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05-03-2009, 10:41 AM   #16
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And a constant focal length allows one to focus on an subject using maximum magnification (i.e. the 200mm end of a 70-200mm lens) and then zoom out to 70mm to take the picture. If the lens is a vari-focal lens, the focus plane will not be what was focused upon at 200mm.

It is easy to find out if your lens if vari-focal or zoom by simply focusing at the long end, zooming out to the short end, and seeing if the focal plane is further or closer (vari-focal) or stays the same (zoom).

Unfortunately, no lens manufacturers make the distinction between them and label everything "zoom".

05-05-2009, 06:35 AM   #17
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QuoteOriginally posted by Jim Royal Quote
I also own the Pentax-A 70-210 f/4 that NaClH2O mentioned above. It a wonderfully sarp lens. And for me, the difference between the Pentax-A and (for example) a lens like the Sigma 70-200 f/2.8 is not just that the Sigma is one stop brighter, but that the Pentax was $550 cheaper.
My Pentax-A 70-200/4's total cost was US$9. How much was that Sig? I also have the Vivitar Ser.1 Ver.1 70-200/3.5 (great heavy booger) but it cost an exorbitant US$33, while the Sears/Ricoh 70-210/4 (PK) and 80-200/4 (M42) were both US$9. I like constant aperture zooms, especially really cheap ones.
05-08-2009, 07:18 AM   #18
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QuoteOriginally posted by SOldBear Quote
I'm confused as what the lens aperture being constant or not has to do with focusing. Can you please elaborate on this.
Agreed, these are separate subjects. This has nothing to do with constant-aperture/variable-aperture zoom properties. Many zooms are "vari-focal" to some extent, where the lens needs to be refocused as the focal length is changed. It is a design goal of zooms to hold focus constant during the zoom but some designs accomplish this better than others. If not varifocal, it means you can zoom in for precise focus then zoom out before shooting.
05-08-2009, 07:21 AM   #19
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QuoteOriginally posted by Cideway Quote
Constant aperture lenses are more likely to be zooms rather then vari-focal lenses.

Definition: Zoom, a lens that is able to change its focal length without changing its focal point.

Vari-focal, a lens that changes focal point when the focal length is adjusted.
Put another way, some folks don't consider varifocal lenses "true" zooms, though the marketplace calls any variable-focal-length lens a zoom.

05-08-2009, 07:25 AM   #20
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QuoteOriginally posted by RioRico Quote
My Pentax-A 70-200/4's total cost was US$9. How much was that Sig? I also have the Vivitar Ser.1 Ver.1 70-200/3.5 (great heavy booger) but it cost an exorbitant US$33, while the Sears/Ricoh 70-210/4 (PK) and 80-200/4 (M42) were both US$9. I like constant aperture zooms, especially really cheap ones.
IMO any Pentax lens in usable condition is a good deal at $9. The Pentax-A 70-200/4 of course is a different lens than the SMC Pentax-A 70-210/4 (different optical design, SMC coatings).
05-08-2009, 08:39 AM   #21
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QuoteOriginally posted by AndrewG NY Quote
IMO any Pentax lens in usable condition is a good deal at $9.
Except maybe a Pentax-110 lens. Or a Pentax CCTV lens.
05-08-2009, 10:07 AM   #22
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I hope this question isn't too naive.
Constant f/4 vs f/2.8-4.0 zoom, all else being equal, which should be more preferable?
Thanks.
05-08-2009, 12:19 PM   #23
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QuoteOriginally posted by RioRico Quote
My Pentax-A 70-200/4's total cost was US$9. How much was that Sig?
I didn't buy the Sigma 70-200 F/2.8. It's nearly $800 on Amazon. Maybe next year. Meantime, the secondhand Pentax will do me very well, even if it is a stop darker.


QuoteOriginally posted by AndrewG NY Quote
IMO any Pentax lens in usable condition is a good deal at $9. The Pentax-A 70-200/4 of course is a different lens than the SMC Pentax-A 70-210/4 (different optical design, SMC coatings).
Ah, I didn't know that there were two varieties of this lens. Happily, I have the SMC version. I paid $200 Canadian -- perhaps a touch high by eBay standards, but it's in perfect condition, and a pleasure to use.

05-08-2009, 12:32 PM   #24
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QuoteOriginally posted by Kguru Quote
I hope this question isn't too naive.
Constant f/4 vs f/2.8-4.0 zoom, all else being equal, which should be more preferable?
Thanks.
That's not a naive question. I chose the constant f/4 of the A 70-210 over the F 70-200/4-5.6 because of the speed at the long end, in spite of the fact that the F lens was AF and the other was not and the price was virtually identical and the camera body I was using was the SF-1.

When it comes to an f/2.8-f/4, that becomes a very interesting question. Why blow a stop at the short end when the long end is just as fast? The only time I think it would be preferable to have the f/4 constant lens would be when using a flash that does not read the exposure through the lens, either TTL or p-TTL. When using a manual flash or an auto flash that uses the sensor on the flash itself to meter, it would be necessary to constantly change the aperture to keep it at f/4 or f/8 or whatever is set on the flash. That would be a pain to work with. One could, I suppose, set the lens to the half stop between and work with 1/2 stop exposure errors. I don't want to.

Don't forget that they only rate the open apertures, but the f/8 on the ring would become f/11 at the long end.
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