An Essay on Lens

Photography technical geeks are always on the look-out for the best lens on the market. One of the points to note in selecting a lens is its image quality and it is quite difficult to quantify the quality by a number. The sharpness of a lens is very often talked about and is a quality which is very subjective.

There are two important parameters that we should look at when we are judging the clarity of the image, or the sharpness: the image contrast and the resolution of detail. A lens can give good contrast but poor resolution and alternatively, a lens can give poor contrast but good resolution. The latter will give better sharpness at a normal viewing distance of say, 30cm; while the former will be perceived to be sharper at 2m away. This is because our eyes can no longer see the fine details produced by the higher resolution lens; contrast of the lens becomes the determining factor for image sharpness at long viewing distance.

Lens manufacturers cannot quantify the quality of their lenses based on subjective characteristics. They publish an extensive test report in a graph known as the “modular transfer function” (MTF) information. In this test, the subject is a test chart of grids. A grid consists of pairs of black and white lines. And the number of line pairs per millimeter (lp/mm) is quoted. The larger the lp/mm means that in 1mm we squeeze more B/W pairs (high spatial frequency) into that space; the ability for a lens to resolve the lines means that we have a high resolution lens. Low lp/mm represents a coarse detail or low spatial frequency. Now we have taken care of the resolution part when it comes to determining the sharpness of a lens.

The next term is modulation. This term is a quantification of the change between the black and white lines. A perfect lens would give 100% black and 100% white at the border of a line pair. In a real world, aberrations and diffraction would make the black line fade gradually into the white line (the black becomes less black and the white becomes less white). The original 100% contrast between a black line and white line on the test subject grid becomes graduated when the light from the subject is transferred through the lens. Modulation is therefore always <100%.

What happens if we plot two lenses MTF on the same graph and plotting percentage modulation against a wide range of lp/mm? It doesn't tell us anything about image quality across the image field, but it makes it easy to see how responses worsens as details become finer. The greater the area under the curve, the better the lens. If a lens is good at resolving broad details, it would make a good video camera lens while a lens good at resolving fine details would make it a better photographic camera lens.

Lens should be bought for a specific purpose in mind. We can always fit an extension tube to a telephoto and focus closer but the image quality will not be as good as a macro lens which is specialized for close-up photography. An aerial survey lens will give results for distant subjects. A lens for enlarging or copying will have optics correction for close subjects. A shift lens will have a very large covering power (to achieve this, the lens aperture is not likely to be large), therefore, likely a wide angle (Have we seen a telephoto shift lens?). By right, digital backs camera is giving very high resolution compared to silver halide films. Therefore, digital optimized lenses should have their circle of confusion (the smallest optical spot) no larger than the distance separating pixels on a CCD or CMOS.

And a tip here, do not clean your lens too often. You will cover your lens with scratch marks. This will more seriously affect the quality of your image than a speck of dust.

I haven't post any images here to illustrate how to read a MTF graph. This is to prevent any infringement of copyrights. We can have a chat if you are really interested to learn how to read one.

posted byWilliam at 1:02 PM  

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