Sunday, October 26, 2008

Apertures, Shutter Speeds, ISO and Exposures Oh My!

This is the very first in a series of articles on Photography Basics. Although this is not the final article on Exposure, Apertures, Shutter Speeds and ISO, I would like the readers of this Blog to understand at least the basic principles of Exposures before I start. I'll try to basically explain 'Exposure', and then you can use the information for doing a little more studying. I usually recommend to people that ask the basic questions, to go out and buy a "Basic" Photography book and that will teach them most of what they need to learn. You can then use the book to guide you along and even a used cheap book works because you can use the book as a reference and guide to do more research on the Internet to find what you are looking for.

F-Numbers, F-Stops, Aperture, Lens Opening – These are refer to the same thing and this is usually the most confusing. But let’s start with Exposure as it deals with the F-Number, The Shutter Speed and the ISO.

Exposure is defined as such, “In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in Lux Seconds, and can be computed from Exposure Value (EV) and Scene Luminance.”

For now lets not worry at all about the Lux Seconds, Exposure Values and Scene Luminance. These are confusing on their own even for professionals and will be subjects of future articles. What we are concerned about here is Exposure with your digital camera, how bright or how dark, you final image will be.

The four factors that determine your exposure are the following:

1 - Available light

This you have no control over in most settings but you can always add lighting or use a flash. This would increase the available light but generally for this discussion, think of it as the natural light available without adding any extra light. It may be a grey day or a sunny day or you may be in the shade... the lighting always varies.

2 - ISO

This translates into the 'Light Sensitivity' of you cameras sensor. In the good old days of film, different chemicals and dyes were used to make the film. Some were more sensitive to light than others. The more sensitive the film, the least amount of light required to give you a 'proper' exposure. Today, with Digital Cameras it is the same except using electricity and different sensor designs they can make the sensor more or less sensitive to light.

The sensitivity of a film was measured in ISO (or in DIN or ASA before ISO became a standard). On a digital camera the ISO is a measurement of a digital camera's imaging sensor's sensitivity to light. This sensitivity can be adjusted.

50 100 200 400 800 1600 3200

Numbering for ISO on a digital Camera is usually 100 ISO to 1600 ISO. 100 ISO being the least sensitive but will give you the best results. The higher the Sensitivity, the more ‘Noise’ or stray pixels that will be introduced into the final image. Generally ISO’s of 100 to 400 are used.

Each ISO lets in half as much light as the next number. Remember that.

3 - F-Stop

This is usually the hardest one to get a handle on for most beginners. The F-Stop is a number that is derived from a mathematical formula that corresponds to the amount of light that is let in through the lens based on the size of the 'hole' in the lens. The 'hole' is usually made up of several blades in a diaphragm that can actually change the size of the 'hole' from a tiny hole to a 'hole' as big as the optics of the lens will allow.


This 'hole’ is actually called the Aperture and it is measured in F-Stops. This is where the confusion comes is. The logic here is simple, the bigger the hole, the more light comes into the camera. Simple. However, the F-Stop numbering system works backwards because of the formula. So, f/3.5 is a big hole (more light) and f/22 is a small hole (less light).

Here is a scale of Apertures - Starting with the largest Aperture, so the most light.


Here is a Diagram showing Aperture for an unspecified Lens. Note that Aperture sizes are dependent on the focal length of a lens.


Each F-Stop Number lets in twice as much light as the next number. Remember that.

So, using the F-Stops or 'F Numbers' you can control how much light comes into the camera. The F-Stops also control what is called Depth-Of-Field. I will save discussing this Depth-Of-Field thing which is not related to exposure until another day.

What adds to the confusion here is that when someone talks about 'Aperture' are they talking about the 'hole size' or the 'Aperture Number' (F-Stop Number)?

