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Monday, June 25, 2007

Digital ISO


Digital ISODigital ISO

Q) How does the ISO setting affect the results in digital capture? My objective is to create a good-quality 16x20 print. Let’s say there’s no movement of the subject involved. How would the image look at ISO 100 compared to ISO 400?

A) ISO is an important consideration in digital photography. Each digital camera comes with a base-line ISO setting. With Canon, it’s usually ISO 100, and with Nikon, frequently ISO 200. This is a function of the sensitivity of the particular imaging sensor and image processor. The base settings are designed to give the very best quality possible from that particular camera because all other settings come from boosting the “signal” (the data from the sensor) from this base. Technically, digital cameras don’t have ISO ratings, but ISO settings, because only the base setting is a true ISO. Some cameras also offer a lower setting, which can be useful when a longer exposure is needed.

If you’re looking for the highest quality possible in a general landscape image, the base setting is the one to use. Using the optional lower ISO (as in the ISO 50 in a Canon camera) won’t improve your quality in that circumstance. To increase the ISO above the base speed may degrade the image quality and noise becomes a factor. With each successive generation of D-SLRs, this becomes less of a problem because of improved technology in the processing of the data from the image sensors.

Depending upon the camera being used, increasing the ISO to 400 will increase the noise factor, which will be seen most in the darker areas of the image, though the degree that this happens is extremely variable and depends on specific camera/sensor combinations. If you’re faced with problems such as the stopping of action or dealing with a low-light situation where a tripod isn’t available, then an increased ISO may help.

Many times we run into a situation where we need more ISO sensitivity to allow a smaller ƒ-stop to be used so we can attain greater depth of field or a faster shutter speed to stop motion, or even a combination of both. On a regular basis, I shoot at ISO 200 to solve these particular problems. With the latest generation of D-SLRs, ISO 400 and even ISO 800 can yield very good results. Lately, I’ve been testing the Canon EOS-1D MKIII and find that ISO 400 and ISO 800 have given me excellent quality while stopping the motion of flying birds. But if your higher ISO setting results in too much noise, you can employ noise-reducing software to negate some of the degradation.

This two-image composite was taken using a tripod under bright lighting conditions and I kept my Canon EOS 5D at 100 ISO for optimum quality. A 24-105mm lens was used at 28mm. Exposure was set to 1/250 sec. at ƒ/16 due to the bright snow and sky.


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