How to best verify there isn't a lens problem

JerryLove · Feb 16, 2014

I have a Canon 70-200 F/4 L lens I picked up used off Amazon. I have been having the problem that image focus isn't as "tack sharp" as I've seen taken by others. I assumed it was something I was doing wrong and, playing with settings (like using high shutter speeds), it has gotten better.

But I pulled out my 100mm F2.8 to shoot with and discovered that I have much better luck with focus. Mistakes I can pin down aside (like letting the shutter speed drop and getting motion blur), I seem to be getting better photographs with the 100mm.

I realize that primes have advantages over zooms; but this just doesn't seem like it's "lens quality limitations" given the 70-200mm f/4 L.

So one possibility is that there's something wrong with my lens (or how it interacts with my T2i). Since I presume I'm well out of warranty (and could not determine original purchase date if I wanted to): what's the best way to get this checked?

Gavjenks · Feb 16, 2014

It's quite possible. I'd simply check by putting the two lenses on a steady tripod and AFing on something clear and high contrast in decent light, then compare, both at 100% magnification and at normal viewing size, and decide based on that. Realize that like you said, the prime is probably going to be noticeably better, and should be expected to be so. I wouldn't assume it was "broken" unless the 70-200 is noticeably missing focus at normal print sizes, and OBVIOUSLY so / dramatically worse at full mag.

It's easy to eliminate the variables of hand shake and motion blur...

tirediron · Feb 16, 2014

And stop down to f8 or so, as no lens is at its best wide open.

Derrel · Feb 16, 2014

Is the rear element of the zoom lens REALLY clean, and grease- and haze-free?

table1349 · Feb 16, 2014

Give this a try: http://www.dphotojournal.com/focus-test-chart.pdf

EIngerson · Feb 16, 2014

My guess is front or back focusing. The problem is the T2I doesn't have a micro adjustment to compensate for it. The 70-200 F4 is a fantastic lens. i had to micro adjust both of my cameras to it though.

JerryLove · Feb 16, 2014

Derrel said:
Is the rear element of the zoom lens REALLY clean, and grease- and haze-free?

I will check that as soon as I get a chance. Recommendations for cleaning those elements? I'll also try pulling off the filter: make sure I don't have a problem with that.

EIngerson said:
My guess is front or back focusing. The problem is the T2I doesn't have a micro adjustment to compensate for it. The 70-200 F4 is a fantastic lens. i had to micro adjust both of my cameras to it though.

Gads: I really hope that's not it. Even if I do get a new body, I would still be using the old one (multiple shooters in the house), and that's my fav lens (until I can save the pennies for a 2.8 IS version).

Dao · Feb 17, 2014

There was an old thread here about soft photos from a Sigma 70-200mm f/2.8 lens. The owner later found that the issue was caused by the UV filter in front of the lens. Once the UV filter was removed, the lens became a gem.