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07-22-2010, 10:52 AM   #1
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focal length

Hallo,
this is just a newbie question.
i just wanna to ask about factor crop and focal length.
so for example i have Pentax K10D which 1,5 X factor crop
SMC 100-300 would be 150 -450 in my camera ...right??
but if put it more KEnko AF DG 1,5 X, then the range should be 200-600mm ...right ?
please correct me if im wrong ... because i just bought new Kenko AF DG with 1,5 multiplier.

regard

ewig

07-22-2010, 02:07 PM   #2
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Crop factor doesn't change focal length - it just gives you a way to compare the field of view of lenses on your K10D with lenses on your film camera. If you don't have a film camera, you can safely forget you ever heard about a crop factor.

Your SMC 100-300 is 100-300mm, period - it doesn't matter what lenses you'd need to get the same field of view on a camera you don't own. Put 1.5XTC on the 100-300 and you've got a slower and lower quality 150-450.

Since this is an *incredibly* common question among newcomers to DSLR's, I'm moving this to the Beginner's forum.
07-22-2010, 07:57 PM   #3
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What Marc has said ... but I'd like to add for clarity that while your lens + TC combo is 150-450, a shot you take at 450mm would be similar in terms of framing to one taken by a film/FF shooter at ~675mm (being 450mm times the 1.5x crop factor).
07-22-2010, 08:54 PM - 2 Likes   #4
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I'll bite back my usual rant about "crop factor" being a bogus, obscene term, with no historical provenance, and just note that a lens is a lens. No matter what format sensor it illuminates, it's still the same lens. Different sized frames (film or sensor) just see different amounts of the projected image, that's all.

Cut out a 4x5 inch picture from a magazine. Draw a 60x90 mm rectangle in the middle of it. That's what a 6x9 camera sees. Draw a 45x60 mm rectangle in the middle of that -- it's what a 645 camera sees. Draw a 24x36 mm rectangle in that -- it's what a full-frame 35mm camera sees. Draw an 18x24 mm rectangle in that -- it's what your dSLR sees. Et cetera. Same focal length, same picture; it's just that smaller frames see less of the picture.

No lenses were transformed during the writing of this post.

07-22-2010, 10:32 PM   #5
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To the OP:
Have you any experience at all with a 35mm format camera?
07-23-2010, 12:50 AM   #6
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QuoteOriginally posted by RioRico Quote
Cut out a 4x5 inch picture from a magazine. Draw a 60x90 mm rectangle in the middle of it. That's what a 6x9 camera sees. Draw a 45x60 mm rectangle in the middle of that -- it's what a 645 camera sees. Draw a 24x36 mm rectangle in that -- it's what a full-frame 35mm camera sees. Draw an 18x24 mm rectangle in that -- it's what your dSLR sees. Et cetera. Same focal length, same picture; it's just that smaller frames see less of the picture.
This is the most clear and simple explanation I've read for focal lengths. Makes me wonder why I ever had a doubt about focal lengths
Thanks a lot.
07-23-2010, 05:23 AM   #7
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QuoteOriginally posted by epidaetia Quote
This is the most clear and simple explanation I've read for focal lengths. Makes me wonder why I ever had a doubt about focal lengths
Thanks a lot.
Don't give him a swelled head.
07-23-2010, 07:40 AM   #8
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The only drag about the (completely accurate) analogy to physically cropping a print is that it gives mistaken impression that pictures from a smaller sensor somehow come out smaller when you view or print them, when there's no particular reason why that would be true. So you have to image that after cutting out the smaller piece, you then stretch it to the same size as the original.


Last edited by Marc Sabatella; 07-23-2010 at 11:30 AM.
07-23-2010, 07:56 AM   #9
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QuoteOriginally posted by epidaetia Quote
This is the most clear and simple explanation I've read for focal lengths. Makes me wonder why I ever had a doubt about focal lengths
Thanks a lot.

