[Lowell Goudge]

I have a question that has been sitting in the back of my mind for some time now.

Has anyone considered replacing the lens mount on their favourite non automatic lens, with one from a junked automatic lens?

I know several stores where you can purchase scrap lenses really cheap, these are things which have been dropped, kicked, smashed, etc...., or low cost kit lenses (35-70 mm typically) but where the mount itself is intact. In principle, you could, with a little fine work, and some technical information on the signals passed from lens to camera, put this mount on a non automatic lens and make it automatic.

I began thinking about this when I picked up a KA mount for my Tameron Adaptall 2 24mm. The mount makes no contact to the lens, and there is not all that much in the mount itself.

[Alvin]

I think that's the equivalent of adding a bolt-on turbo in a car. I'd like to witness this if you make an attempt. Wish I had a cheap lens that we could tear apart, but I can pool in some money.

[jamesk8752]

Mount replacement can be done rather easily with the older K and M mounts, as they are simple mechanical constructions, but the A and later mounts have fiddly spring pins to transmit aperture data and are tricky to handle. The F and later lenses also have a lens-specific microchip to transmit a digital data stream from the lens to the camera body as well as the mechanics of the focusing drive shaft to deal with. I think that there is zero chance of getting a third-party AF lens to work properly on a Pentax body.

All in all, I'd not be afraid to do a K or M mount transplant, but I wouldn't attempt it with an A or later mount.

[Lowell Goudge]

I think that's the equivalent of adding a bolt-on turbo in a car. I'd like to witness this if you make an attempt. Wish I had a cheap lens that we could tear apart, but I can pool in some money.

I will look at at least getting an old lens in a couple of weeks. I found a store that is , as part of a promotion selling the junkers off for $10,

I will try it on a F2 50 mm lense that has absolutely no value.

If that works, I will look at 3 specific lenses I want to change, specifically my 300mm F4, 400mm F5.6 an d my 100mm F4 Macro

[carpents]

Instead of using a real K-mount, I prefer to use the M42-to-K adapters that have the wide flange. They're not really useful for regular use since they don't allow infinity focus, but they have a nice flat bottom that can easily be drilled and mounted onto the bottom of another lens.

Check out the thread about Nikon conversions from earlier today.

[Lowell Goudge]

Mount replacement can be done rather easily with the older K and M mounts, as they are simple mechanical constructions, but the A and later mounts have fiddly spring pins to transmit aperture data and are tricky to handle. The F and later lenses also have a lens-specific microchip to transmit a digital data stream from the lens to the camera body as well as the mechanics of the focusing drive shaft to deal with. I think that there is zero chance of getting a third-party AF lens to work properly on a Pentax body.

All in all, I'd not be afraid to do a K or M mount transplant, but I wouldn't attempt it with an A or later mount.

What I am really looking at is making a K mount an A mount, by setting up the contacts so that I get the automatic apature in minimum F Stop.

If I can figure out the othe contacts, I might also plook at setting focal length, and Maximum apature.

The desire is to achieve the automatic functions on some of my old manual lenses. As I said, in looking at the adaptall 2 mount, I don't think there is a lot to it.

[Skorzen]

What I am really looking at is making a K mount an A mount, by setting up the contacts so that I get the automatic apature in minimum F Stop.

If I can figure out the othe contacts, I might also plook at setting focal length, and Maximum apature.

The desire is to achieve the automatic functions on some of my old manual lenses. As I said, in looking at the adaptall 2 mount, I don't think there is a lot to it.

I think there is more to it than just swapping out the mount, A lenses have the A position on the aperture ring which is a mechanical switch I believe. I think there is more hardware to it than just the plate on the back. I could be wrong, and it would be sweet it I was, but that is my understanding.

[Lowell Goudge]

Just to let everyone know where this is heading, I have aquired 2 scrap lenses, one pentax one sigma, and have begun trying this change.

First, and most important is to note that behind the lenbs mount everything is different from clens to lens.
For example the sigma lens I bought has detents in the back of the mount for the F Stops, where as an older K mount lens I am playing with has the detents internal to the lens.

Biggest hurdles to overcome with trying to convert a K mount to automatic apature will be with respect to the electrical contacts, In newer lenses these take up space that simply does not exist in older lenses. The second issue will be determining which contacts are really needed, which are to have no connection and which can be shorted to the base.

[Lowell Goudge]

As a final update here, I have made some good progress with respect to converting K mount lenses into FA lenses, but not by changing the mounts.

From the two lenses I bought for $10 each, I discovered that one of them, an SMC-Pentax FA28-80 mm f3.5-4.7 power zoom, was in perfect condition except for the power zoom switch. (missing and contacts exposed) As I didn't care about this function, I simply filled the opening for the switch with epoxy and had a good lens.

The other lens was truely junk, a sigma 70-210 with the entire front group missing.

It was too difficult to simply swap mounts because all the other lenses had different things behind the position for the contacts.

The project stalled there for a long time, thinking about the next moves. In searching the web, I found 2 articles on the contact arrangement of KA lenses, which I have referenced below.

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography

Features and Operation of the Ka Mount

These give the coding of 5 of the contacts which are used to tell the camera the maximum and minimum f-stops of the lens.

