Problem with Flats

[JohnnyD333]

Hello SGL,

Ive done much research on taking Flats but I just cant seem to get it right. I have a problem with vingetting so Flats are very important for me. However, I have tried numerous methods and they just dont seem to work. Has anyone had this problem and is there any ideas you guys might have for me. Its starting to bother me, especially since I have great data but my DSLR always produces a bright spot that I cant get rid of. Pixinsight does help, but I feel like Im avoiding the real problem which is my lack of good Flats. Any advice or methods of taking flats would help. Thanks again.

JD

[Psychobilly]

What happens if you stack your master flat with one of the flats? you should end up with a pretty uniform "image"...

I had problems with DSS over or undercorrecting with flats... I now use a EL panel light box and every tiem i use a new setup I take test "flats" at various levels of exposure compensation and see which set produces the best result...

It actually helps to see the effectiveness of the flats if there are some dust bunnies in the flats as well as vignetting... as they tend to pop out if the flats arent quite right...

Peter...

[Psychobilly]

Just a thought.. are they DSLR flats with camera lenses?

If they are they need to be taken at the same focal length and aperture as the "lights"...

I tape of the zoom ring on my lenses using Micropore surgical tape util after I have taken the flats.. exposure for the flats can only be adjusted by varying the shutter speed and not the aperture or ISO settings...

Peter...

[JohnnyD333]

Thanks Peter,

I figured it was the shutter speed that needed to be adjusted. I read several methods of taking Flats but I usually stick to your method, which is keeping the ISO, aperture, focus all the same. My question is when I take the flat, how can I tell that it is a good flat. You mentioned being able to see the dust bunnies, should the Flat have vignetting similar to the Light, or can I check a histogram on the camera to see if it came out correctly. Thanks again Peter for the help.

[Psychobilly]

The flats should show the vignetting.. if you use the "info" palette in PS/CS and move around the image you should be able to see the variation in pixel values...

A stretch of the flat will show up the fall off...

What I found (pity I lost the test images from a few years ago) was with DSS as you adjusted the "exposure" that the flats were taken at you would end up going form under corrected (darker corners) to over corrected flats (brighter corners) the same would happen for dust bunnies...

I used Av mode with various levels of Exposure Compensation to take a series of flats usign an EL panel , These were then seperately applied to the same "lights"... and the resulting image run through an action in CS to make sure identical processing was applied to each image.

Peter...

[JohnnyD333]

Peter,

Made the adjustments you mentioned and it works like a charm. I really appreciate the help. I totally misunderstood the shutter speed of Flats, and after making some major adjustments I got great results. Thanks again.

JD