The Lenses...

In my previous post, i mentioned briefly about the flexibility a DSLR provides with the ability to change lenses. In this post, i will try and explain how i chose my first lens. But first, some basics about DSLR lenses.

Lenses are like the "eye" of the camera. The light enters the camera through the lens and falls on the image sensor after passing through some camera assembly. The lens is undoubtedly the most important part of the camera setup (yes, even more than the DSLR body itself!). The lens specification greatly affects what you can photograph and how good the result is.

Before buying a DSLR (and the lens with it) it is crucial to understand what the lens specifications means. I will try and explain it using the specs of my own lens: AF-S DX Nikkor 18-105mm F/3.5-5.6G ED VR

In this post, I will focus on "Focal Length" with subsequent specs in later posts (watch this space!).

Focal Length (18-105mm) : 

This is the most important specification of the lens. Not going into much optics (difficult for me as i'm from a physics background :D), what this number essentially means is the coverage the lens provides. Take for example, trying to shoot the Eiffel Tower. If you are close to the monument and can't step back any further, with a digicam it is nearly impossible to capture this stunning wonder in its full glory without cutting off the top or bottom. This is because the digicam uses a lens who's focal length is not small enough to provide a "wide angle".

On the other extreme, remember trying to zoom through the window of your hot neighbour and just falling short (ok, this is not recommended :D). Well this is because the maximum focal length of the digicam is not large enough to see far away objects more closely. This requires a "telephoto" lens.

How are these terms defined? The focal length which closest matches the magnification of the human eye is 50mm. Lenses with focal length less than 50mm are called "wide-angle" while those with greater than 50mm are called "telephoto". Lenses which have a variable focal length (like mine) are called "zoom" lenses and those with a fixed focal length are called "prime" lenses. Zoom lenses are very popular because of the flexibility they provide, however it is believed that prime lenses give a better image quality.

However, one important thing to remember in all this is the "crop factor". Let me explain. The numbers that i mentioned above essentially apply only to DSLRs with a full-frame sensor. These are DSLRs with a bigger image sensor eg. Nikon D3, Nikon D700 (much more expensive models $$). Most DSLRs (like my D90) have physically smaller sensors which crop the field of view. This in turn results in the "effective focal length" to be: the the lens focal length multiplied by 1.5 (1.6 for Canon). Hence, my D90+lens has an effective focal length of 27-157 mm. As you can see this can be advantageous for telephoto, but not for wide angle. (consequently super wide angle lenses like 10mm are extremely expensive!)

Because of the popularity of cropped-sensor DSLRs (cheaper), Nikon and Canon offer lenses which are only corrected for this smaller sensor. Nikon calls this lens series as "DX" (hence the DX in the name). Canon calls it EF-S. Disadvantage here is that these lenses won't work on a full-frame sensor DSLR.

Now as you must have guessed already, an 18-105mm focal range is a very convenient range for most photographic scenerios. It provides a decent wide-angle for landscape photography as well as a nice telephoto for zooming in distant objects without having to change lenses. Not to mention that this range is ideal for portrait and street photography. While buying the D90 i had to choose between:

Kit 1: Body + 18-105mm

Kit 2: Body + 18-55mm + 55-200mm (more expensive)

My choice was obvious. With Kit 1 i get the flexibility of range without requiring a change of lens for every other shot i take. With Kit 2, i would have to change lenses to get any telephoto whatsover.  Many newbie's jump on to the opportunity of owning 2 lenses without giving it much thought. Maybe after reading this post more people can make a better, well informed choice :)

The above 2 photographs compare the two extremes in focal length of my lens. Pic1 (above) is taken at 18mm (wide-angle) and Pic2 (below) is taken at 105mm (telephoto). Notice that i have zoomed into the black car in the carpark in Pic1 to shoot Pic2. Impressive huh?

In my next post, I will discuss about aperture, auto-focus and other lens specifications. These are equally important while choosing the correct lens for one's needs.