camera lenses: prime versus zoom

I, along with many of my relatives and friends, belonged to the camp of buying only zoom lenses for our SLR (single-lens reflex) cameras. The bigger and weightier the lens, and the more powerful the zoom, the better. (Needless to say, you serious photographers reading along now realize I am still very much an amateur.) Anyway, in shopping for a new camera, I came across some advice about how prime lenses are better. How could this be? How can one frame a shot if one can't zoom?! Quoting Philip Greenspun's advice:

The cheap zoom lenses that come packaged with a Canon Digital Rebel or Nikon D50 kit are poor tools for building photography skills or making compelling images. The novice photographer who starts with a zoom lens typically uses it in lieu of backing up or stepping forward. An experienced photographer visualizes the scene first, chooses a focal length, then gets into the appropriate position to capture the scene with that focal length. It is much better to get a lens with a fixed focal length, learn to recognize scenes where that lens can be used effectively, and then add additional lenses once that focal length has been mastered. So even if you have the $1000+ to buy a high quality zoom and the muscles to lug it around it is probably a poor choice of first lens.

The $100-200 lightweight zoom lenses that come bundled with digital SLR bodies have some additional limitations. Being a good photographer starts with the ability to recognize a scene that looks attractive under its current lighting conditions. You need a lens with a wide enough maximum aperture, typically f/2.0 or f/1.4, to capture that scene even when the lighting is fairly dim, as it will be indoors or near the end of the day. An inexpensive zoom lenses will have a maximum aperture of between f/4.0 and f/5.6. At an aperture of f/5.6 you will need 8 times as much light to take a picture as you do at an aperture of f/2.0. Thus the cheap zoom will force you to use the on-camera flash, which casts an ugly flat light and in any case will render the scene completely different than it looked to your naked eye. Flash can be useful, especially with multiple strobes in the studio or as a fill-in light to reduce contrast outdoors, but mastering flash photography is a separate challenge.

The final problem with a cheap zoom lens is image quality. There is no free lunch in this world and when an optical engineer cuts cost and weight the sharpness and contrast are reduced. In fact, you might get a better quality photo with a point-and-shoot digicam than with a heavy expensive digital SLR with a cheap light zoom lens attached.

After doing a little more reading, I found that 50mm is the standard focal length for film SLR cameras and really expensive digital SLRs with full-size sensors, while 30mm is the standard focal length for digital SLR cameras with smaller sensors. (Just as a point of reference, standard zoom lenses that come standard with film SLRs zoom between 28mm (f/3.5) and 80mm (f/5.6).

Turns out a 50mm (f/1.8) lens for my trusty Canon EOS Elan II film SLR (which I've given to my sister) cost $80 -- really cheap considering the relative prices of lenses. The online reviews were right: it felt like cheap plastic but worked like a charm. Two samples using the 50mm prime lens (courtesy of my sis, click for larger version):


Yes, this is film! I was pretty surprised by the results -- almost magazine quality -- and my sis's nice composition of the photo helped too. (I realize this is not a controlled experiment by any means.) Compared with my previous photos using the zoom lens on the same camera, they definitely came out sharper than I'd ever seen before.

Now for my take-away point from this whole experience... If you have a zoom lens on either your point-and-shoot or SLR camera, zoom out as far as you can and use that as your fixed focal point. I justify this for three reasons:

1. When the lens is zoomed out, you obtain the smallest f-stop on the lens, which means more light is able to come through the lens. More light through the lens means that you need less light to take the photo, which then eliminates unnecessary use of the flash.

2. When the lens is zoomed out, you are less subject to blurring in your photos (especially in lower light conditions). Without a tripod, there will always be motion when you press the shutter release button. The more you zoom in, the more sensitive your photo will be to motion blur.

3. It's good practice if you plan on getting a fixed focal length (prime) lens in the future. As Greenspun says, it forces you to step backward and forward. However, speaking from past experience, you have to step pretty far forward to capture what you are eye is seeing at its normal point of view (50mm) when you are zoomed all the way out (about 28mm).

I'm not saying you shouldn't use the zoom -- it is a handy feature and it does help one capture shots that wouldn't otherwise work with a fixed focal point, but maybe the lesson should be "don't zoom in more than you have to, especially if you can take a step forward."