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[COLOR=darkgreen][B]No.7[/B][/COLOR][COLOR=black]: Know where your lens performs best.[/COLOR]
The simplest form of lens is formed from one piece of glass, or other refractive material, and is either in focus or not. It's either working at its best, or it isn't.
Typical camera lenses are [B][COLOR=darkgreen]compound[/COLOR][/B] optical designs, utilising several lens elements and an [COLOR=darkgreen][B]aperture[/B][/COLOR] iris to alter the amount of light transmitted through the lens. You can change the [COLOR=darkgreen][B]focus[/B][/COLOR] of the lens according to the distance of the subject. And then we have [B][COLOR=darkgreen]zoom[/COLOR][/B] lenses, where the focal length of the lens can be altered to capture different fields of view.
As you may be beginning to comprehend, camera lenses are quite complex. In fact the job of designing lenses is a huge job, involving vast quantities of computing power in order create an arrangement of lens elements that will result in minimal image distortion and aberrations, as well as maximum image sharpness and contrast.
Lens design is a compromise, dictated by cost, design goals (juggling contrast, resolution, distortion, etc.). Lens performance has also improved a great deal in the last 20 or so years. Inexpensive zoom lenses can now sometimes perform better than fixed focal length or 'prime' lenses from yester-year.
The Digital era has set new challenges for lens designers, too. For compact cameras, the lenses are smaller than ever, matching the frame sizes of tiny sensors that are tiny fractions of the area of old film camera frames. The way the light is received by a digital sensor also requires optimised optics as the further into the corner of the frame you go the less of the light transmitted by the lens is received by the sensor.
Until relatively recently, a typical camera lens was not generally expected to perform particularly well at 'full aperture' - or when the lens aperture was at its widest, and brightest. You needed to 'stop' the lens down one or two f-stops to crisp-en the image up.
Today, lenses often perform at near their best at full aperture, especially so with compact cameras, because there is little scope for stopping the lens down before diffraction softening happens because of the physically tiny apertures these cameras rely on.
So here is a simple guide to discovering the sweet spots of a lens:
[B][COLOR=darkgreen]1. Focal length:[/COLOR][/B] If you have a zoom lens, it's likely that its performance will be best at a particular focal length. You will often find that a lens is sharper at one end of the zoom range than the other. Other quality parameters, like distortion and contrast, will be better at one section of the zoom range than another. You could take test shots at different zoom settings to get a fee for where your zoom works best.
[COLOR=darkgreen][B]2. Aperture:[/B][/COLOR] Cheaper lenses may need to be stopped down in order to avoid full-aperture softness. This is less evident with high performance lenses. But that's not all - stop down too far and you will eventually encounter diffraction softening. Again, take test pictures with your lens aperture altered for comparison purposes and you should be able to detect at which aperture the lens starts to lose sharpness.
[B][COLOR=darkgreen]3. Vignetting:[/COLOR][/B] If you notice that the corners of your picture are darker than the centre, that's 'vignetting', also referred to as 'corner shading'. Vignetting is normally worst when the aperture is at its widest, so this is another good reason for stopping down. Lenses not optimised for digital sensors will show more evidence of vignetting than modern lenses.
[B][COLOR=darkgreen]Tomorrow:[/COLOR][/B] Understanding DPI and PPI
[I]Incidentally, please don't hesitate to post a question about this Daily photo tip if you have one![/I]
I spoke to a well respected expert in camera optics a while back and he felt that Canon's lens design team, for example, excelled at telephoto type lenses, but weren't outstanding when it came to wide angles. I'm sure that other companies will have certain other design strengths and weaknesses to varying degrees. The legacy factor also comes into play. Nikon's 50 year old F-mount has a very narrow diameter in relation to the sensor size and this makes it much more difficult to design lenses that can illuminate the corners of the frame evenly. Canon, Pentax and Sony (Minolta formerly) have significantly wider lens mounts, while Olympus has the widest of all in relation to the sensor size. In terms of specific lenses, no, each will have its own unique characteristics.