Posted By joe rusz on 12/28/2008 12:00 AM
Dave, I went back into my file and found a shot taken handheld at about f 8 and about 4 tenths. It was short on depth of fiedl, because the coach is so long, and thus, not too sharp and showing signs of motion. However, I was able to fix most of that in Elements (thanks Adobe) so the edited image don't look too bad. Interestingly, there is little or no sign of any spots, which makes me wonder if it's dirt on the lens, which became noticeable after I stopped the lens down to f 22 and shot using a tripod. Hmm, since it was a slow speed, do you think the spots are noise? Nah. I vote, dirty lens. How about you? Here's the latest image--I hope.
There are two areas where dirt is problematic on a DSLR - the sensor, and the REAR lens element of the lens. A dirty rear element is more likely to cause spotting than a front element. Generally front element imperfections aren't seen by the "film" as discrete problems (but may cause a haziness), unless they are large and completely opaque. Having said that, you did stop down the lens a lot which *might* make front lens dirt begin to resolve, especially if the lens is designed with some amount of macro capability, or, that was enough to bring rear dirt into the depth of field of the sensor plane (not the same as DOF out at your subject, but related). Clean your lens back and front and shoot some tests on a even surface at f22 or even smaller if you have it. Sensor cleaning is VERY tricky - follow the manufacturer's direction explicitly. Not all sensor cleaning methods can be used on all sensors. They are VERY fragile.
The gray/white background issue is a contrast/brightness/gamma adjustment and if the camera was on auto exposure, the position of the metering sensor on the subject likely accounts for the difference. If the sensor is pointed at the white background it will try to set the exposure to make the white sheet 18% gray (about the gray that's right at the car under the windows in the first picture. The rest of the sheet is slightly lighter because the car itself is dark - averaging, remember). That's what the photo sensor does (which it did). The sensor tries to make the overall brightness of the scene represent an 18% gray luminance. For accurate metering, make sure the center of your viewfinder is sitting on a subject area of medium brightness (grass green, grey tree bark, etc). If the meter is on the extremes - black and white, it'll try to make them gray. The Ford principle - any colour you want as long as its grey

. If your shutter has a "press and hold" feature for locking exposure, then point the viewfinder sensor at a "gray" area of the subject, press and hold, then re-align the composition to what you want and then continue to press down to take the picture. Most SLRs have some variations of this procedure.