Spring 2012

Spring 2012
by ani

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I have the 18-55mm kit lens for my Canon 50D and was wondering if they're still specified against full frame 1:1 crop? Would my 18mm actually be 1.6x?

I ask because I doubt anyone with a full frame camera would even think of using a kit lens of this calibre.

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See also photo.stackexchange.com/questions/1592/… – Hamish Downer Sep 20 '10 at 19:57
Aspects of this are covered at photo.stackexchange.com/questions/103 and photo.stackexchange.com/questions/139 – mattdm Jan 3 '11 at 1:54
up vote 3 down vote accepted

18-55mm is the focal length, which is independent of sensor size.

Your lens is made for APS-C, so the any field of view measurements are likely for APS-C. It would be the equivalent field of view of a 28.8mm-88mm, which we refer to as 28.8e-88e.

On a full-frame camera, it will vignette.

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They are the same. The FOV numbers you have would equal a 28.8e-88e (The FOV of a 28.8mm-88mm lens on a full-frame) – Eruditass Sep 16 '10 at 23:50
So focal length is always going to be the physical focal length and the manufacturer wouldn't ever specify it at it's "crop effective" focal length? – Nick Bedford Sep 16 '10 at 23:52
Correct, except when specifically noted (pretty much just fixed-lens small-sensor cameras like a P&S) – Eruditass Sep 17 '10 at 0:28
I don't think we should use the 'e' notation. Any random reader who comes here from google or bing is not going to know what that means. Its a hyper-niche notation only used here on our site. – jrista Sep 17 '10 at 16:18
Isn't the point to spread it around? I did offer a short explanation as to what it meant. – Eruditass Sep 17 '10 at 20:05

You can't actually mount the 18-55mm lens on a full-frame Canon camera as it's an EF-s lens (not EF) so it's designed to physically not fit. The focal length of a lens is independent of the sensor size (a 50mm lens is a 50mm lens regardless of what camera it's mounted on) however the field of view (how wide the lens appears) will be affected by the crop, so the 18mm lens will be the same field of view as a 28.8mm lens on a full frame camera.

Incidentally I've had very good results mounting the £80 50mm f/1.8 on a £1000 Canon 5D. A lens doesn't have to be expensive to give good results if used in the right way.

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Thanks. I was merely curious to know if they specify the focal length in "effective" focal length. – Nick Bedford Sep 16 '10 at 23:54
SLR lenses are almost always specified using the actual focal length, whereas compact cameras are almost always specified in terms of the 35mm equivalent (the sensors are so small the actual lenses are something like 5mm-15mm). As an SLR owner you are trusted to do the FOV conversions yourself ;) – Matt Grum Sep 17 '10 at 0:17
The videographer at my friend's wedding got some stunning results out of his 1.8 50mm on a 5DII :) – Nick Bedford Sep 17 '10 at 2:46

Because focusing distance is the characteristics of the lens, NOT of the lens + sensor size. Lens are provided with optical characteristics which has nothing in common with sensor, mount type and so on. You need to multiply on the crop factor because of sensor size.

And yes, your 18mm on sensor with crop factor 1.6 will become 28.8mm.

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I figured as much, I just wasn't entirely sure. These answers cleared it up, thanks. – Nick Bedford Sep 17 '10 at 11:37

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