[vietlh81]

I believe that every Pentaxian like to have KA lenses than K,M lenses because of some benefits:
- You can set the aperture on the camera with KA lenses instead of setting the aperture on the K,M lenses. That allows less manual operations when shooting.
- Multi-segment/matrix metering
- Using with P-TTL flash
- The aperture can be displayed in the EXIF of your photo
...

Mark Robert has presented how to enable matrix metering mode for the K-M lens, but this method can work only with the non-digital camera. He believes that the digital data pin transmits the aperture information, therefore it does not work with Pentax DSLR.

I will present how to convert my Pentax-M 50mm f1.7 to KA lens. There consists of two steps:
- Modifying my 50 1.7 lens to KA lens.
- Modifying my K100D. Both can undo if you don't like this

To modify my lens, I follow the principle of the Robert's method (For more details, please read the article of Mark Robert) The max and min aperture of the lens are : f/1.7 and f/22. Therefore the arrangement of insulator on my lens is 101*11, where 0 represents the position of the insulator. In Robert's method, he drilled a small hole in order to make an insulator.
Instead of drilling, I use a corrector to paint on the "0" position which have a distance from itself to locking notch about 2.2 cm (0.87 inch)

But my camera still does not recognize the "new KA" lens. Because the "A contact" of the lens does not touch the "A contact" on the camera. So I need to modify my K100D:

- Unscrew 5 screws on the camera mount
- Put the aluminum foil under the ring mount, cover the "A contact" in order to make it conducting.
- Re-screw the camera.

Voilà, my K100D can recognize the "new KA" 50mm 1.7. From now, I can activate the matrix metering mode, set the aperture on the camera, have the aperture information in the EXIF....

Some test results will be posted.

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Last edited by vietlh81; 06-20-2009 at 11:51 PM.

[jslifoaw]

While it is definitely possible to have it recognized as an A lens, it will probably not function especially well for one reason:

The stop down lever on an A lenses corresponds in a linear fashion to f-stop (in order to make body control easier) vs diameter in M lenses. So for a limited range, the approximate aperture will be similar, but as you get further from wide open, the exposure will probably be much further off.

Since M lenses shoot wide open without any issues, this makes the mod pretty useless unless you absolutely need matrix metering or PTTL flash. It does have uses if you are willing to take the trouble, but unless A lenses are just too hard to find, it might not be worthwhile unless you enjoy hacking

[newarts]

I've done this and find it is useful with Av mode. Disable the spring that keeps the lens' aperture open if necessary*. Set the viewfinder f stop to the lowest possible with the ewheel - now the camera will not try to stop down the lens when you shoot. Instead, you manually stop down the lens when you compose the photo.. the camera will meter off the light that'll be available at shooting time.

There is some exposure error with center weighted metering as I manually stop the lens down which I don't understand; I've blamed it on my split-prism focusing screen.

Iowa Dave

* or you can also partially mount the lens (don't turn it all the way when mounting it.) That'll prevent the camera's lever from interacting with the lens' aperture; drill an indent to positively lock the lens in place.

[stevebrot]

What they (above) said...

Newarts' solution will work, but it is somewhat tedious. Sorry

Steve

[Lowell Goudge]

I did this mod 2 years ago, and posted the mod and results here on the forum.

I had one change, I inserted a spring loaded pin, similar to what is on a camera mount (actually a PCB button contact for a socket array).

This gave A mount identity without camera body mod.

does the camera recognize it as an A lens, yes, and if you paint the correct areas to code the aperture value the camera even sees this,

BUT the aperture operating lever does not have the same motion as an A lens.

A lenses have the area of the aperture change linearly with movement, K lenses it is with the diameter, .i.e a square law.

on the 50mm F2 i modified it under exposed wide open by 1 stop and over exposed by 2 stopped down to F22. It is utterly useless.

[vietlh81]

One of the reasons that I did the modification is that I did some conversions with Konica, Minolta lenses. I hope to have a "K-A" lenses with these modified lenses. Stopping down metering is not the appropriate solution when I shot in low light condition (difficult to focus).

BUT the aperture operating lever does not have the same motion as an A lens.

A lenses have the area of the aperture change linearly with movement, K lenses it is with the diameter, .i.e a square law.

on the 50mm F2 i modified it under exposed wide open by 1 stop and over exposed by 2 stopped down to F22. It is utterly useless.

I agree with Lowell, but it will be more interesting if you change the maximal-minimal aperture by modifying the code.(very easy to do with the corrector)

Firstly, I post some results with the code 101*11 corresponding to (f/1.7-f/22)

@1.7

@2.2

@4

@5.6


The photos have correct exposure with f/2.2 and f/4 but underexpo with wide aperture f/1.7, overexpos with f/5.6

@8:

@11

@16

@22


At f/8, f/11, it is still overexposure. At f22, it goes again to underexposure.

I am not quite clear why at wide open, the photo is underexposure.
But I think if we change the maximal aperture to be faster (e.g f/1.4), the results will be better

[vietlh81]

I use the corrector to change easily the code to 111*10, corresponding to the lens f/1.4-f/22.

@ f/1.8 (I cannot set f/1.7 on the camera)


@f/2.2


@f/4


@f/5.6


@f/8


@f/11


The new result are better. The aperture f/16 and f/22 are not used frequently, so I don't test them here.

