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/411-canon_55250_456is_50d?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.

Makes sense.  If you used the 55-250 on the 450D, then got the 7D and added the 15-85, the 15-85 would definitely appear to be the better lens when compared to how the 55-250 performed on the 450D.

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!

Well, when it comes to your pictures, it's your opinion that matters in the end, right?  That said, maybe sometime you could post a pic of the same subject with the same settings (focal length, aperture, and shutter speed) -- might be interesting to compare.

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!

Yep -- cropping robs a lot of resolution.  Something to keep in mind when discussing primes vs zooms.

It sounds as though the 55-250 would probably be a wiser choice given the circumstances.

The resolution loss from cropping is not quite as severe as I made it out to be, since you'll be keeping the central portion of the photo which resolves better than the edges that are going to be cropped off.  So let's say you'll retain 75% of the resolution, rather than 65%.

That said, assuming the PZ test is representative of your lenses and also representative of perceived resolution in the photo, the 55-250 at 130mm will still be the better choice, by far, than the 15-85 at 85mm cropped to a 130mm FOV.

Thanks again for your time and useful suggestions..

Of course, you could always test it for yourself.  Choose a scene at distance that has a lot of detail in it, and take a pic at 85mm with the 15-85, then at 130mm with the 55-200, crop the 85mm pic to the same framing, display (or print) both at the same size, and compare.