Back to Basics – ISO and exposure wrap up

isoIn the previous few articles, I have discussed some basic aspects of photography. The first subject to get a close look was exposure, and I have discussed two of the three components that control it: shutter speed and aperture. In this article, I will bring in the missing piece – ISO (or film sensitivity). After that I will conclude the exposure subject.

We have learned that the sensor (or film) can get the same exposure if we prolong the duration the shutter is open, but use smaller aperture (or shorten the duration that the shutter is open, while using a bigger aperture). If we want to be absolutely honest (which, at least for now, we do), we have to include the third part of the equation: film sensitivity (AKA ISO).

In short – ISO sets the impact that light will have on the sensor. High ISO will make our exposure brighter, while low ISO will make our exposure darker.

So how can we use ISO to produce better photographs?

I’ll try to show that by giving an example. Say that there is a picture that we want to take. Alas the location is dark, and the exposure meter tells us that with our widest aperture, we need to set the camera to a shutter speed of 1/60. Oh yea – I forget to mention – the zoom lens is set on 80mm to get the action just in the right framing. That means that we need to set the shutter speed to at least 1/120 to avoid camera shake. How can we get out of this tight spot?

We can boost the sensitivity of the sensor. Double it from 100ASA to 200ASA. to keep the exposure the same we will need to reduce the shutter speed to 1/125.

So the thing that I have been hiding from you on the last two articles is that you can also play with the ISO setting on your camera to balance your exposure. Let’s take a closer look:

One of the basic rules of photography is called “the rule of reciprocity” or in “non Latin” the exposure triangle. It means that there are three parameters (or controls) you can use to set a picture’s exposure. (Three… as the number of angels in a triangle…). So now when you want to get more Depth of field (use smaller aperture) you can prolong you exposure or increase your ISO setting. Similarly, if you want to get a longer exposure, you can either stop down your aperture or decrease your ISO setting.

Luckily ISO setting usually doubles as you for each dial up. So opening one stop (say from f/11 to f/8) means setting one ISO lower (say from 400 to 200). Similarly shortening the shutter speed in one step (say from 1/125 to 1/250) means setting one ISO rating up (say from 100 to 200)

But nothing comes without a cost. You can ask the bar tender at your local pub – (s)he will approve. The price that we pay for increased sensitivity is noise. The higher you go with your ISO setting the more noise your image will have. BUT! Have no fear because you can either use this noise or try and eliminate it. More on this soon.

ISO and Granularity – Maybe the most noticeable aspect of high ISO images shot of film is the high granularity they display.


Image by Michael Grayson (Flickr ,Gallery) (ISO = 1600)

ISO and softness – When taking pictures with a high ISO setting, the granularity of the image increases and even when in focus objects become soft. Objects that are outside the depth of field are even softer. This effect can be use to artistic means, and is very popular with black and white photographers.

Iso and softness
Image by Arkady Renko (ISO = 1600)

ISO and sharpness – this one is easy to remember – the lower the ISO the sharper the image.

ISO and eliminating digital noise – as I mentioned before using High ISO setting gets some noise into your image. One of the more popular ways to eliminate this noise is to use noise removing tools like noise ninja or neat image. See how noise ninja removes the digital noise from this 1600 ISO image. (You pay some sharpness, but sometimes it is worth it)

High ISO and Noise

High ISO and long exposures – There are two major factors that contribute to noise in digital images: sensor getting hot and High ISO. Combine them both, and you are on your way to noise fiasco. The rule of thumbs is that you should try and take long exposures with the lowest ISO setting possible.

ISO and live shows – when you go to shoot a live show, you have to deal with very “bad” lighting conditions. Usually you will need to squeeze every photon of light. To capture both your main subject and the atmosphere, set your camera to relatively high ISO, wide aperture and low shutter speed, along with a rear sync flash.

iso and live shows
Image by dreadfuldan (flickr, gallery) (ISO = 800)

This is it!!! Pat yourself on the back. After covering ISO, Shutter Speed and Aperture, you are now educated about the three most important controls of exposure.