What tricks is Canon playing with the mm of its Lens?

[sv2dgi]

I was using an 20D for more than 4 years. I owned the EF-S 18-55 (original - no IS), bought a EF 28-135 IS and later a EF-S 17-55 f/2.8 IS.


Yesterday my new 5D Mk II arrived (body only) and I plugged the 28-135 to its body. I took a picture of my living room at 28 mm, took another picture with my 20D and the 17-55 at 17mm and the 28mm one looked wider. That's weird I thought...


The difference was visible even from the viewfinder, but I said ok, the 20D viewfinder is old, less coverage etc. etc. but the pictures showed the same thing.


I compared the two sensors' size (36x24) to (22.5x15) and they are EXACTLY 1.6x. I thought maybe the factor is not exactly 1.6x and they round it but... it is exactly 1.6X


17mm * 1.6 = 27.2 mm


28mm * 1.0 = 28.0 mm


So the 17 is wider! But not in actual life, where my 28 is wider than the 17. Both pictures are taken from the same position, with the same f/3.5 and the same focus point...


Then I plugged the 18-55. Ok its distortion at 18 is terrible and it is less wide than the 17, which makes sense.


What happens here? Where is the trick? Why the cropped frame lens is not exactly 1.6x the normal lens?

[Joel Bookhammer]

That is wierd, could you post some examples of your findings?

[sv2dgi]

See the images:


a) Canon EOS 5D MkII 28-135 at 28mm: http://users.otenet.gr/~sv2dgi/IMG_0007.JPG


b) Canon EOS 20D 17-55 at 17mm: http://users.otenet.gr/~sv2dgi/IMG_0606.JPG


They are just processed to JPG and scaled down (no cropping of course) from DPP. Compare the gaps left of the wooden cupboard and right of the rightmost corner...


You can also see the EXIF for further info...

[Keith B]

The 17-55 at it's widest covers78°30' and the 28-135 covers 75º. And then if you factor in the crop.


I would think Canon would have taken the crop factor on the EF-S in account when the list it's field of view but possibly not.

[sv2dgi]

Yes... maybe, but the photos disagree...

[mpphoto12]

the 5DMKII is a full frame thats the reason and the kit lens would normally fit on any body but i read an article in pop photo with the same problem. Its becasue it fits but its not suited fpr full frame bodies.

[Mathew Wilson]

Lenses are rated at infinity, so closer objects can appear wider at the same focal lengths with different lenses (the 24-105 is wider at 24mm than the 24-70 with close subjects).


Plus the 17mm on the 20D is a tiny bit wider at 27.2mm (not by very much but it adds up).

[Daniel Browning]

There are at least three factors in play here:

Breathing
Unit tolerances
Marketing



Breathing. is how the focal length of the lens changes as you rack focus. Try it. Watch the viewfinder as you focus from the minimum to infinity: you'll see it "zoom", even on a prime. This is normal for a still lens. The focal length printed on the lens is usually based on being set to infinity focus. So, getting back to your test, breathing could come into play in two ways: A) The 17-55 at 17mm has more breathing than the28-135 at 28mm. Or B) You did not have them on the same focus point (I bet you did, but even if you didn't it likely wouldn't be significant.)


Unit tolerances. A little known fact about lenses is that focal length varies with unit production. Three copies of the same lens may each have slightly different focal lengths. Your 28-135 may have been slightly wider than normal, and/or your 17-55 slightly longer than the average.


Marketing. It's pretty common for Marketing departments to exaggerate a little bit. 17.9mm may be enough of an excuse for them to call it 17mm. 380mm is close enough to call it 400mm, etc.

[sv2dgi]

Thanks a lot for your answer. It makes sense.


My only objection is that if the third reason, "Marketing" is playing its tricks, then the sites that test the lenses, should in addition to all the other parameters checked, check the ACTUAL length of the lens. This is the only way to keep companies' marketing departments in-line.

[ShutterbugJohan]

Thanks a lot for your answer. It makes sense.


My only objection is that if the third reason, "Marketing" is playing its tricks, then the sites that test the lenses, should in addition to all the other parameters checked, check the ACTUAL length of the lens. This is the only way to keep companies' marketing departments in-line.
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Pop Photo (www.popphoto.com) tests actual focal length. They also test the effective aperture. (I think.) :-)


Hope this helps.