Actually, his post had nothing to do with focal lengths. I think that was his point.

It WAS, however, a very good illustration of how the so-called "crop factor" (a term that I hate, as well) affects FIELD OF VIEW.

And that is the point. Crop factor has absolutely no effect on focal length. It only affects field of view.

Someone asked if the OP has any experience with 35mm. If they do not, then the term crop factor is entirely irrelavent. It only has meaning when comparing an APS-C camera to another image size, usually 35mm full-frame. We could just as easily compare APS-C to 6 x 6cm medium format and arrive at a crop factor of 2.8.

When I got my K10D (my first dslr), I learned about the crop factor, figured out what it meant and then promptly forgot about it. I just know that approx. 33mm is "normal". Shorter than that is wide angle; longer than that is telephoto.

To me, its a little like converting from fahrenheit to celsius or miles to kilometers. You don't need to do the math. In fact, doing the math just confuses you. You just need to learn a couple of new reference points. Twenty degrees celsius is a comfortable room temperature. Water freezes at zero and boils at 100. Your morning commute isn't 30 miles; its 48 kilometers. New York isn't 1,000 miles away; its 1,600 kilometers. The speed limit isn't 60 mph; its 100 kph.
07-23-2010, 03:40 PM   #10
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ok,
first table, thanks for the answer and explanation,
and what about the table in this site ?
Lenses for SLR and DSLR cameras - Wikipedia, the free encyclopedia

i think its also explain how to get the right focal lenght ?

am i right "marc" ...
07-23-2010, 06:37 PM   #11
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It's a correct chart for performing a conversion that the previous responses should make clear you have no reason to need to perform.
07-23-2010, 09:32 PM   #12
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Let's look at a nice big 'ideal' 100mm lens. It has a wide mount and can be fitted to a LF (large format) camera lens board. With adapters, we're going to put it on some cameras of various formats.

First, we must know that a 'normal' lens has a focal length close to the diagonal of the frame (film or sensor) of whatever format camera it's on. So we'll see what's 'normal' for each format, and compare those with our 100mm lens. Our reference will be the 'normal' of a 35mm (135 film cartridge) camera, whose 24x35mm frame has a diagonal of about 43mm.

A 4x5" view camera has a diagonal of 162mm. A 100mm lens is 62% of that 'normal'; its 135 equivalent would be 43x0.62= ~27mm. That's rather wide, the same as 18mm on an APS-C dSLR.

A 6x9cm medium-format camera has a diagonal of 101mm. The 100mm lens is dead-on normal.

A 6x6cm MF camera has a diagonal of 79mm. The 100mm lens is 127% of that; its 135 counterpart would be ~55mm, a classic "long normal" size, like 37mm on APS-C.

A 645 MF camera's diagonal is ~70mm. The 100mm lens is 143% of that normal; its 135 equivalent would be ~61mm, just a little longer, a short tele, like 41mm on APS-C.

A 135 FF (full-frame) film cam's diagonal is 43mm, as I said. The 100mm lens is 233% of that; it's a good, moderate tele, like 67mm on APS-C.

Your APS-C dSLR has a diagonal of 30mm. The 100mm lens is 333% of that; its 135 FF counterpart would be a longer tele, 143mm.

And an m4/3 cam has a diagonal of ~22mm. The 100mm lens is 455% of that; its 135 equivalent would be ~195mm, almost a birder's tele, like 130mm on APS-C.

So, that same single 100mm lens ranges from moderate wide-angle, to normal, to long tele, depending on what size frame it illuminates. The lens hasn't changed. Its focal length, and the perspectives of objects it sees, remain exactly the same. EXACTLY THE SAME!! When I wrote of 'equivalents' and 'counterparts' above, I mean the AoV (angle of view) that each frame sees. That lens projects exactly the same image, but the different-size frames see various portions of that image. That's all there is to it.

I hope you're all thoroughly confused now. Yes, you're welcome.
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