In the Menu of every Canon DSLR is a function always designated as
Format. Selecting Format will do two things: (1) It will completely erase all
data, including any protected images, from the CF (Compact Flash) card, or the SD (Secure Data) card. (2) It will change a card that has been used
in another camera for use in the camera you have in your hand.
Note that Formatting in camera simply deletes the fi le directory structure
and allows that images on the card to be overwritten. In most cases, the
images can still be rescued after an accidental format as long as the card
is not overwritten.
Let’s look at each in turn.
First, think of a clean CF or SD card as a straight line. When you take two
(or more) consecutive pictures on a clean card the camera writes them
in sequence, one after another, using as much space on the card as is
necessary for a comfortable fi t. When the straight line is exhausted, the
card says, “Whoa, Cowboy!” (your display may say “CARD FULL”), and
won’t allow you to take any more pictures until you swap out the card for
a fresh one.
Let’s assume that, instead of swapping out the card, you choose to go
back through the fi les and eliminate those that are clearly inferior by
using the Erase function. Unlike a motored hard drive, which will put bits
and pieces of a fi le wherever there is a little room, your camera’s card can
only write in a straight line, which means it will place smaller images into
whatever larger slots are available or truncate (cut off ) larger fi les which
it cannot fi t into an existing slot. Aside from fi les being truncated, a big
enough problem by itself, placing smaller fi les into larger spaces means
that some memory space, those little bits and pieces, will not be used.
When that happens, there’s no way to realize the full memory potential of
the card, and you won’t be able to get all the images you should.
In practical terms, what does this mean? Simply put, you should re-format
each card between uses to maximize its potential. Although you can use
the Erase function in the fi eld when you fi nd yourself without enough
card space, just erasing individual images, even selecting Erase All, will
ultimately hamper the performance of the card.
ISO Speed
ISO is a number that indicates the chip’s sensitivity to, and is based on
formulas presented by the International Organization of Standards (who
couldn’t get their initials right, but at least did a good job fi guring out
exposure). Based on a set series of tests, an exposure under a controlled
light source at a given shutter/aperture combination will yield a perfect
exposure. When the ISO is increased, the same perfect exposure will be
attained at a predictable (and constant) shutter/aperture combination.
Generally speaking, setting your camera to a higher ISO will allow faster
shutter speeds under dimmer light, while lower ISO means slower
shutters to record the same exposure.
I say generally, because with any ISO, you can manipulate the shutter/
aperture combination for creative eff ects (FIGS 4.43 and 4.44).
You can set your camera’s sensor to record a wider range of ISO speeds
than were ever available to fi lm shooters, even those who manipulated fi lm stock by “pushing” or “holding” the exposure in processing, and with
incredible precision. Just as you can a
djust shutter and aperture values by
thirds of stops, so can you adjust ISO.
All Canons begin their default ISO range at ISO 100, with most capping
out at ISO 1600 (the Mark III default caps at ISO 3200 but is expandable
to ISO 6400!). Except for the Rebel, the ISO on all Canon models may be
expanded. Using the correct Custom Function will expand the ISO range
to 3200 for most cameras.
ISO expansion works the other way, too, lowering the minimum ISO to 50.
This is the solution for when your shot will look better with less depth of
fi eld or more blur. Please note that, although the rule of thumb is always
to shoot at the lowest ISO possible for maximum image quality, using
ISO 50 will actually decrease the dynamic range of the image. This is why
Canon puts 50 outside of the normal range and accessible only via a
Custom Function. The highest ISO setting, 3200 on most and 6400 on the
Mark III is noisier than Canon generally considers acceptable. This is why
these ISO settings are not included in the regular selections.
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