Blue glow

[tijmz]

Hi all,

I took some snapshots today, including one of a statue. However, part of the image (shown below) shows a blue glow on the top right side. In the background, you can see the same eerie blue emanating from the branches. The left shoulder was also a bit bluish (not shown).

http://hangmat.etv.cx/rembrandt.jpg

Now my question is: what causes this effect? And how do I prevent it from occurring? Is it just a poor choice of lighting conditions (I am completely new to photography)?

[sdommin]

That's called "chromatic aberration" (CA) or "purple fringing" (PF), and it's common in digital cameras. Some cameras/lenses suffer from it more than others.

Definition here:

http://www.dpreview.com/learn/?/key=chromatic+aberrations

You can usually minimize it by using a higher f-stop.

[aam1234]

According to our friend here "Jon" it's mostly from the lens.

[maderito]

I like the term "blue glow" -- makes it seem like an intended consequence of our photographic work. Certainly seems like an appropriate label for the posted image which has one of the most serious cases of CA I've seen. What camera/lens/f-stop was used?

[tijmz]

The picture was made with an A75 and a TC-DC52 lens mounted on top. The f-stop I don't know, as I shot the picture in auto mode. Since there was a lot of light (broad daylight at around 10:30 AM) I can imagine the f-stop being very chosen very low by the camera.

[maderito]

Chromatic aberration (CA) is caused by differential refraction of the wavelengths in light. It has to be corrected for in lens design so that the lens is "achromatic" – i.e. so that the different wavelengths arising from a point source in the object converge to a point source in the image.

I would not pretend to know more - it's pretty complicated. Suffice to say that it's worse with wide-angle lenses (especially zooms) and cheaper lenses and is exaggerated in digital photography, especially with the small CCD sensors of P&S cameras. Finally, it is common to see it most strikingly in edges silhouetted against the sky.

My guess is that the image quality of your A75 – reportedly quite good – was degraded by the teleconverter.

I try to avoid CA by using good lenses, avoiding the shortest end of a wide angle zoom, especially when shooting objects against a sky, and by shooting RAW since Photoshop has pretty good corrections for CA as part of its RAW conversion options. There are also third party software solutions.

In your case, you might want to take some test shots with that teleconverter to see how much you gain/lose in image quality.

[BOBinsane]

I have a Powershot S50 and it is an expert at creating this effect. :lol: Otherwise it's still a decent cam. :roll:

[tijmz]

I thank you all for the explanations! Today I will try and shoot similar images using different f-stops both with and without teleconverter.

[Jon]

Chromatic abberation and purple fringing are actually two different things. CA is a product of your lens refracting the different wave lengths of light slightly differently; you'll see blue and red fringes because the lens is acting like a classical prism; it's correctable by careful lens design and is more likely in long focal length lenses, and around the edges (centers are easier to correct).

Purple fringing is a sensor issue, essentially a sensor cell where there's an abrupt change in density may get overdriven. It's most common with wide angle lenses and wide apertures on bright, contrasty scenes (like yours). IIRC.

You can check the EXIF information stored with your original pictures to find out the aperture your camera used. It's available through ZB, or in WinXP's Explorer (right-click the file, choose "Properties", then "Summary" and "Advanced".