Difference between revisions of "Lens"

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=== Related Lens Topics ===
 
=== Related Lens Topics ===
 
* [http://wiki.photoblogs.org/In_Praise_of_the_Standard_Prime_Lens In Praise of the Standard Prime Lens]
 
* [http://wiki.photoblogs.org/In_Praise_of_the_Standard_Prime_Lens In Praise of the Standard Prime Lens]
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Revision as of 00:32, 21 June 2005

The Lens

Fixed Lens Fixed or Interchangeable?

A Fixed lens is simply a lens that is permanently fastened to its camera as opposed to a system camera that allows you to change its lens easily. Fixed lenses are commonly found cameras aimed at consumers, from old box, TLR (the Mamiya C systems being the exception) and folding cameras, through the consumer range finders of the 70's and through to today's point and shoot film and digicams. There are certain advantages to having a fixed lens on your camera:

  1. As no mechanism for changing lenses needs to be built into the camera design it can help keep the camera smaller and lighter.
  2. In digital cameras a fixed lens means that there is less chance of introducing dust to the sensor surface.
  3. Fixed lenses are designed for a specific camera model and so less compromises have to be made in the lens design.
  4. Cost - if your camera comes with a fixed lens you don't have to worry amount buying a lot of additional glass to build a system.
  5. Portability - a fixed lens should be enough for most situations you encounter so you have less accessories to carry and you will waist less time changing lenses.

Interchangeable lenses are more commonly found on cameras aimed at professionals and enthusiasts including large format, SLR (medium format and 35mm) and high-end range finder cameras. The advantages to interchangeable lenses include:

  1. A larger range of focal lengths and specialties (shift, macro, etc.) are available than you are likely to find on any fixed lens camera.
  2. Each lens can be designed for a specific kind(s) of working situations and specialties without the compromises a generalist fixed lens has to be designed for.
  3. Longevity - you can upgrade your camera body without losing any investment you have made in additional lenses if your new camera choice is in the same family as your old camera.

Auxiliary lenses - if your camera has a fixed lens there are accessories available that allow you to enhance your fixed lenses range. These included close-up lenses that allow your camera to focus closer than it naturally can. They also include wide-angle attachments that allow your fixed lens to capture more of a scene than it otherwise could. They also include popular telephoto attachments that allow your fixed lens to reach further than it otherwise could. Telephoto attachments include extreme digiscoping lenses. Digiscoping is the practice of mounting a digicam on a spotting scope of telescope to create extreme focal lengths.

As with all photographic equipment, auxiliary lenses range in quality from the truly dreadful to the professional. Bear in mind that any auxiliary lens that you attach to your fixed lens is adding more glass between the subject and the film. As such it is bound to affect image quality and the amount of light passing through to the film plane. Cheap auxiliary lenses add horrible distortion and purple fringing to your shots. Auxiliary lenses are a compromise solution to extending the range of a fixed lens that can provide good results but there appear to be no bargains in this niche market place and you will get what you pay for,

Zoom or Prime?

In general, non-zoom lenses let in more light than zoom lenses, which allows you to take photos in more difficult situations.

Contrary to what many people think, Zoom lenses have no effect on perspective. All they allow you to do is to modify the cropping before you take the shot. This can add great flexibility, nevertheless. Zoom lenses are bulkier than fixed lenses, and they introduce an extra adjustment you must make before taking the picture. The vast majority of digital cameras come equipped with Zoom lenses.


Lens Speed

Lenses that let in a lot of light are called 'fast' lenses. This quality is indicated by a number which is called the maximum aperture or maximum f stop. The smaller the number, the faster the lens. A very fast lens will have a maximum aperture of f2.0 or less. As lenses get faster than about f2.0, they become much more difficult to make, much larger, and much more costly. All very fast lenses are interchangeable lenses, so if this will be important to you, you will need an interchangeable lens camera. Very high speed would be important if you wanted to take photos in dim light without flash, and without a tripod.

Depth of field

The depth of field is a way of describing how much of your image is in focus. When the camera is focussed at a certain point, it will remain in focus rof objects slightly in front of that point, as well as slightly behind. The distance between the closest object that is in focus, and the most distant one, is the depth of field. The depth of field is dependent purely on the geometry of the lens, and cannot be changed by the manufacturer. Generally, the shorter the focal length of the lens, the greater its depth of field. Since all small digital cameras have lenses with very short focal lengths, they tend to have very large depth of field. This has many benefits, and generally makes the job of the autofocus mechanism much easier. On the other hand, certain aesthetic effects become more difficult when the lens has too much depth of field. Finally, the faster the lens, the lower the depth of field. This means that while using a very fast lens will allow you to photograph in dim light, it will be very difficult to adjust the focus when you do this. Moral: if you think you want a very fast lens, you will pay for it in cost, weight, bulk, and poor depth of field. Zoom lenses have more depth of field when set to their shortest focal length, then when set to the longest.

Angle of view

In general, the shorter the lens, the wider it's angle of view. It's much easier to get very wide angle lenses for film cameras than for digital, unless you are talking about very expensive digital cameras. Many digital cameras suffer from a lack of wide angle ability, and if that's important to you, that will affect your choice. If you must have very wide angles, you will need to get a camera with interchangeable lenses, whether film or digital. Some lower cost digital cameras can be fitted with add-on lenses that increase their angle of view. In 35mm cameras, a 50mm lens is said to give an angle of view similar to the human eye, though many folks dispute this. nevertheless, this has come to be called a standard lens, for 35mm cameras.

Zoom ratio

The zoom ratio is the ratio between the lenses shortest focal lenght,and its longest. The majority of modern zoom lenses are about 1:3, meaning that their longest focal length is 3 times the shortest. For example, there are many 35-105 lenses available. As the ratio gets bigger, the lens becomes much harder to manufacture, and more expensive. Some modern digital cameras have zoom ratios of 1:10, or even 1:12. It may be that such a camera could lessen the need for interchangeable lenses, and perhaps these will become more of the norm. Currently, they represent the bleeding edge of consumer optical technology.

Frequently, lenses for digital cameras are labelled with the focal length they would have if they were 35mm cameras. This gives a way of comparing zoom ratios between film and digital cameras. In any case, divide the larger number by the smaller. If the result is less than 3, then it's unimpressive. If it's about 3, then it's a normal, conservative design. If it's much greater than 4, some testing might be in order.

Distortion

This is when the lens represents straight lines as bent. Often, especially with zoom lenses at the wide end of the zoom range, straight lines at the edge of the frame will appear slightly curved. There seems to have arisen a kind of accepted dogma that this is a bad thing, though nobody seems to explain why. Let's just say that if you think that's a bad thing, then check carefully for that behavior in any camera you are choosing. In the majority of photographs, it simply doesn't matter. Once again, it seems to be difficult for lens manufacturers to achieve very low distortion in conjunction with all the other good features they want their lenses to have.

Related Lens Topics