The ever increasing megapixel count in modern digital cameras has reached a stage where the resolution of your picture is actually limited by the quality of lens used and not by the resolution of the camera. So lens quality has assumed new significance, you need higher quality lenses to exploit the full potential of your digital camera. Higher resolution sensors amplify any minor flaws the lens has and thus makes using lower quality lenses with them absolutely unacceptable.
There are a number of things that determine the quality of a lens, here are some of the important factors to consider while evaluating the quality of a given lens.
Resolution
Resolution is one of the most important factors to consider when evaluating lenses. Resolution determines how much details can the lens resolve. Some lenses are very sharp and produce much detail and some are not that sharp, typically this is very much in proportion to the price tag of the lens. Only exception to this is some prime lenses like the 50mm 1.8 which is extremely sharp but moderately priced.
Chromatic Aberration
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| Photographic example showing high quality lens (top) compared to lower quality model exhibiting lateral chromatic aberration (seen as a blur and a green edge in areas of contrast.) Credits: Wikipedia |
Chromatic aberration is a common optical problem caused by lens dispersion. It is also known as color fringing” or “purple fringing” and occurs when different colours of light travel through the lens at different speeds and gets focused at different positions in the focal plane. In other words the lens is unable to bring light of all wavelengths (colour) to the same focal plane and as a result the image looks blurred with noticeable coloured edges with bluish green, purple, red, yellow or magenta tint. Chromatic aberration occurs mainly in high contrast situations especially when shooting subjects with a bright background. Post processing software’s like Adobe Photoshop could fix the problem to an extent.
Distortion
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| Lens distortion. Image credits: Wikipedia |
We have seen that fish eye lenses are not corrected for distortion and they produce images with a great deal of distortion in them, in reality many lenses are not perfect and even lenses which are not fisheye do suffer from distortion. This causes straight lines to be rendered with a curve. Lens distortion is most noticeable when straight lines run parallel to and near the edge of the frame. The amount of curve will vary from lens to lens and also from focal length to focal length within the same lens.
The most common kinds of distortion are:-
Barrel distortion which appears as a bulge in the center of the image (commonly found in wide angle lenses) and it makes it hard to get horizon straight as it kind of bends.
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| barrel distortion courtesy wikipedia |
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| camera lens barrel distortion courtesy wikipedia |
Pincushion distortion is the opposite of Barrel distortion (mostly found in tele photo lenses).
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| pin cushion distortion courtesy wikipedia |
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| camera lens pincushion distortion courtesy wikipedia |
And Mustache or Wave Distortion; which combines both barrel and pincushion distortions. Barrel distortion in the center and pincushion distortion towards the corners. It is called mustache distortion as it bends the lines and creates the shape of an old fashion mustache shape in the lines that follow the edge of the top of the frame.
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| moustache distortion courtesy wikipedia |
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| Camera Lens Mustache distortion courtesy wikipedia |
Distortion is something that can be fixed in software by removing the distortion from the image (though wave distortion is harder to correct). But, this correction is a destructive process. You lose image quality during the process of remapping the pixels in the image. A better alternative is a distortion free lens which delivers a better final image.
Vignetting
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| vignetting image courtesy david ball |
Vignetting (or light fall-off) is the darkening of the corners of an image. This effect is caused by more light reaching the center of an image than reaching the edges. All lenses let in a little less light at the corners. It is only that in case of better quality lenses the effect is not that noticeable. Vignetting is most noticeable when you shoot some subject that is very bright (e.g. sky) using wide open apertures.
Vignetting is not always bad, in certain cases it enhances the look of the image by focusing the viewers attention more towards the center of the frame where the main subject is located, but it could look really bad in certain other situations. Vignetting can be fixed with some in camera software and also in image editing softwares.
Flare
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| Lens flare image courtesy wikipedia |
Flare is caused when lights from just outside the frame (non image forming light) are coming in and bouncing around inside the lens causing problems. Flare is mostly caused by bright light sources.
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| camera lens flare image courtesy wikipedia |
When the bright light source is within the image frame, it produces visible artifacts. And when the light source in not within the frame but just outside; it causes haze. Haze makes the image look "washed out" by reducing contrast and color saturation (adding light to dark image regions, and adding white to saturated regions, reducing their saturation). Using a lens hood is a highly effective technique for reducing flare.
Bokeh
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| bokeh image courtesy wikipedia |
Bokeh is the aesthetic quality of the blur found in the out of focus areas of an image. Differences in lens aberrations and aperture shape (no of aperture blades) cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is rather unpleasant or distracting—"good" and "bad" bokeh, respectively. Bokeh occurs in areas outside the depth of field and photographers sometimes shoot at shallow depth of fields to create images with pleasing out of focus regions. Bokeh is most visible around small background highlights, such as specular reflections and light sources, which is why it is often associated with such areas. However, bokeh is not limited to highlights; blur occurs in all out-of-focus regions of the image.
Lens Sharpness
As photographers we need to be aware of the fact that even lenses with identical focal lengths and features could be of very different optical quality. So if you are to figure out which lens is better from a given set then you need to be aware of certain characteristics of lenses.
Most major manufacturers have some way to distinguish their pro series lenses from their standard lenses. In case of Canon they call their premium lenses L, L stands for luxury and all top of the line lenses from canon are marked L and they feature a red band on the barrel. in case of Nikon they do not have a specific category like Canon but they do put a gold stripe and a gold letter N on their best lenses. N stands specifically for Nano Coating, a type of coating they put on lenses to reduce flare. But it’s not the only thing that they do to the lens to make it good, there are several other factors that result in the quality difference between these top of the line lenses and other ordinary lenses.
What’s the best aperture for maximum sharpness?
We know for a fact that lenses are sharp in the middle aperture range. But precisely which aperture setting will work best can only be determined using some tools like the lens MTF charts, and the data in those charts needs a little bit interpretation.
If you look at the center and corners of an image you could find that in most cases center sharpness is not really a big issue, most lenses are sharp in the center, but some lenses are very very weak when it comes to image quality especially in the corners and that is where the real quality difference is felt. So there are ways of rating lenses in a very technical manner which we'll discuss next.
