K mount lens on SD15

[guzzibreva1]

I have a really nice F1.7 Pentax lens that works on my SD15 with one small problem.

The lens will focus at infinity just fine but it will travel beyond infinity and hit the hot mirror. I have to be very careful to not turn the lens farther in.
Is this normal for a K mount lens?

I really like the lens because it doesn't create magenta hot spot when stopped down. My other M42 lens (Helios M44-4) works well as long as it it not stopped down. It looks like the IR that is being reflected off the hot mirror shines on the aperture blades and creates the red area in the center of the image. I am wondering what is different about the Pentax lens.

 

[pntbll248]

I don't imagine it's the SMC Pentax-M 50 f/1.7? If so, I use the same lens and have the same results. You may notice that you also lose just a bit of close focus. I do, at least.

If I remember correctly, the Pentax K mount has a slightly longer registration distance, but is otherwise mechanically quite similar to the Sigma SA mount. The result is an offset focus profile -- goes past infinity, loses some close focus. My personal experience with the SMC Pentax-M 50 f/1.7 (no idea what you have, just a guess) is that it will hit the IR filter, but even with considerable force it won't break the IR filter. Sits evenly along the plastic rim of the IR filter, never touches the delicate glass. Of course, you should never ever ever test this, any contact with the IR filter can be trouble, I found out by accident

My experience with manual lenses has been spotty. Some work great, some give strange color such as green vignetting or magenta hotspotting. Can't really say what causes the magenta, but your explanation sounds plausible. I'm running through a collection of about 60 M42/Pentax K/Nikon F manual focus lenses over the next few weeks on the SD14 to see which work and which don't -- it'd be great to have a lens list of which manual focus lenses work, which ones have weird quirks, and which ones just aren't usable.

 

[DaSonyGuy]

I have a really nice F1.7 Pentax lens that works on my SD15 with one small problem.

The lens will focus at infinity just fine but it will travel beyond infinity and hit the hot mirror. I have to be very careful to not turn the lens farther in.
Is this normal for a K mount lens?

The registration distance of PK is about 1.5mm more than SA so you can fine tune it to give infinity at the infinity stop on the lens by adding one or more shims between the PK plate and the lens. Even one or more rings of thick paper/card would work as shims.

I really like the lens because it doesn't create magenta hot spot when stopped down.

My other M42 lens (Helios M44-4) works well as long as it it not stopped down. It looks like the IR that is being reflected off the hot mirror shines on the aperture blades and creates the red area in the center of the image. I am wondering what is different about the Pentax lens.

I only have one lens that has the magenta centre cast issue...The Pentacon 50mm f1.8...I put the issue down to the Pentacon having very shiny chrome plated iris blades, so causing internal reflections within the lens, the magenta colour coming from the magenta coatings on the elements.

The Meyer Optic Oreston 50mm f1.8, which is supposed to be optically identical to the Pentacon, has non-reflective matt black iris blades and it does'nt have the problem which confirms my hypothesis.

The vast majority of lenses, your Pentax included, have non reflective matt black iris blades, and hence why they dont have the problem either.