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Monday, March 27, 2006

What's a 'Prime Lens'?

A "prime" lens is simply another name for a single-focal length lens.

A few other common lens terms:

A "zoom" is a lens of variable focal length that maintains its focus as the focal length is changed. Its counterpart is the "varifocal," which changes focus as the focal length is changed, and has to be re-focused before shooting. Varifocals were quite common in the early days of variable-focal-length lenses and are increasingly rare today, if indeed there are any left.

A "constant aperture" lens is a variable-focal-length lens with an automatically-adjusting aperture such that the set apertures (including the widest one) remain constant over the lens's entire focal-length range. The counterpart of this is the "variable aperture" lens, in which the lens opening stays constant and, thus, the ƒ-stop changes as the focus length is changed.

A "retrofocus" lens is a wide-angle lens designed to focus further back than a lens of the same focal length normally would, usually to make room for a reflex mirror. In extreme retrofocus designs, the optical center of the lens can actually be closer to the focal plane than the rear element of the lens itself.

A "telephoto" lens is the opposite, you might say, of a retrofocus lens: it is a lens the optical center of which is beyond the objective (outermost element). Put another way, when focused on infinity, the objective is closer to the focal plane than the focal length measurement of the lens.

"Long-focus" lenses are long-focal-length lenses that are not telephotos. Usage has confused this issue so much that "telephoto" or just "tele" is now commonly used for all long focal length lenses whether they're telephotos (most now are) or not.

A "spherical" lens is one in which all the elements in the lens have surfaces that are sections of a sphere, or are flat. An "aspherical" lens is one in which one or more element is not spherical. So-called "spherical aberration" occurs when the rays collected from the outermost edges of a simple lens cannot be brought to the same plane of focus as the rays collected closer to the center of the lens. Aspherical lenses are designed to correct this aberration.

"Depth-of-field" is the region on either side of the plane of focus in which the circle of confusion is small enough that the object can be mistaken for being in focus. Where exactly this limit falls is a judgement call. The higher the system resolution, the smaller the "acceptable circle of confusion" may be, making d.-o.-f. less. Also, curiously, most photographers are taught that d.o.f. is a function of aperture but not the equally important fact that d.o.f. is a function of focus distance: d.-o.-f. increases as focus distance increases. It's always amusing to hear people worrying that their lens may not be focusing perfectly on infinity—as if any lens could get infinity out of focus, for practical purposes, when set at or near its farthest focus distance.

Zooms vs. Primes
There are visual and conceptual implications to this comparison, but in physical terms, primes are usually faster and smaller than zooms that include their focal length, and can sometimes focus closer. They almost always require fewer elements, which in turn affects flare and veiling glare (the two main kinds of non-image-forming light transmission). Zoom lenses have made huge strides in recent years, to the point that a well-designed and well-made zoom lens can equal or surpass the optical quality of an average prime. Close-focusing with zooms has greatly improved over the past two decades as well. Generally speaking, a zoom lens is as large, as heavy, and as slow as the largest, heaviest, slowest prime lens its range covers, and is more susceptible to flare.

Quality: "Bunching"
The state of the art in camera lenses is still reached only by the best primes, and progress continues to be made in improving the SOTA. However, the increments of improvement have gotten steadily smaller, to the point that they are now mostly beyond having any appreciable practical value. Also, increasingly, the "practical quality" (i.e., image quality in actual picture-taking) of cost-constrained and size-constrained lenses, and zooms, are "bunching up" closer and closer to the high end of the scale of optical quality. The average today is very high indeed.



Anonymous Anonymous said...

Great and informative post. The link to this page has been put up on the "Gear & Equipment" section of

3:04 AM  
Blogger Mike Johnston said...

Thanks for the link, Daniel, I appreciate it.

3:09 AM  
Anonymous Anonymous said...

Incidentally might it be worthwhile to make a note that most digicam lenses are varifocal? i.e. IME generally with digicams the right order is to zoom/set focal length, pre-focus/recompose, shoot

With my dSLR I can use my Sigma 18-50/2.8 zoom at 50mm to set focus I want and then pull back to as wide as I like and the focus point stays put.

3:38 AM  
Anonymous Anonymous said...

The resolution of these explanations has exceeded the capacity of my scanner but that is my problem and I am up-grading. Thank you very much! -N8

8:12 AM  
Anonymous Anonymous said...

Great site! I greatly enjoy reading this blog each day.

In considering depth of field and the relevance of focus distance, one should keep in mind the hyperfocal distance of a lens:

Focus your lens at infinity and the leading edge of the area that is in focus is the hyperfocal point for that lens.

Focus on that point instead of infinity and you'll have the greatest range of focus from infinity back toward your location.

Prime lenses used to (some may still) specify their hyperfocal point. Calculating the hyperfocal point for a zoom lens is more complicated, as it will vary with the focal length. Cheat sheets, slide rules and even computer (and Palm) programs are available to help do this.

12:58 PM  
Blogger Motto! said...

The hyperfocal distance is not just a function of focal length but also aperture. I figured I should mention that since some may be confused.

Obviously the deeper the depth of field, the closer the hyperfocal distance is.

3:37 PM  
Blogger Doug said...

I don't know about other brands, but most of Canon's SLR "zoom" lenses are varifocal.

Here is an old list (almost 3 years old) of Canon's true zoom lenses.

Also, Canon's current "zoom" lenses usually can focus well beyond infinity in order to allow for temperature variations. When one of these lenses is "set at or near its farthest focus distance", infinity is usually quite out of focus.

11:55 AM  
Blogger Mike Johnston said...

"infinity is usually quite out of focus."

There's a name for lenses of which that might be true: they're called "defective." (s)


2:52 PM  
Blogger Doug said...

Mike, there's nothing defective about it. It's an intentional design feature that assures that the lens can be adjusted to focus at infinity even in extreme temperatures. On lenses with a distance scale, the "room temperature" infinity point is clearly marked, but it is not at the end of the focus range. This is described in the owner's manual for the lens.

Take a look at this photo. You can see the "infinity" mark in the focus window. It's got an L-shaped line extending to the left. The hook on the L (which is aligned with the white focus marker in this photo) is the room temperature infinity focus. The infinity mark is at the extreme rotation point.

Canon is not alone in this practice. Some Pentax lenses (at least the 400mm) also provide room for temperature changes. As do some Nikons (at least their ED telephotos).

1:20 PM  

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