Out damn spot! HELP!

[Sambo]

Hello all... recently purchased a 28 - 75 mm zoom off of ebay. It's a genuine Minlota lens, (one of the old ones) not an off brand. The condition is great, no scratches, or moisture. My problem with it is a distinct but hazy, large blue spot in the middle of the image when I take a photo with it. It seems to be the most apparent at slow shutter speeds and small apertures. I'm sure its not the camera, as I have taken photos under the same conditions with my other lenses (50mm fixed, 70-210 and 75-300zooms) with no blue spot. I am at loss as to why this is happening! If anyone out there knows what this is, or how to fix it, I would be eternally gratefull! You can see an example of the "ghost spot" at
http://www.bytephoto.com/photopost/d...04bluespot.jpg

Thanks.

Sam

[famjad21]

Hi Sam,
I'm not sure what the exact problem is. If it only occurs with that lens then it may need an overhaul where the lens elements are cleaned of debris, fungus, etc.

Hope that helps,
Frank

[bytethis]

G'day Sam.

Frank's suggestions are certainly worth checking out.

Could also be a phenomenon called 'ghosting'.

This is a scenario (which I imagine is most prevalent with small apertures - slow shutter speeds and small apertures = lots of light coming in) whereby light allowed though the lens as normal, can 'bounce' off the sensor to the back element of the lens and then back onto the sensor again. Just out of interest, does your lens elements have a bluish hue?

Research I've done indicates that this is most often caused by utilising lenses built for SLR film cams, where the reflective properties of the film is not as dramatic as with sensors (or their protective glass coating. So basically, you never see this issue with film cams.

(good quality) Lenses made specifically for digital cams nowdays have multi coatings on the rear element to prevent this occurence.

Having said all that, I'm not sure if there's a solution (if this is the cause of the prob you're experiencing) Hopefully it's fixable!

[Sambo]

Thanks Graham and Frank...

I don't think the lens is damaged, since at shorter focal lengths and faster shutter speeds, the lens works quite well. (I took the Eye of the Dog pic and several others in my gallery with it, and plan to post a landscape this week). I think you've hit on it Graham. I also did some research and have found that the light bouncing off the sensor does describe my problem. My lens does have a slight blue tint to the optics. It is an older one designed for film cameras. One of the reasons I like it so well is that it is so robust, not plasticy (is that a word?) like so many of the newer ones. I guess I'll have to live with it untill my budget increases!

Thanks so much for your responses and advice.

Sam