In studying these articles, and also the sigma mount and a tamron adaptall 2 mount, it became clear that to convert the functions a spring loaded contact was needed in the A position (3rd contact from the right when looking at the back of the lens) which would short the recessed contact in the camera mount to the lens flange.

Further observations and experimentation with the sigma mount indicates that the data contact, (left most contact) also needs to short to the lens flange to recognize the lens as a KA.

Armed with this data, I modified an XR Rikenon f/2.0-16 50mm lens. Photo below. I elected to mask off thearea with nail polish, as opposed to drilling a hole and filling each insulator area with epoxy as some articles suggest, just to tri it out.

It works!!!

I have done some trial shots over the entire F stop range. I am going to take a close look at linearity of exposure, as one of the articles mentions that the apature mechanism changed from linear diameter change to linear f-stop change between the K and KA mount lenses. So there might be a little exposure drift over the entire range.

Edit note, I am still checking the validity of the grey pattern coding as the results seem to be a little variable, (good thing I used nail polish)

[dantuyhoa]

Do you have some photos to show ?? I am really curious . Thanks .

Pentax users are so talented : carpents , roy and you among others!

[Lowell Goudge]

Do you have some photos to show ?? I am really curious . Thanks .

Pentax users are so talented : carpents , roy and you among others!

I will post photo's and also any linearity test results I have when I get final mod done on the f/2.0 50mm.

I am having some difficulty with registration of the insulating areas for the F-stop decoding.

I have got a junk 2x TC that I will sacrafice the mount from to make a template for all contact points. That way I can modify my lenses (5 of them) more accurately

[Lowell Goudge]

Final post, please note exposure curve vs f-stop, and photo of mount with insulated spots and pin for Automatic apature. COnversion of this lens was a pain because there is a screw to attach the lens mount right where there should be an isulator.

It was an interesting experiment, however exposure is a little off due to the linear change in diameter of the irus with the apature activation lever, as opposed to KA mount linear change in f-stop (i.e. irus area) with lever.

The only way I can see this working is if we cvould get pendax to allow us to program an exposure compensation curve for each lens into the camera.

this was done on my *istD which has previously been checked and found to be very linear in manual focus.

EDIT - I Tried on the K10D just to see how it works there, with the K10D metering.

[jslifoaw]

That graph is interesting.

With original K-mount lenses the lever which opens/closes the aperture is proportional to the diameter of the aperture, while on A lenses it is proportional to the area. I.e. on an A lens, the lever would move twice as far to stop down 2 stops as it would 1 stop to make body controlled aperture simpler.

The problem with K lenses is that how much distance to shfit the lever by 1 stop is undefined (though follows a logarithmic pattern, just with an unknown basis); all the lever does is keep the aperture wide open when mounted. so the results may not be the same from lens to lens.

Very interesting though and if you use the lens wide open it would work fine since wide open metering is the reference.

[Lowell Goudge]

That graph is interesting.

With original K-mount lenses the lever which opens/closes the aperture is proportional to the diameter of the aperture, while on A lenses it is proportional to the area. I.e. on an A lens, the lever would move twice as far to stop down 2 stops as it would 1 stop to make body controlled aperture simpler.

The problem with K lenses is that how much distance to shfit the lever by 1 stop is undefined (though follows a logarithmic pattern, just with an unknown basis); all the lever does is keep the aperture wide open when mounted. so the results may not be the same from lens to lens.

Very interesting though and if you use the lens wide open it would work fine since wide open metering is the reference.

Thanks for the response, and yes I already was aware of the difference, i.e. proportional to diameter as opposed to area (or F-stop)

Having seen the result, I have decided not to modify my other lenses just yet. What I have done, is asked Pentax if they would consider allowing an exposure compensation curve, based upon apature, so that user's could fine tune the metering. If they come back with something positive, I will reconsider.

The reason I went this way in the first place is as follows:

Although I have some good modern lenses (all 5 arew zooms covering 10mm to 200mm), I have some good manual focus K mount lenses, including an SMC-M 100 F4 macro, a Vivitar 70-210mm Series 1 F3.5 zoom, an SMC-300 F4 and a Vivitar 400 F5.6.

While I can use these lenses quite well on my *istD both with and without a flash, on the K10D there are two issues, metering on these lenses is NOT as good as my *istD, and obviously P-TTL flash does not work since apature is unknown.

My test lens was not important to me (XR Rikenon F2 50mm) as I have an SMC-50mm f1.4, so I modified it to prove it could be done, and then checked how exposure worked out.

[Ben_Edict]

The problem with the step-down distance is so severe, that a K-lens with A-mount won't close the aperture down fully. You are at least 2 f-stops off. Also, as the step-down distance is proportional to the area, the exposure difference varies with the f-stop and gets worse the farther you stept down. I tried it with a 3.5/15 mm K-lens. Fitting the A-mount wasn't that bad. I just had to route a small captivity to accomodate the sping below the A-contact. Also you won't need a fixed A-position, remembering to have the aperture ring set to its smallest aperture is enough.

But anyway, it is of no use, to have an A-lens-modification, if the metering is useless...

BY THE WAY: manual metering with the K10D can be improved by using an istDS matte screen. I am using now a istD screen with grid and get somewhat better results than before.
regards
Ben