issue behind the metering issue on M42 lenses is because the lens contacts on the A series lenses made the aperture ring position coupling redundant. This change was made in about 1978/79 with the introduction of the first ME Super Program body, (I think) but they did not delete the aperture coupling until the late 1990's.

With the DSLRs they added the stopped down metering function to retain their film base of users. Many people have complained long and hard about the present "crippled" K mount but I doubt pentax will ever change back and add the aperture lever again
11-13-2010, 01:44 AM   #33
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Would it be difficult for Pentax to add support for K and M lenses? In the past it has been mechanical but it should be possible to do this through software right? Or am I missing something vital here?
11-13-2010, 06:04 AM   #34
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QuoteOriginally posted by Jimfear Quote
Would it be difficult for Pentax to add support for K and M lenses? In the past it has been mechanical but it should be possible to do this through software right? Or am I missing something vital here?
It is certainly possible for software to compute different aperture lever motions for different type lenses, but I think there might be a few interface problems with the lens and also with the user.

As a critical example, an F1.2-F22 A type lens not set to "A" on the aperture ring looks to the camera exactly the same as all type K or M lenses (all pins grounded except "A"). How would this case be handled?

It might be that a single case of potential confusion like the above is the reason Pentax has not added more complete support for legacy lenses.

Dave

PS I've found it marginally useful to make my camera think all lenses are A type; this enables
P-TTL flash for all lenses (which I use infrequently).


Last edited by newarts; 11-13-2010 at 06:11 AM.
11-13-2010, 09:17 AM   #35
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QuoteOriginally posted by cyclone3d Quote
...
BUT... when using it as an A lens, but in M mode, the color reproduction is a whole lot better then when using it as an M lens.
Me too. Both the attached pictures have been taken with the A50 1.7 the only difference with the aperture ring being in the A-position (#1), and @f=8 (#2). Light should be identical, a single tungsten spotlight (there is a window in the room, but it was dark outside). These have been taken in M-mode (f=8, t=1/2s), from the same camera position (tripod); I took #1, turned the camera (k-x) off, moved aperture ring to f=8, turned it back on and took #2.

The pictures were shot RAW and converted to JPEGs and scaled down to 0.3mpix for posting in the camera with the same parameters.

Conclusion: the body treats A and M lenses differently (#2 is has a stronger yellow tint).

Last edited by jolepp; 03-16-2011 at 08:57 AM.
11-13-2010, 09:28 AM   #36
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The key difference with the pictures in my above post seems to be with AWB. Setting WB to the tungsten preset gives more equal results (again #1: A-position, #2: @f=8). Also, with the corresponding RAW files ufraw reports the camera WB figures as 3319K / 3880K. However, it seems as if #2 had also been more generously exposed although the actual parameters are the same (IS0200, f=8, t=1/2s).

Last edited by jolepp; 03-16-2011 at 08:57 AM.
11-13-2010, 10:21 AM   #37
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Hi Cyclone,I'm a dental lab tech , so I wax and cast things into metal so if you need me to make you any thing to make it work let me know .
11-13-2010, 10:54 AM   #38
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QuoteOriginally posted by Jimfear Quote
Would it be difficult for Pentax to add support for K and M lenses? In the past it has been mechanical but it should be possible to do this through software right? Or am I missing something vital here?
QuoteOriginally posted by newarts Quote
It is certainly possible for software to compute different aperture lever motions for different type lenses, but I think there might be a few interface problems with the lens and also with the user.

As a critical example, an F1.2-F22 A type lens not set to "A" on the aperture ring looks to the camera exactly the same as all type K or M lenses (all pins grounded except "A"). How would this case be handled?

It might be that a single case of potential confusion like the above is the reason Pentax has not added more complete support for legacy lenses.

Dave

PS I've found it marginally useful to make my camera think all lenses are A type; this enables
P-TTL flash for all lenses (which I use infrequently).
i think it would be relitively simple but not how you are thinking.

Start by programming max and min aperture in the same way as you do focal length, then use the green button to let the camera meter open and stopped down, it will then know the aperture setting of the lens, and it can use this in P-TTL flash.

alternatively, with the camera could with manual aperture lenses in P-TTL mode you could stop down before the preflash metering, then flip the mirror, in this way it would still use the cameras metering for flash.

