Of Focus, Focal Length and Optical Zooms
You see someone sitting across the table because you focus on her. If you don't focus enough, you can see right through her, even right through the walls of the restaurant, wherever your fancy takes you. Since cameras are an imitation of the eye they also can see something only if they focus on it. If a camera is able to see everything right from a vase at a meter's length to the point where the land meets the sky with uniform clarity, the focus is set to infinity. But if a camera has a specific focus on an object situated anywhere in the visual spectrum, everything else assumes secondary importance and is viewed less vividly. Say you want to photograph your group of friends surrounding a table in the restaurant. You require a lens which has a wide field of view so that it can include in its focus objects in the immediate vicinity. A lens that allows you to do this has the appropriate focal length for such a range, and is referred to as a 'wide angle' lens. Typically, for focusing closer and wider, a lens with a focal length of 35mm or less is required. Lesser the number, wider the field of view of the lens. But what if you want to capture a bunch of ferries 500 meters across the creek? A parakeet perched at a tall structure? You probably need a lens with a focal length of 100mm or beyond. Does that mean you need different lenses for shooting objects at different lengths or fields of view? Yes and no. Yes, if you are dealing with a lens of fixed focal length. No, if you are using a lens which can change its focal length, referred to as a 'ZOOM LENS". A zoom lens can achieve different focal lengths by moving various optical elements in relation to each other and the censor mechanically. A lens normally consists of a group of lenses having different optical qualities. So a 35mm-105mm zoom lens can have a wide field of view able to include a bunch objects in the immediate vicinity when set to a focal length of 35mm. When set to 105mm, the same lens will be able to fill the field of view with the objects at a distance from, say the ferries which are 500 meters away. In effect, this lens will have a zoom factor of 3X (35 X 3 =105). Thus a lens can have a zoom factor of 2X to 20X or even beyond. Remember, this is the only type of zooming which is possible within the laws of physics and it is called Optical Zoom.
But haven't you noticed cameras proudly wearing tags of Digital Zooms which purport to extend your reaches to the moon and even beyond? Digital zooms of 5X, 500X or even 5000X ? This is the first thing you should be wary of when choosing a camera. Digital zooming is the biggest hoax that was set upon the unsuspecting masses by the manufacturers of cameras. Digital zooming is simply increasingly closing in at the center of the picture achieved by the optical prowess of the camera. Most of the times, the inevitable loss in quality is glossed with introducing artificial pixel elements in the picture too. Thus, any picture you see can be zoomed to infinity and before long you will be left with blotches of colour and you do not require a camera to that. Stick firmly to what your target camera can perform optically. So hereon, whenever I'll talk of "zoom" it will imply "optical zoom".
About 3x optical zooms are standard on current day consumer cameras. 6X zooms are sweeter. there are models which offer 18X zooms but at these magnifications camera shakes will pose huge problems, but that is a subject we need to return to later. What I'd like to dwell on at the moment is where exactly should the focal length of a camera start.
Human field of view corresponds roughly to that produced by a lens with a focal length of 50mm. That is the reason why a lens having that focal length is called a 'normal lens'. A lens having a wider field of view will be able to include more to the immediate right and left. A lens having an extremely wide field of view, say having a focal length of 10mm or even less, is called a 'fish-eye lens'. On an average, the focal length of fixed lens zoom cameras starts at 35mm as it is both expensive and complicated to produce lenses of variable focal length starting at wider angles. While 35mm may be good for shooting a group of 6 to 10 persons from a distance of 3 to 4 meters, it may get tighter if more people were to assemble horizontally or more view of the area were required. Similarly, if you were shooting landscapes you may find the vista to the immediate right and left out of the frame. The only way to include more people or more of the scenery in the frame would be to move backwards, which will not always be possible. A good point to start is 27mm which will put that extra room in the frame, and the landscapes and party photographs will start having a wider canvas. Anything wider is a bonus, but that is where 'barrel distortion' starts creeping in, and we can talk about that later.