Lower quality zoom lens or cropping higher quality wide angle?

HappyHiker wrote:

Great Bustard wrote:

Assuming your lenses perform anything like the lenses PZ tested:

http://www.photozone.de/canon-eos/465-canon_1585_3556is?start=1

http://www.photozone.de/canon-eos/465-canon_1585_3556is?start=1

the 55-250 at 130mm will deliver significantly more resolution.

The 15-85 at 85mm resolves (lw/ph -- center / edge) 2362 / 1846 at f/5.6 whereas the 55-250 at 135mm resolves 2157 / 2069. So, the lenses resolve essentially the same.

However, if you crop 85mm to 130mm, you retain only 65% of the resolution, which means that the 15-85 cropped to 130mm will have only 65% the resolution as the 55-250 at 130mm.

I note that you say how the 15-85 has "much higher IQ" than the 55-250. The possibilities are:

• Your 55-250 is substandard.
• The 15-85 PZ tested was substandard.
• PZ's test was either jacked or the MTF-50 measurement does not accurately represent IQ.
• The AF of the 55-250 is spotty compared to the 15-85, causing lower IQ in many photos.
• Your assessment of the relative IQ of the two lenses is in error.
• The types of pics you take with the 15-85 have more appeal to you than the types of pics you take with the 55-250.

In any case, if I had to bet, I'd certainly bet on the 55-250 delivering the better photos in the circumstances you describe.

Wow - thank you for the detailed and number-heavy reply! It was very useful. I think it's quite possible that my assessment could be partly to blame - I bought the 7D and 15-85 at the same time, which gave me a pretty big jump in IQ from the old 450D anyway, so that may also have influenced my perspective. Mind you, I've tried taking photographs of the same subject using both lenses and I still personally feel as though the 55-250 gives a 'duller' raw picture than that with the 15-85. It could also be that the 55-250 I own is substandard but I'm probably not enough of an expert to judge effectively!

Given that I'm using Lightroom, I would hope that any photos taken on the 55-250 for the display could be given the extra punch - crunching the numbers to show that I'd only be at 65% resolution if I cropped the 15-85 made me realise quite how much of the resolution I stood to lose! It sounds as though the 55-250 would probably be a wiser choice given the circumstances. Thanks again for your time and useful suggestions.

BengtS wrote:

Assuming your lenses perform anything like the lenses PZ tested:

http://www.photozone.de/canon-eos/465-canon_1585_3556is?start=1

http://www.photozone.de/canon-eos/465-canon_1585_3556is?start=1

the 55-250 at 130mm will deliver significantly more resolution

Based on PZ, I also bought the 55 250. My findings are similar to yours. The pictures looked flat. For that reason, I returned it. If that was just coincidence, I dont know.

It makes me feel a little better that I'm not the only person that thought that!

I think that is not unusual. Recently posted comparisons of the 18-55 IS to the 15-85 also showed the 18-55 IS be "flatter".  The two kit lenses are good for the price, and the resolution figures are good. But I think one area where they are inferior to lenses such as the 10-22, 15-85, or 17-55 f2.8 IS is the contrast. The 18-55 IS and 15-85 handle contrast rich situations less well and tend to flare more. Images will be more "flat", i.e. less contrast rich when bright light comes from the front (e.g. overcast cloudy white sky can be enough for this). You can correct for this to some extend of course in postprocessing in the computer.

Given the limitations of the kit lenses (18-55 IS, and 55-250 IS), you might perhaps be better off with the 15-85 for fireworks in terms of contrast (not resolution wise), because there you will have strong light against black sky (presumably). I have never tried my 55-250 IS with fireworks, but presume you should be able to find samples on the internet.

see also http://forums.dpreview.com/forums/post/26136829