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Old Oct 25, 2004, 10:57 PM   #1
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I am buying a 20D and need to get wide angle for interior shots. What I don't understand is why I constantly see the EF-S10-22 discussed with the 35mm conversion next to it i.e. 16-35mm. If the lens is designed for the C sized sensor why is there any crop issue? Isn't this lens simply 16-22mm as in 35mm?? If not what is thenew lensdesign designationfor??Definitely confused!:-)

I would actually prefer to get a regular EF lens so I don't get locked into the C sized sensor forever. But, this seems impossible with a 1.6 crop factor for regular lenses.

Thanks for any clarification!
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Old Oct 27, 2004, 3:19 PM   #2
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The 10-22 lens has a range of focal lengths, and when put on an optical bench these numbers can be measured. Therefore, the 10mm to 22mm range is in accord with optical theory and practice.

The 10-22 lens can only be mounted on EF-S lens mounts. The only cameras available on this lens mount have a 1.6 crop factor (or focal length multiplication factor). So, what you see when looking through the view finder "looks" like what you would see from a 16-35mm lens on a full frame (or film)35mm camera. In effect, since the sensor is only 15mm by 22.5mm the sensor looks like a 1.6 crop of what would have been recorded on the center of a 24*36mm film/sensor.

This cropping makes the lens look like it has the longer focal length, but at the same time, the lens looks to have the depth-of-field of a lens with the focal length and aperture it actually has (10-22). So, if you metric is image field, it looks to be magnified; but if your metric is DOF then it looks to have the DOF of its printed FL and aperture.

The only realistic alternative to Canon's 10-22 is the Sigma 12-24 that is available in EF mounts. I, myself, am waiting for the reviews to come out on the 10-22 before buying either the 10-22 or the 12-24.
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Old Oct 27, 2004, 8:14 PM   #3
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Thanks for the response!

I "think" I understand the crop issue but I don't understand why Canon developed these "digital" lenses ie the EF-S lenses specifically for the Rebel and 20D if it was not to solve the crop problem. I've read that they are designed to be closer to the sensor plane which I thought was to solve the crop problem but that's not it. Why buy these lenses if they operate the same as a regular EF lenses? Still confused :-?
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Old Oct 27, 2004, 11:46 PM   #4
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Hi there,

I will have stab at this one. Ihave also thought about this beinga 20D owner. However I am no expert so these are just some ideas and could be incorrect.

Yes there is still a crop, but, it isa only a logical crop. The 35mm comparison with an EF type lense is a standard and when we compare to the 35mm EF lens standard the EF-S lenses on a 20D perform differently.The image circle itself (the circle of lightrefracted in the lense onto the sensor) is not cropped with an EF-S lense, the image circle fits correctly the size of the smaller sensor ofthe 20D. But as a comparison to an EF lens on a 35mm cam, we can think of the EF-S lense on a 20D as a crop (this is what 'appears' to occur, although optically this has not occured).

For example at focul length 20mm with an EF-S lens on a 20D, the image circle is not cropped at all. But this lens would have the same field of view as a EF lens at 32mmon a fullsize sensor(1.6*20). This is a comparison point, a standard, but there is no optical crop.

Why do this? Becuase of the smaller sensor size on the 20D and 300Dthe image circle required from the lens is smaller.Smaller image circle means less glass required to build any givenlens which equates to aless costlybuild for lenses.

Hence, yesless field of view from our lensegives a crop in acomparitive sense (i.e. when compared to the 35mm EF lense standerad), but with the EF-S lenses weget alenses at a lot lower price than if there were no EF-S lenses.

Hope it helps (and hope its right...?).

Cheers, Tim

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Old Nov 10, 2004, 7:50 AM   #5
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If you compare the same lens on a budget DSLR and a full-frame SLR, you'll see that the magnification is the same, but that the budget DSLR loses the outer parts of the image. This is what I'd call the crop. It's imagining that you had the potential for the whole 36x24 mm image on the sensor, but the manufacture cropped off the edges of the sensor to make it 22.5x15 mm (perhaps they used the rest of the sensor to make a couple of other digicams OK, I'm kidding about the literal crop, but you should clearly understand the figurative crop -- the sensor is simply smaller, so you get less image with the same lens. You get the same center, but not the edges -- they're "cropped".

36/22.5 = 24/15 = 1.6. This is where the "1.6x" number comes from.

Now, if the sensor is 1.6x smaller linearly, then why do people say that you get 1.6x "focal length multipler"? Because that crop acts just like what a 1.6x larger lens would give you in a field of view - you would lose the edges when zooming in closer. When you post-process thebudgetDSLRimage, to give it the same print/image size as you would have with a full-frame camera, you must magnify the image the same extent to reverse the crop -- i.e. 1.6x. So this is like getting a 1.6x greater lens, but you should clearly understand that this is essentially post-processing and loss of field of view, and not a magnification of the sensor.

It is important to understand that the focal length is actually not changed, despite the so-called "focal length multiplier". The depth of field depends on the actual focal length, not the "effective focal length". (There is some consideration of the print re-magnification, but that's less important than the actual focal length.)

So this is why they can't say that it's a 16-35, because it's not. It's actually a 10-22, but it has a field of view similar to a full-frame 16-35. Now you can completely ignore the "35mm effective" if you wish to -- the only reason for mentioning it, for all types of cameras with different image sizes -- is because it gives a point of reference of field of view that many of us are used to / can use.

My position is nearly in complete agreement with Mitch's. The difference is that I would not say that looking through the viewfinder of a 10-22 budget DSLR viewfinder is the same as looking through a 16-35 full-frame viewfinder. I prefer to say that looking through the same lens using the two different cameras would look the same, except for the crop of the edges. This is a slight distinction of perspective, but one that I think is helpful for clearerunderstanding.
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