Fixed vs. variable focal length lenses.


No longer a newbie, moving up!
Jan 26, 2006
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Tottenville, Staten Island, NYC USA
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Or 'prime' vs. 'zoom.'

I've seen many references to both types of lenses recently. Perhaps it's time to mention a few of the strengths and weaknesses of both. [Please note that the following statements are not absolutes. I'm sure, for example, that you can find a specific fixed focus lens [Diana] that isn't as sharp as a certain [Vivitar] variable focus lens.]

Fixed focus lenses are sharper and have greater apertures than variable focus lenses. This can be important in low light situations or when maximum sharpness/image detail is important.

Variable focus lenses provide the ability to 'crop' without moving camera position. They also provide the ability to vary the relative size of a distant object to a nearer object without changing the size of the nearer object on the film/sensor.

The choice of the 'ideal' lens type depends on the particular photographic situation. In low light where longer exposures are not an option, the fixed focus is probably the one to use. In situations where camera movement is restricted, the variable focus may be the best bet.

I tend to use fixed focus lenses. However, if faced with the choice of improving a composition with a variable focus lens as opposed to an increase in sharpness with a fixed focus lens, I'll use the variable.
I might add that evaluating DOF tends to be slightly trickier with zoom lenses because they exaggerate the z-axis (forward or backward) distance between objects in the field of view when you zoom.
The very best fixed focal length lens will outperform the best zoom.That being said they have evolved to the point that the best zooms will outperform anything less than the best fixed length lenses.

My 80-200 2.8 EDIF Nikkor AF is only inferior to my 300 4.5 EDIF Nikkor AF by the slightest amount.

Primes can be 'faster' glass and free from the degrees of barrel & pin-cushion distortion found with do-it-all telephoto-'macro' lenses.
From what I've seen/read, repeated testing by multiple sources seems to suggest that even cheap primes, such as the standard 50mm f/1.8 in most name brand lines (usually under $100) will outperform the top zooms of the same brand in resolution of small details, contrast, and controlling diffraction and flare. These tests have been done in the last year in almost all of the major photography mags. I know that Pop Photog just did one comparing the top of the line Canon L zooms to their $70 50mm f/1.8, and the cheap prime beat the $1000+ zooms.
It depends on the job. If I need f/2 or less, then I have to go with a prime, because I don't (and probably won't ever) have a zoom that fast. So I have to make due with the focal lengths I have in primes. I tend to use my zooms at only a few focal lengths anyway, and I still shoot many cameras that don't have zoom lenses as an option, so it's how I work. As long as I can buy 3 faster primes for less than 1 fast zoom, I'll probably go with the primes.

It's almost never an issue of sharpness vs. composition for me. Whatever lens I am using, the act of hand holding is the great equalizer. I'll admit that to see the difference in sharpness/fine details between fancy zooms and primes I'd probably need a tripod, mirror lock-up, time to make everything perfect, a good loupe, etc... And if I'm going to do all that, then the 20D goes back on the shelf, and I break out the big film gear (and I don't have a single zoom for any of my medium format or 4x5 gear).

Zooms are more convenient. If the world went my way, I'd have a bag full of primes, and always have the time to change to the lens I need. Unfortunately I do find myself in many situations where I know I don't want to be fumbling around switching lenses everytime I want to go from wide to tight. When I'm on a portrait shoot I usually use primes; there's no rush, and I've probably had the opportunity to think about the shot beforehand. When I'm wandering around shooting candids at a wedding I need the ability to change focal length fast.

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