There is a lot of literature available about this type of lens, but several people have asked me about them recently, so I thought I’d share my experience. The ideas below are definitely not an “expert” opinion, just some hard learnt lessons which I admit are partial and limited, but I hope of some use to digital SLR owners.
1. Lens focal length is measured in millimetres from the sensor to the aperture diaphragm. Wide-angle is considered anything below 30 mm. The latest fisheye from Sigma has a focal length of just 8 mm.
2. The sensor in most digital SLR’s is smaller than a 35 mm exposure and is expressed as a proportion known as the “multiplying” or “crop” factor. The crop factor varies from 1.3 X to 2X. For example, the sensor in an Olympus dSLR has a crop factor of 2X, which increases the focal length of a telephoto lens (a 200 mm offers the same focal length as a 400 mm on a full-frame camera), but also reduces the field of vision of a wide-angle lens by the same proportion. A 24 mm lens is therefore almost the same as a 50 mm on a full-frame if your camera has a crop factor of 2X. A wide-angle lens is thus not as wide as its rated focal length would have you believe.
3. Some lenses are specifically designed for smaller sensors and will
vignette badly on a full-frame or conventional camera. Check with the vendor before buying.
4. Wide-angle and fisheyes distort the edges of your image – sometimes to extremes. It’s advisable not to place human subjects near the edges of your shot if you want them to look vaguely normal.
5. Wide-angle lenses usually require large (i.e., expensive) filters.
6. It is almost certain that your wide-angle/fisheye is ‘soft’ at its widest aperture and shortest focal length. Post-processing will therefore be necessary and may not overcome this problem. If possible, try to use the lens’ sweet spot.
7. Both wide-angle and fisheye lenses are particularly prone to
flare, i.e., coloured rings or specks reflected from a light source. A lens hood (or shading with your hand) may partially help when using a wide-angle lens. There is no solution for fisheyes other than post-processing.
8. Wide-angles and fisheyes are also more likely to suffer from
chromatic aberration: coloured edges or “fringing” which can be corrected using free software such as the Adobe Camera Raw plug-in, etc.
9. You can have a lot of fun with fisheyes and their extreme distortion, but remember that the photographer’s feet or elbows, etc., can sneak into the image. Also, always have a blower to hand. The final lens element protrudes from the barrel and therefore collects dust.
10. Both wide-angle and fisheye lenses offer many exciting ways of expressing your photographic skills, but will make more demands on your post-processing abilities. There is, however, one golden rule:
try before you buy. Check for vignetting, CA and visible softness in the lens’ sweet spot before you part with your hard-earned cash. Reputable camera dealers usually allow this.
Brand names are quoted here for information purposes only and are not included as an endorsement of any kind.
