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12-26-2008, 08:30 PM   #1
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DA lens confusion

I am considering buying a KM but am confused at the new lens selections from Pentax. I'm currently using my trusty Super Program with a A 28 2.8 and a A 50 1.7. Everything indicates these will be quite usable on the new camera, except of course auto focus, no problem. But after checking out the Pentax site, I see the new DA 40 2.8. It looks like a great lens to use with the KM. My confusion is on the focal length. Even though it is being advertised as designed for DSLR's, the specs show it to be a 60mm equivalent. If it's designed for digital, why the continued use of the crop factor? 40mm for $250 would have been great for indoor use, especially on vacation, since I usually find 50mm not wide enough. Next option is the DA21 3.2. I'm surprised at the price difference since it is slower. I'm stuck deciding between the KM or a D40 at this point, since it looks like using modern zooms is the way to go. Any help is most appreciated. Thanks!

12-26-2008, 09:08 PM   #2
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40mm is still 40mm, cropped sensor or not. 60mm is the 35mm equivalent field of view, which has become the conventional way to compare lenses.
12-26-2008, 09:33 PM   #3
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QuoteOriginally posted by reniegreg Quote
If it's designed for digital, why the continued use of the crop factor?
All interchangeable lenses for SLR and DSLR cameras are labeled with their actual physical focal length. This focal length (or range of focal lengths for a zoom) is fixed and does not change depending on whether you mount the lens on a film camera or a cropped sensor digital camera.

Even the 4/3 people (many of whom claim there is no "crop factor" for their lenses because they're all designed specifically for use with 4/3 sensors and cannot be used on full frame cameras) use the actual, physical focal length of the lens. Thus a 20mm lens for a 4/3 camera has a field of view equivalent to a 40mm lens on a 35mm camera.

When people talk about a 40mm lens being a "60mm on a digital camera", they are being lazy and careless. The lens is still a 40mm lens. What's changed is only the field of view. If they use "60mm" in describing the lens they should phrase it using explanatory language such as "a field of view equivalent to a 60mm lens on a 35mm camera", or (more usual) "60mm equivalent field of view".

Careless use of "equivalent focal lengths" is the cause of no end of confusion among folks who are new to the concepts (and in many cases among those who should be better versed in them as well).
12-26-2008, 09:38 PM   #4
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To put it another way, buying a lens was designed for digital doesn't magically make your sensor bigger. And that's where the "crop factor" comes from": having a sensor that is smaller than 35mm. It doesn't matter whether the lens is designed for film or digital. When shooting on a camera whose sensor is 1.5 times smaller than a piece of 35mm film, *any* lens needs to have it's focal length multiplied by 1.5 to figure out what the "equivalent" focal length would have been on film to get the same angle of view.

When you say 50mm is usually not wide enough, do you mean, it wasn't wide enough on film, or that it is no longer wide enough on digital? No matter how you slice it, a 40mm lens is wider than 50mm lens. If the 50 was wide enough on film but not on digital, you may find the 40mm perfectly wide enough on digital after all - it's only a little longer than 50mm was on film. But if you're saying 50mm wasn't wide enough on film either, then consider just using your existing 28 - it's going to be the equivalent of 42 when used on digital, so it will indeed be a little wider than the 50 was on film.

12-26-2008, 09:46 PM   #5
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Another thing that people often say is that the crop helps with long throw...ie their 300mm lens magically becomes a 450mm and they get the equivalent magnification (or in 4/3 land, the 300 becomes a 600). That of course is ridiculous as glass is glass. All you're doing is cropping the field of view.

But it is funny how these terms get tossed about on the interwebs then they become gospel and highly misunderstood.
12-26-2008, 10:13 PM   #6
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QuoteOriginally posted by Marc Sabatella Quote
To put it another way, buying a lens was designed for digital doesn't magically make your sensor bigger. And that's where the "crop factor" comes from": having a sensor that is smaller than 35mm. It doesn't matter whether the lens is designed for film or digital. When shooting on a camera whose sensor is 1.5 times smaller than a piece of 35mm film, *any* lens needs to have it's focal length multiplied by 1.5 to figure out what the "equivalent" focal length would have been on film to get the same angle of view.
Marc - I know you know this - you're one of the smartest guys here. Crop factor makes me (and all of us) crazy!

Don't you mean 2/3 the area of 35mm film? (the sensor is 1/3 smaller than 35mm film, not 1.5x smaller than 35mm film; if the image area is 1/3 smaller then the objective FoV must be as a lens 1.5x longer).

