Why Do Some of My Portraits Have High ISO’s?

There are two kinds of light – the glow that illuminates and the glare that obscures.”
James Thurber

I want to answer a question I recently received from a readers regarding a post I did on Flashpoint’s 13-inch fluorescent ring light. Bernie asked, “Why are some of my portraits shot with such high ISO settings?”

erica.ring

I’m sure he is concerned because everybody knows that higher ISO settings produce more digital noise than lower ones but as cameras get better and better in their ability to shoot at high— 12,800 or higher—ISO settings that assumption may need to be reassessed. (See my review of the Pentax K1 for Shutterbug magazine where I shot some images at ISO 204,800.)

As I mentioned in a previous post, all artificial light sources obey the inverse square law, which states the intensity of illumination is proportional to the inverse square of the distance from the light source. This means that an object that’s twice the distance from a light receives a quarter of the illumination. What it means to us photographers is that if you move a light from five feet to 10 feet away from a subject, you will need four times the amount of light to produce the same exposure. You can do this by opening the lens aperture two stops, increasing the power of the light source, or raising the ISO setting.

erica.ISOFlashpoint’s Ring Light is rated at 70 Watts at 4500 Lumens. I received a number of e-mails urging me to develop an “ Equivalent GN” to evaluate the output of this light and although I tried, in retrospect, it just didn’t make sense to do it. Mea culpa.

For any continuous light source or under any given available light situation, I tend to shoot at ISO settings that produce a hand holdable shutter speeds that would create the least subject motion and a modest enough aperture to maintain appropriate depth-of-field, while providing acceptable low noise levels. The portrait of a Laura Bachmayer was made with a Panasonic Lumix GH3 with Lumix G Vario 12-35mm f/2.8 (at 35mm) and an exposure of 1/50 sec at f/2.8 and ISO 640.

Ultimately it all comes down to the photographs: When looking at an ISO 1000 image, for example, under extremely high magnifications the noise is negligible and you could deliver a 16×20-inch print to a customer with no apologies. There is no one perfect ISO setting for every photograph, so you need to make your tests using your own camera. (If you have any questions you want to see answered, please click the Contact button above and drop me a note.)

Screen Shot 2016-03-26 at 9.13.43 AMIf you’re interested in shooting portraits, please pick up a copy of “Studio Lighting Anywhere” which is available from your favorite book or camera stores as well as Amazon.com, where your purchase helps this blog.

Author: Joe Farace

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