How to choose the lens for your SLR

Lens is the eye of your SLR camera. You may have an SLR camera of any make, it may be manual focus or autofocus, analogue or digital but the quality of the captured image essentially depends on the quality of the lens that you are using. So carefully choose the right lens for your SLR camera. These days utmost care is taken by the manufacturer to make lenses for the SLR cameras. But there are some differences in optics and other features. There are some simple rules that you must keep in mind while buying a lens.
 

  1. Focal length
    By focal length, we can divide lenses into categories such as Fish eye, Ultra wide angle, wide angle, normal, medium telephoto, super telephoto, macro, etc. You have to decide which focal length you want to buy according to your need. Normally, to start with an SLR body (Digital or Analogue) you need at least a wideangle, a normal and a telephoto. The cheapest option here is to buy a zoom (wide to tele). For full frame DSLRs or analogue SLRs 29~105mm & for APS sized sensor DSLRs 18-70mm is the ideal.
     

  2. Maximum Aperture:
    By the maximum aperture value of a lens, you can actually judge the quality of the lens. For normal lenses, 50mm f/1.2 is considered to be some of the best lens by the manufacturer but with wide open aperture it will give you an extremely narrow depth of field and it is very expensieve. So I suggest you  to go for a 50mm f/1.4. For wide and telephoto f/2.8 is considered to be pretty good. But for the zoom lenses this maximum aperture is a bit fuzzy. Let’s consider two lenses:
    i> 28~105mm f/2.8
    ii>28~105mm f/2.9~4.
    In the first one the maximum aperture throughout the zoom range (i.e. 28mm to 105mm) is f/2.8 only. In the second one, at 28mm the maximum aperture is f/2.8 but when the lens zooms to 105mm the maximum aperture avalue decreases to f/4. Actually it decreases gradually as focal length varies. Between these two lenses obviously the first one will be much more expensive than the second one because the first one has much superior optics. So the image quality will differ.
     

  3. A few words on coating:
    Coatings are treatments applied to the outer surfaces of lens elements to reduce reflections. They are a layer (single coating) or layers (multi coating) that are a fraction of a wavelength of light thick.
    Single coating: dimmer blue or sometimes amber reflections when looking straight into the glass
    Multi coating: much darker reflections of many colours, especially green or dark red.
    Hint: multicoated Schneider lenses are usually marked “MC”.
     

  4. Special Lens Elements:
    In the professional quality lenses, the manufacturer incorporates special quality optics, which make the lenses very expensive. To make wide angle lenses every manufacturer uses Asperical lens elements tto avoid distortioins in wide angle. Aspeherical lens is just a hybrid lens made with glass and plastic. As regards distortion, ideally straight lines would stay straight in a photograph. In reality they often curve a tiny bit in or out when they run along the sides of an image.
    If they curve out from the center of the image they are called “barrel” distortion, as theses lines would mimic the shape of a wooden barrel.
    If they curve inwards, this is reffered to as “pincushion” distortion, since the shape mimics a pincushion or pillow as seen from above.
    Another distortion, which is very disturbing in colour photography is called chromatic aberration. Let us not go very deep into the scientific aspects of chromatic aberration. This chromatic aberration prevents us to achieve exact colours and sharpness. This was last seen in photography with fast telephoto lenses and was cured with Nikon’s ED glass. These days every lens manufacturing company has used this kind of low dispersion glasses in their lenses. Nikon says it ED and ED ll, Sigma named it PO and according to Tamron it is LD. So the bottom line is that if a lens consists of ED and Asperical elements its picture quality has to be superb. Distortion and aberrations will be minimum in that lens.
     

  5. Number of Diaphragm blades:
    The number of diaphragm blades in the lens actually makes a lot of difference to the capture. Most professional lenses have 9 blades in the diaphragm. It makes the diaphragm like a circle. Lenses other than professional one have only 6 to 7 blades in their diaphragm. The more round the diaphragm opening is, the better the “Bokeh” will be.
    Bokeh, from Jaanese boke, means unsharp. The appearance of the out of focus areas of a phtoographci image. Also, the rendition of out of focus image areas by a particular lens, i.e. a lens with a pleasing out of focusi renditions is said to have “good bokeh”. Actually speaking the out of focus highlights takes the shape of the diaphragm opening. If the diaphragm is a 6 blade one the out of focus highlight will be like the shape of a hexagon. 
    The lenses with 9 blades in their diaphragm are very costly but your images will be better. For super telephoto focal lengths cheaper mirror reflex lenses are alos available. These lenses give pretty crisp images but with theses lenses only you can achieve the ‘doughnut’ shaped out of focus highlights.
     

  6. Special purpose lenses:
    There are some special purpose lenses like wide-angle PC (perspective control), Macro TS(tilt shift) & fisheye (super wide angle). Though these lenses are very expensive one has to buy them to do special kind of photography. While buying a macro lens always remember to buy a lens with a macro ratio of 1:1,
    If you have an APS sized sensor DSLR (not the full 35mm sensor), do know the multiplication factor of the focal length written in your camera manual. This will help you to find correct lens for your DSLR.
    when buying a lens always look after the supplied accessory with it like lens-hood, front cover, rear cover, carrying puch etc. always use Multicoated filter in front of a lens because normally available cheap filters deteriorate the image quality.
     

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