In my last two articles I talked about why Minolta Glass is still popular and which of those lenses are still popular today. Before decided which lenses to buy to add to my two kit lenses I did some research on lens catogories. There are dozens of different types of lens available, designed for use in a wide range of circumstances. The most important factor in any camera lens is its focal length. This determines which type of lens it is, and what subjects it will be able to photograph. Focal lengths range from just a few millimetres up to over a metre, and can be loosely grouped as follows:
| Focal Length | Lens Type | Common Subjects |
|---|---|---|
| 8mm – 24mm | Ultra wide angle (fisheye) | Wide panoramas and skyscapes, artistic |
| 24mm – 35mm | Wide angle | Interiors, architecture, landscapes |
| 35mm – 85mm (50mm common) | Standard | General purpose |
| 85mm – 135mm | Short telephoto | Portraits, candid |
| 135mm – 300mm | Medium telephoto | Close sports, action |
| 300mm+ | Super telephoto | Far sports, wildlife, nature, astronomy |
Lens focal lengths are specified for a camera with a “full-frame” 35mm sensor. My Sony A65 use a smaller sensor, and this has the effect of cropping off the edges of the photograph, resulting in an image which is more “zoomed in” than it would be on a full-frame sensor. This cropping makes it seem as though the lens has a longer focal length than it really does. The Sony A65 has a crop factor of 1.5x, meaning that a 50mm lens actually has an effective focal length of 75mm when fitted to this camera.
The table below shows the focal length for a 35mm FF sensor, the APS-C (A65) equivalent and the smaller FF length required to get the 35mm equivalent:
| Category | FF mm | APS-C | APS=FF |
|---|---|---|---|
| Ultra Wide-Angle | 8–24mm | 12–36mm | 5–16mm |
| Wide-Angle | 24–35mm | 36–52.5mm | 16–23mm |
| Normal | 50mm | 75mm | 33mm |
| Short-Telephoto | 85–135mm | 127.5–202.5mm | 57–90mm |
| Telephoto | 135–300mm | 202.5–450mm | 90–200mm |
| Super-Telephoto | 300mm+ | 450mm+ | 200mm+ |
In addition to focal length there is another important measurement called aperture. Aperture determines how much light a lens lets in. When a lens has a wide aperture, designated by a small number called an f-stop (e.g. f/1.8), it captures more light. When a lens has a narrow aperture, designated by a larger number (e.g. f/5.6), it captures less. Wide apertures allows photos to be taken with less light because the lens can see more of the light. With a narrower aperture less light could pass through. Also, narrower apertures provide a greater depth of field, meaning that more of the image, such as a landscape, will be in focus. A wider aperture would make the landscape appear less sharp.
Now with an understanding of focal length and aperture as well as an understanding what different types of lens categories there are I was ready to pick additional lenses, more about that in my next article.
