Help ! Purple spot/disc in image (Sigma 18mm 3.5)

[gilroy]

I have a Sigma 18mm 3.5 that seems to create a purplish spot at the center of the image especially shooting against the sky or something. Is this normal for this lens? Will a polarizer help?
Thanks for any info.

[rogprov]

Its a problem with the rearmost lens element not having a coating designed for use with digital cameras. Light is bouncing between the sensor's anti-aliasing filter and the rear element. I don't think you can do much about it. This a problem common to many lenses which were made originally for film cameras where the problem does not exist. A different (anti-reflective) coating is used on lenses designed for digital cameras.

[gilroy]

Thanks for the info Roger! I guess I can only use it without issue on a film body then

[rogprov]

There may some change with different apertures or when strong direct light is not entering the lens. I had a lens which sometimes but not always exhibited the problem - but it's while ago and I can't remember the details. Of course you may be able eliminate the spot in Photoshop

[David Kilpatrick]

I had one of these lenses when I got the KM 7D. It was fine - sharp and generally good colour and contrast - on any 'normal' scene but produced a distinct sharply focused flare spot whenever a light source appeared in the pic or just out of the frame. It had to go, because too many pictures were spoiled by the flare patch.

David

[gilroy]

Thanks for the response David. I might also try it for just night time slow exp shots.

[David Kilpatrick]

That's why I sold mine - I tried some night time shote and every single light in the image was mirrored with a reflection...

David