Anatomy of a LensOne of them ain reasons for getting an SLR/DSLR is o be able to change the leneses. Here is what you should consider when you go and look for the new glass of your dreams.
Lens Speed
The front element can be a good clue to the speed or a lens (but there are some exceptions), with this lens shown it has a very large front element so this would make it a fast f/2 or f/2.8 lens. These lens can let in more light to enable a much fast shutter speed or use in lower light conditions.
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Rotating front element
Some cheaper lenses have a front element that rotates as the lens is
fosused.The only issue with with is when you have a filter attached,
this can cause problems if you have a circular poloriser as as you
focus you poloriser filter will change the amount and direction, you
need to try to avoid these types of lenses if using filter is something
you want to try in the future.
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Filter thread
This is the thread that stands just in front of the front element
glass. This thread enables you to fit a filter to the front of your
lens. This will enable you to use filter like a Polariser lens, or ever
a Cokin screw-infilter holder. These threads range from as small as
48mm to as large as 83mm.
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Distance Scale
This is a useful for calculating Depth of field in order to maximise
front-to-back sharpness, and so is particuarly good for landscape
photography. Many modern lenses dont have this, but it is not accential
for a good working lens.
Minimum Focus length
How close you can get before you lenses reaches its near
focus limit if very important especially when you are considering doing Macro
photography. It is all very well getting a lens that offers a large zoom range
with a fast aperture, but if you want to take images of small close up shots
and you lens will not allow the camera to focus then you will find yourself not
being able to get the shot you want.
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Lens Hoods
Not all lense come with a hood (built in or bayonet fit)
so it is worth lookng that this as it could cost more at a later
date, or find hard to source
Fixed or extending lenses
Zoom lenses have to move the elements further apart from each other to create the zoom effect in the camera. To do this some more compact zoom lenses extends the inner barrel to lenghten the lens and gives a zoom effect this is called a "Trombone style" lens. This is something thing you need to remeber if you need a compact zoom for traveling as these lenses reduced length is much smaller than a fixed lenght zoom.
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Optical Quality
It's only by testing a lens in a camera shop and reading reviews that you'll know if a lens quality will meet your expectations. Sharpness is key, but so is the lenses ability to handle flare, Vignetting and optical aberation. Most of those letters after a leneses Focal length (APO, L, ED, ASP) arn't marketing gimmicks, they do actually signify better glass has been used.
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Lens fitment
You need to make sure you have the right fitment on you lens to fit the DSLR.SLR body that you are using. This night sound like a stupid ting to say as you might say "well mine is Canon or Nikon" but you have to remeber that even Canon and Nikon have changed there fitment over the years and older lenses might not work.
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Aperture
Another factor that determines whether a zoom lens is cheap or expensive is whether it keeps the same aperture throughout the zoom range or if the aperture gets smaller the further you zoom in on a subject. The downside of this kind of "floating" aperture is that in order to maintain the same exposure the shutter speed need to become slower as the aperture gets smaller.
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