[chse]

I have several Pentax PKA adaptall2 mounts, but none of them works perfectly. Most show about 0.5 stops miss-exposure when stopped down, compared to wide open. With all of them it was even worse when I got them (2 did not stop down more than f8, too) I could adjust them but never got it completely right. After taking them apart I realized that design is flawed and prone to faults.

So my idea was to get the much cheaper Ricoh RI mount and adapt it by setting the contacts on the camera mount so it would be recognized as the right A lens. I did this with some aluminum foil (for the A contact) and tape as insulators (see here for details Features and Operation of the Ka Mount ).
My idea was that the RI mount, which is made for the Ricoh 'A' bayonet should have the same linear aperture lever movement as the Pentax PKA mount. But apparently it has not: After the modifications the camera recognized it as A lens, the widest and smallest aperture was right, but when setting the aperture on the camera, I got increasingly overexposed picture with increasingly closed apertures. At the f32 setting the aperture only stopped down to about f8.
For me this is hard to understand since there are several Pentax A mount lenses which can also be used on the Ricoh 'A' mount.
Does anyone know how the Ricoh A mount works? And why the movement of the lever is different?

[Lowell Goudge]

I have several Pentax PKA adaptall2 mounts, but none of them works perfectly. Most show about 0.5 stops miss-exposure when stopped down, compared to wide open. With all of them it was even worse when I got them (2 did not stop down more than f8, too) I could adjust them but never got it completely right. After taking them apart I realized that design is flawed and prone to faults.

So my idea was to get the much cheaper Ricoh RI mount and adapt it by setting the contacts on the camera mount so it would be recognized as the right A lens. I did this with some aluminum foil (for the A contact) and tape as insulators (see here for details Features and Operation of the Ka Mount ).
My idea was that the RI mount, which is made for the Ricoh 'A' bayonet should have the same linear aperture lever movement as the Pentax PKA mount. But apparently it has not: After the modifications the camera recognized it as A lens, the widest and smallest aperture was right, but when setting the aperture on the camera, I got increasingly overexposed picture with increasingly closed apertures. At the f32 setting the aperture only stopped down to about f8.
For me this is hard to understand since there are several Pentax A mount lenses which can also be used on the Ricoh 'A' mount.
Does anyone know how the Ricoh A mount works? And why the movement of the lever is different?

If you research back starting about a year ago, you will find a thread that I started, looking at modifying a K mount lens to a KA mount. I did this because I was (am) frustrated wityh the inability to use P-TTL Flash or automatic modes on my collection of K mount primes.

To make a long story short, although you can make the camera electrically control the apature, the movement of the apature of K mount lenses is not the same as KA mount. K mount lenses have a linear relationship between the movement of the apature lever and the diameter of the apature, KA mount lenses have a linear relationship between the movement of the apature lever and the area of the apature. If you recall the high school geometry, area of a circle is a square function of the diameter. As a result your intermediate apature exposures can give you several stops of over exposure

As to why the tamron mounts are inconsistent, I don't know. I only have one adaptall lens (a 24mm f2.5) and have both K and KA mounts but have abandoned the KA mount. I use it strictly in manual mode.

as a note, I have been after pentax for a long time now to modify the functions relating to manual apature lenses. If they allowed you to input minimum and maximum apature when the camera detects a manual lens, they could modify the apature control function (for linear diameter with movement) and let youo control the lens by letting it sit on mionimum apature.

This would not only give you automatic apature, but also help in correcting the very poor performance of the K10 metering with manual lenses, and permit P-TTL flash

[chse]

If you research back starting about a year ago, you will find a thread that I started, looking at modifying a K mount lens to a KA mount. I did this because I was (am) frustrated wityh the inability to use P-TTL Flash or automatic modes on my collection of K mount primes.

That is an interesting idea. So does it work on DSLRs, I remember a website which did this to allow matrix metering. Does it change anything if you do not have the contact for the A position.

as a note, I have been after pentax for a long time now to modify the functions relating to manual apature lenses. If they allowed you to input minimum and maximum apature when the camera detects a manual lens, they could modify the apature control function (for linear diameter with movement) and let youo control the lens by letting it sit on mionimum apature.
This would not only give you automatic apature, but also help in correcting the very poor performance of the K10 metering with manual lenses, and permit P-TTL flash

Unfortunately, I believe, this will never happen, even the real (uncrippled) KA mount will never come back since the new lenses just do not need it.

[Lowell Goudge]

That is an interesting idea. So does it work on DSLRs, I remember a website which did this to allow matrix metering. Does it change anything if you do not have the contact for the A position.

The modification worked, with the contact in the A position, without it you can;t do a thing, and you need a spring loaded contact in the A position because this contact on the camera body is recessed. That's how they sense a K ount lens, the contact cant reach the lens mount to connect it to ground.

In terms of working, the mod worked as intended, just not as wanted.

the linear change in diameter with movement of the apature control caused a 2 stop over exposure in the middle ranges, (due to the difference between area and diameter). As I did not wish to get into the redesign of the apature coupling on the lenses to change the opening to linear area, I abandoned the project.