Nikon D300 Complete Manual page 240

Size is in K (e.g. 6000 is 6MBs) and some of these sizes are the average I've
seen using my D300, not necessarily Nikon's listed sizes.
Note: Not all CompactFlash cards labeled of a particular size
actually have the same capacity due to differences in file
allocation and marked bad sectors. Moreover, a card
marked 1GB often has slightly less than 1GB of space. With
larger capacity cards, actual capacity is typically about 5%
less than stated size.
Thus, the numbers in the above table are an approximation
only. If you shoot a scene with a great deal of detail and
Sharpening set to 7 on a CompactFlash card that
overstates its capacity, you may get far fewer images than
listed in these tables, especially with JPEG images
(sharpening adds more detail that JPEG has a hard time
compressing). Conversely, if you shoot non-detailed scenes
with Sharpening set to 0 on an efficient CompactFlash
card, you may get slightly more images than shown. Like
EPA mileage labels on cars, what you actually achieve may
not be what the manual indicated.
To set Image quality (and Image size):
õ
1. Press the MENU button to show the menu system.
2. Use the Direction pad to navigate to the SHOOTING
menu (camera icon tab) and press the
Direction pad to select it.
3. Use the Direction pad to navigate to the Image
quality option. Press the > key on the Direction pad to
see the sub-options.
Thom Hogan's Complete Guide to the Nikon D300
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