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10-16-2007, 01:02 PM   #1
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So, which is wider? And does it matter?

I'm really leaning towards the Sigma 10-20mm lens over the Pentax 12-24mm lens because of the price difference and the Sigma has a wider field of view. However, on the product page for the Sigma it says "equivalent range of approx. 15-30mm when used with a Pentax digital SLR camera". Soooooo, does that mean that it's really NOT a 10-20mm lens? I already have a lens that covers from 18mm and up, so to drop that much $$$ for not much wider doesn't seem to make sense to me.

So tell me this. Why is it labeled as a 10-20mm lens for the Pentax, but it really translates to 15-30mm? Why don't they label it as a 15-30mm zoom?

Sorry if this has all been hashed out before.

10-16-2007, 01:12 PM   #2
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QuoteOriginally posted by azcavalier Quote
I'm really leaning towards the Sigma 10-20mm lens over the Pentax 12-24mm lens because of the price difference and the Sigma has a wider field of view. However, on the product page for the Sigma it says "equivalent range of approx. 15-30mm when used with a Pentax digital SLR camera". Soooooo, does that mean that it's really NOT a 10-20mm lens? I already have a lens that covers from 18mm and up, so to drop that much $$$ for not much wider doesn't seem to make sense to me.

So tell me this. Why is it labeled as a 10-20mm lens for the Pentax, but it really translates to 15-30mm? Why don't they label it as a 15-30mm zoom?

Sorry if this has all been hashed out before.
the lens is really 10-20mm.

the statement "equivalent range of approx. 15-30mm when used with a Pentax digital SLR camera" really means when compared to the field of view of a 35mm camera (some of us are old enough to remember what film was),

as a result, the lens is wider than the 12-24, because the 12-24 has an "equivalent range of approx. 28-36mm when used with a Pentax digital SLR camera".

The reason lens manufacturers quote both the real focal length and the equivelent to 35mm for different cameras is because the equivelent focal length does change from camera maker to camera maker. if you had a 4/3 system like the olympus, the equivelent focal length woudl be 20-40mm because the 4/3 system has a smaller sensor.

It would just be much better to only quote the real focal lentgh.
10-16-2007, 01:21 PM   #3
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The focal length doesn't change, the Field Of View does, because your sensor is 1.5 times smaller than 35mm film.

When you use that 10-20mm on your DSLR, you'll see an angle comparable to what a 15-30mm looks like on a film camera.

When you use your 18-55mm, you see an angle that looks like what 27-82.5mm would look like on full frame...

10mm is a whole lot wider than 18mm

Tom
10-16-2007, 01:22 PM   #4
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The Sigma lens is a 10-20mm lens, and the Pentax one is 12-24mm. That's tells you the focal length of the lens, but that doesn't actually tell you the field of view (or angel of view) that you get. Focal length is one half of the story.

The other half is the size of the sensor (or film) in the camera. These two factors—focal length and sensor size—combine to give you the field of view, to what degree is it wide angle, telephoto, and such. So if you get one lens at 10mm, the view will be different if it is a small sensor, or a large one, where the larger one will produce a wider view.

For decades, the only prominent film size for SLRs was 35mm film, which actually measures 36mm x 24mm. For a long time, this was the only film (or sensor) size that many people had to deal with, so a 10mm lens on one 35mm camera produced the same view as a 10mm lens on someone else's 35mm camera. So everyone (including myself) got stuck with the idea that focal length = field of view since the film size didn't change.

Today, we have digital SLRs, which don't use film, and most have a sensor size that's smaller than film, about 24mm x 16mm. Remember that field of view is determined by both the focal length and the sensor size. Since the sensor size on most dSLR cameras are smaller than film, a 10mm lens on a film SLR will produce a wider view than the same lens on most dSLRs.

Now, 36mm x 24mm is about 1.5 times bigger than 24mm x 16mm on each size, this is known as the 1.5x crop factor. So a 10mm lens on a dSLR will have a field of view similar to a 15mm lens on a film SLR (10mm x 1.5 = 15mm).

That is why the Sigma lens mentions that their 10-20mm lens on a digital SLR produces the same view as a 15-30mm lens on a 35mm film SLR. The same thing with the Pentax lens, the 12-24mm lens on a dSLR has a view similar to a 18-36mm lens on a 35mm film SLR.

So I hope that this makes sense. I think that a illustration will make things more clear, but I haven't gotten around to making one. Cheers.

