Apertures, Shutter Speeds and ISO Exposure Revisited Apertures, Shutter Speeds and ISO

EXPOSURE TRIANGLE – Remember this?

ISO/Film Speed ISO = International Standards Organisation It is a ‘scale’ which denotes how sensitive a film or digital sensor is to light The higher the ISO number the more sensitive to light the film or chip is. Film Camera = ISO setting is determined by the film rating. Digital Camera = can be adjusted to suit needs.

Understanding how to use ISO Choosing ISO setting or film speed is primarily dictated by available light levels. Low light levels will require a high ISO (1600 - 6400) setting (unless it’s possible to use a tripod). In good light photographers would most often use a lower ISO setting (100 – 200).

Why use those settings? Low ISO settings (100 – 200) Positive - less image noise / grain and finer detailed images, especially in larger prints. Negative – Reliant on good light conditions so may need to use a tripod and / or a larger aperture. High ISO Settings (1600 – 6400) Positive – Allows you to use a faster shutter speed if required, allows you to photograph in low light Negative – Has an adverse affect on image quality.

Effects of ISO setting on image quality This illustrates the difference between High and Low ISO settings. At 3200 ISO, note the higher levels of noise. This can degrade the detail and soften the image.

Setting the Camera – Command Dial

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Shutter Speed

Modern Camera Shutter speeds Apertures ISO Setting Correct exposure and exposure compensation Indicator Exposure Mode Indicator

Shutter Speed Comparison 1/8 1/125 1/500

Fast Shutter Speed (1000th Second)

Fast Shutter Speed

Slow Shutter Speed (1/2 second)

When to Use a Tripod To take images with a slow shutter you will need to use a tripod otherwise you will get camera shake A general rule of thumb suggests that you need to use a shutter speed at least equivalent to the focal length* of the lens used to avoid camera shake. *For example, a 50mm lens or 50mm setting on a zoom lens would require a shutter speed of at least 1/50th second to avoid camera shake.

Aperture & f numbers

Aperture One of the Key controls that allows us to obtain ‘correct’ exposure, whilst allowing us to control the image characteristics Correct ‘exposure’ is based on available light levels and must be measured using a light meter. Based on those light levels, we can then control the exposure through a selected combination of ISO, Aperture and Shutter Speed. We can also vary those settings as the they are designed to be interchangeable.

Aperture Outlined here are the standard aperture settings on a film camera lens. Here they range from f2 to f22. What does this mean? You might ask…

What are ‘F Numbers’ F1.4 f2 f2.8 f4 f5.6 f8 f11 f16 f22 ‘F numbers’ are a set of numbers (ranging from ‘1’) that represent the size of the ‘aperture’ to be used. The sequence with ‘F numbers’ seems odd when compared to shutter speeds as they don’t initially appear to represent such a logical sequence (i.e. an exact halving or doubling of exposure). If any of you do maths, this is because ‘F numbers’ are worked out based on the surface area of a circle. For example, if ‘1’ is our ‘maximum aperture’ starting point, to double the surface area of a circle we have to multiply it by the square root of 2 (which = 1.414).

F Numbers Continued The resulting numbers are rounded off starting from 1, so: 1 x 1.414 = f1.4 1.4 x 1.414 = f2 2 x 1.414 = f2.8 2.8 x 1.414 = f4 4 x 1.414 = f5.6 and so the sequence goes……

If we consider that changing the ‘standard’ aperture setting by one stop, say from f5.6 to f8, allows only half as much light to pass through the lens. Consider the following: What would happen to our exposure if we changed our aperture setting from f16 to f11? What about if we changed it from f2 to f4? Does the aperture setting have another effect on the image besides controlling exposure levels?

Depth of Field

Depth of Field

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Be a ‘Thinking Photographer’ All of the exposure settings are interchangeable and if we understand how to use them we can make better pictures. Perhaps the most important consideration is whether the ‘Shutter’ setting has priority or the ‘Aperture’ setting has priority. So, what will determine this choice? That’s right, the nature of the subject matter and our creative intentions will determine whether we are more concerned with ‘movement’ (showing it or freezing it) or ‘Depth of Field’ and how much of the image is in ‘sharp’ focus.