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11-04-2007, 07:03 PM   #1
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Fixed Focal Length.

Hi gang,

Being fairly new to all this Jazz, I have been wondering why people buy fixed focal length lenses with all the zooms on the market. Some fixed being very expensive. Is this simply a clarity issue? I could see having say a 50mm for studio portrait work but other than that I'm afraid I don't understand.


P.S. At this point I only own 18-55mm, 50-135mm 2.8f, and 70-300mm.

11-04-2007, 07:37 PM   #2
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Although that some zooms approach the image quality of prime lenses (the DA*s, the Tamron 28-75mm f/2.8 for example), you just can't beat prime lenses for sheer speed. They are also nice and compact compared to comparable zooms.

The fastest zoom lenses are f/2.8, whereas many primes are f/2.0 or faster. If I don't need that speed, I am generally fine with my Tamron 28-75mm f/2.8. But once the light levels drop and I can't use flash, nothing beats f/2.0 or f/1.4.
11-04-2007, 07:43 PM   #3
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Quick reply for primes.

- Resolution (esp corner and edge res) is often better in comparable price ranges..
- Brighter (I am yet to see a Pentax mount f2.0 zoom)
- I don't find I need all the range offered by a zoom, I like that primes make me think more.
- Size for long range, my 300/4.5 is pretty small compared to a zoom that would offer that quality at 300mm and f4.5.
- Often (in pentax) better CA/PF and vignetting performance in the primes.
- A lot of zooms I have used/owned suffer from more distortion than my primes.

Its all personal preference though...
11-04-2007, 08:40 PM   #4
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DA40 vs. 18-55

QuoteOriginally posted by magnum1 Quote
I have been wondering why people buy fixed focal length lenses with all the zooms on the market.
The physics of optics dictates that any zoom lens is a compromise. Fixed focal lengths are optimized exactly for that focal length. As such, they are smaller, faster and cheaper for much higher quality. Compare the size and speed of a DA40 to your 18-55. Then, look at some galleries.

11-04-2007, 08:56 PM   #5
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All good points mentioned above. Let me add that in general, a zoom is only as fast as its "slowest" focal length and only as compact as its "largest" focal length. Wide angles (especially inexpensive ones) are often pretty slow and bulky, after all. The result is that on the kit lens, the widest (i.e. fastest) aperture at 55mm is f/5.6, compared to a 50mm prime lens, at f/1.4. The prime is 4 stops faster than the zoom at that focal length, meaning that a shot you could take at ISO 200 on the prime would have to be taken at ISO 3200 on the zoom (assuming both are taken at the widest aperture).

Even on a pro-level zoom like the DA* 16-50mm, the widest aperture is f/2.8, which is 2 stops slower than the prime. And the DA* is 5 times the price. (Yeah, yeah, that's kind of unfair. You'd also have to buy a $600 DA14 or $400 DA21 to fill in the wide end of the zoom, and maybe an $800 FA31 or $200 FA35 for the "normal" range.)

For size, speed, and (usually) image quality, it is primes all the way. For me, anyway.
11-05-2007, 02:15 AM   #6
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Not to 'zoom' bash or anything, but I had the exact opposite question - Why make all the trade-offs for a bulkier, slower lens with lower IQ when I could do most of things a zoom could do and more with my feet and a prime?

Don't get me wrong, I can definately see the DA* for example being necessary for a person requiring fast focal length changes at events or the like, but for casual shooting, I just want maximum image quality. Personally, I carry 4 or 5 primes peaked for their particular focal range and make up the difference when necessary with a little creative framing or a little legwork - works for me, but then again, I also prefer to manually focus all my shots too, heh.
11-05-2007, 04:02 AM   #7
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Just yesterday I shot an event using nothing but a 105mm prime and a 55mm prime, swapping lenses at least a couple dozen times.....and screwmount at that.

I didn't find it particularly burdensome or limiting. Good thing, too, since neither of the auto-focus primes I own (kit 18-55 and FA J 75-300) could have handled the f4 the lighting required I do every shot with.

11-05-2007, 04:52 PM   #8
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Original Poster
Thanks folks. You guidance is much appreciated.

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