The lens component is arguably the most important component of our cameras. The glass will time and time again be chosen over any other piece of equipment in a photographer's kit, and many a photographer will tell you they would take a dodgy body with a good lens over a top of the range body and a dodgy lens, and will fork out the money to prove this theory.
We know there are hundreds of different type of lenses you can invest in (fish eye, wide angle, fixed, macro, telephoto, tilt shift, PC etc) for hundreds of types of cameras (canon, nikon, pentax, sony etc), so it is important to do research on the lens before purchasing it and make sure it is right for your specific needs. We've learnt that each and every style of photography will have a lens that can enhance the photograph and compliment the style so you get the photograph you visualised every time.
The main component that differentiate lenses is their focal length. The focal length of a lens determines the amount of scene that is recorded. The bigger the focal length (1200 mm) the less image is recorded (close up) as apposed to smaller focal lengths (15mm) which capture a wide angle of view. Different focal lengths are appropriate for different styles of photography (however boundaries can always be pushed).
Portraiture generally utilises lenses with a focal length of around 80mm and above to ensure the frame is filled, however lately zoom lenses have become increasingly popular and are a good choice as they guarantee that you can stand back and not be in the subjects face (which some subjects find intimidating) and use a greater depth of field to blur out the background and ensure the focus is always on the subject. Focus is essential in portraiture lenses as the eyes of a model are often the most important aspect of the portrait, so you want them to be sharp.
For interior architectural shots, wide angle lenses (not fisheye) are appropriate as they have the ability to capture a whole scene at once. We've learnt that prime lenses will give the photographer an advantage in low light situations as they have the ability to shoot 'faster' and control linear distortion. External architecture is similar to interior but a little different. Wide angle lenses are an obvious choice to capture the whole scene and prime zoom lenses are good to control convergence of lines.
Landscape photography is easy to please in the lens department. Basically anything can be used depending on what type of shot you're after. Wide angle lenses are good to get sweeping shots, a zoom lens to capture a section of the landscape that you find interesting and to create depth of field or a standard 50mm lens to create multiple shots that can be stitched together in a panoramic way.
Additional to these generic sort of lenses, there are specialist lenses that appeal to a niche type of photography. These lenses can become quite expensive so it's important to be able to justify buying one of these by using it regularily. Perhaps just borrow one if that's not going to be the case ;).
Lenses have the ability to change perspective of an image, you can compress an image or create the illusion of space. It's important to know your lenses limits and it's faults (if any) and how to correct these. Of course as photographers, we are taught to try and fix everything on camera and use photo editing software as a last resort. So learn to counteract your lenses weaknesses.
Lenses can come into contact with many types of abberations: Chromatic abberation, spherical aberration, barrel and pin cushion distortion, resolution, transmission and vignetting.
There are two types of chromatic aberration: Axial and lateral. Lateral aberration is more common in longer lenses and harder to correct. You can't just stop down to reduce this aberration but it can be minimised on photoshop. Symmetrical lens design can reduce aberration, but let's be honest. There's no such thing as a perfect lens.
Spherical aberration is where the lens cannot bring marginal and axial lines to the same point. This results in a loss of contrast but can be reduced on camera by stopping down. Soft focus lens utilise this aberration.
Barrell distortion is where the image appears to be barrelled out from the center. This is because the lens cannot maintain the same shape across the image plane. This type of aberration does not affect sharpness just the shape and appearance of the subject and cannot be reduced on camera by stopping down.
The opposite of barrell distortion is pincushion distortion where the image looks like it has been squished in the middle. Funnily enough a lens can create both ends of the spectrum and cause both barrell and pincushion distortions. Again, this type of aberration does not affect sharpness and cannot be reduced on camera by stopping down. However, both of these distortions can be fixed in post production.
Transmission is concerned with the accuracy of the f-stop stated on camera. It effects exposure.
Vignetting is the darkness that appears in the corners of an image, can be corrected in photoshop however it does appeal to some people aesthetically and can even be added in photoshop for those who's boat it floats.
The shiny nature of a lens can affect it's performance. Light gets trapped and bounces around the glass and can cause ghosting and lens flare. This can be corrected by angle of view you choose to take but again, pleaases some people aesthetically and can be added in post production.
Lens testing is an important way to test for your camera's "sweet spot" It can be quite easy and beneficial, this is the general process.
Place 5 targets on a black flat surface, one in each corner and one in the center of the frame.
Set the lighting and make sure the exposure is set correcting.
Set up your tri-pod and ensure that it is balanced and level and then place your camera on it ensuring it's levelness on both planes aswell.
Focus auto or manually on the scene.
Using cable release take exposures at different stops and different zooms and critically analyse all your images.
We've learnt that you can find tonnes of information on different abberations and how to correct them or at least minimise them. This time of lens test is always going to produce different results because of different camera settings and different physical settings in which it is taken. You can utilise manafacturers resources which generally provide a means of testing your lens for its sweet spot as per maufacturers settings. As previously discussed no one lens is going to be perfect, it's basically impossible, but there are means and measures we can use to minimize these aberrations both in camera and in post production editing.