Tuesday, October 29, 2013

Tools of the Trade ~ Optics, Lenses and their physical properties

If you spend any time over on the message boards and forums around the 'net reading about lenses and cameras while trying to keep up with the volume of "information" regarding the tools of the craft, you might be a little confused.

It appears that people feel they need a "better" lens to make a "better" photo.  There are folks who are convinced that Zeiss or Leica lenses are demonstrably better than, say, something Canon or Nikon or Sony might offer.  Some folks have strong feelings about the selections they have made and will defend them to the ends of the earth.  Many websites offer frankly misleading information about lenses and how to select something that will work well for you.

What I would like to do here is start with a basis for understanding optical properties.  I will begin with a set of definitions and their practical effect.  I would like to do this in a non-scientific language manner so as to keep this accessible to anyone who has an interest in furthering their understanding of what is really going on when we talk about lenses, optics, and imaging systems.  Future blog entries will cover the reality of modern optics and compare them against marketing perception and some people's beliefs.

To begin with, the function of a lens is to take light rays bouncing off or emanating from a subject, pass them through glass elements of various shapes, and to send those rays of light on to a blank surface or light sensitive material.  Traditional materials have include canvas (for artists who worked inside a darkened room - see Vermeer's work as a good example), photographic film (including wet plate collodion and dry plate film - see Kodak's revolutionary work in this area), and the current widely available digital light sensors.

The challenge is how well light rays that pass through glass are "focused" onto the intended medium.  It is this simple, fundamental act of making sure that an image is free from as many "un-desirable" optic artifacts that the entire conversation of "lens quality", product prices, and who makes the "best" optics arises.

Resolution -
When people think of image sharpness, they are thinking of optical resolution.  When a scene has a transition from a light area to a dark one, resolution is how quickly and accurately that transition can be captured by our imaging system.

While I promised not to throw too much science into the discussion, it is important to realize that there is a natural, physics based limit to how sharp a lens can be.  It is called optical diffraction.  Lens designers know these limits and, in some cases, try to build optics that come as close as possible to these limits (given time and cost of materials and manufacturing).

We, the common human, can measure resolution if we so chose.  The classic method is to use a United States Air Force (USAF) Resolution Test chart.  For years I used this one from Edmund Optics in the US.

You may notice from the theoretic limits that it is expected that a lens will more accurately preserve light to dark transitions for lines that radiate away from the center of the field of view than such transitions made perpendicular to those rays of lines.  This is an interesting property of optics and one that is good to keep in mind as we look at the following optical effects.

This approach to measuring optical qualities has fallen from favor as a stand-alone test method.  However, it is used as the basis for the most commonly used current method of describing optical qualities, and this is as follows.

Modulation Transfer Function (MTF) curves -
When you look for lenses from current manufacturers you many times see MTF curves offered as a proof of demonstration of quality.  This test method is useful because the human eye sees "sharpness" in terms of contrast, not resolution.

The MTF test method expresses how much contrast is preserved by an optic as a scene transitions from light to dark areas at various levels of resolution.  These levels of resolution are taken from the resolution test method for radial and tangential lines (see prior section).  The thickness of these lines as well as the focal length of the lens under test and the distance between the lens and test chart are what predetermine those levels of resolution.

Typically you will see two different levels of resolution used in published test results.  If comparing lenses from different manufacturers, it is important to know what resolutions were used.  Different results will be reported for different levels of resolution.  Further, it might make a difference to you to learn which manufacturers publish expected design results (nearly everyone) and which offer actual test samples (Zeiss in some cases and Sigma).

If you would like to better understand MTF, how it works, and how to read MTF curves, Cambridge in Color's tutorial might be a good place to start. They provide a nice overview of the relationship between resolution and contrast, as well as providing a good understanding of various test methods and physics involved.

Chromatic Aberrations -
An optical effect that you might read about in lens tests is Chromatic Aberration (CA).  This is where a lens fails to successfully align colors across the visible spectrum.  This effect is typically more difficult to control near the edges of a scene, which is why that is where testers look for the effect.

Additionally, you will read test reports that measure the amount of CA various lenses have at different apertures.  In practice, you will see CA as color "fringing" of portions of a scene rendered near the edges of a field of view.  The amount of CA can vary with the size of a lens aperture.  Optical designers work to control, if not outright eliminate CA.

Field Flatness -
An important design element in creating a lens is to come close as possible to ensuring that elements in a scene arranged on a flat plane are accurately reproduced.  In other words, objects arranged along a line in a scene are in focus across the scene after passing through a lens.

In many cases, lenses are sharp along some kind of curve.  The practical effect is that if you were to take a photograph of a painting, for instance, the edges of the painting may not be sharp in your photograph, but subjects slightly in front of or slightly behind the painting would be sharp.

Lens Distortions -
Another design element in lens creation is controlling spatial distortions.  Said another way, lens designers work to ensure that straight lines along the edge of a scene are accurately reproduced.  This is why you will read in lens test reports the amount of distortion they were able to measure.

If lines near the edge of a field of view bow out and away from the center of an image, it is called barrel distortion.  If these lines are reproduced in a curve shape leaning toward the center of the field of view, it is called pincushion distortion.  If there is no distortion, the lens must a gift from the gods.

This pretty much sets the stage for future blog entries where I will rant and rave about how people perceive their lenses, the prices they are willing to pay for them, and try to compare these subjective "feelings" about lenses and lens "quality" against physical reality.

If you find this kind of information fascinating and if you would like to delve further into common photographic optical properties, take a look at Zeiss' primer on the subject.

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