One give away is the term 'Lower' and Smaller'. A 'Lower' Aperture is a reference to the F-Stop Number. A Smaller Aperture is a reference to the Aperture or size of the hole. But, if I say Bigger, you do not know what I am talking about. I could be talking about a bigger hole or bigger number? When discussing Aperture it is very important to state what parameter you are discussing.

4 - Shutter Speed

Shutter speed is measured in Seconds and it controls how long the cameras sensor is exposed to light. Also a very simple concept. Un-like the inverse results of the f-number vs. the amount of light, shutter speeds are easy to understand. 1 second, 2 seconds and so on, these would be considered as long exposures. On the shorter side you would have 1/60 of a second, 1/125th, of a second and so on, easy to understand.

Here is a Shutter Speed Scale in seconds - Starting with the fastest Shutter Speed, so the least amount of light.


Each Shutter Speed allows half as much light as the next number. Remember that also!

In the end, this also controls how much light goes into the camera based on time. This Time Setting can determine if a photo is blurry or not. If you used a 5 second exposure, could you sit perfectly still for 5 full seconds?? You shutter speed also controls "Motion Blur" or rather blur caused by moving objects.

Back to Exposure

So given a specific scene, say a House Sparrow on a branch. Based on 1) the available light and based on 2) the Sensitivity of the Sensor (ISO 200 for this example) a correct exposure would be calculated by your camera and usually given to you in an Automatic Mode.


To calculate this Exposure your camera doesn't know it's looking at a Bird on a Branch, but rather thinks it is looking at a whole scene that is perfectly 18% Grey. (I will write another blog about this 18% Grey as it is the basis for understanding exposure).

So, for this example lets say the camera suggests using an F-Stop of f/8 and a Shutter Speed of 1/125th of a second. Those are the settings you would need to use to get a correct exposure or a ‘Properly Exposed Photo’. Not too light and not too dark.

This leaves you two controls. You could leave the shutter speed at 1/125th second and change the Aperture. Changing the Aperture to f/4, Bigger Hole, would let in more light and would lighten your photo. An Aperture of f/11 or f/16 would make the hole smaller and would let in less light making your photo darker.

Conversely, Leaving the Aperture at f/8 and changing the shutter speed you could darken the scene by using a faster shutter speed (less light) or brighten it by using a slower shutter speed (more light).

This is where the fun comes in and where you as a photographer and artist can control what is going on. Remember I suggested that the Aperture controls the Depth-Of-Field and that the Shutter Speed can control "Motion Blur"? Well, the camera lets you take a ton of different settings while ALWAYS making things perfectly exposed.

Back to our example. If f/8 at 1/125th of a second is a correct exposure, then we could keep the exposure (the total amount of light going into the camera) the same by changing the shutter speed higher or lower, and then adjusting the Aperture to make sure we have the same amount of light in the end.

What is nice is that the Full F-Stops and Full Shutter Speeds each allow half as much (or twice as much) light as previous or next numbers. (You remembered this right?)

So with our example of f/8 at 1/125th of a second. We could change the settings to f/4 (twice as much light) and to 1/250th of a second (half as much light) and still have a perfectly exposed scene (or bird in this case). By making these changes and then appropriate corresponding changes to keep the exposure correct you can control the shutter speed and apertures you want so you have full control of Blur caused by movement or Depth-Of-Field.

This then lays the ground work for many upcoming blogs. Hope you enjoyed it.

© 2008 Francois Cleroux

(Version 1.11 - October 2008)

Please feel free to leave comments, corrections, ideas, thoughts or suggestions.

1 comment:

  1. Hi Francois
    Great post and very helpful. I always found the exposure as a very subjective aspect in photography. Theoretically I don't believe in the concept of a good exposure. We can talk about a correct exposure from a technical point of view but since the exposure is so subjective, what you might think is correct for a particular subject could be entirely different for someone else.


I value thoughtful comments and suggestions. If you like or dislike this post, please let me know. If you have any ideas or suggestion, comments or corrections (I do make mistakes) please also let me know. Thanks.

- Francois Cleroux