Alex

10-16-2007, 01:50 PM   #5
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QuoteOriginally posted by gerbilbox Quote
That is why the Sigma lens mentions that their 10-20mm lens on a digital SLR produces the same view as a 15-30mm lens on a 35mm film SLR. The same thing with the Pentax lens, the 12-24mm lens on a dSLR has a view similar to a 18-36mm lens on a 35mm film SLR.
No, that makes sense. If they had said that it was comparable to a 15-30mm lens on a 35mm film camera, I would have gotten it. But they didn't. They just said that it was 15-30mm when used with a Pentax, which I had taken to mean that on other brands, such as Canon or Nikon, that it would be 10-20mm, but for some reason (maybe having to do with the build of the Pentax) that the same lens was equiv. to 15-30mm. No where did it say that it was comparing it to a 35mm film camera.

Thanks for the responses. That obviously helps a great deal!
10-16-2007, 02:56 PM   #6
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Just be aware that what the other posters have said in reply applies to All DSLR's, not just Pentax, they all share the same 1.5 factor.
The best way to satisfy yourself is to try the lens on your camera, you won't be dis-appointed.
And LBA does not like to be kept waiting.
PS: full frame & Oly excluded.
10-16-2007, 03:16 PM   #7
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QuoteOriginally posted by GWP Quote
PS: full frame & Oly excluded.
Canons are 1.6x factor
10-16-2007, 03:30 PM   #8
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The 10mm will give you 102.4 degrees and the 12mm will give you 99 degrees.
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10-16-2007, 05:22 PM   #9
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Hi azcavalier

When you said "I'm really leaning towards the Sigma 10-20mm lens" I suddenly became very envious.
You see I've been trying to purchase one of these astonishing lenses for quite some time here in the U.K., but Sigma's main warehouse in Hertfordshire regrettably experienced TWO major break-ins in the last few months and a LOT of inventory was stolen. Hence NOBODY over here has them for sale in Pentax mount until the next shipment arrives from Japan.
Once you've tried this lens out, you seriously ain't gonna wanna take it off your camera again.
I am soooo jealous right now !

Best regards
Richard

P.S. Serious Warning - Please check THOROUGHLY for de-centerring/out-of-focus images on this particular lens BEFORE parting with your cash ! It's a known issue with some examples of the 10-20mm Sigma !
10-16-2007, 05:56 PM   #10
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QuoteOriginally posted by Deni Quote
Canons are 1.6x factor
Well, for the current DSLRs that are built like 35mm film cameras, I know of the following crop factors:

Canon 1 DS MkIII and 5D and Nikon D3 are "full frame" (same as the 24x36mm 35mm film frame)
Canon 1D MkIII and Leica M8 are a 1.3X factor (multiply by 1.3 to have the equivalent field of view of full frame)
All Pentax DSLRs, Sony, Fuji and all Nikon other than the D3: 1.53X factor
All Canon other than 5D and 1 D series: 1.6X factor
Sigma SD14: 1.7X factor
All "4/3" system (Olympus, Panasonic, and Leica branded): 2X factor

I also think the old Kodak DSLRs with the Nikon F-Mount were 1.3X, but I'm not sure. And there may be others that I've forgotten.

And there are also a number crop backs for various medium format cameras.
10-17-2007, 01:27 PM   #11
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QuoteOriginally posted by Confused Quote
Hi azcavalier

When you said "I'm really leaning towards the Sigma 10-20mm lens" I suddenly became very envious.
You see I've been trying to purchase one of these astonishing lenses for quite some time here in the U.K., but Sigma's main warehouse in Hertfordshire regrettably experienced TWO major break-ins in the last few months and a LOT of inventory was stolen. Hence NOBODY over here has them for sale in Pentax mount until the next shipment arrives from Japan.
Once you've tried this lens out, you seriously ain't gonna wanna take it off your camera again.
I am soooo jealous right now !

Best regards
Richard

P.S. Serious Warning - Please check THOROUGHLY for de-centerring/out-of-focus images on this particular lens BEFORE parting with your cash ! It's a known issue with some examples of the 10-20mm Sigma !
Sorry. Makes you wonder where the inventory went...haven't seen anything much for sale on eBay!

*Anything* that I buy is going to have to come off of the internet anyways. I don't think that anyone within 200 miles of Blacksburg, Virginia has even heard of Pentax cameras. It's all the "big 2". We have aone bare bones Ritz Camera at the mall, but the guy in there said that he had heard of the K10D, but they didn't carry anything for it. I'll probably just buy it from B&H online. I hope I get one that doesn't have any out-of-focus issues...I hate dealing with mail order returns.
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