Consultant's Corner, A Lens is a Lens...Right??

Peter H. Putman, December 26, 1999

As you go about researching literature and talking to dealers in your quest for the "perfect" projector, you'll hear plenty about brightness, contrast, weight, and signal compatibility. What you won't hear much about is lenses, and their importance in the chain of projector specifications.

It isn't by chance that some models of projectors make better images than others. Even the best projectors can be laid low by poor optics! A better understanding of lenses and how they work will help you in your quest. Let's take a closer look at some common questions about lenses:

Q. What's the difference between a zoom lens and a prime (fixed-focal length) lens?

A. A prime lens can only be focused - its focal length cannot be changed. Zoom lenses have several moveable glass elements, allowing their focal length to be adjusted over a specific range. If you use a projector with a prime lens, you must physically move the projector to make the image larger or smaller. This isn't necessary with a zoom lens - a simple twist of the barrel changes image sizes.

As a rule, prime lenses are optically sharper than zoom lenses, and are used in applications where the projector placement, projection throw, and screen size don't change. Zoom lenses are more useful in situations where the projector is moved around, projection throws vary, and screen sizes will change.

Q. Aren't most lenses similar in quality?

A. Not really. Lenses with premium multiple optical coatings will produce better colors and improved contrast. Premium lenses will have minimal spherical aberrations, a fancy term for flaws in the optical glass.

One common flaw - curved-field distortion - will show up as an inability to keep uniform focus across the image. The center may look sharp, but one or more edges may be soft. By the way, this can happen with both prime and zoom lenses!

If you want to be sure your lens is sharp, test it with a page of small text (12 point or smaller) and make sure the projector is square to the projection screen. Zoom the lens back and forth and look for even focus across the entire image. If you see a focus problem, try another model!

Q. Should I make sure my portable projector uses a zoom lens? Do I need a power zoom lens?

A. Yes, and no! It makes no sense to buy a portable projector without a zoom lens if you plan on using it in different locations. Moving a projector to change image size is unnecessary work and a pain in the neck! As far as power zoom goes, that's really a "bell and whistle" feature that does nothing to improve the performance of your projector, but will add substantial cost and mechanical complexity - something to think about if your projector gets damaged in transit.

Q. What do those numbers in the lens specifications mean?

A. Lens specifications (when you can find 'em) tell us two things: First, the focal length of the lens, usually expressed in millimeters (example: 50-75mm). Second, the aperture, or glass surface area that can pass light, usually expressed in f-stops (example: f2.5-3.0).

While focal length numbers may not be useable by themselves, the ratio of the longer number to the shorter number is very useful. In our example, the zoom ratio of 75mm to 50mm is about 50%, or 1.5:1. This means we can vary the size of the projected image by 50% without moving the projector. Larger ratios give us more control over image size, while smaller ratios mean less adjustments will be possible.

F-stop numbers are useful guides to relative brightness of the image when comparing two or more projectors with similar focal length lenses. (Did you get all that?) Simply put, a projector with a 50-75mm, f2.5 lens will produce images brighter than one with a 50-75mm, f3.5 lens. Some manufacturers will give an f-stop specification over the entire focal length of the lens, i.e. 50-75mm, f2.0 - 2.6. Make sure you compare apples to apples!

Q. What effect does zooming a lens have on the brightness of my images?

A. The light output from a zoom lens will vary as you make the image larger or smaller. The increase or decrease can be predicted by the zoom ratio. For example, there will be a 50% decrease in brightness when zooming from 75mm to 50mm, or about 1 f-stop for you photography buffs.

One caveat: You've probably also guessed that since image brightness varies with the setting of a zoom lens, the ANSI lumens rating will also vary. When I test image brightness on a given projector, I always conduct the tests with the zoom lens set half-way through its range. Most manufacturers do this, too (fortunately), although there may be a few that still "fudge" the readings and use the brightest zoom setting.

©1999 Peter H. Putman/PHP Communications

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