Macro help

[Chris P]

Hi all

I was asking in a previous thread about reversing rings and macro photography and a forum member very kindly donated an old M42 mount 50mm f/2.8 lens to my cause.  Now I've finally got the time to play around with it and I've noticed a light spot appearing in shots taken at larger apertures.  I've posted a couple of test images below.  Any ideas as to what this might be?

I've checked and it's nothing on the sensor (it doesn't happen with any other lens).  I wondered if it might be diffraction of incident angle light but I've also noticed a small scratch on the (normally) rear element (which is, of course, poking out to the front.

[Oly Paul]

I wondered if it might be diffraction of incident angle light but I've also noticed a small scratch on the (normally) rear element (which is, of course, poking out to the front.

Proberley a combination of both Chris as the rear element is usally coated as well.

[Chris P]

I was playing around with this at the weekend and I think I might have worked out what this is.

I know it's not the small scratch on the rear element because it's off centre and if I rotate the lens the bright spot doesn't move.

But... the 50mm f/2.8 has shiny metal aperture blades and the bright spot gets smaller but better defined as the aperture gets smaller (you can't really see it on what I've posted above sadly).  However, I tried reversing my Nikon 35mm f/1.8 which has black coated aperture blades and this doesn't happen.

Because it's a G series Nikon lens I can only shoot at f/1.8 or f/22 (as I have to hold the aperture lever open with a finger) but I get no "bright spot" at all regardless of aperture.