Defects in Lenses – All that you need to know !!!

Defects in Lenses – All that you need to know !!!

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In the real world we live, a perfect lens is almost impossible to design and manufacture. Almost all lenses that exist produce images which have one fault or the other. These are caused by optical defects that are divided into two classes called distortions and aberrations.

Remember that while both distortion and aberration are optical defects, the first deals with the quality of reproduction or fidelity of an image. That is, straight lines not being rendered straight thus distorting the shape. Aberrations are normally associated with effects that cause loss of sharpness of image.

First, let us see how an “ideal” distortion free lens behaves. If you photograph a grid pattern with such a lens, the image it has captured will look exactly like the original pattern. In reality this is not what you get. The grid changes shape or in other words gets distorted. The most commonly occurring distortions are called barrel, pin cushion and moustache. Let us look at these in more detail.

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Barrel Distortion: Here the image looks as if it has been stretched over a sphere causing the edges to bulge. Wide angle lenses are especially prone to barrel distortion. A high quality wide angle lens should be free from this defect. You need to consider this aspect if you are photographing subjects with linear features (straight lines) like buildings, interiors, etc. So, if architectural photography is going to be your main area of interest, then you need to look for a lens that has minimum distortion as buildings with curved walls look unnatural. There are software packages now that can correct this distortion to a great extent and some cameras can do it themselves once they know the lens profile. However, it is best to start with a lens with low distortion if your work is critical.

An ultra wide angle lens with no distortion – a truly “rectilinear” lens (as opposed to a fisheye lens which has severe built in barrel distortion) would be very difficult to design and costly. The Zeiss Biogon super wide lens used on the Hasselblad SWC camera is for all practical purposes distortion free. It has a price tag to match that performance!

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Here is a picture taken with a lens that has barrel distortion. Note how the two columns are bent like the sides of a barrel.

Pincushion Distortion: This is the opposite of barrel distortion. Here objects at the edges look as if they have been tucked in towards the center. This sort of distortion is most prevalent in telephoto lenses.

Moustache Distortion: This sort of mixes both barrel and pin cushion distortions and is somewhat less common. Here the distortion is first of barrel type towards the center of the image but turns into pincushion towards the edges.  Normally zoom lenses exhibit this.

That is about distortions. Now, let us look at aberrations that you will come across. The common ones are chromatic aberration, spherical aberration and coma.

Chromatic Aberration: As you know light is composed of components of several different wave lengths. These different components focus at different points causing chromatic aberration or color fringing. This is especially visible at high contrast boundaries as a light purple colored band.

There are several ways of reducing this. One is to use low dispersion glass for the elements. This type of glass is called by several names commercially as ED (extra low dispersion), LD (low dispersion) and so on. Some manufacturers use fluorite elements for the same purpose.

It can also be reduced by certain design techniques called achromat and apochromat designs in which multiple elements are used together to correct this aberration as well as spherical aberration.

You can now use software programs to reduce chromatic aberration. Some cameras can even correct this when they capture an image.

 

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Spherical Aberration: Lens elements have spherical surfaces as they are easy to manufacture. However, this gives rise to spherical aberration. Due to this light rays parallel but offset to the lens axis (that is, those which are at the edges of the lens) and rays closer to the lens axis come to focus at different points. This causes loss of sharpness. One way to rectify this problem is to use “aspherical” lenses. (Note: you cannot make out if a lens has aspherical elements or not by looking at the lens unless it is explicitly written so, on the lens) These were once very difficult to manufacture. Now, modern techniques like glass molding, hybrid elements where an aspherical resin based element is combined with a spherical glass element have brought the costs down. Lenses that use aspherical elements are called aspherical lenses. There are other ways to cure the spherical aberration but at present use of aspherical elements is the most popular one.

Coma: This is another type of aberration that is closely related to spherical aberration. In this, points especially point light sources take a comet like appearance. It occurs when rays are incident on the lens at an angle. Once again rays that pass close to the center and those who which pass through the edges do not come to focus at same point causing this aberration. The remedy is similar to the one that is used to rectify spherical aberration.

Interestingly there are many software packages that can correct various distortions and aberrations. The correction can be done manually by the user or even automatically. For the latter, the manufacturer of the lens needs to provide the lens characteristics (called profiles). Once the profile for a particular lens is known, the software package can use this and correct the defects  precisely and automatically.

Here are a few more issues that concern the performance of a lens:

Flare: A modern photographic lens will at least have four optical elements. While this is done to reduce optical defects mentioned, use of multiple elements can give rise to the problem of internal reflections. These can cause what is called a flare. In one form this results in light being scattered throughout the lens and has the effect of reducing the overall contrast of the image.

Another form of flare, sometimes called “ghosting” results in bright patches with the same shape as the diaphragm. This is due to the light reflecting off the diaphragm blades and striking the sensor.

Flare frequently occurs when a light source is shining on the lens but is not visible in the frame. Extreme wide angles lenses are more prone to flare due to the large area they cover and also due to the large number of lens elements used in their design.

The lens elements are normally coated to reduce flare,. Coatings are extremely thin and cause the colors you see when you look into the lens. Coatings improve the picture quality significantly. Another simple way to reduce flare is to use a lens hood.

Diffraction:  This is caused by the light rays dispersing when they hit an opaque object like the diaphragm. This becomes more acute with smaller openings (small apertures). Due to this a light beam will register as a small circle rather than a point on the sensor with subsequent loss of sharpness. This is the reason why the smallest aperture is limited to around f/8 on short focal length lenses used in digital point and shoots and to about f/22 in the case of D-SLR lenses. Large format cameras have much larger focal lengths and therefore a bigger opening for the same f/ number. Hence, in these cases you can stop down further (up to f/64 or even more) before diffraction effects become more prominent.

Diffraction Limited Lens: Every lens will be limited by diffraction effects causing loss of sharpness. The effect will be less for lenses made for large format as explained, but is nevertheless there. This just cannot be avoided because a lens has finite opening even if the aperture is wide open. This is a theoretical fact based on laws of physics and is not a defect. Added to this will be the aberrations which will further reduce the sharpness. So, the point you need to remember is that while aberrations can be reduced or even eliminated in highly corrected lenses, diffraction cannot be avoided. A lens that has been manufactured so that it is only limited by diffraction (that is – it has no aberrations) is called a “diffraction limited” lens. Such lenses are not normally manufactured photographic purposes. However, there are some photographic lenses (examples are, Leica 50mm f/2.0 APO Summicron, and Zeiss 135mm f/2.0 APO Sonnar, etc.) that are regarded as being (or close to being) diffraction limited lenses. Expect to a pay hefty premium for such optics!

Sweet Spot of a Lens: There is a general contradiction in handling diffraction and aberrations. For example small apertures (large f numbers) increase diffraction while aberrations reduce.  To get optimum performance, it is advisable to use the lens at its “sweet spot” which is normally, but not always at about two stops stopped from its maximum aperture (smallest f number).  You can conduct a few tests at different apertures and find out the sweet spot of your lenses and then use these apertures for maximum sharpness whenever conditions permit.

The points mentioned in this article will give you a much better understanding of the various comments made by testers in the reviews that are published in magazines and on the Internet. They will also help you to make a more knowledgeable decision when you purchase your next lens.

 

All text and image © Ashok Kandimalla

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