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All that you need to know before you buy a lens:
Before you make a decision about buying a lens (and to understand how photos are affected by various settings), you need to understand the basics. If you don’t understand something, just keep reading as it will become clearer towards the end.
Types of lenses:
- – Prime lenses – fixed focal length (no zoom – just one angle of view). Why? Because you get very high quality for a relatively low price. Achieving the same quality with a zoom lens is a big challenge, so the price goes up.
- – Zoom lenses – pretty obvious (variable focal length, but lower quality)
Zoom lenses can be classified as with:
- Fixed maximum aperture (or f/ number) – the same amount of light falls in across the whole zoom range.
- Variable maximum aperture – amount of light falling in decreases with zooming in (getting close to the object, so faster shutter speed is required to avoid motion blur)
Wide - A wide angle lens is a lens which has a shorter focal length than that of a ‘standard’ lens, allowing a much wider area to be brought into frame. To obtain a wider view, the centre lens has to be closer to the sensor (or film if you are using a film camera) of your camera, which means the lens must be shorter in overall length. A lens with a lower focal length (in millimetres) is always wider angle than a lens with a higher focal length. For example, an 18mm focal length is wider angle than a 55mm focal length. Wide angle lenses for digital cameras typically range from about 8mm to 35mm.
Standard - A standard lens is one which is in between wide angle and telephoto. Standard lenses for digital cameras typically range from about 35mm to 80mm.
Telephoto - A telephoto lens allows you to zoom in on far away subjects. The telephoto range is the widest range of focal lengths, ranging from under a hundred to over a thousand millimeters. These lenses are also the longest and bulkiest of all lenses, and it is not uncommon for a telephoto lens to weigh a pound or more. Telephoto lenses for digital cameras typically range from 80mm to 1200mm or more.
Fisheye - A fisheye lens is an extremely wide angle lens which captures at least a 180 degree field of view, and takes distorted pictures which bulge outwards in the middle, similar to a fish eye. Fisheye lenses were originally developed for use in meteorology and astronomy, but became popular with photographers for the unique images that they produce.
Macro - A macro lens is a lens of any focal length (but typically standard or short telephoto) which allows an extremely close minimum focusing distance to the subject. True macro lenses also feature a 1:1 (life size) reproduction ratio. Macro lenses are used for extreme close-up photography.
Parameters:
Focal length – is the distance between the sensor (or film in film cameras) and the lens (the nearest piece of glass to the sensor) or how far you zoom in or zoom out, that’s why zoom is given in mm as it reflects the focal length, which is measured in mm.
Focal length at Wikipedia
Aperture / f-number: Size of the opening inside of your lens that regulates the amount of light falling in, and also, as a side effect, it regulates the depth of filed (area of sharpness in the picture). The higher the f-number, the smaller the aperture (like that makes our life easier, but f-number is a mathematical value of focal length divided by aperture diameter in mm). Aperture can be set both in the camera and on the lens (although many lenses do not have that option), but the change happens in the lens only.
Aperture at Wikipedia
Depth of field – the length (distance) which will be in focus in the photo. It depends on the aperture and the size of sensor. Compact cameras have very small sensors, so the effect is difficult to observe.
![[Depth of field]](http://www.starshipmodeler.com/tech/cj_photo_DoF.JPG)
At f/32, the background is distracting. Â Â Â At f/5.6, the flowers are isolated from the background.
Depth of field at Wikipedia
Sharpness—or the lack of it—is immediately noticeable when you look at a photograph. If you are making a portrait, you want only the person to be sharply focused, but not a distracting background. In a landscape, on the other hand, often you will want everything sharp from close-up rock to far away mountain. Once you understand how to control depth of field, you will feel much more confident when you want to make sure something is—or isn’t— sharp.To control depth of field, you have three factors to work with.
- Aperture size. The smaller the aperture, the greater the depth of field. The larger the aperture, the shallower the depth of field.
- Camera-to-subject distance. As you move father from the subject you are focused on, you increase depth of field. As you move closer, you decrease it.
- Lens focal length. Using a wide-angle lens or zooming out increases depth of field. Using a long lens or zooming in decreases it.
Each of these three factors affects depth of field by itself, but even more so in combination. You can get the shallowest depth of field with a lens zoomed in on a nearby subject using a large aperture. You get the greatest depth of field when you are far from a subject, with the lens zoomed to a wide angle, and using a small aperture.
Stop – this usually refers to one step between one f-number and another (just another way of describing aperture changes). Each stop up increases light intake by 100%, and each stop down decreases light intake by 50%. If in automatic mode, the camera will compensate this by slowing down or speeding up the shutter speed. Cameras and lenses can usually operate in steps of 1/2 or 1/3 stops.
Resolution – similarly like the camera, a lens has resolution, but it’s more difficult to measure and it changes between the centre of the lens and its edges. Kit lens might have resolution of around 10M pixels or less, while good lenses will have up to 50M pixels (don’t quote me on that as I’m not an expert). You won’t find any info on lens resolution on the box as it’s not that important and subject to debate.
Other factors related to the lens, although not regulated by the lens:
Shutter speed – how long the sensor is exposed to light while the photo is taken. The shorter the time, the less blur in the photo, but less light falls in, so you need to have more light on the object photographed. If the photo is still dark and you cannot slower the shutter speed, you can decrease the f/number and/or increase ISO. This is set in the camera and does not affect the lens directly, but your camera might decide to change aperture of the lens to reflect the shutter speed changes (depending on the camera mode currently in use).
35mm equivalent – this matters only when coupled with a camera. It’s a standard that tells you how big or small angle of view is. You can compare it to different currencies; to know the value of something in another currency, you need to know what the exchange rate is, but in if everybody used, for example, only Euro, you would not have to calculate. So in photography we use the equivalent, so we do not have to calculate it ourselves.
E.g. with some cameras 18mm lens will give you the same angle as 28mm with other cameras, that’s why we use the equivalent, so 35mm equivalent of 28mm is always the same on each camera, although real distance will be different (I know, the more I explain, the less you understand). 35 mm is the size of a standard film exposure; hence this has become the baseline for other sizes. Digital SLR’s usually have smaller sensors (with factor of 1.5 or 1.6 – so called APS-C size), but there are some full frame (35mm) DSLR’s – they are usually more expensive and for professional use. All Pentax DSLR bodies are APS-C, so 18mm will always be equivalent of 28mm on 35mm (18×1.5=28). Check Wikipedia article if you need more explanation: 35mm equivalent
Please, let me know in the comment if you need more explanation or if you need answers to some other issues.
Regards
Kamil
