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After considering what kind of lens you're looking for and what the options are from various lens manufacturers, you're ready to start looking at the specifics of your individual candidates. There is only one single feature that should be your make-or-break kind of deciding feature for a lens and that's image quality. No matter what extra features the lens has, if it can't deliver a good image, you should eliminate it from your range of choices. Evaluating image quality can be broken down into just a number of simple kind of steps and parameters. And here are the things that you want to consider. Sharpness. Sharpness is a tricky thing because there is no objective standard for sharpness. In optical circles, they often refer to sharpness as acceptable sharpness and what's acceptable to you is really all that matters. Contrast. And I am going to relate this to color also. Just how good does the lens deliver rich colors and nice contrast with a lot of pop. This again is a somewhat ineffable indefinable quality. You have just got to look at some images and see what you like. Lens flare. These are those weird circular reflection patterns that you get if you point the lens into a light. Wide-angle lenses are far more susceptible to flare than telephoto lenses. And the way to check for this is to put the lens on the front of a camera and watch the edges as you move the lens towards the light source. See if you get any bad flare problems. It can be difficult thing to really force the lens into flaring, but if it happens very easily, you know that that lens probably has a flare problem. Distortion are those weird spherical geometric things that happen mostly with wide-angle lenses, and we saw examples of that before. That's where the corners and the edges get all warpy. Horizontal and vertical lines start to bend. Spherical distortion like that is certainly something you really want to check for when you're looking at a prime lens, but it's also true with zoom lenses. If a zoom lens has a really wide angle end to it, you want to test for distortion and flare there also. So, you are going to want to take your zoom lens, zoom it out all the way, see if you can point it at some light to get it to flare. Pan it around the room, watch the corners, and see if you see distortion. If a lens is not good enough to focus all wavelengths of light to precisely the same point, you get something called chromatic aberration. You may have seen this with the kit lens that came with your camera because they very often have these problems. Where you will see it is shooting a high contrast situation. So maybe leaves up against a bright sky, telephone wires up against the bright sky, the line of a rooftop up against the bright sky. Rather than just a nice smooth edge, you will have an edge with purple or magenta, sometimes green or cyan fringes around it. That's because those wavelengths of light are scattering, rather than being focused to the same point. You can fix that in your image editor, but you really don't want to count on your image editor to solve problems. You wan to shoot good images in your camera. It's difficult to test for that on your own, but lens review sites will very thoroughly test for and tell you about chromatic aberration. Bokeh, as we saw earlier, is the softening that happens with-- lens softening in the background that happens when a lens is open to its widest aperture. So, if you are buying a really fast lens, you want to evaluate the bokeh, if you are the type of person who's picky about that sort of thing. If you're buying a fast lens, you're probably spending a lot of money so you might as well be picky about bokeh. Just take a look at it and see if you like the shape of really bright highlights in the background. If you like them round, if you like them more polygonal, that's going to be the difference in bokeh that may help make your buying decision. So how you evaluate all this stuff? Obviously reading lens reviews is one thing. The web is full of lens review sites and camera review sites. A lot of those sites will let you download full-res images shot with particular lenses. That's a nice way to get images into your image editor where you can look at them up close. Best thing to do though is, if you can, rent the lens that you're considering or even a couple lenses that you are considering. That gives you a chance to take them out and shoot with them on your own, really take them for a test drive. Online rental places like borrowlenses.com are great for that. Your local camera store might have a rental house attached to it that will let you get a lens out the door and shoot with it. Ultimately, the thing to remember though is, image quality, image quality, image quality. That's what should define your buying decision and that's what you want to consider very carefully before you make your final purchase.
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