Can any one explain.....

Hi all,ive just noticed im getting a blue and green colour in most of the images taken with flash,its normally green but sometimes its blue. Its every image,granted im having to view them at 100% but its there none the less. And now ive noticed it its driving me mad!,any ideas whats causing it?

Sorry, but blue and green colour where? Do you mean white balance? I can't see anything wrong with that picture so it may be you monitor calibration. I assume you have not calibrated your screen to match prints or real colours.

Im as calibrated as can be,I 1st spotted it on the screen on the camera,look in the reflection of the flash in the pupil....

Can you circle it in red or something as I really am confused as to what you are referring.

I cant see anything amiss either

ken

I see a slight amount of ca around the flash's highlight in the reflection of the pupil.
the overall color balance seems ok, though.

Betty, do you mean the purple fringing around the flash reflection?

a little chromatic aberation (ca)is normal with digital.
you'll notice it in high contrast areas of your images. ie green leaves against an overexposed sky....or in your case, bright overexposed highlight against a dark pupil. different colors of light have different wavelengths and will bend through glass at different angles. the effect is that different colors of light will focus at different distances from the sensor plane. this results in colored halos. it's normal. somes lenses will control it better than others, but lighting can control it also.
here's an example of how to avoid that in a similar shot:


Notice how I've avoided ca by using a larger, more diffused lightsource, closer to the eye.
now granted, I've still got alot of work to do to clean up the red party lines, and the skin tones.

I mean this


I guess its normal from all your comments It just seems excessive to me. I finally stump up for a nice shiny sigma EX lens and it seems worse than my cheaper kit

Probably down to the difference in coatings. Pentax SMC is rather nice

It's called "purple fringing" and is pretty easy to get rid of - especially in this case. I would plump for this "quick'n'dirty" method in Photoshop:
* Add a new layer
* Set the layer blend mode to "color"
* Pick a small, soft brush and Alt-click to pick the "neutral" color you want and paint over the purple

As it's a pupil you could do the same by desaturating the color - add a hue/sat adjustment layer, sat to -100, then fill the mask with black. Then paint soft white where you don't want the purple.

There are also some automated methods. "dSLR Tools" has an action for just that

Matt

In certain lighting conditions, purple fringing is common to most DSLR's and has very little, if anything, to do with the lens.

This phenomenon is a function/limitation/inability of the DSLR's sensor to properly resove high contrast parts of the image. Given the same lighting conditions almost any lens would exhibit this to some degree.

CA is generally seen as a discolouration, displayed at an angle to the incident light hitting the lens. This is down to different colours taking slightly different paths through the lens's optics and they don't always arrive at the same place on the sensor. Much in the same way that a prism splits white light into it's component colours of the spectrum.

If you think of a lens as a complex series of complicated prisms which ideally should recompose the various colours and place them at their same origin at the sensor. Unfortunately it doesn't always get it right and cheaper lenses can get it wrong much more often.

Purple fringing differs in that it appears all round a particularly bright, or overexposed, high contrast area of the sensor.

It appears that the latter is the case with the OP's flash reflection of his model's pupil. The green cast he mentions may well be CA, but the purple (blue) fringing seems to me to be just that, fringing.

"It should also be mentioned that the lens exhibits also a fairly severe degree of purple fringing at large aperture settings which cannot be completely reduced during post processing. Unlike "normal" CAs purple fringing resembles diffuse halos which pollute into darker areas. Stopping down helps to reduce fringing."

- www.photozone.de/8Reviews/lenses/sigma_1850_28/index.htm - on the Sigma EX 18-50mm

And all this time, I've been lumping my purple fringing and chromattic aberations into one pile......lol!
I stand corrected, but the sloution still works!
even getting your flash further awy from the lens will help, if only by getting the catchlight out of the pupil....

Thanx guys you've given me the answer i was looking for. Oddly upstairs in the attick i get purple and downstairs i get green The new EX lens is a 50mm macro but ive noticed it now on all my lenses. Ill have to play around some more in cs3.

I have noticed considerable difference in the amount of purple fringing between lenses. So I think it does have quite a bit to do with the lens. I'm not saying some doesnt happen in the camera too though.