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Old Dec 14, 2005, 8:40 AM   #1
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Hello all;

I find myself going around in circles on this....

I had an Option 550, with a 5x zoom. The lens was listed as equivalent to 37.5mm to 187.5mm. With film, 50mm was usually a 1:1 magnification ratio. Does that mean that the lens on the 550 when at maximum zoom was equivalent to 3.75x magnification?

Now, I have a DS2 and use my old Pentax lenses, in particular a Pentax-F 70-210mm. I also have a Sigma digital lens 18-50mm.

As I understand it, the mm ratings for the Sigma match the crop factor of the camera. In other words, 18mm really is 18mm. But, for the older lens, my 70-210 becomes a 105-315mm. Based upon the 50mm 1:1 standard, does this mean my old lens at maximum zoom gives 6.2x magnification?

Thanks for your help. At my age I confuse easy.

Regards, Rich
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Old Dec 14, 2005, 10:22 AM   #2
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Nothing to do with age ...

37.5mm to 187.5mm is 37.5mm to 187.5mm on a P&S.

18-50mm is 27-75 on any SLR I could afford.

The 3x,5x, 10xjustrefer tothe ratio of the lens at Wide angle (lower number) to full zoom (higher number), for instance;

38-380 is 10x. It's easy for consumers to undertand they're getting a lot of zoom when it's 10x, but not very accuratecompared tothe obscure focal length, which has no relation to anything a common person could reference.
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Old Dec 14, 2005, 12:13 PM   #3
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Pastor Rich, Panzergnome is correct, the Sigma lens is designed for the APS sized sensor, so it will vignette seriously on a film SLR, but 18mm is 18mm, and an 18mm lens on an APS sensor camera will give the equivalent field of view of a 27mm lens. Sigma is not technically lying to their customers but they are allowing the misconception to continue.

As for "magnification ratio" that is not the correct way to look at it. The diagonal length of the sensor is equivalent to the most "natural" perspective lens, in 35mm that is about 43mm, but the 50mm became the close equivalent. With a Pentax *istDS2 the diagonal is somewhere around 30mm so a lens of around 30-35mm should be about "normal".

Ira
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Old Dec 14, 2005, 1:23 PM   #4
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i'll probably get hate mail for this but...

rich, if i were you I'd stay away from the lenses that have the smaller image circle. i know i am. what happens when full size sensors are the norm?? the lenses that are made today with the smaller image circle will not be compatible
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Old Dec 14, 2005, 1:51 PM   #5
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Roy, I agree with you on sticking with the standard lenses, I think I've already figured in doing that... next year will see the Samsung-Pentax alliance bring new product and sometime after that will be the full-frame models.

I'm quite pleased with using my old lenses, I'm just trying to understand what magnification I'm getting when I put on my 70-210mm lens.

Also, it is too cold for this right now but come the spring I will have my 600mm telescope outside and that will be a fun challenge.

Rich
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Old Dec 14, 2005, 1:55 PM   #6
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Hi Ira;

Yeah, that makes sense. I also have a pentax-F 35-70mm. So with the 1.5 factor that becomes the 52mm kind-of standard we used to have.

Thanks, Rich
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Old Dec 14, 2005, 2:09 PM   #7
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I hear you about the full frame lenses.

Did you see the new Sony P&S with a SLR sensor????

Imagine this, new series of lenses necessary due to the mirror being eliminated.

IOW, the lens on that camera is 2mm away from the SLR-like sensor, so conventional and cropped SLR lenses will be useless anyway.


Granted, it doesn't have interchangeable lenses, but anyone want to bet its coming??
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Old Dec 14, 2005, 3:31 PM   #8
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This is one more point about the magnification issue, if 35mm is about normal view than any multiple of 35 will be another magnification factor. For example, to equal the magnification of an 8X pair of binoculars the camera would need a focal length of about 8 X 35mm or 280mm, a 35mm camera would need 8 X 50 or 400mm. This is only a rule of thumb, not very scientific.

Ira
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Old Dec 15, 2005, 12:34 AM   #9
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I don't buy digital lenses because I still use my MZ-S and I want lenses I can use on both my DS and the MZ.

The APS sensor (I wonder why they don't call it half frame) also take advantage of the center of the full frame lenses, giving less image softening at the edges that the same lens will show with film.

There is no magnification change using a film lens on the DS, just a crop factor of the image.

Tom
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Old Jan 9, 2006, 3:14 PM   #10
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The reason people speak of "35mm-equivalent focal length" is that so many of us have used 35mm cameras for so long that we're accustomed to what a given focal length number represents in that format. It's like a common language that conveys what field of view we can expect. Of course, every film size or sensor size has its own "standard" and "wide" and "tele" focal lengths. I'm very much enjoying using my SMCP-M 50mm f1.4 as a moderate "portrait" telephoto on my DS2, where it's equivalent to a 75mm in the 35mm format. It's real fun mounting my 75-150mm zoomwith a 2X converter, where the resulting 300mm on my DS2is the equivalent of 450mm in 35mm terms! Of course, my 28mm wide-angle becomes the equivalent of a 42mm, so I bought the Pentax kit 18-55 lens just to get back some wide-angle capability...

Dan
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