Optimum Lens performance
“Sweet Spot” , the Optimal aperture on a lens
All lenses have an aperture setting, or range, that is called a "sweet spot" for sharpness. Each lens is different, but as a general rule of thumb, the sharpest images are taken with the aperture two f/stops down from the widest opening. So if a lens is wide open at f/2.8, then the "sweet spot" would be two stops down at f/5.6.
But what one does not realize is that one will also get "softer" images with the aperture at its smallest opening, due to diffraction (light rays get slightly "bent' as they squeeze through very small apertures, resulting in you getting progressively less sharp images beyond a certain aperture). When you use smaller apertures (towards f/22) to achieve a greater depth of field, at some point the aperture size will cause some softening due to the effects of diffraction.
Most lenses will not give you their best performance when wide open, and thus will generally give you sharper images at the area of “critical focus” - the point of focus, not factoring in depth of field, when stopped down about 2 stops. Once you reach a certain point your images do not get any sharper as you continue to use smaller apertures. If you go too far, then diffraction becomes a significant problem. This is the reason that again, as a general rule, the “sweet spot” for most lenses in the f/4 to f/11 range. This of course is dependent on the quality of the camera and the lens. Having said that, some of today's quality pro lenses perform optimally when wide open.
Top quality lenses are vital to creating “tack sharp” images. This usually means buying the Pro-grade lenses by the manufacturer of your camera. There are a lot of good third party lenses out there and some will give you some good results. But if you are looking for better than good, then your best bet is to go with the camera's brand name. There may be a few exceptions, but very few.
Top quality lenses are much more expensive and there is a very good reason for that--they are that much better. They will generally give you better image quality, faster auto focus speed, better detail, sharper focus at all apertures and focal lengths (if it is a telephoto lens), minimal image distortion, better quality of color, flare resistance, vibration correction, and build quality.
Most lenses, from the least expensive to the most expensive, are sharper in the middle of the image than they are on the edges or the corners. Pro lenses will give you better results, but the sharpness will still drop down in the corners, even if it is dropping from exceptional down to excellent. So if you have an image that requires the entire photo to be “tack sharp”, you may want to consider leaving extra room on the edges so that you can use your photo editing software to crop off the portions of the image (edges and corners) that are not “tack sharp”.
Zoom lenses also have a "sweet spot" when it comes to focal length. I talked about a "sweet spot" with regards to the aperture, but it also applies to Zoom lenses, which tend to be “softer” at the longest and shortest focal ranges, and sharpest in the middle range of the zoom. Towards the shortest focal length the lens will have the largest amount of distortion, although it may be minimal in some lenses. At either end of the focal range, the lens will be more prone to chromatic aberrations than in the middle of the focal range.
Micro-Adjust Your Lens
If you find that your tests do not meet your expectations, you can use an adjustment feature that is found on many newer, high to medium end cameras, called AF Fine Tune by Nikon and AF Micro-Adjustment by Canon. When you focus on a subject, but image is focused a little in front of the subject it is called "front focus". When you focus on a subject, but the image is actually focused a little behind the subject, it is called "back focus". When this happens, the AF Micro-Adjustment on your camera may help fix the problem. Each of your lenses may be off by the same amount or they may be off by a different amount for each lens.
Get your Lens Calibrated by the manufacturer
If you find that your lenses are off more than can be corrected by the Micro-Adjust feature, you should send your lens in to the manufacturer for re-calibration. I know it's hard, but you should also send the camera with the lenses so that they can be calibrated together. If you have the results from the tests you took, send those along as well.
Test & Practice
If you have your lenses calibrated or feel comfortable with their existing results, it’s now time to practice more. Work with the various f-stops and see what kind of results you get. Be sure to take notes on each of your shots, so you’ll know what the camera settings were—what worked and what didn’t work for each of your shots.
Most Important of all
Enjoy your photography ! Its takes an eagles eye to differentiate the sharpness of one good lens from another good lens. And to make out a sharper image between the images shot at two different apertures, you need two images to begin with. But as a photographer, while capturing the split second moment you only take one shot, just at the right moment. If you are using professional gear, bother about the sweet spot and the other things only as much as you can without missing out on the moment.
There are a few things which you can control and keep in mind
Make sure the lens is clean. Not just from the front but from the rear as well.
Do not attach a cheap poor quality filter in front.
Have a lens hood, when shooting against the light. Also, the quality of the UV filter will influence the image quality while shooting against light
Do not focus closer than the minimum focusing distance of the lens
Focus carefully and at the most important area in a photograph.
Keep the camera / lens steady and follow the shutter speed equivalent to the focal length.
Keep the ISO low.
use the widest aperture at which aberrations are minimum, which generally means stopping the lens down to 2 or 3 stops from the widest aperture.