[jslifoaw]

The coding will not matter; all it does is assign numerical values to your aperture.

So if you code wide open to be F2, and set to F4, it should expose exactly as if you coded wide open at F2.8 and set to F5.6; in both cases you're telling the camera to stop down two stops.

Yours results are pretty good so I wouldn't worry too much though

[vietlh81]

So if you code wide open to be F2, and set to F4, it should expose exactly as if you coded wide open at F2.8 and set to F5.6; in both cases you're telling the camera to stop down two stops.

Yours results are pretty good so I wouldn't worry too much though

That's only true when you're shooting with A lens, whereas K-M lens does not stop down linearly

As you suggested above, an other benefit of A-lens is to get the proper exposure with PTTL flash. With M lens, the PTTL flash always fires full power. I post some other images taken with built-in PTTL flash (sorry, I have not enough money to buy a true PTTL swivel flash )

@1.8


@2.2


@4


@5.6


@8


From f/11, the background will be darker.
@11


@16


@22


I used self-timer shooting (2s) and found that there are 2 pre-flashings. 1 when mirror locks-up, an other when the shutter releases. I am not clear why there are the first one.

[rsBayArea]

Just tried a modified version of this with a vivitar 35-70mm 3.5-4.8. Reason for choosing this lens: somehow I ended up with 2 copies and it's a very fun walkaround lens on my k2000. Instead of modifying the camera, I drilled out for the A pin and put a spring-loaded pin removed from a ricoh lens in the A position. Shorted it to the flange with a bit of aluminum foil (I may replace this with wire but it's working well as is) and connected it to the body. At first, no luck, but then I jiggled the lens a bit and the pin dropped into place, giving me body control of the aperture with the lens set to f22. With the camera in Av mode, I ran a few sequences of flowers in the backyard; histogram shows a nice, level exposure at f3.5 through f16 and the aperture is correctly recorded in the exif. I still need to apply isolators to tell the camera it's a 3.5-22 (it currently thinks it's a 2.4-16).

Seems that my biggest problem is with pin placement; I think it's only about 10 thousandths off, but that's enough to require the jiggling to get the pin to engage. Once it falls in place it works great. I will probably do a little drill-bit-cutting on the hole to oversize it a bit more and see if that helps it fall in place without jiggling, but this mod would probably be best performed with a 1:1 printed template to guarantee that the pin centers are in the right place.

Behavior of the lens with the body flash: not too hot. I suspect that the overexposures may have something to do with the fact that the camera body has absolutely no clue how far the lens is focused so it's just shooting 1/180 at full flash no matter what aperture I set it to. It's not a huge deal; my primary reason for trying this modification was to get the aperture into the exif data automatically (works perfectly) and I'll probably continue to use the lens in M mode with a flash, maybe Av in daylight, so all in all I'd call it a success. The proper exposures in Av at all stops (3.5 through 16) was a nice surprise.

If anyone else comes across one of these vivitars, it'll likely have oily blades. It's an easy fix: 4 screws on the silver flange, rear optics thread out, wipe the blades with zippo fluid on a q-tip, blow it dry, and replace the optics/flange. Only part to be careful of is that the aperture ring has a tiny ball bearing to snap into stops; when the flange is removed, the aperture ring comes right off and the ball will likely be lost. Holding down the ring while doing other work will keep it in place. I've had to clean the blades about 3 or 4 times but the time between cleanings increases, so I suspect the oily residue is almost all gone.

[Lowell Goudge]

In a nutshell here is what I see are the issues with the mod based upon my experiences

You can code the lens and get auto and p-ttl functions. But....

Exposure linearity is an issue due to the aperature linkage

You can play with the aperture coding to try and sort this out but then you have the issue that the camera gets the wide open aperture wrong and this screws up P-TTL flash

As I concluded all along it is just not worth it

[PiDicus Rex]

Has anyone used this sort of modification when shooting video - I'm curious as to if it would help to prevent the auto-gain ramping of a K-01 when used with KM lenses.

[klaus123]

Has anyone used this sort of modification when shooting video - I'm curious as to if it would help to prevent the auto-gain ramping of a K-01 when used with KM lenses.

I just did a A-Modification for a K24/2,8 - but especially the K01 behaves completly different than the DSLRs. I have not tested Video-Mode so far....

[stevebrot]

Has anyone used this sort of modification when shooting video - I'm curious as to if it would help to prevent the auto-gain ramping of a K-01 when used with KM lenses.

This is known to simply not work with the "A" contacts on any Pentax body that has them. The issue is the linearity of the aperture mechanism (lever travel vs aperture opening). No amount of foil, paint, or Dremel work will change the fact that the body will not be able to accurately set the aperture to the set value. The K01 has additional issues (see comment above).


Steve

[PiDicus Rex]

Steve, that's really not an answer to my question, it's a comment on a limitation that the lens will have if modified and used.

I'm less concerned with the linearity of the Iris operation, on a lens that is pretty much only ever going to be used in wide-open, or nearly wide open, f-stops when shooting video, then I am with the camera thinking it's a KA lens and switching off the Auto ISO that occurs in all modes when a K series lens is used.


Klaus, I'd love to hear if the ISO remains fixed according to user setting, if you have access to a K-01.