Thursday, March 6, 2014

Photo Tip Thursday: Investigating ISO

Hi there.  It's me again.  This post, we'll be going over the last component of the exposure triangle, ISO.  This one is a bit tricky to conceptualize so I'll try and break it down in an analogy.  First thing's first though, ISO refers to the sensitivity of the image sensor to light.  It has nothing to do with the physical structures that control aperture or shutter speed, but it does indirectly influence how they work.

Now for the analogy.  Think of yourself in a dark room. Your eyes represent the image sensor.  Obviously they are not able to form an image of the room because there is very little ambient light and your eyes are not sensitive enough to pick up on it.  To be able to form an accurate image of your surroundings, you need to increase the sensitivity of your eyes so that they can better detect and capture the small amount of light around you in the room.  This is where ISO comes into play.  Imagine ISO as tiny little helpers that work for your eyes, and that the ISO number represents the number of helpers available to your eyes.  When you open your eyes, these little helpers fly out and each grab a single photon of light and bring it back to your eyes before you close them.  At ISO 100, you have 100 little helpers that retrieve 100 photons of light.  That's not much light at all.  At ISO 6400 however, your eyes have 6400 little helpers that can retrieve 6400 photons of light. That's 64x the amount of light captured at ISO 100! This will allow your eyes to form a much brighter image. 

Obviously your camera doesn't have microscopic "little helpers" that fly out and capture photons of light, but the sensitivity of the image sensor to light can be increased to better detect the light hitting it by raising the ISO.  ISO 100 is typically the lowest ISO on most D-SLRs, meaning that the image sensor is the LEAST sensitive to light. At ISO 200, the image sensor is twice as sensitive to light as it is at ISO 100.  At ISO 400, it is twice as sensitive to light as it is at ISO 200, and four times as sensitive to light when compared to ISO 100.  Most D-SLRs can reach ISO numbers of 6400, some even going as high as 25600!  

Raising the ISO has two major impacts on your photos:

1. When aperture and shutter speed are kept constant, raising the ISO increases the brightness of your photos.

The aperture and shutter speed were kept exactly the same in these photos.  The only setting was changed was the ISO.  Notice that when the ISO was increased {the photo on the right}, the brightness increased as well.
2. At a constant aperture, raising the ISO allows you to use faster shutter speeds.

In the photo on the left, in order to achieve the proper exposure, I had to use a full second long shutter speed to let in the proper amount of light. This is entirely too slow of a shutter speed to use while hand holding a camera {I was using a tripod here}.  By raising the ISO in the photo on the right, I was able to increase the shutter speed to a level which would be acceptable for hand holding the camera.

You might now be thinking that the ability to adjust ISO on your camera is the best thing since sliced bread, giving you the ability to conquer any low light situation that you should encounter.  Well, yes and no.  Relying solely on ISO to brighten your image is a bit like making a deal with the devil of the camera world.  By increasing the ISO, you will indeed achieve brighter photos and you can use faster shutter speeds as well, but at the same time, you'll be sacrificing image quality.  An unfortunate consequence of increasing ISO is that you also increase the amount of noise in your photos.  Noise is that nasty grain that appears in photos and isn't very pleasant to look at.  Due to this, the ISO level should be kept as low as possible if it can be helped, and only increased if you can't achieve the desired exposure by first adjusting the aperture and shutter speed.

The lower the ISO number the higher the image quality.  This is a cropped section of two similar photos, notice the large amount of noise in the right image when compared to that of the left image.  Higher ISO means more noise and decreased image quality.

When are appropriate times to raise ISO then?  Indoor shoots, and shoots near dusk are the two most common times that you'll find yourself having to increase ISO.  These are scenarios where you'll have already selected an aperture of choice to achieve the desired depth of field, and you've made the shutter speed as slow as possible without having to worry about camera shake, but your images are still coming out underexposed.  In these scenarios, increasing the ISO speed will be the only way to achieve the desired exposure without using a flash.

Thanks for reading!
"The Boyfriend"


  1. Thank you for this post! Very informative. I am going to share with my fiance as well!


  2. I salute your boyfriend. He did good. Proud photographer over here. :)


  3. Thank you for sharing this information I am always on the look out for simple camera instructions and explanations:)

    Aloha, Kathleen | House of Polynesia


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