Getting rid of Red Eye and Noise


How many times has a great photo of a subject been ruined by unreal, staring red eyes in the person?  There are a few causes for “red eye”.  Ignoring the obvious ones of late nights with excessive alcohol intake and scratchy contact lenses, the photographic cause of “red eye” is the flash burst illuminating the back of the eyeball!  This is particularly a problem with cameras that have their own in-built flash.  The startling look of staring red eyes can certainly spoil an otherwise pleasant portrait.

The reason for this is that the beam of light from the flash is very close to and parallel with the axis of the lens, so the lens “looks” directly into the back surface of the eyeball as does the flash beam.

One way to get around the red eye phenomenon in photography. One way to get around the red eye phenomenon in photography.

To get around this problem, professional photographers will use a flash gun mounted off to the side of the camera.  In this way the flash actually comes across the subject’s eyes at an angle and “red eye” is less likely.

Another reason for the prevalence of “red eye” is that in low light situations (and that’s the times when you have to use flash illumination) the subject’s pupils are naturally dilated and it becomes even easier to see into the back of the eye.

Many camera manufacturers have now begun incorporating a “pre-flash” before the main flash to make the pupil contract, so it is less likely that you will see inside the eyeball.  The only problem here is that many people imagine that the “pre-flash” going off means the picture has been taken and move away just as the main flash fires.  If you are using a camera with this facility it is best to warn the subject that there will be two flashes, with the real one being the last one!

If all else fails, there are computer programs to change the color – or use sunglasses!

Noise and ISO

In the old days of film, we bought film stock according to its sensitivity to light.  This was called the ASA rating.  All things change, and ASA terminology was changed to ISO scales.

ISO sensitivity expressed the ‘speed’ of photographic negative materials, but since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given to represent sensitivity to light.  The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations.

This ISO setting heralds a new approach for users of digital technology.  Where once you needed a tripod if you were trying for photos in poor light, by increasing the ISO rating, you are ‘fooling’ the camera into thinking the light is better than it really is.

Where once you would have needed to physically change to a different roll of film if you wanted a different ISO speed, digital technology allows you to simply dial one in.  In this way, you can record images taken at different ISO speeds on the same memory card.

The camera’s ISO setting or ISO ‘speed’ is usually listed as ISO 50, ISO 100 and ISO 200 and on through 400, 800, 1600 and even 3,200 with some cameras.

Higher numbers represent greater sensitivity and the ratio between two ISO numbers represents their relative sensitivity, meaning a photo at ISO 200 will take half as long to reach the same level of exposure as one taken at ISO 100 (all other settings being equal).  Put another way, by doubling the ISO you are halving the shutter speed necessary.

This is accomplished by boosting the electronic image signal in the camera; however, this also amplifies ‘noise’ and so higher ISO speeds will produce progressively more ‘noise’.

Here is the root of the problem; all this increase in sensitivity does not come free.  When you boost the sensitivity of your image sensor by selecting a higher ISO, the image sensor is now able to record a fainter light signal.  However, it is also true now that it will record fainter noise, where noise is any signal that is not attributed to the light from your subject.

For me, I set my camera on ISO 100 for every day use and have no noise problems.