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01-08-2008, 08:16 PM   #1
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Why is this lens so cheap?

At B&H photo this lens $129.00US


Sigma Zoom Wide Angle-Telephoto 28-70mm f/2.8-4 DG Compact High Speed Zoom Autofocus Lens for Pentax USA

Does anyone have this lens?
Just curious!
Thanks
Shannon

01-08-2008, 09:15 PM   #2
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Shannon,

It's an older, less digitally-optimized lens. Take a look at Sigma's listing of lenses, here:

Sigma - Lenses
You may find it interesting to read the web site's explanations of designations like APO, EX, etc. The lens that you referenced is DG-only lens -- not EX or APO. It's also not a fixed-aperture lens. B&H also carries the Sigma 28-70 f/2.8 (fixed) EX DG. It's $340.

Will
01-08-2008, 09:24 PM   #3
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Thank you Will
I am going to check out your link. I wasn't sure about the 2.8 - 4. F stop.
I appreciate your time
Shannon
01-08-2008, 09:51 PM   #4
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QuoteOriginally posted by norco5 Quote
Thank you Will
I am going to check out your link. I wasn't sure about the 2.8 - 4. F stop.
Shannon,

You're welcome.

That "f/2.8-4" aperture-range specification in the name/description of the lens is one of the most important details you'll see. If the lens is listed with a single f-stop, in other words, if it says simply "f/2.8", it means that the lens has a fixed aperture throughout the zoom range. When I shoot with my Sigma 18-50 f/2.8, I can set the aperture to f/2.8 and zoom in and out without worrying that the aperture needs to change or is going to change on me willy-nilly. On the other hand, when the name/description of the lens specifies a range of apertures, like "f/2.8-4," the larger aperture (say, f/2.8) is available only at the shortest focal length, and the second aperture listed is the widest aperture available at the longest focal length. So if you're shooting with that Sigma lens in low light:

-- at 28mm, you can go as wide as f/2.8; but
-- at 70mm, you can only go to f/4.

Fixed aperture is better, but more expensive. Apparently it requires more sophisticated design and engineering.

By the way, if the lens is a "prime" -- that is, if it has only one focal length -- then you'll always see just one aperture listed.

Will

01-08-2008, 11:29 PM   #5
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This is such great information Will! Thank you!
i would like to get a lens that will fit in between my kit 18-55 and My tamron 70-300 both F4
A lens that I could use inside and out (low light) without breaking the bank. I have looked at both fixed and zoom. I only take photos for myself, and to annoy my family
So I need to stay within a certain price ... not over $350Can
So if you have any suggestions ?
01-09-2008, 12:43 AM   #6
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I've got a Sigma 28-70 EX f2.8 (non DG) which is pretty poor really. Colour & contrast are lacking, not very sharp, CA a big issue. Have managed to get a couple of OK photos out of it but I think in general the older non DG lenses should be avoided.
01-09-2008, 04:40 AM   #7
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QuoteOriginally posted by norco5 Quote
This is such great information Will! Thank you!
You're welcome. ;-)


QuoteQuote:
i would like to get a lens that will fit in between my kit 18-55 and My tamron 70-300 both F4
Forgive me for correcting you here, but these details in the description of the lens are important, as there are often lenses that are quite similar and distinguished only by some small detail in the description. Sometimes, there are two lenses with the same zoom range but one lens is f/2.8-4.5, while the other is f/2.8 (constant). To distinguish the one that does not have the constant aperture, you must give the aperture range for the other one.

So, to say simply "both f/4" suggests that they both have a constant max aperture of f/4. Not true of these lenses.

The Pentax kit lens is an f/3.5-5.6, if I recall correctly, not a constant f/4, indeed, not f/4 at all. Remember, this means that the max aperture at 18mm is f/3.5, while the max aperture at 55mm is f/5.6. For the Tamron 70-300, the aperture range is f/4 (at 70mm) to f/5.6 (at 300mm).


QuoteQuote:
A lens that I could use inside and out (low light) without breaking the bank. I have looked at both fixed and zoom. I only take photos for myself, and to annoy my family.
So I need to stay within a certain price ... not over $350Can
I don't have a specific recommendation, but you can look and see what B&H in New York City has. CLICK HERE to view their Pentax-mount lens listings, sorted by price (cheapest first). Note that there are a number of lenses with focal length ranges of 28-80 or something like that. I notice that someone really likes the Sigma 28-70 f/2.8-4 DG ($129!). This is not to be confused with the Sigma 28-70 f/2.8 (constant aperture), which I used to own and thought was pretty good. The 28-70 f/2.8 from Sigma is comparable to the Tamron 28-75 f/2.8 but both are a bit more expensive than your budget allows. Constant max aperture usually means higher price.

Will
01-09-2008, 06:22 AM   #8
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QuoteOriginally posted by WMBP Quote
When I shoot with my Sigma 18-50 f/2.8, I can set the aperture to f/2.8 and zoom in and out without worrying that the aperture needs to change or is going to change on me willy-nilly.
Will,

I've been itching for that lens. How do you like it? Does it scale up on your K10D?

Thanks,

01-09-2008, 06:35 AM   #9
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QuoteOriginally posted by jshurak Quote
Will,

I've been itching for that lens. How do you like it? Does it scale up on your K10D?
It seems a pretty good lens. Not sure what you mean by "scale up".

Will
01-09-2008, 07:09 AM   #10
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QuoteOriginally posted by WMBP Quote
It seems a pretty good lens. Not sure what you mean by "scale up".

Will

Crop factor or Magnification factor? For example my 70 - 300mm acts more like a 100 - 400.

Last edited by jshurak; 01-09-2008 at 07:15 AM.
01-09-2008, 07:48 AM   #11
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Any effective focal length on a lens on an APS-C camera is multiplied by 1.5 - it will be a 42-105 equivalent. It's more like downscaling than upscaling.
01-10-2008, 10:59 PM   #12
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I have heard real good things about the new Tamron 28-75mm 2.8 zoom. Constant 2.8 through the complete zoom range. You can get your hands on one for about $379.00. Suppose to be one of the hotest new lenses.
01-10-2008, 11:45 PM   #13
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QuoteOriginally posted by Mapleleaf-Mick Quote
I have heard real good things about the new Tamron 28-75mm 2.8 zoom. Constant 2.8 through the complete zoom range. You can get your hands on one for about $379.00. Suppose to be one of the hotest new lenses.
Yep, a great lens
01-11-2008, 07:17 AM   #14
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Yep, the Tamron 28-75mm is a great lens, don't know how "new" it is though, I suspsect it's been around for a few years already. Also full-frame compatible!
01-11-2008, 07:24 AM   #15
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Some Tamron shots -






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