Modifing older K and M lenes into an A. Possible?

[scott_verge]

Reading over Bojidar Dimitrov's Pentax K-Mount Page. It seems to me

that you could take an older K or M lens and modify it so the camera

would think that its an A lens and treat it like an A lens, pretty

much giving those older lenes a new life on the new cameras like the

*istD and such.

 

The only hiccup I see is the Linear Diaphragm Actuator. I'm not

quite understanding the differences but it seems to me if you can use

an A lens on an older body not made for one and a M lens on a newer

body and they both with fine then there isn't a problem. I could be

wrong though.

 

As for the transformation, I would say drill out a small shallow hole

an fill it with epoxy for the noncondoctive contacts and leave the

conductive ones alone. As for the pin that comes out to tell the

camera the lens is in A setting, drill from the back of the mount

(inside of the mount removed from the lens) and put in a spring loaded

ball bearing. Then set the aperature to the full closed posistion

Usually f22 on my common lenes.

 

Then the lens would still work as a normal K or M lens on older

Cameras and as an A lens permanently set to A on newer cameras.

 

I'm pretty much ready to try this experiment on one of my lenes, but

I don't have a Camera that doesn't like the older lenes, I have a

Z-1p, but at least my older lenes would be let me set the Aperature

through the software and use the matrix metering mode.

 

So what do you guys think? I'm just not sure if the linear part

screws up the whole idea.

[belina_manalang]

Yes it's possible. Check out Mark Roberts' website:

www.robertstech.com/matrix.htm

[greg_erker]

The linear part is a problem. In the pre-A cameras the lever that

holds the aperture open moves the full amount when you take a photo.

The lens aperture ring controls the aperture the lens ends up

closing down to.

 

In a Program camera the body needs close the aperture to a value

it determines. So the body moves the lever a certain number of mm

to get the aperture it wants. If you trick it into thinking a non-A

lens is an A lens the exposure will likely end up way off from what

is correct. I'm not sure how the non-linear curve looks so you

may end up with overexposure over part of the range and underexposure

over the rest, or it may over(under)expose over the whole range,

with the amount of over(under)exposure varying with the aperture

the camera is trying to get.

 

So shooting in Program mode may not work well since the amount of

exposure adjustment you'd have to make would vary with scene

brightness. But it might be usable in Aperture priority. Set the

aperture you want, take a test shot, look at the LCD and histogram

to adjust the exposure compensation. Then it would work until you

pick another aperture.

 

If I could affort an ist D I'd probably put my effort into building

a jig to drill that extra hole for the locking pin. Then I could

shoot with the lens stopped down. For me that would be fine since

my non-A lenses are fast primes that I normally shoot wide open

or stopped down only 1 or 2 clicks.

 

Greg

[scott_verge]

I'm still not sure there is a curve in the older lenses.

 

From the K mount page.

 

"All K-mount lenses have identical stop-displacement."

[greg_erker]

The Mark Robert's page is about adding contacts to get matrix

metering. He says that it won't "fix" lenses for the ist D.

 

If you don't believe that the old lenses are different maybe you

should round up two 50 mm lenses of the same speed. One non-A and

the other an A lens. Set them both to f22. Move the lever on each

an equal amount and compare if the two apertures look the same

size over the whole range.

 

Greg

[scott_verge]

Yeah I would try if I can find one.

 

I'm going to at least do the first step and get matrix metering working on all my lenses.

[scott_verge]

Well I converted my 28 and 50mm lens so they have matrix metering.

 

Definatly cool.

 

I'm going to try to get my hands on another 50mm lens to compare to this weekend.

 

"All K-mount lenses have identical stop-displacement." is haunting me.

 

I guess if I fooled the Camera into thinking it was an A lens the only way to confirm if it was metering right would be to take some test shots on slide film and get them developed right? Or is there another way?

[greg_erker]

Here's the full quote "The least-noticeable change is in the movement of the diaphragm actuator. In the original K-mount its displacement is proportional to the diameter of the diaphragm opening. In the KA mount, it is proportional to the area of the diaphragm opening, and thus to the selected f-stop. This leads to an identical stop-displacement between any two consecutive f-stops, and thus greatly simplifies the operation of the body in the Tv and P operating modes. All K-mount lenses have identical stop-displacement".

