[ivanvernon]

I am using Chinese Pentax 645-to-K-mount adapter to fit Pentax 645 lenses on to my Pentax K-1 and K-5ii cameras. There is no glass in the adapter. I am unimpressed with image quality in terms of sharpness, though otherwise acceptable. Based on my experience to date, there is not much point to the adapter exercise.

However, I have noticed one curious behavior that I am unable to explain. A light blur/vignette appears near the center of many of the images. I have noticed this using both the K-1 and K-5ii cameras, and both the Pentax 645-A 55mm f 2.8 and the Pentax 645-A 80-160mm. I am attaching a few photos where this effect appears. I will appreciate any explanations or theories on why this effect occurs.

Thank you.

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[pepperberry farm]

just a WAG, but could the adapter be allowing light into the mirror box?

[35mmfilmfan]

Or could it be internal reflections within the body of the adapter ? Or, indeed, the adapter not fitting snugly to either the lens or the body ?

[photoptimist]

The image circle of a 645 lens is quite large compared to the sensors on the K-1 and K-5 and all that collected light has to go somewhere. If the inside of the adapter is shiny (even if it is black), you'll get this kind of center fogging (what 35mmfilmfan mentions).

The best solution is an aggressive lens hood to cut down light entering the lens.

[grahame]

A light blur/vignette appears near the center of many of the images

I think it is the reflection from the sensor bounced back to sensor by the rear element, since the rear element is not coated to reduce reflection from that direction.
reflection from the inner side of the adapter might cause more defused haze, not that center-weighted.

[RAART]

It is the internal reflections bouncing back same as on some older regular lenses, just google blue dot lens reflections... however that does not have nothing to do with 645 lenses. I also suggest that you invest in really good quality 645 to K adaptor.

I use 645 Mamiya 210mm F4 lens on K-3 and I love it... remember that K-3 has higher pixel per inch ratio as K-1 and it will be very critical in terms of sharpness.

Here is example of the lens resolution.

[ivanvernon]

Thanks for all the helpful comments. I will experiment accordingly and report progress or lack thereof.

[mattb123]

If the inner surfaces of the adapter are shiny, you can black them out with a flat black enamel pen for very little money. I did this with a cheap shiny 67/645 adapter and it helped.

[ivanvernon]

Thanks for all the helpful comments. I used black construction paper to fashion a lengthy hood, same diameter as lens exterior measurements, and shortened it to about 6 inches, just less than the length that produced vignetting. I taped the hood onto the lens and made numerous test shots. The photos produced were mosly too poor to display because of the bright midday sun. However, I was unable to reproduce the center area vignetting, although a better test will be made in less bright light later today.

I am attaching a few other shots to illustrate the lens's characteristics as used on the Pentax K-3 camera. These seem to me to be of reasonable sharpness and quality. However, I will greatly appreciate any comments anyone may be able to offer on these shots--not from an artistic standpoint, just from the standpoint of lens performance,.

Many thanks to all who take the time to respond.