Photography Lesson: The Lens

Apart from the bells and whistles, the part that greatly contributes to the image quality is the lens.  Almost all the interchangeable lens available is composed of the lens, iris diaphragm, focus ring and aperture ring.  I’ll discuss them in turn.

The Lens

In the old days, if you needed a 50mm (normal) lens, a 10mm (ultra wide angle) lens, a 200mm (telephoto) lens, you had to buy them individually.  Modern optics has progressed to the point that you can buy 1 single lens that has macro, wide-angle, and telephoto.  For example, my old Tokina lens had focal length range from 28mm to 200mm, and had macro capability.  This was made possible by combining differently shaped lens, called compound lens. 

Sample Compound Lens (Tokina)

The objective of the lens element is to focus the image as sharply as possible

The differently shaped lens that make up a compound lens are called lens elements. The example above (left) is a Tokina 16.5-135mm Ultra Wide-Range Zoom lens made up of 15 elements in 9 groups.  The barrel is just about 3 inches long!

So why do you need all these lens shapes? Well, each of these elements aims to direct the path of light rays such that they recreate the image as accurately as possible on the film plane or the digital sensor. The goal is to minimize aberrations, while still utilizing the fewest and least expensive elements.

Optical aberrations occur when points of the image do not translate back onto single points after passing through the lens, and fall into two general classes: monochromatic and chromatic.

Monochromatic Aberration

Monochromatic aberrations are distortions that affect image quality that has nothing to do with color:

Spherical aberration is a distortion of the image caused by the shape of the lens.  Thus far, even the most sophisticated optical telescopes have yet to creat a lens free from imperfections.  Only perfectly shaped lens don’t have optical aberrration. As I’ve said, this is very difficult to achieve.

When light rays are refracted (or reflected) by the lens, the amount of refraction (or reflection) is different at the edges than in the middle.  Mostly, refraction at the edge may suffer from positive refraction (light being bent too much), and refraction near the middle may suffer from negative refraction (too little bending).  Both result in spherical aberration—the light rays do not converge at the same point.

Perfect lens (above) and an imperfect one (below) resulting in spherical aberration

An Achromatic Doublet

Aspheric Lens

The top image on the illustration above (left) illustrates the effect of a “prefect” lens wherein all the light rays are focused on a singular point.  The image at the bottom on the same illustration above (left) shows the refraction of light through an imperfect lens.  Note the sharp refraction (positive refraction) of the light passing through the edges resulting in a shorter focal length.  Also note that the refraction decreases as you go to the center where, the refraction is too weak (negative refraction) that the focal point is well beyond those formed by the light from the edge.

To correct this, lens makers use either a doublet, a special combination of convex and concave lenses (see example above center), or they use aspheric lens (see example above, far right).  The shape of aspheric lens counters the positive refraction at the edges so that it coincides with the middle.

A coma is a type of optical aberration results in images that appear to have tails (progressively decreasing afterimages) just like comets.  The head of a comet, from which spew the tail is called the coma, hence the name.

Pincushion and Barrel Distortion

Geometric aberrations is a distortion of the shape of the object rather than the sharpness or color.  These are either positive (commonly referred to as pincushion distortion) and negative (commonly referred to as barrel distortion).  Pincushion distortion is normally an effect of zoom lenses.  Barrel distortion is normally associated with wide angle lenses (most obvious in fisheye lens).  The distortions are not obvious in most cases except when the image contains a lot of straight edges, like planks, wall edges, and the like.

Chromatic Aberrations

This type of optical aberration is caused by the nature of color.  Remember that visible light is made up of different colors.  Each color has its own wavelength—from red (the longest) to blue (the shortest).

Wavelengths of different colors

When visible light (composed of different colors) get refracted through the lens, blue gets refracted most (shorter focal length), red gets refracted least (longer focal length), and green in the middle. This phenomenon is called chromatic aberration.  Lens makers create low dispersion lens to correct this.  In the example below, you’ll see some tint of color on the edges of the feathers in the second picture.  That’s what chromatic aberration looks like.  The first picture of the Bald Eagle has none.

Correct image

Color Aberration

Standard Lens vs Low Dispersion Lens to correct chromatic aberration

The resulting focal lenghts created by the different colors is called secondary spectrum.  Low dispersion lenses greatly reduce the length of the secondary spectrum to something hardly noticeable.

Why do you need to know all these things?  These things are items for consideration when you buy lenses.

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