A S C R A P B O O K O F S O L U T I O N S F O R T H E P H O T O G R A P H E R
Reviews of photography products that enhance the enjoyment of taking pictures. Published frequently but irregularly.
14 January 2013
Last week a reader asked for a simple definition of a prime lens. That didn't seem to be too much to ask, so we hit the Reply button and ... froze. Simple?
Try answering that question without resorting to any jargon. You can't use technical terms like "focal length." You have to put it in plain English.
It's remarkable that in his 1980 volume on The Camera, Ansel Adams didn't use the the term (as far as we can tell) in his chapter on Lenses. And even today, in NK Guy's The Lens, the term is used but not defined.
A prime lens is one that, like your eyes, doesn't zoom.
If you want to zoom with a prime lens, you do it with your feet. Get closer, move back. Zoom, zoom.
A prime lens used to be everyone's first lens. You'd buy an SLR and pick a prime lens, typically a 50mm (except for Pentax, where a 55mm was favored, or Konica, which offered a wider angle than the standard 50mm).
But everyone's first lens today is a short zoom. When you buy a dSLR, it comes with an 18-55mm kit lens. Or, if you want the whole hand-holdable range, you opt for an 18-200mm kit lens with everything in one lens you never have to remove. Why would you need a 50mm lens? You can hit 50mm in either kit lens.
Well, there are reasons. Good ones.
ONE GOOD REASON
In Adams' day, zoom lenses were more famous for their compromises than their quality. He warned:
Unfortunately, some zoom lenses do not provide the highest image quality; I recommend careful checking at all focal lengths and focus distances before purchasing one.
And while zoom lenses have come a long way in the multi-coated, computer-designed and manufactured era, trying to be everything to everyone is still a good deal harder than doing one thing right.
So what's the one thing primes do right?
Thanks to a simplified optical design, one thing primes do right is provide much larger apertures than zoom lenses can. A zoom may be able to open up to f2.8 at wide angle but, as the barrel extends, end up at f5.6 at full telephoto. A prime can sit there at f1.4 all day.
So if natural light photography is your game, you'll be very happy working with a fast prime lens.
'NORMAL' HUMAN VISION
Primes have a few other advantages.
You may give up the ability to compose your image by zooming, but with a prime, you select the angle of view you want to capture. Adams explained it this way:
A "normal" lens is defined as one whose focal length is about equal to the diagonal of the film format. Such a lens will have an angle of view of about 50 degrees to 55 degrees, comparable to what we consider normal human vision.
So lenses with a focal length of less than 50mm (or the 35mm equivalent of 35mm if we're not talking about a full-frame sensor) show us more in the frame than normal human vision would take in. And lenses that are longer than 50mm show us less.
That, as it turns out, makes some focal lengths better for some things than others. For example:
- If you want to do some discrete street shooting, a 35mm f2.8 prime is going to be very forgiving.
- If you want to capture roughly what you're seeing, a 50mm is going to see the back of the head of the person in front of you just the same way you do. Or the car crossing the intersection. Or the building across the street.
- If you want a nice portrait that 85mm moderate telephoto will give you some breathing room between you and your subject while defocusing the background to nicely put the emphasis on your subject.
- If you just happen to be on safari, hear a roar and think better of getting off the bus to chase a predator, that 400mm telephoto is going to be your friend. And may even save your life, come to think of it. It will go where you should not.
Or as Adams put it:
A short-focus lens makes possible exciting near-far images, exaggerating the differences of subject scale and depth. A long lens favors more accurate "drawing" of the features of a portrait, and gives a quasi-abstract impression of two-dimensionality with distant objects. On the other hand, some photographers prefer to use only the normal lens (sometimes for reasons of economy!), and visualize all their images in reference to its properties of focal length and angle of view.
Primes are also lighter and more compact than zooms. You can carry a couple of them in a small camera bag. You can work with one on your camera all day. Some very small wide angle pancake lenses make it seem as if you don't have a lens on your camera at all.
Primes tend to enforce a discipline, much like writing a sonnet instead of free verse. When you pop that 28mm wide angle lens on your camera, you look at the world differently than when you use a 135mm telephoto. You sweep, you don't peer. You see nearby things in relation to their surroundings not a blurred background.
But they also inspire a new perspective. Shoot with a 50mm for one day, then try a 35mm the next and you'll be thinking about your photography in a different light.
It's a different way of seeing. Which is why Adams observed at the very beginning of his chapter on lenses that "there is something magical about the image formed by a lens."
Adams, as a member of the f64 Group, championed sharpness. But his mentor Dassonville felt quite the opposite, looking for the dreamy look of soft images.
We're not comfortable in either box. We've seen images whose sharpness was indeed impressive but which actually distracted us from the subject. And we've seen soft images in which we really wished a little more information had been conveyed.
Sharpness, then, is an attribute not a goal. It's a tool in the photographer's belt to be used as you see fit. Blur the background on a portrait. Use a selective focus lens to draw attention to only a part of the scene. Dramatically sharpen some texture to raise the hairs on your viewer's back. You decide.
In that light, primes have always been among the sharpest lenses. But zooms are no long far behind with modern multi-coated surfaces and special glass. They're sharp enough to let you play with sharpness.
A REAL DEFINITION
So the real definition of a prime lens is that, unlike a zoom lens, it's a lens with only one focal length. It's a 50mm prime lens, not an 18-55mm zoom lens. It will have a wider maximum aperture than that zoom lens so you can work in lower light and at faster shutter speeds. And it may even be noticeably sharper.
When you tire of your zoom, consider a wide-angle prime or a short telephoto prime with macro to recapture the magic of making images.
If I am reading you right, with an 18-55mm lens zoomed to 55mm, I would be in the normal range of human vision -- is that right?
-- Richard Banks
Thanks, Richard. Yes, roughly speaking, that's right.
One could argue that on a subframe camera that 55mm is really more like 82.5mm (Nikon) or 88mm (Canon) so already into telephoto range, but the assumption in the article is that we're talking about a 35mm frame. -- Mike