[GUB]

This is the first time I have consistently got an artifact in my image from what I presume is sensor reflection of the rear element.
The lens is an old Soligor Miranda Bellows 135mm 3.5. The rear element appears slightly concave.
The artifact only appears when the lens is stopped down. (here f8)
The effect was visible in LV and constant.
I can't think of anything else it could be - any ideas?
One shot f8 and the other wide open (captioned).
On the K-1

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[disasterfilm]

Have you looked down into the lens with a penlight? Localized contrast loss can be a symptom of haze or fungus on one of the rear elements in front of the aperture blades. It'd also become more prevalent on images the more you stop the lens down.

[GUB]

Have you looked down into the lens with a penlight? Localized contrast loss can be a symptom of haze or fungus on one of the rear elements in front of the aperture blades. It'd also become more prevalent on images the more you stop the lens down.

Cleaning a mark on the internals made no difference. And wouldn't its effect be too blurred to see ie loss of contrast.?
Anyway I found the problem. I was using a m42 pentax bellows unit 11 in conjunction with a non infinity (flanged) adapter. There is a slot in the adapter at the 4 oclock point (looking at the camera.). It was letting a sliver of light in aggravated by both the bellows surface and camera surface being chrome. Even worse with a Mark 1 Pentax bellows.
It is important to note here never to use an infinity adapter with its spring in place in this situation because to remove the bellows it must be rotated off the adapter and the camera body fouls it.
This just leaves the question why I had never noticed the problem before and I think it must be because I usually use a flash with the bellows.

[Lowell Goudge]

Look for any shiny parts , lens securing rings, part of the mount etc.

[yucatanPentax]

Glad you found the problem!
Yes, adapters can be an issue, especially with older equipment and lenses that do not cover a larger circle than the mount itself.

I wish there were some type of foil (meaning very thin) plate or cover to go between older lenses and the M42 adapters to better close off light leaks. Anything with much thickness at all would case a loss of infinity focus, but for macro work, even a thicker cover wouldn't matter too much.

[GUB]

Does the slot actually have a purpose in a modern DSLR ?

[kypfer]

I've "adapted" my generic M42 bellows to PK simply by affixing a flanged (non-infinity) adaptor permanently to the rear of the bellows.
It took a "bit of a jiggle" to get things to sit upright initially, now they just clip on and off like any bayonet lens and don't foul the flash/prism housing … using the flanged adaptor means the bellows lock onto the camera, so reducing the risk of accidents. I've not noticed any light leaks, but now it's been mentioned I'll fill the little slot with something suitable, just in case
For light leaks when using M42 lenses with a small rear diameter on a conventional M42-PK adaptor, a black elastic hair "thingy" seals any residual gaps quite nicely … I keep one both with my Helios-44-2 and my Lydith

[UncleVanya]

I use an original Pentax adapter, but I add an m42 extension tube to the body before mounting the bellows. Also my bellows has the removable lens and body mounts, so it’s only awkward not impossible to remove it even if I don’t use the extension tube.

[GUB]

I've "adapted" my generic M42 bellows to PK simply by affixing a flanged (non-infinity) adaptor permanently to the rear of the bellows.

Same here because then there is no risk from using an infinity one in a blonde moment and the flange has the lock pin slot to firmly locate the setup.

It never occurred to me that it could leak light because I use the flange type to avoid leaks with the Helios 44 series.

I use an original Pentax adapter, but I add an m42 extension tube to the body before mounting the bellows.

It appears that the infinity type has the same potential to leak light at this point with my Pentax Mk11 bellows. It has an unfortunate bevel at the edge of it's mount surface. And a chrome surface that reflects.

[yucatanPentax]

For light leaks when using M42 lenses with a small rear diameter on a conventional M42-PK adaptor, a black elastic hair "thingy" seals any residual gaps quite nicely … I keep one both with my Helios-44-2 and my Lydith

I'll have to figure out what kind of "hair thingy" works ... I guess it's a trip to Target for me.

[GUB]

I'll have to figure out what kind of "hair thingy" works ... I guess it's a trip to Target for me.

I have gone far more upmarket than that A large rubber o ring repurposed from a farm trough valve.!

[UncleVanya]

I'll have to figure out what kind of "hair thingy" works ... I guess it's a trip to Target for me.

Look for a thick hairband.