Railfan Technology > Telephoto Question

I still get out my SLR and shoot some slide film now and then. I've noticed that when I use my telephoto the area around on the outside of the image seems darker compared to the center and compared to slides shot without the telephoto. If you hold the slide up to a bright light and magnify it that effect seems to go away some, and the slide looks properly exposed. I just can't figure out what causes this dark area around the edge of the image. Any thoughts? Thanks for your help.  

WichitaJct Wrote:
-------------------------------------------------------
> I still get out my SLR and shoot some slide film
> now and then. I've noticed that when I use my
> telephoto the area around on the outside of the
> image seems darker compared to the center and
> compared to slides shot without the telephoto. If
> you hold the slide up to a bright light and
> magnify it that effect seems to go away some, and
> the slide looks properly exposed. I just can't
> figure out what causes this dark area around the
> edge of the image. Any thoughts? Thanks for your
> help.

What you are describing sounds like "vignetting," where the outer perimeter of the lens is darker than the center.  Certain lenses are more prone to "vignetting" than others.  Adding filters can worsen the amount of vignetting you see.  Nearly all lenses will show it to some extent, but a lot of it can be cleaned up in digital images.  In fact, most digital image processing software will have a profile for a given (newer) lens, that will automatically correct distortion and vignetting before you even see it.

As the previous poster said, "vignetting."

Is the lens designed for your camera? If you're using
a DX (digital less than full frame lens) on a FX (full
frame) camera, you'll get the effect big time. If the
lens is a cheapie delux, you might see vignetting.

Some times just stopping down an f stop or two will manage
the problem.

HTH.

John.

Since you're shooting with a film camera, you can pretty much discount the digital problems.  The vignetting could be from any filters you might have put on your lens.  If you're using a lens shade, that could cause the problem, also, though that occurs more often with wide angle lens.

Vignetting has one fundamental physical optics cause that can't be eliminated: the photon flux through a flat aperture drops off as you move away from on-axis rays. This is why it is more apparent in very-wide angle lens photos.

It can be compensated for in image processing, however.

Ray

Two things come to mind right off the bat. Many newer digital cameras correct for vignetting, something your film camera can't do, of course.  Also, as mentioned earlier, if your tele lens is designed as a DX or APS-C crop lens, it will show vignetting on a film camera due to the bigger area of coverage needed.  The image circle of lenses designed for APS-C crop sensors (Canon Rebel and Nikon D-3xxx, 5xxx, and 7xxx series, for example) will usually not cover the whole image area of the film frame of 35MM film.  If you are using a full-frame digital with an older lens, it may be correcting this automatically in the JPEGs.  

Reminds me of my Argus C-3.  I bought a telephoto lens for it ( in 1962) that REALLY left a vignette on the slide.  You can still induce some vignetting in modern cameras, but it is hard to do.

Ed