The math is 3/2, or three halves (.5 + .5 + .5)
12-26-2008, 10:51 PM   #7
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Three halves is 1.5, isn't it? I'm not being serious, but isn't it just another way of saying the same thing?
12-26-2008, 11:06 PM   #8
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QuoteOriginally posted by nostatic Quote
Another thing that people often say is that the crop helps with long throw...ie their 300mm lens magically becomes a 450mm and they get the equivalent magnification (or in 4/3 land, the 300 becomes a 600). That of course is ridiculous as glass is glass. All you're doing is cropping the field of view.

But it is funny how these terms get tossed about on the interwebs then they become gospel and highly misunderstood.
Well, cropping the field of view is all you're doing if you change from a 50mm lens to a 100mm lens without moving the camera, too. So, they're right, a 300mm lens does magically become a (lower-resolving) 450mm lens. If I took a tiny, say, 200x200 pixel crop of a full-size image, depending on the lens it was taken with it may 'magically' be the same thing as a 2000mm focal length lens -- but there's not much resolution

12-26-2008, 11:09 PM   #9
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[QUOTE=Sean Nelson;430339] (many of whom claim there is no "crop factor" for their lenses because they're all designed specifically for use with 4/3 sensors and cannot be used on full frame cameras)[QUOTE]

Do they really? Hilarious.
12-27-2008, 01:57 AM   #10
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QuoteOriginally posted by Sean Nelson Quote
(many of whom claim there is no "crop factor" for their lenses because they're all designed specifically for use with 4/3 sensors and cannot be used on full frame cameras)
QuoteOriginally posted by clm Quote
Do they really? Hilarious.
Some of the 4/3 folks vehemently deny that there's a "crop factor" and instead use the term "focal length multiplier". Of course they're really referring to the same thing - the fact that in order to quote the equivalent field of view of a lens on a full frame camera you have to multiply the focal length of a lens used on a smaller sensor by some value.

In my lexicon, whether the lens can be used on a full frame camera is irrelevant, the point of distinction is that the sensor is "cropped" to a smaller size than full frame. Nobody seems to dispute the use of the term "cropped sensor" when referring to the 4/3 system.

But it seems to be rather like arguing how many angels can dance on the head of a pin...
12-27-2008, 02:01 AM   #11
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QuoteOriginally posted by monochrome Quote
Don't you mean 2/3 the area of 35mm film?
Marc is talking about the length of the diagonal, not the area. The diagonal length is the metric that tells you what factor to use for multiplying the focal length to get an equivalent field of view.
12-27-2008, 08:24 AM   #12
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QuoteOriginally posted by monochrome Quote
Don't you mean 2/3 the area of 35mm film?
Could be. I didn't actually do the math (and I have a degree in mathematics!) - I was speaking kind of colloquially there. Measuring area versus diagonal would indeed make a difference, and actually, I'm not sure whether APS-C is 1.5 times smaller than 35mm, or if 35mm is 1.5 times bigger than APS-C. Anyhow, its not worth worrying about as long as we understand the effects.
12-27-2008, 08:26 AM   #13
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QuoteOriginally posted by nostatic Quote
Another thing that people often say is that the crop helps with long throw...ie their 300mm lens magically becomes a 450mm and they get the equivalent magnification (or in 4/3 land, the 300 becomes a 600). That of course is ridiculous as glass is glass. All you're doing is cropping the field of view.
Which indeed gives you the greater magnification when you actually view the images at any given size. In any *practical* sense, 300mm lenses on APS-C *do* magically become similar to 450mm lenses on 35mm.
12-27-2008, 09:00 AM   #14
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Perhaps this might explain it:

Focal length is focal length. Crop factor is not (and never will be) a substitute for optical magnification, or for the magnification/resolving power of a lens.

The subject is still the same distance away in your viewfinder as before. Only the FOV has decreased. A 200mm lens on a DSLR (with a 1.5x crop factor) does not give that lens a focal length/resolving power of a 300mm lens. It does give the 200mm lens the effective FOV of a 300mm lens, but not the image quality of a 300mm lens (assuming the lenses are equal quality).

Hope that helps.

Regards,
Marc
12-27-2008, 12:13 PM   #15
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QuoteOriginally posted by Marc Sabatella Quote
Could be. I didn't actually do the math (and I have a degree in mathematics!) - I was speaking kind of colloquially there. Measuring area versus diagonal would indeed make a difference, and actually, I'm not sure whether APS-C is 1.5 times smaller than 35mm, or if 35mm is 1.5 times bigger than APS-C. Anyhow, its not worth worrying about as long as we understand the effects.
35mm film (full-frame) has exactly twice the area of APS-C. Its diagonal is 1.5 times longer. It is correct to say that APS-C is 1.5 times shorter than full-frame, or to put it another way, APS-C is 2/3 of full-frame.

You were right every which way, Marc
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