A Quick Guide
To Some Common Terms
Associated With Gear That You Might Buy
Probably
the most common letter I get from readers goes something like this:
“Dear HelpLine, Your column is the best in the magazine.”
(Okay, so I threw that in. They don’t always start that way;
sometimes they mention Rick Sammon’s column. But I digress…)
“I’m looking to buy a new digital camera. I’ve narrowed
it down to the RG6-AZ by Nifty Optical and the KMG365 by pixGrab.
Which would you recommend?”
The only answer I have for this letter is “Whatever works best
for you.” Everyone has different needs and different priorities
when it comes to equipment, otherwise there would be only one kind
of camera, one type of image-editing application and one model of
printer.
I can make a recommendation based on only what I think is important.
You might be more concerned with zoom capabilities or shutter lag,
while I might put more emphasis on macro capability. That’s why
we create a Buyer’s Guide every year—so you can get
the information you need to help make your own personal buying decision.
(Besides, if I tried to answer all of the product recommendation letters,
I’d never have time to write this column. Although I’ve
heard that pixGrab makes some wonderful equipment.)
Another frequent HelpLine letter centers around all the lingo that
pops up in digital photography. In this Buyer’s Guide,
I thought I’d cover, in alphabetical order, some of the terms
that people ask about most often.
Bit-depth: Sometimes referred to as color depth, this term
represents the number of bits or binary digits used to indicate the
color value of each pixel. One-bit color can represent two values—zero
or one—or in color terms, black or white. Eight-bit color can
represent 256 different colors. Twenty-four-bit color uses eight bits
for each value of red, green and blue. Think of it in terms of 256
different possible values for the amount of red in each pixel, as
well as 256 for blue and 256 for green. Twenty-four-bit color can
represent millions of colors.
Buffer: This is a term used with digital cameras that’s
similar to memory in your computer. Think of a buffer as a waiting
area, where each image is temporarily stored before it gets written
to your memory card. The larger the buffer in a camera, the more images
you can capture until the camera stops you and has to finish writing
the data to the memory card.
Dispersion: You hear this term when dealing with optics. Manufacturers
talk about low-dispersion (LD) glass. Dispersion is the separation
of light into its different color components as it passes through
an optical medium. Put another way, Judy Garland was singing about
dispersion in “Somewhere Over the Rainbow.” Manufacturers
use LD glass to reduce color fringing and improve the overall sharpness
of the image.
Dot (or pixel) pitch: This term is used to indicate the sharpness
of a computer monitor. Manufacturers can’t use the word “resolution,”
since that setting is determined by the video card (1024 x 768 will
display as 1024 x 768 on all monitors capable of displaying that resolution,
for example). Dot pitch is the measurement, in millimeters, of the
distance between phosphor dots (or LCD pixels) of the same color in
a display. The smaller the number, the sharper the image. Keep in
mind that dot pitch is only one factor in determining display quality.
Dynamic range: This is a term often used when discussing scanner
performance. Dynamic range is the measurement of the scanner’s
ability to resolve the full range of tones. The scale used for this
specification goes from zero to 4, with zero being full white and
4 being full black. Dynamic range can be shown as two numbers, represented
by the terms Dmax and Dmin, or it can be noted as a single number,
which is the difference between Dmax and Dmin. The higher the dynamic
range number, the greater the ability of the scanner to see details
in both the lighter and darker areas of the image.
EXIF: The acronym stands for exchangeable image file format,
which allows for the storage of image data in the image file itself.
A whole host of information, including the shutter speed and aperture
you used for exposure, as well as the lens focal length, is stored
in a special location in the image file. The “exchangeable”
part means that even though the data was created in the camera, image-editing
software from different manufacturers can read it.
Exposure compensation: In a digital camera, this function allows
you to override the exposure recommendation given by the built-in
metering system. For example, if a scene you’re capturing is
repeatedly exposed as dark, you might use exposure compensation to
lighten up the image.
Focal-length multiplier: This is one of the most misunderstood
terms used with digital SLRs (it’s also referred to as the magnification
factor). Most digital SLR cameras use an image sensor that’s
smaller than a 35mm film frame. Because of this, the image sensor
is seeing only part of the image presented by the focal length as
compared to that same focal length on a full-frame 35mm SLR. The most
convenient way of describing how the smaller image sensor affects
the performance of the lens is to say something like “a 100mm
lens will act like a 160mm.” What’s actually happening is
that the field of view of the 100mm seen by the sensor is changing,
not the focal length.
Histogram: This is a graph displayed on the LCD of many cameras.
It’s a diagram of the dark to light range of the captured image.
Typically, the left side of the histogram is the dark part of the
image and the right is the bright. If all of the data on the graph
is on the left of the scale and it’s clipped (cut off sharply)
at the left edge, the image is underexposed; if the data is on the
right side and also clipped, it’s overexposed. You’ll also
see a histogram pop up in many image-editing programs so that you
have a guide while making image level adjustments for setting black
and white points.
IS or VR: Image Stabilization (Canon) or Vibration Reduction
(Nikon) is technology in the optical system that compensates for tiny
camera movements. (A stable camera is key to getting sharp photos.)
This technology involves sensors in the lens that detect movement
and a mechanism for making slight adjustments to a movable lens element.
Some systems make those corrections by moving the image sensor (called
AS for anti-shake). In some video cameras, the corrections are made
on a frame-by-frame basis by shifting the active pixels on the sensor
to compensate.
Metering systems: These include Evaluative, Honeycomb and
Matrix. They’re how manufacturers have set up their cameras for
determining the best exposure for a scene. They all sample light readings
in various parts of the image frame (usually a minimum of eight, but
they can go up to several times that), then compare them to determine
a possible exposure. Some cameras take this process a step further
by examining the data from the light meter and comparing it to a database
of image scenes. These systems can be successful in achieving good
exposures in difficult light situations.
Photosites: This is where the action is in a digital camera.
While you might think that an image sensor is comprised of millions
of pixels, in actuality, it’s made up of photosites. Each photosite
contains a light-sensing device (called a photodiode) and a storage
area to hold the charge created by the photodiode. When light hits
the photodiode, it converts the light into electrons, or a charge.
The more light that hits the photodiode, the greater the charge. Further
processing either at the photosite or elsewhere in the camera turns
this charge into digital data that, in turn, is combined with other
photosite data to create a pixel.
Picoliter: Inkjet printer manufacturers use this term to describe
the droplets of ink that are produced from the print head. Printers
don’t print pixels on the page; they use droplets to simulate
the colors and tones contained in the image file. A picoliter is a
millionth of a millionth (or a trillionth) of a liter. It’s a
measurement of volume, not of area. The area that’s covered by
a picoliter of ink depends on many factors, including the formulation
of the ink; smaller droplet sizes usually translate into better photo-quality
tones, gradations and colors.
