Latest posts:

• Ottawa Tulip Festival Photos 2011

• Topic 3: Understanding “ISO”
• Topic 2: Understanding “Shutter Speed”
• Topic 1: Understanding “Aperture”
  

Upcoming Posts:

• Topic 2: Review of the Light Craft Workshop ND500 Filter. The true 9 stops ND density Filter!
• Topic 1: How to photograph with a Kodak slr/n at 12:00pm using ISO 6.

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If all films and digital cameras had the same sensitivity to light the information above would become constant and easy to learn. But there is a third element for creative control of your photographic exposure. It is this third element that changes the shutter speed and aperture settings. It is called ISO

ISO is an acronym for International Organization for Standardization. In photography is designates the sensitivity of film to light. For digital users it designates the sensitivity setting of the image sensor to light.

When you buy film you should be concerned about its ISO rating. The most popular films are rated as ISO-100, ISO-200, ISO-400. Some film is rated at ISO-800. Why?

Number one if you are shooting in bright light you’ll want a low ISO rating i.e. ISO-100. If you are shooting in low light, perhaps indoors or an evening setting, you’ll want film with a high ISO rating i.e. ISO-400. Once you have inserted that roll of film into your camera you will have to use it up or take it out mid-roll before you can change to another ISO rated film.

Walla, the wonderful world of digital photography! You can change the ISO setting on your camera at anytime you want. Wow! What a difference that has made to my photography experience.

How do ISO ratings affect the exposure of your photograph?

Some digital cameras have ISO settings that range from 50 to 3200. That is a range of 6 stops in the exposure category. It works this way. Every time you double the ISO rating you essentially add one full stop of exposure. I.e. 50 to 100 is one full stop of exposure, 100 to 200 is one full stop, 200 to 400 is one full stop, 400 to 800 is one full stop, etc.

What does that mean in the practical sense? If you have your ISO setting at 100 and your light meter informs you that your camera should be set at f/4 and 1/60 second then you can take a well exposed picture at those settings. But what if your subject is moving or your lens only opens to f/5.6? 1/60 of a second will not stop the motion of a moving object and if your lens only opens to f/5.6 you are likely to have an under exposed and blurry photograph. Now is the time to step up the ISO setting. If you step up to ISO-200 then you can move one of the other elements one full stop. Your setting would now be f/5.6 and 1/60 or f/4 and 1/125. Only one setting can be increased by a full stop. If your camera allows for 1/2 stops you could increase both the aperture and the shutter speed by 1/2 to equal 1 full stop.

But in the case above we’ll probably need more than one stop to freeze the motion so we will set our ISO to at least ISO-400. This will give us the ability of setting our aperture at f/4 and the shutter speed at 1/250. This will really help out the end result of our photograph.

Limitations of ISO settings

There is one limitation to setting your ISO at the higher settings. In the film world we called it “grain” which sometimes was employed very creatively but usually was considered a degradation in the quality of a print, especially for a large blow up. In the digital world we call it “noise.” “Grain” and “noise” are, in a practical sense, the same thing. They each have a different quality about them but they both are essentially degradation in the quality of the photograph.

Higher ISO ratings tend to create more grain or noise in your image. I.e. ISO-100 will have much a smoother finish than ISO-800. So the rule of thumb is to use as low ISO rating as possible and still have enough light to obtain a proper exposure.

Below are variations from ISO 200 to ISO 2EV above 1600 taken from a Nikon D100.

Understanding Aperture
Every camera lens has a piece of glass on the front that covers a hole in the lens that lets light into the body of the camera. In the barrel of the lens is a set of blades that make that hole larger or smaller. The size of that hole is called the aperture. The smaller the hole the less light comes through at once. The larger the hole the more light comes through. It’s like turning on your water faucet at home. If you turn it just a little a small amount of water comes out at once. If you turn it all the way a lot of water comes out at once. This is the aperture setting.

The size of the aperture is designated by numbers called f stop settings. Usually those numbers will be settings in the range somewhere between 1.4 to 32. The smaller the number the larger the aperture. As the f stop number increases the aperture decreases. (The reason for this is a mathematical explanation which is very complicated and is different for every type of lens. It has something to do with lens size and its proportions. The explanation is not relevant to this article.)

The picture below illustrates the relationship between aperture size and f stop number. The f/1.4 setting shows that it is a much larger hole than f/16.

Apertures from f/1.4 to f/16

Aperture is measured in a scale called “stops.” Here is a scale of full stop increments for a very fast lens that has an f/stop rating of 1.4. Many lenses will start with 4.0 or less.

• 1.4
• 2.0
• 2.8
• 4.0
• 5.6
• 8.0
• 11
• 16
• 22

Many camera have settings at 1/2 stops and even 1/3 stops in between each of the setting above but to keep things simple I’ll not cover that chart in this article.

Most of our electronically controlled cameras will automatically detect the f/stop limits of the lens installed.

Two Important Reasons for controlling Aperture Size

• Number One: It allows control of the volume of light that comes into the camera at one time. When your subject is bright you need to use a smaller aperture setting (larger numbers – f/8, f/11, f/16, f/22). When the light on your subject gets dim you will need to set a larger aperture (smaller numbers – f/5.6, f/4, f/2.8).

• Number Two: Aperture settings control another very important element in photography called “depth of field.” “Depth of field” is the term used to describe how much of your photograph will be in focus. Here is how it works. The smaller the aperture (the hole in the lens) the greater the depth of field. That means that f/16 or f/22 will provide a photograph that has almost everything in focus from the foreground to the background. An aperture setting of f/1.4 or f/2.8 will cause some portion of your photograph to be out of focus. There are times when this is very advantageous, for instance if the background is distracting it is a good time to use a large aperture, focus on your subject and let the background be out of focus.

Here is an illustration of using a larger aperture to blur the background that would otherwise have been very distracting if a small aperture had been used: