focuss??

i have a reoccuring, though intermittent problem when taking wide-field shots by piggypacking my film SLR (Canon Rebel 2000) with a 28-80mm lens.

often, but not always, my photos show stars not as pin-points but as coloured cirles several millimetres across, often with blurbed edges.

i assumed that this was because i had not focussed to infinity but vigilent checking prior to taking a shot did not solve the problem.

next, i considered that perhaps the focusser was slowly rotating downwards (and away from the infinity position) with gravity and
mount motion during the duration of the shot. so i started using an elastic hair-tie to hold the focusser in the infinity position once the shot was composed.

this 'appears' to hold the lens in position, however, i am still producing photos with blurbed, round circles of light rather than
pinpoints. the cirles are indeed circular so i believe that tracking is accurate enough and not the source of the problem. especially
since other shots taken during the same session will be ok.

i know that a picture says a thousands words but i not currently in a position to access a slide scanner.

i am hoping that the experience of the group may recognise my issue and give me any pointers, other possible points of failure, or a list of tests to perform to solve this problem.

thanks in advance,
ian

The only thing that comes to mind is if you are not using some sort of dew shield then perhaps the lens dewed over. Hard to tell without a pic.



Robert W.

thanks for the tips. testing infinity in daylight is a good idea, i''ll try this.

i think i need to do a series of test shots at different focal length and apertures and see if i cant find a point that works.

thanks,
ian

Ian,
If your film is buckling, you may notice that stars are more focused on one side of the image, but are blobs on the other side. I have had this problem with my 35mm on some occassions. The real solution is to keep moisture out of the film chamber, but this seems like a whole lot of hassle to me (Nitrogen purging, etc.).
Best of luck!
Tim Cross