|
Leon,
Vignetting is a function of the optics of lens design.
There is a law called the Cos4 law that describes this effect, which basically says the further from the centre of axis - the greater the light fall off.
Wide angle lenses are worse than telephoto lenses.
CCDs (which are 3 dimensional wells) can exaggerate this effect compared to film (which is nearly perfectly flat and almost 2 dimensional) as incident light on the sensor furthest from the centre strikes the CCD at progressively larger angles and this light can bounce around (off the microlenses and the walls of the well) and not all of it makes its way to the actual sensor at the bottom or if it does it results in various types of aberrations.
This is a function of individual lens design and the CCD.
Most telephoto lenses are however less prone to this problem than wide angles, but it exists as a problem in all but the very best Telecentric designed lenses - as a far I know these are primarily being made only by Olympus for their E series digital cameras, but no doubt starting to be emulated by others.
As far as I know Vignetting cannot be eliminated (even at a design stage - except by dramatically oversizing the lens compared to the image size but it can be improved by stopping down, the smaller the aperture the less the effects of vignetting.
The compromise is longer exposure times - double for each f stop.
Be careful not to stop down too far (if the lens even allows it) so that you greatly exceed the diffraction limit of the particular lens and get image quality degradation.
Stopping down 2-3 stops in almost all cases will also assist in image quality and sharpness. The exception here is with some of the very top quality lenses where they remain excellent wide open - but a new 300mm f2.8 lens will cost you about $10-15,000
After all that - the only real solution is to take flats and compensate in post processing - as has been stated.
Cheers Rally
|