Compensation versus ISO


Today’s Question: Do you have a preference for using the “increase/decrease” exposure control in the camera over changing the current ISO value that you are using? It seems (on my Nikon) that using this control is easier (faster) than selecting a different ISO number.

Tim’s Quick Answer: To me the most important consideration here is how you are impacting the final exposure settings. Ultimately, any technique that enables you to quickly achieve optimal settings for the overall lens aperture, shutter speed, and ISO settings is a good technique as far as I’m concerned. I would just caution against allowing one setting to become problematic just because changing a different setting was faster or easier. For example, I wouldn’t want to let the shutter speed get too slow just because it was easier to apply exposure compensation rather than change the ISO setting.

More Detail: I find that most photographers understand the basic concepts related to the “exposure triangle” of lens aperture, shutter speed, and ISO setting. However, I also find that many photographers get confused about when to adjust specific settings, including exposure compensation.

Exposure compensation enables you to apply an adjustment to the overall exposure settings when using an exposure mode other than Manual mode. For example, if you are photographing a snow-capped mountain, the camera’s meter may produce an exposure where the areas covered in snow are completely blown out. You could then apply a negative exposure compensation value to reduce the exposure and retain highlight details in the snow. For example, if you applied a one-stop exposure compensation value you might cause the shutter speed to go from 1/250th of a second to 1/500th of a second, creating a darker exposure.

The ISO setting enables you to have the camera apply amplification to the signal being captured by the image sensor. If you are in the Manual exposure mode, then increasing the ISO setting will indeed cause a brighter exposure, assuming you didn’t change the settings for the shutter speed and lens aperture.

However, in one of the automatic or semi-automatic exposure modes, changing the ISO setting will not change the overall exposure. For example, let’s assume you had the camera set to an aperture of f/8 and a shutter speed of 1/250th of a second at 100 ISO while in Aperture Priority mode. If you then increase the ISO setting to 200, the shutter speed would change to 1/500th of a second. You would have changed the shutter speed along with the ISO setting, but you would not have actually changed the overall brightness of the exposure.

Taking all of this into consideration, my recommendation is to find the best way to achieve optimal overall exposure settings with minimal compromise. In general, I find that involves prioritizing the most important exposure setting based on the circumstances, but reviewing all of the exposure settings to ensure you aren’t creating problems. For example, if you want lots of depth of field you might want to stop the lens down completely. However, that could result in a shutter speed that is too slow. And, of course, you also need to consider the effect of increased ISO settings on the noise levels within the photo.

There is much to consider when establishing the overall exposure settings, but with practice it does get easier to evaluate and decide on those settings more quickly. And again, any technique that enables you to achieve optimal settings quickly is a good technique as far as I’m concerned!