As for normal metering, and AV mode, you would have to accept somewhere an error, for lenses taken out of the A mode, you can't have it both ways

11-13-2010, 11:11 AM   #39
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QuoteOriginally posted by Lowell Goudge Quote
...
Start by programming max and min aperture in the same way as you do focal length, then use the green button to let the camera meter open and stopped down, it will then know the aperture setting of the lens, and it can use this in P-TTL flash.
...
As for normal metering, and AV mode, you would have to accept somewhere an error, for lenses taken out of the A mode, you can't have it both ways
How about a number of user presets from which to select at power-up? Also, the camera might offer to run a calibration routine assuming constant light when creating a preset and maybe assume M-style aperture lever mechanics as a default. Also, these could perhaps be imported from the memory card.
11-13-2010, 12:52 PM   #40
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K and M lenses will ALWAYS shoot wide open in AV mode. To use them, you need to use M mode with either the Green Button, or a hand held meter. M42 lenses can be used in AV mode. But more than likely you'll have to experiment a little and set some exposure compensation to get correct exposure.
11-13-2010, 01:12 PM   #41
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An interesting quirk of M/K lenses in Av is that the body actually stops the aperture down, but somehow still manages to expose with it fully open. This can be seen by looking into the lens when pressing the shutter with the aperture ring set to a smaller aperture. Exposed images come out the same regardless of the aperture ring setting. (This experience is with the k-x.)
11-13-2010, 01:59 PM   #42
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QuoteOriginally posted by Lowell Goudge Quote

OK That's the theory behind the conversion, now the reality.

The conversion works, but, the aperture mechanism is nor mechanically the same between A llenses and M/K lenses. A lenses have the area of the aperture change linearly with movement of the aperture activation lever, K and M lenses have the area change as a function of the square of the movement of the lever. As a result, the camera does not select the aperture it thinks is required and you get the exposure error I plotted on the first page of this thread.

therefore although it is possible to do the conversion the exposure error that results makes the conversion useless.

Correct, but there are a few exceptions. A few M lenses had their apperture action modified from square to linear in the very end, while still finnished of as M lenses without the A contacts, I think the M28/2.8 and the M80-200/4.5, perhaps more. These lenses would be ideal to convert. Not the most exciting lenses though...

Doesn't this belong to beginners corner? There must be hundreds of threads asking the same questions?
11-14-2010, 12:05 AM   #43
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QuoteOriginally posted by jolepp Quote
Me too. Both the attached pictures have been taken with the A50 1.7 the only difference with the aperture ring being in the A-position (#1), and @f=8 (#2). Light should be identical, a single tungsten spotlight (there is a window in the room, but it was dark outside). These have been taken in M-mode (f=8, t=1/2s), from the same camera position (tripod); I took #1, turned the camera (k-x) off, moved aperture ring to f=8, turned it back on and took #2.

The pictures were shot RAW and converted to JPEGs and scaled down to 0.3mpix for posting in the camera with the same parameters.

Conclusion: the body treats A and M lenses differently (#2 is has a stronger yellow tint).
This is the same sort of thing I saw.. except I was testing outside. When the lens was used as an M lens, I was getting too much blue hue. When I switched it to be used as an A lens, the color was spot on.

I saw you second post, and it really does seem to be a white balance issue between M and A code.
11-14-2010, 12:06 AM   #44
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QuoteOriginally posted by pentaxk10duser Quote
Hi Cyclone,I'm a dental lab tech , so I wax and cast things into metal so if you need me to make you any thing to make it work let me know .

I'll keep that in mind. I may have something coming down the line that may need a special part or two made... but I usually make stuff by hand.

Thanks for the offer
11-14-2010, 02:22 PM   #45
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QuoteOriginally posted by cyclone3d Quote
.
...
I saw you second post, and it really does seem to be a white balance issue between M and A code.
Not only that though: it seems that there is 1/2 EV difference in exposure too. That was how much I had to adjust #2 after setting the white balance to 3319K like #1 had to get a similar appearance with ufraw (with the rest of the parameters the same). A difference in exposure is clearly visible in the histograms too.

As far as I can see this is not a fluke: it would be nice if someone else tried this under controlled conditions (as I have attempted to do after noticing this as I were casually comparing the sharpness of 50mm A / M which happened to be in tungsten light).
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