Lower quality zoom lens or cropping higher quality wide angle?

HappyHiker wrote:

Hello everyone - first time posting here so apologies if there's a better place for this question.

It's a simple enough question - and probably a stupid one - but I'd love it if someone more experienced than I am can advise!

I've got a 7D with three lenses: the Canon 15-85mm, the Canon 55-250mm and the Canon 1.4 50mm prime. I'm hoping to take some photographs of fireworks on New Year's Eve, as I got some fairly spectacular ones before using a borrowed Sigma 70-300mm on an old 450D a few years ago. However, my vantage point for the firework display is quite a long way off - previously I've got great pictures from the same spot at around 130mm, so I'm trying to work out which lens would be my best choice for Monday night. My tripod is nice and sturdy, so I'm not worried about that.

The 15-85mm has a much higher IQ than the 55-250mm (which seems to produce 'flatter' and less punchy pictures in comparison), but obviously it won't zoom as far. Am I better off using the 55-250 and zooming to approximately 130mm, or should I use the 15-85 at full zoom and just crop the final images? I'd obviously like to keep my final images as large as possible while maintaining image quality. The 7D produces pretty big pictures to begin with - but since you only really get one shot at photographing fireworks I'd like to get it right!

Any advice would be hugely appreciated! Many thanks in advance.

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.