AboutSteve Pearson Expertise Try me for 35mm or darkroom issues. I am a professional photographer in Australia. I work solely in 35mm digital format, after 35 years of colour and monochrome film in many formats. I have darkroom experience and for many years belonged to my local camera club. I conduct photography workshops around the state on any and most subjects relating to photography and often judge photography exhibitions and competitions.
Experience I hold a Diploma of Applied Science in Forensic Investigation, with part of my studies involving photography. I spend most days with a camera and am into digital photography in a big way. I was given my first camera in 1965 and have not been without one since. I spent nearly 20 years as a forensic investigator with my state Police Force, and since retiring a couple of years ago have concentrated on commercial photography - especially weddings.
Expert: Steve Pearson Date: 7/13/2005 Subject: lenses
Question ,What does it mean when a lense is labled f/4.0-5.6 30-80mm AF? ( What do all those specifcations relate to, like can you only stwitch between those two f-stops or what?)
2, Is a telephoto lense just a really big zoom?
3, Whats a wide angle lense?
Thanks.
Answer Trevor. Thanks for your question/s.
An f/4.0-5.6 30-80mm af lens is an auto-focus(AF) zoom lens (30-80mm) that has a maximum aperture of f4 when set at 30mm and a maximum aperture of 5.6 when set at 80mm. The 'f' refers to the maximum aperture at the particular zoom setting. It all becomes an exercise in fractions. For the f4 @ 30mm, you divide the 4 into the 30 to get 7.5. That 7.5 means that the aperture (or opening in the lens to let the light in) is 7.5mm when the lens is set at infinity. Conversely, if you divide 80 by 5.6 you get 14.285 which means that the maximum aperture is about 14mm when set at 80mm. You don't switch between these two f stops. The maximum aperture available will vary between the two, depending on the focal length you set. Of course you can set the aperture, but it will be no bigger than f4 when at 30mm, and no bigger than f5.6 when at 80mm. These maximum f stops also give you the rated 'speed' of the lens. If you could get an f/2.8-f4 30-80 lens, you would have a much faster (and more expensive) lens than the one you mention. It's a bit like film ISO ratings. The 'faster' the film - say ISO 800 - then the less light it needs to work in successfully. A faster lens - say f1.4 - also needs less light to work in successfully than an f2.8 lens. Confused? Hope not.
A telephoto lens is normally a fixed focal length, as opposed to a zoom lens. A telephoto lens can be 130mm, 500mm or similar. Nowadays however, telephoto is commonly used to refer to zoom lenses.
A wide angle lens is one that has a focal length shorter than the focal length of a lens for the same camera that gives a 'normal' view. Most 'normal' fixed lenses for 35mm cameras today are 50mm. That is very close to what is a normal view of 43.26mm. This is arrived at by using Pythagoras theorum. A 35mm negative is 36mm x 24mm. The 'normal' focal length for that is the diagonal. So, to get the diagonal we use Pythagoras formula of 36 squared (1296) + 24 squared (576) = 1872. The square root of 1872 is 43.266615305567871517430655209646 or about 43mm. Simple when you know how. Therefore a 35mm focal length lens is a medium wide angle. A 28mm focal length length is a short wide angle lens. Once you go beyond 28mm (say 24mm) you are getting into really short and really wide angles. You are also getting a lot of distortion. If you are doing landscape work and don't keep the horizon in the centre of the shot, it will be curved. The shorter the lens, the greater the curve.
