What is this blue outline? How to get rid of it?

Hello, I rented a Canon 17-55 f2.8 lens and have it mounted on a Rebel XS. I am going to be going to new york city and wanted to take a few shots with it just to practice. However, I am having some problems getting my photos coming out clear and have this weird blue outline I noticed.

Here is a link to the full size image: http://i.imgur.com/m7Q​ED.jpg

I had it set to f20 so I figured that most of that flower would be in focus, but it doesn't appear like it. Then there is this blue outline on the flower as seen here:

Looks like Chromatic abberation

I would have thought this grade of optics wouldn't suffer from this problem. At least not to this degree.

It's CA as mentioned above. It can generally be corrected in programs like DPP and Lightroom (especially if shot in RAW) and a few other ways in photo shop.

The flower may appear soft/out of focus b/c f20 may cause too much diffraction and cause the image to appear soft. Try something along the lines of f8 or so and see.

All lenses have some level of CA. Generally, the better the lens, the lower the level. Althought the 17-55 is a pretty sharp lens, you would see a different levl of CA with the 17-40L or 16-35L

CA can show up in a variety of lenses. The 85L has a bit of a reputation for that!

ihaveworms wrote in post #10134109
Can someone tell me what is going wrong?......I had it set to f20

There is part of your problem. F20? Try and F stop below F11 with a shutter speed over 1/50th and see what you get.

Hello ihaveworms,
As someone else mentioned, part of the problem is that you have stopped the lens down to such a small F stop. Try taking the photo using an F stop using F5.6 to F8. With such a small F stop, you are using a very small area of glass at the center of the lens. I believe it is safe to say that the sharpest images (on Modern Lenses) are going to occur in the middle of the
F stop range.

This link might say it better than my attempt.
http://en.wikipedia.or​g …#Effects_on_ima​ge_quality

Regards,

110yd