Lenses: Focal Length
One of the benefits of owning a DSLR is having the impudence to change lenses according to your needs. We’ll look at the basics of what a lens is all close by from focal length, to hole, to demystifying all those strange symbols. Today, let’s look at central length.
Focal LengthWhen upon rays hit a lens, they converge at a individual focal point. In it’s simplest brains, the distance from the middle of the lens to where the lines meet is the focal length of the lens denoted in mm. Lenses traverse from 10mm at the wide end up to 300mm for a consumer telephoto lens.
There are two types of lenses:
Prime - These lenses entertain a fixed focal length. As such, they almost always have a larger aperture than zoom lenses. Some acclaimed lenses include 30mm f/1.4, 50mm f/1.8 or f/1.4, 85mm f/1.8 or f/1.4.
Zoom - A zoom lens has the gift to vary its focal length and as a consequence, can give you different angles of views.
Lens manufacturers grow many different ranges of convergent length but they usually collapse under three categories:
Encyclopedic Angle - Lenses wider than 50mm (eg. 10-22mm, 14-24mm)
Healthy - 50 mm
Telephoto - Lenses longer than 50mm (eg. 70-200mm)
You may bear seen the words “35mm match” when talking around focal length. This is unreservedly a comparison to 35mm film cameras, not to be screwed-up with focal length of a lens. (The film utilized in 35mm cameras were, surprise shocker, 35mm wide!)
So why do we need to disorder with knowing the 35mm (full construction) equivalent? It’s used more as a sample so we can make comparisons. Most DSLR’s own a crop sensor (except for rich frame DSLR’s like the Nikon D3, Canon 1Ds and 5D) content compared to a 35mm camera, we only see a crop of the verified field of view. Different camera companies use disparate crop sizes. The crop value is referred to as the Central Length Multiplier(FLM). My Nikon D300 has a crop lender of 1.5. This means that compared to a filled frame camera like the D3, I see 50% less with my D300 than I would with a D3.
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