Hours of Idleness-A Photographer's Journey in St. Louis

Lenses

Posted in 35mm, photography, technique by Jason Gray on July 25, 2010

“A painter is in love with painting, not scenery.” -Ad Reinhardt

And, if you are an aspiring photographer, you are probably in love with lenses (whether you realize it yet, or not).  A Lens is a disc of transparent material, usually ground glass, that has one or more curved or sloped surfaces.  Lenses project an image into the camera by bending (refracting) rays of light as they pass through the surface of the lens.

Example:

In photographic practice, there are two types of lenses:

1. Simple Lenses

2. Compound Lenses

Simple lenses are the kind you are most likely to encounter for most types of photography.  They consist of two categories:

1. Converging Lenses (also, Positive or Convex Lenses)- are thicker in the middle of the ground glass discs than at the edges.  When rays of light that are parallel to each other in wavelength pass through these lenses, they are refracted inward to converge at a small point on the film or digital sensor; this point is known as the focus.  The distance from the center of the lens to the focus point is called the focal length.  There are three types of converging lenses:

a) Plano-Convex Lenses, which have a lens’ surface organization that looks like this ->  ( I

b) Double-Convex Lenses, which have a lens’ surface organization that looks like this ->  ( )

c) Concavo-Convex Lenses, which have a lens’ surface organization that looks like this ->  ( (

2. Diverging Lenses (also, Negative or Concave Lenses)- are thicker at the edges of the ground glass discs than they are in the center.  When parallel rays of light pass through the lens, they are diverged outward.  This means that diverging lenses produce only virtual images, or images that are always right side up and on the same side of the lens as the object (like a mirror).  There are three types of diverging lenses:

a) Plano-Concave Lenses, which have a lens’ surface organization that looks like this ->  ) I

b) Double-Concave Lenses, which have a lens’ surface organization that looks like this ->  ) (

c) Convexo-Concave Lenses, which have a lens’ surface organization that looks like this ->  ) )

Compound Lenses are generally more expensive than their simple lens counterparts, and serve a more specialized purpose.  They consist of two or more simple lenses fitted together, and this combination helps to correct some of the problems concerning aberration that simple lenses produce on their own.  For this reason, compound lenses are often used in architectural photography, where lens aberrations, like distorted straight lines, are at a premium to be resolved when longer focal lengths are not an option.  You might also find compound lenses in certain microscopes and telescopes where acute focus points generating tiny depths of field are necessary.

My essay on cameras lists many of the different types of lenses commonly used in general photography.  However, I do think that it is worthwhile to consider some random lens facts.

1. Most lenses produce their sharpest image two to three full stops less than wide open, so a Nikon 50mm f/1.8D lens is ostensibly sharpest at between f/8 to f/16.

2. Depth of Field, or the area that is in sharpest focus within a picture, extends for 1/3 of the total area in front of the plane of sharpest focus, and 2/3 behind it. You can maximize your depth of field within your image area by placing your focus point about 1/3 of the way in.

3. A wider lens opening (smaller f-number) means that more light will be reaching the film or digital sensor, and that less time will be required for the shutter to stay open. Additionally, a wider opening equates to a smaller depth of field.

4. By standing in one spot and zooming in or out to frame your subject, you are not changing your perspective. For instance, say you want a head and shoulders shot of a man. If you are using a 50mm lens, it will require you standing closer to your subject than if you were using a longer focal length; this distance from your subject, despite equal framing of your composition, means that your perspective changes and the man’s features will appear slightly different in either photograph (for instance, the closer you get to him, the larger the closest parts of his face will appear. ie. the nose). By relying only on the zoom function of a lens to frame your subject, you are not varying your perspective, which means that you are limiting how you record what you see. It is therefore important to move around, and prime (or fixed focal length) lenses can help you do this.

5. Determine a “normal” focal length (the focal length that admits light wavelengths within an angle of view similar to the human lens, the eye). The focal length of a normal lens for any sized picture format is equivalent to the distance across the diameter of the format in question. You can determine this by following this two-step equation developed by Ansel Adams. Step one: Height² + Width² = x; Step two: √x = Normal Focal Length. For instance, a Nikon DX sensor camera has a format height of 15.6mm and a width of 23.7mm. The first step results in “243.36 + 561.69 = 805.05”. The second step results in “√805.05 = 28”. Therefore the normal focal length lens on a Nikon DX camera is 28mm.

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  1. […] Cameras 2. Film 3. Lighting Equipment (coming soon) 4. Filters 5. Lenses Tagged with: Basic of Photographic Equipment, Jason […]

  2. […] is very interesting about this is that, between lenses of different focal lengths, the total amount of light transmitted to the digital sensor or film […]


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