 

I think the last sentence is a typo. It should say all KA-mount lenses

have identical stop-displacement. That would make sense. A program

body wants to pick an aperture one stop down from wide open. So

it allows the lever to move a specific amount during the mirror up/

stop down phase before the shutter opens. And since all KA lenses

are the same N-stops per mm this will work with any KA lens. If

the stop-displacement isn't constant on all KA lenses then the

Super Program wouldn't know how far to move the lever to get the

aperture it wanted.

 

You could email Boz to get a confirmation that this is a typo.

 

Greg

[scott_verge]

Yep you could be right, that does make alot of sense, I have emailed him so hopefully he will confirm.

 

Thanks for the feedback.

[greg_erker]

Thanks Scott. Keep us informed.

 

Greg

[Jochen_S]

I haven't tested anything myself, but I can remember some article about Chinon cameras which offered Program-modes with ordinary K-mount lenses. As far as I can remember exposure was about at least 2/3 of a f-stop of and these cameras were only recommended for people never using slide film.

[scott_verge]

So it would have been a constant 2/3rds off? That doesn't sound that bad, they could have compensated for that in the programming if you told it the lens.

 

Hell they could have had an area in the programming to put in a lens name and all the specs and then you could select from a list of programed lenses and it would know how to use them.

 

Any idea where you saw this? You could just use exposure compensation to compensate for that differnce if it was a constant one.

[scott_verge]

Ok I got an email back from Bojidar

 

"I should have said "All Ka lenses have the same stop-displacement." By

this I mean that the lever displacement between f/2 and f/2.8 is the

same as, say, the displacement between f/11 and f/16, on that lens or

on

any other A-series lens.

 

The displacement with K and M lenses is "potentially" different between

any two lens stops and between lenses. In fact, that lever is only

used

to fully open up the aperture. So the displacement between any two

apertures is not important.

 

In M and Av, the body simply allows the lever to move ALL THE WAY. It

is the internal lens mechanism that stops at the selected aperture.

This works like this for ALL K-mount lenses, also A lenses.

 

In P and Tv (only possible with A lenses), the body allows the lens

lever to move only a specified amount. The lens is fully closed ("A"

is

beyond f/32), but the body does not allow it to close fully."

 

 

Now it all makes more sense to me.

 

Still wondering about that chinon body thing though, I'll do more research into that.

[scott_verge]

Found some stuff on those chinon program cameras, I want to know how they do it but we can't.

 

"Although the Chinon CP-X incorporates a single program AE, it still is the most versatile program system camera available. Unlike other program AE SLRs, the Chinon CP-X does not require special mount lenses and works perfectly with the universal "K" or "KA" or "KR" mount lenses.

 

This breakthrough in technology not only preserves your investment in lenses but also provides an extensive variety of future lenses which will couple precisely to the Chinon CP-X."

 

"Unlike other program cameras, CHINON CP-X camera accepts any existing "K" mount lens even in the program mode. Generally, you should set the aperture at its minimum setting (largest number on f-stop aperture ring]. Although a perfectly exposed picture will result at any selected aperture setting, the program range is limited by not "stopping down" the lens to the minimum aperture. For example, using the 50 mm V1.4 lens with its f/stop set at 5.6, the camera is controlled by the program characteristics of f/1.4 through f/5.6. Beyond f/5.6, the camera is controlled by the aperture priority automatic exposure mode (See Table B)."

 

 

Very interesting, somehow they pull it off.

[greg_erker]

Scott,

 

I wonder if they did a meter reading after stopping down the

lens. Then they could tweak the exposure time based on what

the aperture acutally ended up being.

 

It would mean that the shutter lag would be longer in Program

mode.

 

What vintage are those Chinon camaras? I might be able to find

a review in my old magazines.

 

Greg

[chris_eve]

This Chinon system sounds something like the method Fujica used on the AX-5 to allow the use of "ordinary" 42-screw auto-aperture lenses in shutter-priority and program modes (with the XD adaptor). There is a VERY slight delay between pressing the button and the shutter firing, only really noticeable if the aperture is noticeably sticky when operated manually. Basically the camera seems to say : "OK, you want this shutter speed, I'll close the aperture down 'till the light is right then release the shutter".

Simple yet effective !!

[Jochen_S]

As far as I remember the should have been made in the mid of the 80s. As far as I know they were build because Chinon was unwilling or unable to pay for a KA-mount license. According to the metering error mentioned above they just dared to shit on precision. Pentax didn#t so they created A-lenses.

[greg_erker]

I'll take a look at my Pop and Modern Photo mags from that

era (when I get some time). If there's a review of the Chinon

body they might explain what (if anything) they did to get around

the non-linear aperture issue.

 

Greg