View Full Version here: : Any Tips to stop Vignetting
Can anyone suggest some practical way to stop, or at least reduce vignetting, to be honest i didn't think that the Cannon 300 mm would have that problem, would stopping it back a bit help.
05-12-2007, 09:17 PM
You will get vignetting in just about every situation when imaging Leon. Just take flats. Taken correctly they will deal with your vignetting problem, no worries. :thumbsup:
If you want to do it mechanically then (from memory it think) you will need to invest in a curved chip, and I don't think there are any available at this time.
Ah, Paul why did I know you would help me out, ;) thanks heaps, oh well, flats it is then.
05-12-2007, 09:54 PM
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.
06-12-2007, 05:34 AM
C'mon Leon, get the light-box built, even try an ugly one like Huffs or mine. It really is the next hurdle for you. Once you master it you will wonder why you even faltered. In the meantime try a flat coloured wall, as long as it is evenly lit.
06-12-2007, 07:36 AM
If you don`t want to bother with flats or a light box just stop your lense down to about f/4. I don`t bother with flats with my 200mm lense and I push the processing real hard and I don`t really notice any vignetting at f/3.5-f/4 but there is some wide open.
Nearly all fast lenses will suffer from vignetting wide open and the shorter focal length ones are even worse. A stop or to closed down will reduce it significantly.
Thank you for your helpful comments, it seems that flats are the go for some and stopping down for others, looks like i will have try both.
Ok Gary, I'm off to the shed :lol:
I usually take my flats at pre-dawn, Leon after an all nighter, of a clear patch of blue sky.
That's another quick option for you.
Andrew, when you do the flats at that time, do you use a white cover of sorts, or just the open tube.
Just open tube Leon.
Try shooting away from the east, where there's no gradient in the sky.
You'll need to work quickly though.
Thanks Andrew, I will give that a go next session .
06-12-2007, 01:14 PM
Leon a Canon Super Telephoto does not vignette It merely follows the laws of physics to almost perfection! Seriously though flats are a must. What I do is wait till just after sunset and pointing the lens vertically take an image on the Av setting (the camera adjusts the exposure automatically for the set aperture) and then I set the camera to manual and for that aperture TRIPLE the exposure the camera used. This gives you a much better flat to correct with. I generally do 16 exposures and use ImagesPlus to combine with median stack and then convert to a grey image by using Luminance and Gamma (under Color) by using the default settings.
Next time I get down to Ballarat I'll try and get to your place to meet etc. You must be OK as we are both clog wogs! Dutch to those that do not know.
Thank Bert It seems all comments for my question are basically the same, flats and more flats, I will heed this information and get serious.
Bert you are welcome any time, it would be great to meet you.
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