Canon 100-400mm L f4-5.6 IS How can I tell if my lens is sharp?

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This is my real first lens so I dont have much to compare with.

How do I know if my lens is sharp and what can I do if its not? Can it be serviced?

For a few months I have thought that many of the picutres are hit and miss on the sharpness. Some pictures look nice while others look like it looks like a still from a video camera.

Today just for comparisson, I took a picture of a bird feeder with my 100-400mm at 400mm and then with my dad's Sigma 100-300mm f/4 EX DG.

When I looked at both pictures uncropped it seemed the picture from the Sigma was a little sharper. I blew the Sigma image up to 100% and the Canon image to 80%, this made them the same size on the screen and the Sigma image was much sharper.
Putting the Canon image to 100% and the Sigma to 130% (So the same size on the screen again) even though the Sigma image is now enarged 30% it still sharper.

Is there any test I can do to properly test the sharpness of the lens? If it is found to be a little soft can it be "serviced"?

P.S The camera I use is a Canon 50D.
 
try micro-focus adjustments.

Lenses can be serviced and re-calibrated but from the 50D onwards they introduced microfocus adjustments which allows a little bit of calibration to be done by the user.
 
andrewc said:
try micro-focus adjustments.

Lenses can be serviced and re-calibrated but from the 50D onwards they introduced microfocus adjustments which allows a little bit of calibration to be done by the user.
Click to expand...


Hi Thanks for your quick reply.
I have played with the micro adjustment and it was at its best on default.

I may have found the answer myself.
The quality of the lens is terrible at 400mm. I noticed another pic of the bird feeder which wasn't a part of my test which was very sharp (well compared to the others) But I did notice it was shot at 370mm.
I tested this by taking picture of the moss growing on the garden fence from the bedroom window. One pic is at 400mm and the other was 360mm and the quality difference is huge.
Both of these shots are a 100% crop


4757120007_e39967ee80_b.jpg

@400mm

4757120133_1dacd75993_b.jpg

@360mm

Is it me or is there quite a difference in quality?
 
might be worth posting an uncropped image as well so we can see what the whole image is doing...
 
That's a Canon L lens. Unless it's old and abused, there's unlikely to be anything wrong with it. Long lenses are much harder to use than some people think, needing very critical focus and much higher shutter speeds to avoid camera shake.

But it's always possible that something's amiss, and it's very easy to tell if you have another similar lens of known good quality to compare it against. Which you do! Just shoot the same target, at exactly the same settings on the same camera. Focus very carefully, ensure there is absolutely no danger of camera shake, and make sure the light doesn't change between shots as the diference between sun and shade will throw everything. If there's anything wrong, you'll see it immediately.

If you want to check focus accuracy, use a target at a reasonable distance. If you shoot an artificially close target (like the one linked to above) especially with a long lens, you will quite likely find problems where none exist, and make adjustments that will actually make the focus accuracy worse at normal shooting range.
 
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