photography essays: part 3 - iso & the exposure triangle

i - s - o

means international standards organization. wait, what? this is a post about photography, right? yes; iso sets standards for many things including film speed, which is the film's sensitivity to light. with digital cameras, there is no film, but everyone knows iso so we still call it iso.

the lower the iso, the less sensitive to light; the higher the iso the more sensitive to light. at a lower iso, more exposure to light is required to create the same image density than if the camera was set at a higher iso.

iso is measured using the ISO arithmetic scale (ASA scale): 100, 200, 400, 800, 1600, 3200, 6400

changing from a normal rating of 100 (used outside on a sunny day) to 200 will get you one more stop of light. why do you care? say you are inside, it's late afternoon and you really want to shoot at a greater DoF (maybe at a f11 aperture). but, with the available light you stop only stop up to f5.6 before your shutter speed is too slow shutter speed that it picks up handshake (unintentional motion). if you increase the iso, you can gain more stops of light and can increase your aperture number without decreasing shutter speed.

if you need stops of light, you get through either:
increasing aperture size (bigger hole to let light through),
decreasing shutter speed (keep shutter open longer to let more light in), or increasing iso (more sensitive to available light).

too complicated? an illustration...



i've tried to find just the image, but this online tutorial has a great moving image of the exposure triangle towards the end. actually the whole video is quite helpful, but it's about an hour and a half, so set aside some time to watch it.

another reason you might want to alter the iso is to introduce grain as an artistic tool. the higher the iso, the higher the grain. gives the photo a nice artsy feel, especially if combined with black & white.


a post wouldn't be right without a photo.

set the iso high on my nikon coolpix point&shoot; shot is grainy, but i like it.

on a cobblestone path in the castle district
budapest, hungary









good articles: cheat sheet, cheat table & pictures, highly recommend! excellent illustrations on relationships in the exposure triangle, inverse square law, exposure definitions, intro to flash

next up - white balance