Focal Length

The use of lens is to focus an image to the sensor. Image is the subject you are shooting. Focus means to zoom in or out, sharpen or blur an image. Lens comes in different range for shooting different scenes. For this reason, the lens of a SLR/DSLR camera can be detached and changed.

Lens is mainly categorised by focal length, measured in mm. A lens may have a fixed single focal length (e.g. 50mm, 85mm). Or a range of focal length (e.g. 17-55mm, 18-200mm).

When I was making purchase for my camera with another friend, this guy at the shop showed us a few different lenses and told us many things which we don't understand. One of the thing he mentioned was crop factor which puzzled me totally. Anyway we just went WAH and WOW at the photos taken. I asked about what those mm numbers of the lenses means. He told me they don't mean much and asked me not to worry about it. How can?! I want to know what they mean and how it relate to the types of photos taken!

I came home and started reading on focal length. I realised it was something I've learnt in school. OMG! I've totally forgotten about it! So, it's back to school time.

As you can see from above, the distance between the lens and sensor is the focal length. This is what the mm numbers are all about. Next, we have to understand how different focal lengths affect the angle of view.

Take a look at the comparison diagram above. Given same distance between the sensor and the subject (tree), as the focal length changes, the angle of view also change. Short focal length gives you a wide angle of view (where you can see more than just the tree). Long focal length gives you a narrow angle of view (where you can only see the tree). In layman terms, zooming OUT means to shorten the focal length to focus a wide scene. While zooming IN means to lengthen the focal length to focus on a particular object in a distance.