Last night someone asked me what causes red-eye during flash photographs. My simple explanation at the time was that the light from the flash was reflecting off the back of the retina. Red-eye is more prevalent at night when the eye’s iris is widest open to let in the most light. Some camera manufacturers try to overcome this with a special red-eye-reduction flash; those annoying multi-flash, red-eye-reducing cameras attempt to fool the eye into thinking that there’s an abundance of light and to subsequently close the iris.

Photo © Francheska Ashley Doll

Naturally, I consulted my medical references when I got home. I was pretty darn near close, although it makes me feel a bit like Cliff Claven when I get these things even slightly wrong.

No, it’s not technically the iris that opens and closes. The iris is the colored part of the eye, the muscles that surround the pupil, and is technically one of the body’s many sphincter muscles. Its purpose is to open and close the pupil, the hole in the center of the eye that lets in the light.

Yes, I said “sphincters”. Go ahead and get over your giggling so you can go back and re-read that and maybe learn something.

“My, what a nice pair of sphincters you have!” — isn’t exactly an opening line you’re likely to hear often. Stick with “My, what pretty red eyes you have!” You’ll get much farther with girls like Cheska.

You did notice her red eyes, didn’t you? Or were you unable to focus your eyes on anything other than her round objects?

So, why red? I couldn’t remember at the time, but if you remember what’s back there in the retina you can easily figure it out: rods, cones, and blood vessels. The light is reflecting off the blood vessels in the retina right back to the camera. And since everyone I know has red blood, well, you “get the picture”…

Solution

The simplest methods to prevent red-eye? Use an off-camera flash. When the flash is too close to the lens, as on most point-and-shoot cameras, you get that lovely red demon-like reflection.

Second best? A little touch-up with Adobe Photoshop, iPhoto, GIMP, or Pixelmator. Doesn’t actually prevent red-eye, but any of the major image-editing software packages can easily remove the redness, or have readily available free plugins to do the job near automatically.

But What About Dogs and Cats?

Thanks to ‘Sir Matthew of Wembly‘ for pointing out that while red eye in humans is caused by reflections from red blood vessels at the back of the eye, other animals reflect colors other than red. The explanation may require a higher level of vocabulary than Matthew is accustomed to.

Photo © therealmalingerer.com

The epithelium or retina in many animals (including dogs, cats, and deer) have a special reflective layer called the tapetum lucidum. It acts almost like a mirror at the back of their eyes and enables the animal to see in dimmer light than it would otherwise be able to.

Consisting of a compound called guanine, the tapetum lucidum reflects light outward and thereby allows a second chance for its absorption by visual pigments at very low light intensities. The visual pigments present in the eyes of these animals produce the non-red eyeshines often seen from nocturnal animals.

Humans, of course, do not have a tapetum lucidum layer in their retinas — therefore, the reflection off the retina shows the red color from blood vessels. As I clearly stated above.

Interestingly, dogs and other animals that have complete heterochromia iridum (or bi-colored eyes) usually give off flash reflections of different colors, too. Humans with heterochromia (such us Kate Bosworth, Christopher Walken, and Mila Kunis) still reflect blood red in both eyes.

Photo © Canon U.S.A., Inc

Just over a week ago I purchased a Canon PowerShot G2, my first digital camera. It comes with a 32MB CompactFlash card, but I upgraded it immediately to a 256MB card. I knew I’d need the extra memory storage for the high-quality images.

• At the worst possible image quality setting, 640×480 with the highest compression, it will take 2,714 pictures on my 256MB CompactFlash card of a quality that is still better than most product displays on eBay, or 338 pictures on the included 32MB CF card. A bit pixelated, perhaps, but acceptable for low-end web usage. Fine if you need to take 75 rolls of 35mm film. Wouldn’t print it, though.
• At 640×480 with the best quality compression, you can take 960 pictures, again with the 256MB CF card. That’s almost 27 rolls of 36-exposure 35mm film.
• The setting I use most often is M2 (Medium #2) with the best quality compression. That gives me 430 pictures at 1024×768 pixels, or the equivalent of 12 rolls of 35mm film, great for producing high quality photos that I can publish on the web. Sure, I could use the smaller 640×480 format, but with Adobe Photoshop 6.0, I can crop and size the image anyway I want to, eliminating the need for additional costly zoom lens attachments. Plus, printing 4×6 prints should work well with this format, although I haven’t tried it myself.
• I don’t use the M1 (Medium #1) setting much because the M2 setting meets my average daily needs. It takes 246 pictures at 1600×1200.
• The L or Large setting takes 123 pictures at a whopping 2272×1704 pixels, resulting in a terrific quality 3.8 megapixel image. A bit under the 4.0 megapixel that is advertised, but I’m pleased with the results. That’s only 15 pictures with the supplied 32MB CompactFlash card, so you can see why the memory upgrade is necessary.

There are a few things I dislike about the camera:

• The camera won’t turn on in picture-taking mode with the lens cap on.
• There’s a flimsy, bendable, rotating piece of plastic that acts as a cover over the DC adapter plug. I know it’s going to break off someday.
• Picture-taking and picture-viewing modes are totally separate modes, so if a picture opportunity comes up while you’re viewing pictures, you’ll miss it.
• The lever that controls vision correction when viewing through the viewfinder is too easily moved.

And there are a few things I love about it:

• Good exposures with the auto flash, even at night.
• Macro capabilities are excellent. Turn off the flash, though.
• Onscreen menus are very simple and easy to use. I expected them to be much more complicated.
• I have a USB Microsoft keyboard with built-in USB ports in the back. I bought a USB adapter for $20 that accepts CompactFlash cards. So, no picture downloads for me! I remove the CF card, plug it in the adapter, and it appears as another drive on my Windows XP computer. I was even able to completely avoid installing any of the Canon software. No software updates, no drivers, no problems! Note that these adapters are available for almost any digital camera that uses CompactFlash or SmartMedia.
• It’s a point-and-shoot simple enough for the entire family to use, although because of its $700 price tag I’m a bit paranoid to let the kids try it out in case they drop it, or do something equally damaging.

According to the internal counter, I’ve taken 273 pictures already. Most of them were discarded immediately after taking the photos, but that’s one of the cool things about digital cameras — no costly film or extra film developing expense. I’m thinking about taking this gem with me everywhere so I’ll never miss a moment (as long as it’s in picture-taking mode).

The Canon PowerShot G2 gets an A in my book. Why not an A+? That’s reserved for beauties such as the Nikon D1X or the new D100, but at their several-thousand-dollar price tags, they’re way out of my league…