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Old Mar 19, 2005, 6:43 AM   #21
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The thin lines are contrasts and the green do look better as well

... but very true you are comparing an 85mm to a 125mm MTF (who know where it's at 85 for Sigma) - but bear in mind where the cost is between this two lenses :?

i.e. is it good enough for the price and convenience (MTF's do not measure vignetting - see any)?
http://www.fredmiranda.com/forum/topic2/143791
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Old Mar 20, 2005, 5:20 AM   #22
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I stand corrected on which line pairs mean what, a bit, but the Sigma curves are still better. The Sigma curves for 10 & 30 line pairs are higher than theCanon black pairs up to 11mm, and 11mm is the far corners a Canon sensor. I'll try to insert here a diagram of a 11mm radius circle of the Rebel XT sensor (not sure if I know how), in any case that sensor is 22.2 x 14.8mm, and only the far corners are more than 11mm from center:


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Old Mar 20, 2005, 8:06 AM   #23
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... You can always take the square root of the sum of the 1/2 distance(s) squared (~13.34) :lol: :-) :G

which is what peripatetic was talking about:

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Old Mar 20, 2005, 5:57 PM   #24
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Please explain, is my diagram incorrect? My understanding is that the MTF charts are radii from the center, and for APS-C lenses they go to 12.5 because that is appx where the picture ends (varies from camera to camera). Why would you need to square the distance?
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Old Mar 20, 2005, 6:55 PM   #25
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the xt sensor is 22.2x14.8 mm... the diagonal is squarerootof(22.2*22.2+14.8*14.8), or 26.7mm... half of that is 13.35...
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Old Mar 21, 2005, 3:05 AM   #26
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It's easy to get carried away with comparing the MTF of one lens to another.

One reason I find the Canon charts useful is that they do show the performance at f8. I'd like to see Sigma include that info too, not least because at wide angles one frequently stops down to f8 or less to get better DOF.

Anyway - if the thick lines stay above 0.8 that's very good performance. If one lens is 8.5 and another 8.8 it would be hard to tell in most real-world situations. For the thin lines you'd like to stay above 0.6 if you can.

The only thing that concerns me slightly about the chart of the sigma is the degree of drop-off at the edges, but I don't own this lens and the samples posted here show that it doesn't seem to be much of a problem. Certainly not visible under most circumstances.

These are both good lenses. Most of the difference in results would be down to the person behind the lens rather than the lens itself.
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Old Mar 21, 2005, 9:44 PM   #27
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I see what you were calculating. But we aren't interested in the four pixels in the four corners of the picture. My diagram is accurate; it shows the MTF dropoff of the Sigma lens at 18mmaffects only the far corners of the picture.
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Old Mar 21, 2005, 9:48 PM   #28
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Yup, both excellent lenses. They are probably close enough optically to be a wash.

style="BACKGROUND-COLOR: #000000"Canon has IS, Sigma has longer reach + lower price. If they were the same price I'd go for Canon, but instead I'll pocket the extra $350 and wait for the a less expensive superwide zoom or the coming 30mm f1.4 prime.
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Old Mar 21, 2005, 10:04 PM   #29
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... or pocket the extra $200 and spring for the tamron 18-200mm?
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Old Mar 22, 2005, 5:24 AM   #30
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Really, is it out? Is it good? (the Tamron 18-200). I was amazed that is basically the same weight and size and speed as the 18-125.
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