Nikon D300 User Manual page 146

Card Capacity Reporting
Why does my memory card's remaining image capacity
seem to stay the same in NEF Lossless compressed and
Compressed modes as in Uncompressed mode? Shouldn't
it show lots more capacity in compressed modes since they
make the image smaller by 20 to 55 percent? The reason
your camera does not show any increased image capacity
on the Control panel in compressed modes is because the
D300(S) has no idea how well it will be able to compress a
particular image.
An image with a large amount of blank space, such as an
expanse of sky, will compress a lot more efficiently than an
image of a forest with lots of detail. The camera shows a
certain amount of image storage capacity in NEF (RAW)
modes—around 300 images with an 8-gigabyte card. You'll
find that in the two compressed modes, the D300(S) does not
decrease the image capacity by 1 for each picture taken as it
does in Uncompressed
This means that the camera will decrease the number of
available images only every two or three shots, according to
how well it was able to compress the images. When the card
is full, it might have more than twice as many images stored
as it initially reported that it could hold. Basically, your
D300(S) deliberately under-reports storage capacity when you
are shooting in either of the NEF (RAW) compressed modes.
My Recommendation: Since I shoot in
RAW most of the time, I'm concerned with maximum quality
along with good storage capacity. The
mode.
146
Lossless compressed
Lossless compressed