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Optical Focal Length for beginners

Focal length and lenses: A quick overview

As you progress in photography you find yourself pushing back the boundaries of what you want to achieve. Depending on how quickly you advance you are one day going to get to the point where the standard lens that you bought with camera is not quite going to cut it. The good news is that you probably bought a digital SLR (instead of a point and click) because you were the kind of photographer who was going to shoot all kinds of images in all kinds of situations (or at least progress to that level).

Improving your photography: Investing in lenses

Unless you are a pro, earning you living with your photographic budget and having a pro budget to match, then a huge factor in your choice of lenses is going to be cost. Unlike some areas of photography the amount you spend is directly going to affect the quality of your images. Rule number one: Optics are expensive! Rule number 2: optics plus brand name makes for very expensive! As we've said before this is why people tend to stick to the same manufacturer once they have started (See Nikon or Canon in the August 2009 blog).

Understanding focal length

Cost aside, your lens is all about focal length and you're going to need to reduce or increase focal length as you start to push the boundaries of a standard lens. Basically focal length can be broken into two areas; less than 35mm and greater than 50mm. The standard stock lens on a Canon mid SLR has (in basic terms) a focal length between 18 and 55mm, hence the 18-55 written all around it! Lenses less than 35mm are classed as wide angle and are excellent for landscape shots, while above 50mm is telephoto and will magnify. The 18-55 lens is hence a good multi-purpose lens as it can cover a good mid focal length. Moving up the focal length ups the magnification. As a rule of thumb up to around 135mm is best for portrait work (it gives good depth of field) and anything above 200mm is good for distance; sports or wildlife type applications.

Telephoto stability

However, if you're going large telephoto then you're going to have to spend some money. These are big, bulky lenses with a lot of optics. They are also heavy so you're going to need to invest in something to steady the camera. A lightweight monopod will do for most applications and is far more mobile and less cumbersome than a tripod. The Manfrotto 680B Monopod is a good start and light enough to not impact on your baggage allowance if you're flying to do your photography shoot.

Cropping Factor

As with all photography the camera has an influence on the components and this is especially true with lenses. Most mid range, 'enthusiast' or 'semi-professional' digital cameras do not have a full frame sensor. A full frame sensor is the central imaging semiconductor and is one of the reasons the top of the range cameras are so expensive. In order to bring the costs down, the sensor is reduced and a cropping factor is electronically built in to compensate. The result is less image in frame. So the same lens, on different manufacturers' models, can affect the image taken. The math's is relatively simple. A cropping factor of x1.6 (That offered by most mid range Canon cameras) used on a 70-300mm lens would give a field of view equivalent to 112-480mm on a 35mm camera.

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September 5

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