OTHER SPECIAL LENSES

Macro Macro lenses can focus very close allowing real size, 1:1 image ratios, ie an object 10mm in size will appear 10mm on the 35mm frame. Excellent for nice close ups of insects or flowers.

Fisheye Lenses Distort the perspective to create a circular "fisheye" 180° image. A very specialised lens. Picking the correct subject is necessary but when you do can produce some memorable images. Focal lengths vary, 7~16mm.

Super Wideangle Lenses (<24mm) Like wideangle but more so, but not as much as the fisheye. Great for exaggerated perspective or scenes from restricted vantage point. Favoured lens of the estate agent!

Super Telephoto Lenses (>300mm) Longer telephotos and an eye-watering price tag to match. Can be heavy due to the amount of glass they contain. Often they have a tripod mount on the lens. You will need to tripod mount to reduce camera shake and weight of lens (unless you're after a work out!) Favoured by tabloid journalist when spying on celebrities!

OTHER TERMS

Fast Lenses A fast lens is one that has a large minimum aperture and is often a good thing. The minimum aperture might be f/1.4 or f/2.8 or whatever is appropriate for the lens compared to other lenses of the same focal length. Obviously the larger minimum aperture requires larger glass elements and is consequently heavier and maybe bulkier than a lens one or two slops slower. They are often higher quality as a side-effect of the lens maker justifing the extra expense.

Mirror or Reflex Lenses It is possible to make lenses using mirrors to fold and focus the light rather than glass and are also known as catadioptric lenses. Many telescopes are like this. The advantages of this type of lens are compactness and reduced weight. Long glass telephotos are big and heavy beasts. The reflex equivalent is compact and lighter making hand holding possible. Like big telephotos, they usually have built-in rear-mounted filters. Catadioptrics also produce characteristic doughnut shaped out-of-focus highlights, or bokeh, which can be quite pleasing.

Apochromatic Lenses An apochromatic lens is designed to focus three wavelengths of light, corresponding to the colours red, green and blue, onto the film plane. This reduces chromatic abberations, or the phenomenon of different wavelengths being focused at different distances or different point of the film plane. Chromatic abberation appears as coloured fringing around high contrasts objects typically a red fringe on one side and a purple fringe on the other. Normal lenses are called achromatic and they are designed to focus two wavelengths (red and blue) onto the film place and the designer assumes that everything between will be similarly focused. Apocromatic lenses are also designed to focus two wavelengths at the edges to reduce spherical abberations. Spherical aberrations show up as unfocused portions of the frame usually at the edges and at larger apertures. To achieve these feats some or all of the optics in an apochromatic lens are made from special (expensive) glass. Apochromitic lenses can be expensive!