Lenses for SLR Cameras
There are numerous different types of lens for SLR cameras. The three main
types that are most likely to be of interest to the amatuer are -
- Wide angle - for taking pictures where a wide field of view is required e.g. buildings
- Telephoto - for zooming in on the subject e.g. wildlife
- Macro - for focussing very close up e.g. insects, flowers etc
Understanding Lens Jargon
If you look at lenses in magazines you'll see various numbers and strange names
attached to them. I'll try to define a few in brief here to make it a bit
clearer -
Focal Length
This is effectively the distance that a simple lens would need to be from the film/detector
in order to get a sharp image. The value is given in mm and only really applies
to single lenses. Most lenses for cameras actually consist of a number of
individual lenses working together. The individual lenses are reffered to
as elements. When you see the focal length given for a camera lens the number
is really the equivalent focal length.
But what does it mean?! It is a measure of how much the lens magnifies.
The bigger the number the more magnification. As a side effect, the bigger
the number the narrower the view it will give. Hence, when you want to take
a photo of a landscape or a building you'll often want a lens with a short focal
length (a wide lens) to fit it all in.
To make it a bit clearer here's the way lenses are often split down into ranges
for different uses -
- 0-30mm Very Wide Angle. Sometimes called
fish eye lenses. Make the image very distorted
- 30-49mm Wide Angle. Useful for landscapes
as mentioned previously. Much less distorted
- 50mm Normal. This is the focal length
which for a 35mm camera gives roughly the same view as your eye
- 51-300mm Telephoto. The range over which
most 'zoom' lenses operate. Make the subject look bigger.
- 300mm+ Super Telephoto. The sort of thing
professional wildlife photographers might use. Very big, very heavy, extremely expensive!
All of what I've said so far is correct for a 35mm film camera. If you're
using a digital camera things aren't necessarily right. The size of the film
or sensor that the light is being projected onto by the lens has an effect on how
much magnification you get from a particular lens. Fortuantely it's pretty
simple - if you half the size of the sensor you double the amount of zoom.
To keep life simple it is usual to talk about the 35mm equivalent focal length (35mm
efl for short). All this means is that if I have a 35mm film camera and you
have a digital camera with a sensor half the size of my film we can both take a
photo with 300mm efl lenses and get the same results. So I would use a lens
with an actual focal length of 300mm and you would use a lens of 150mm actual focal
length (x2 because the sensor is half the size of my film = 300mm efl).
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