Extract from
http://forums.dpreview.com/forums/read.asp?forum=1029&message=24452081
I have compared the MTF charts of the new 55-250 IS vs the 70-300 IS. Unfortunately, the Japanese site has it's chart load in java and I can't link to the chart and show them here with side by side comparison.
For those who know how to read MTFs, at 55mm vs 70mm, at with being mindful to look at the 70-300 only up to 15mm on the chart, the 55-250 is hands on sharper than the 70-300. At their longest end, the 70-300 is better, sharper at the center. The 55-250 however, at 250mm doesn't lose sharpness either. The 70-300 IS even is sharper at the edges at 300mm though the spike can't be seen in a crop camera and in most instances, it's the center of the frame that needs to be sharp. The 70-300 IS overall is 1 notch better in this area, but it does lose it's sharpness rapidly going to the edge, and it's S and M lines tend to drift apart more.
The performance of the 55-250 is akin to the non-EF-s 55-200 mk2 in terms of the S and M lines staying close together (w/c is a good sign and a sign of sharpness all around). But the newer lens is much better. It is sharper (above 7 vs 6) and better in contrast. The contrast is really much higher and this will reflect on the images of the new lens. Overall, the 55-250 wins hands down even compared to the 55-200 mk2. I believe those UD glass has a lot to do with it.
At 55mm, if you look at the 100 f2 or 85 f1.8 at f4 (extrapolate it), you will find that the 55mm of the new lens is almost like a prime in sharpness. It's almost unbelievable that a consumer zoom can do it this. But if you look at the 70-200 f4L IS, you will find that it's the same thing.
I didn't compare it with the 50 f1.8 mk2 at f4 because the f1.8 diagram isn't representative and is hard to extrapolate from that.
Also, like the 55-200 mk2, the 55-250 IS, also tend to maintain it sharpness and doesn't break down at the longer end, though the 55-200 mk2 is more consistent at f5.6 as it holds the S and M lines together (a great feat if you ask me), ,while the new lens has a loss in sharpness at the long end in the corners as far as the M line goes. The S lines stays on top and is very high, higher than at 55mm, so as far as we will see it from pictures, in most likelihood, without pixel peeping, the images at 250mm will likely be as sharp or just slightly off compared to the 55mm capture. Contrast is way better with the 55-250 IS at the long end compared to the 55-200 mk2.
Also to remind everyone, MTFs don't tell us and can't show vignetting, CAs, AF speed, AF accuracy, handling, where the apeture change points occur as we zoom, colors, and other field performance variables. However, MTFs are a good way to gauge a lens performance on paper, when the we don't have an image or sample pictures. Even analyzing after the fact, I find that most lenses do behave similarly as their MTF charts show. If we say a lens is sharp based on experience, I find that MTF charts usually explains it especially with lenses at 50mm and above. I
It is harder to read MTF charts with wide angle lenses as the lines usually swing also wildly all over the chart, not to mention that the S and M lines usually stay far apart. But medium to long zooms or primes are easier to analyze. Look at a 24mm or even the 16-35L lenses vs say an 135 prime or 100-400 and its easier to read the latter 2 lenses.
So based on the the MTF charts, I'd say the new 55-250 is sharper than the 70-300 IS on the wide end. The 70-300 is slightly sharper at the center at the long end. But the 55-250 isn't really far behind. The 55-250 also exhibit the same MTF performance of the 55-200 mk2 throughout the range, meaning the S and M lines relatively don't break apart. Assuming this is so, I expect the 55-250 to be very good at at 55-180mm. Overall, the new lens also has a better sharpness and contrast than the 55-200 mk2.
Conclusion?
Unless there are other side issues like the 70-300 IS having sharpness problems when tilted vertically, or some motor or other problems that might come out later, vignetting, etc., based on the MTF charts alone, the 55-250 IS is an excellent lens that will surprise many. In fact, if the MTF charts aren't lying, many will find that this lens will likely be very sharp at 55-135 wide open that one will find it hard to believe that a consumer grade lens took the shot. Canon may not have given us a ef-s 50-135 f2.8 IS, but if only if sharpness is as the basis alone, we might start having people comparing it with the 70-200 f4L IS instead of the 70-300 IS.
MTF comparison samples can be found here:
http://www.pinoyphotography.org/forum/index.php?topic=19730.15
P.S. The 70-300 is no "hidden L". I find it laughable that Photozone could make such a ludicrous comment...