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07-31-2007, 05:08 AM   #1
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Since I've never owned a zoom, I'm trying to understand the limitations of them. It seems the biggest problem is the Vignetting and distortion that occurs at the ends of the focal lengths. To avoid this an answer seems to be overlapping the range of the zooms that are owned. The first two zooms I'm considering are:

Pentax SMC-FA 24-90mm f/3.5-4.5 AL
PENTAX SMC-A 70/ 210 mm F 4

My question is what is the effective focal length of these lenses if you were to minimize the problems. Is it in the area of say, 30-70 and 90-180 or is it greater?

Thanks for any help on this.


07-31-2007, 06:22 AM   #2
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Don't worry about vigneting as it is DEAD EASY to fix. Distortion is a bugger however so watch the wide end carwefully and stop down.
07-31-2007, 06:42 AM   #3
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QuoteOriginally posted by Alfisti Quote
Don't worry about vigneting as it is DEAD EASY to fix.

Please elaborate. I have traditionally cloned vignetting out, but if there is a better way...
07-31-2007, 06:50 AM   #4
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You shouldn't see any significant vignetting whatsoever in zoom lenses sold today, especially the two you listed. With extremely wide angle lenses (zoom or fixed focal length), because of the very wide viewing angle, you may see the edges of a filter or lens hood attached to the front of the lens. Of course, the solution is to use thinner filters or remove the filters entirely. The same with removing the lens hood. However, you should not see that problem with the two lenses you listed - not that wide.

Barrow distortion, on the other hand, is a common problem with zoom lenses. This is a slight distortion in the corners of the images, mostly objectionable only at the longest focal length settings (many actually desire it with very wide angle lenses). However, this is only a minor problem, almost undectable in most real-world images, in all but the very cheapest, off-brand, lenses. If absolute perfection is desired, one can simply not zoom to the longest zoom setting - drop back about ten percent from the maximum. The same with the widest setting, drop back roughly ten percent. You shouldn't see much barrow distortion with the two lenses you listed.

The other common issue with zooms is the often overall slowness compared to better fixed focal lengh lenses. Of course, this is only a significant issue when you intend to shoot in lower light (late evening, dusk, dimly lit rooms, etc) without a tripod or additional lighting.

Zoom lenses have improved dramatically over the last couple of decades, competing pretty well today, image quality wise, with fixed focal length lenses. I use zoom lenses for about 95% of my photography, even sessions with advertising clients. The only fixed focal length lens used is an extreme telephoto (1000mm for wildlife), only because a zoom at that focal length would be so large as to be truly combersome and too slow for the purpose. However, my 500mm is a zoom lens (100-500mm) which works very well beyond it's general slowness (f:6.3 at 500mm). Of course, a tripod is virtually required at those focal lengths, so a slow lens are not really that much of a problem.


Last edited by stewart_photo; 07-31-2007 at 07:01 AM. Reason: clarification
07-31-2007, 07:06 AM   #5
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Hi Stewart,
Thanks, that clears up a lot. Since I haven't gotten into wide angle shooting yet these 2 lenses look like they will do just fine. Which 100-500 lens do you have? That looks like the next lens I should think about.


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