Sunday, August 31, 2008

September 2008 HelpLine

Damian Greene Published in HelpLine
September 2008 HelpLine

This Article Features Photo Zoom


HELIOPAN CIRCULAR POLARIZER
Beam-Splitting Headache
Q) While searching online for a mid-priced, 67mm circular polarizer for my Nikon D200, I came across this filter description that has me puzzled: "Used like and gives the same effect as [a] polarizer, but designed for cameras with beam-splitting metering." Is this filter a polarizer? How do I know if my D200 has beam-splitting metering, and if I use this particular lens with my D70 or something new later on, will this filter no longer work? Is this a concern for all circular polarizers, or is this model by Tiffen something special?

Judy Maki
Thunder Bay, Ontario, Canada

A) Thanks for the question-and for letting me know from what part of the world you're writing. I can't get many people to include their location when e-mailing!

I think your confusion is caused by the choice of words in the description. When polarizers first came out, they were linear. Since there was only one kind of polarizer, calling it a linear polarizer was unnecessary. Today, there are two types: a regular (or linear) polarizer and a circular polarizer. The description says the circular polarizer "gives the same effect" as a polarizer, but it IS a polarizer. It's sort of like saying a Ford acts just like a car. (A Ford is a car!) Aside from the grammar...

When you buy a polarizer, you have to make a choice between a linear polarizer and a circular polarizer. The choice is easy: Always get a circular polarizer. Your digital SLR uses a reflective optical device called a beam-splitter that sends light coming from the lens in two directions. It sends most of the light to the viewfinder so you can view the scene and also so the sensors in the viewfinder can evaluate the light level in the scene in order to set exposure; that's the light meter in your camera. But the beam-splitter also sends light to the focus sensors so the camera can detect and set proper focus; that's your camera's autofocus system.

Without delving too deep into a discussion of optics (some would say "too late!"), the beam-splitter in your camera doesn't expect light that's polarized. And that's exactly what a linear polarizer produces. A circular polarizer essentially adds another optical element to a linear polarizer. It mixes up the light so it isn't polarized, but it does this only after the light has passed through the polarizing elements to get rid of glare and other "artifacts" that you use a polarizing filter for.

So the simple answer to your question is that if you stick with a circular polarizer, it will work on all your digital SLRs, even your future ones.

Note: You could use an old linear polarizer on a D-SLR. You'd have to use manual focus and probably manual exposure or at least exposure compensation.

The RAW Deal
Q) I'd like to know about shooting with my camera set to RAW mode vs. JPEG mode. Sometimes I think I should leave it in JPEG mode and shoot away, and sometimes I think that I'll have less to worry about if I leave it in RAW. Then my memory card starts filling up.

Tim R.
Via the Internet

A) If you want to start a lively discussion amongst photographers, ask about JPEG vs. RAW. There are some who say JPEG is a file format for amateur photographers while RAW is a format for pros. I don't think it's as simple as that. Aside from the blurring of pros vs. non-pros these days, there are professionals who use JPEG and amateurs who use RAW.

First, JPEG is a compression scheme, not a file format. Second, there's no RAW file format, per se, as each manufacturer has its own proprietary data file that contains data captured from the image sensor, with very little processing. It's helpful to know these facts, even if it's easier to simply refer to JPEG and RAW as file formats. I don't expect any change in how the terms are used, but I like to be "technically" accurate a few times a year. Nonetheless, I'll follow the masses and refer to JPEG and RAW as file formats.

There's no question RAW offers distinct benefits for the photographer who needs them, including the ability to make greater changes to the file before the image degrades from overprocessing. However, RAW isn't for everyone. It requires more work and time to process than other formats. For the photographer who likes to work quickly and wants to spend less time at the computer, JPEG offers distinct advantages and, depending on your skill in the digital darkroom, it even may give better results. This may sound radical considering what some "experts" say about RAW vs. JPEG.

It's important to understand how the sensor processes an image. It sees a certain range of tones coming to it from the lens. Too much light, and the detail washes out; too little light, and the picture is dark. This is analog (continuous) information and must be converted to digital (which is true for any file format, including RAW or JPEG). The complete data is based on 12 or 14 bits of color information, which is changed to 8-bit color data for JPEG or simply placed virtually unchanged into a 16-bit file for RAW. (A bit is the smallest piece of information that a computer uses—an acronym for binary digit. Data of 8 bits or higher is required for true photographic color.) This occurs for each of three different color channels created by the image sensor: red, green and blue. RAW files have little processing applied by the camera. The fact that they contain 12 or 14 bits of color information is a little confusing since this information is put into a file that's actually a 16-bit format.

Both 8- and 16-bit files have the same range from pure white to pure black because that range is influenced only by the capability of the sensor. If the sensor can't capture detail in areas that are too bright or too dark, a RAW file can't deliver that detail any better than a JPEG file. It's true that RAW allows greater technical control over an image than JPEG, primarily because it starts with more data (12 or 14 bits in a 16-bit file), meaning there are more "steps" of information between the white and black extremes of the sensor's sensitivity range. These steps are especially noticeable in the darkest and lightest areas of a photo. So it appears the RAW file has more exposure latitude and that greater adjustment to the image is possible before banding or color tearing becomes noticeable. But don't fall into the trap of using RAW as an excuse for not paying attention to exposure. Watch the histogram!

Card space is an issue, too. JPEG format compresses (or reduces) the size of the image file, allowing more pictures to fit on a memory card. The JPEG algorithms carefully look for redundant data in the file (such as a large area of a single color) and remove it, while keeping instructions on how to reconstruct the file. JPEG, therefore, is referred to as a lossy format because, technically, data is lost. The computer rebuilds the lost data quite well as long as the amount of compression is low. RAW is uncompressed and produces the largest file size that the camera can create.

Which format works best for you? Your personal way of shooting and working dictates that decision. If you're dealing with problem lighting and colors, for example, RAW gives you a lot of flexibility in controlling both. If you can carefully control your exposures and keep images consistent, JPEG is more efficient.

If you have questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or This email address is being protected from spambots. You need JavaScript enabled to view it.. Visit our website at  www.pcphotomag.com for the web-exclusive HelpLine Weekly and past HelpLine columns.

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