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Canon EOS Crop Sensor for HD
APS-C sensor cameras including the 80D, 70D, 7D Mk. II, 7D, EOS M and Rebel models for HD video recording.


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Old March 5th, 2011, 08:42 PM   #1
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10x zoom and the lens sweet spot

Pondering the T3i's 10x digital zoom,
and beginning with the assumption that all lenses have sweet spots (the higher the quality, the less dramatic the difference is in 'sweetness' across the lens), and the sweet spot is typically away from the edge and toward the center...

can we expect that the 10x zoom will be cropping the edges of the lens and utilizing the center portion, thereby eliminating the weaknesses of the lens found toward the outer edge eg soft focus, chromatic aberration, and other distortion?

[if I understood lens optics better I would attempt an answer, but it's beyond me for now :) ]
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Last edited by Scott Brickert; March 6th, 2011 at 12:15 AM. Reason: added new opening phrase
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Old March 5th, 2011, 09:24 PM   #2
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Re: 10x zoom and the lens sweet spot

I'm not a lens expert but I don't believe that's how it works. Do not take this as gospel at all, this is only my very elementary interpretation of all the information i've read.

In my understanding, when a lens zooms, the distance between the elements is increased, thereby reducing the angle of view. So the way I understand it, it will still be using the full surface area of each lens element to generate the image circle.

I also understand that as the distance between the lenses increases, the margin for error becomes greater, which is why chromatic abberations usually become worse at the longer end of a lens zoom range.

Keep in mind that if you are only using the centre portion of the lens, you might get rid of the softness and CA at the edges but the crop will make the centre portion of the lens less sharp as well.

(edit - just saw that you were referring to the crop/digital zoom of the t3i, as opposed to a 10x zoom lens. Apologies for the somewhat unhelpful post)

Last edited by John Wiley; March 6th, 2011 at 01:56 AM.
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Old March 5th, 2011, 10:41 PM   #3
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Re: 10x zoom and the lens sweet spot

Hi Scott,

Cropped sensor cameras do use the center of the lens' image circle - they suffer less vignetting with full-frame lenses because of that - but remember that just by doing the math it seems like any further than 3x will severely cut into your resolution. The 60D/7D/T2i/T3i have a pixel count of 5,184 x 3,456. They normally use the whole sensor to produce the image, but at 3x it will already be down to only using 1728 pixels across. Or am I wrong in that figure? And by 10x its down around 518 pixels across? It seems that the option to choose an area of 1920 pixels across for a magnification of 2.7x would be ideal. Or maybe that is in fact what they are doing and just rounding it up to 3x in the specs??


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Originally Posted by Scott Brickert View Post
Beginning with the assumption that all lenses have sweet spots (the higher the quality, the less dramatic the difference is in 'sweetness' across the lens), and the sweet spot is typically away from the edge and toward the center...

can we expect that the 10x zoom will be cropping the edges of the lens and utilizing the center portion, thereby eliminating the weaknesses of the lens found toward the outer edge eg soft focus, chromatic aberration, and other distortion?

[if I understood lens optics better I would attempt an answer, but it's beyond me for now :) ]
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Old March 6th, 2011, 12:22 AM   #4
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Re: 10x zoom and the lens sweet spot

I just ordered a T3i, so when it arrives in a couple weeks I'll attempt to answer my own question :)
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Old March 6th, 2011, 02:10 AM   #5
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Re: 10x zoom and the lens sweet spot

Sorry Scott, I misunderstood your original question. I thought you were referring to a 10x zoom lens as opposed to the t3i's digital zoom.

The t3i's 3x zoom funtion is apparently very good from what I've heard so far, but beyond that it is essentially a digital zoom and you lose quality.
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