Free Photography Tips & Tricks for digital SLR camera owners

Step by step photography instruction on how to take great photos with a DSLR (Digital Single Lens Reflex) Camera

What is ISO?

Your DSLR camera's sensor has different degrees of sensitivity to light. The more sensitive your digital sensor is, the more it can capture images in low light conditions. This sensitivity level of the digital sensor is referred to as ISO. In the Shutter Speed & Shutter Priority Mode article, it was stated that photos that are taken with slow shutter speeds are susceptible to become blurry. If the ISO setting is increased then a faster shutter speed can be used without having to change the aperture in order to get the same exposure without the risk of blurring.

Remember this rule of thumb regarding the relationship between ISO and shutter speed.

  1. Double your shutter speed for each full-stop decrease in ISO OR halve your ISO for each full-stop increase in shutter speed
  2. Halve your shutter speed for each full-stop increase in ISO OR double your ISO for each full-stop decrease in shutter speed

How do we measure ISO?

ISO measurements are represented by numbers. The example below shows full stop increments although newer cameras allow for less than full stop increments. Double the number means an increase of one full-stop.

ISO: 50, 100, 200, 400, 800, 1600, 3200, 6400 and up

The higher ISO numbers allows your digital sensor to capture more light. Higher ISO are normally used in low light conditions like night photography or indoors.

To show you the effect of varying ISO settings on the exposure of a photograph, refer to the illustration below. The aperture and shutter speed remained the same all the way to isolate the effect of ISO.

ISO Effects
Figure 1: Effects of ISO on Exposure

The image in the middle taken with ISO 800 has the optimalexposure. Given that the aperture and shutter speed remained constant, when the ISO is increased, the image is overexposed. When the ISO is decreased, the image is underexposed.

ISO settings for varying lighting conditions

Depending on the lighting conditions of a scene, here's a general rule of thumb for what ISO number to use:

  • ISO 100-200: Outdoors (Sunny)
  • ISO 400: Outdoors (in the shade or overcast (cloudy)); Indoors (when there's lots of light)
  • ISO 800 and above: Indoors (with minimal light); Low light conditions, sports or action shots, night shots

What is Digital Noise?

In digital photography, digital noise are random specks of red, green and blue dots in an image that makes the image grainy. Digital noise generally lowers the print quality of the image and is normally undesirable unless it is an intentional for creative purposes. However, todays digital camera have advanced so much that ISO 800 and below are generally free of digital noise. Professional level digital SLRs are even more advanced and can go up to ISO 1600 or 3200 without dimishing the photo quality.

I would advise you to do some testing of your DSLR camera's ISO setting and determine how high you can go before you lose image quality where the photo is no longer acceptable to you.

Below is a sample image showing the same photo at ISO 200 (high quality) and ISO 3200 (low quality).

ISO and Digital Noise
FIGURE 2: ISO and Digital Noise

The photo taken at ISO 3200 has lost a lot of the image quality and the digital noise is clearly visible.

Adjust Exposure by changing ISO setting

When changing exposure settings, most photographers usually resort to adjusting the aperture or shutter speed setting. Most forget that there is a third element in the exposure triangle, the ISO. Changing the ISO sensitivity setting is another way to adjust exposure.

Using higher ISO settings

It is normally not advisable to use higher ISO settings unless you are faced with the following obstacles:

  1. If you are in a low-light situation and you have have maxed out on your aperture and shutter speed settings and you still cannot get enough light to get a proper exposure without using a flash or additional lighting.
  2. If you are too far away from your subject that even a flash doesn't give you enough light to get a proper exposure.
  3. If you intentionally want to use digital noise for artistic effect on your photograph.
Related Topics
  1. Online Course #1: Understanding Exposure
  2. Shutter Speed & Shutter Priority Mode
  3. Aperture & Aperture Priority Mode