How DSLRs calculate the number of remaining photos?

Question

My Nikon D7000 has a 16 GB SDHC card. The DSLR is configured to shoot in RAW only. When the card is empty, the camera displays that I can shoot 449 photos.

The size of RAW files vary from 17 to 22 MB. So 16384 / 22 gives 744.7, far from the displayed 449 photos.

Why such difference? Is the maximum possible size of a RAW file made with this camera is 36.5 MB (16384 / 449)? Or will the camera use only 10 GB of memory (449 × 22)? If it's the second case, why is it using only a part of the SDHC?

Answer 1

The only sure thing I know is that it will use most of the whole card.

The number has to be an estimate since the size of files is variable as you noted. They probably account for other issues too like fragmentation and prefer erring on the safe side.

As you advance, the estimate usually gets better. You may notice that sometimes you take a shot and the counter does not decrease.

Answer 2

As the files vary in size, the camera can only guess how many photos there are room for, and different cameras of course have slightly different methods of guessing.

What I have noticed with my Canon DSLRs is that they use some kind of projected average size to calculate the number of photos left based on the memory amount left, and the average size is different depending on the ISO setting that you have chosed. If you choose a higher ISO setting, the photo count goes down somewhat. Generally the actual average size is somewhat smaller than the projected average, so the card ends up having room for a few more photos than the camera initally guessed.

Answer 3

Because the file size is variable, the camera can't calculate how many pictures will fit in the remaining card space. In my experience with the Nikon D7000 and Olympus E-510, these cameras seem to devide the remaining space on the card by a predetermined file size that depends on the image quality settings (RAW, JPEG, compression, size). This predetermined file size seems to be quite a bit bigger than the actual files sizes these cameras produce. My guess is that camera manufacturers have their cameras overestimate the size of the images so that the remaining space indicator becomes an indicator of at least how many images will fit on the remaining card space. It would be much more annoying if your camera tells you it has room for 100 more pictures and then runs out of space after 80 shots than that it tells you it has room for 20 more pictures after you've taken 100 pictures. Better to bring too many memory cards than too few.

Answer 4

Another factor is that you don't actually get the full 16GB; there's two reasons for this.

1. Storage manufacturers often quote capacity where GB = 1,000,000,000 (10^9) bytes, while in many other uses GB = 1,073,741,824 (2^30) bytes. (Purists want the kilo, mega, giga, etc. prefixes to work strictly in powers of ten, proposing instead the kibi, mebi, gibi, etc. prefixes for powers of two.)
2. There's nontrivial overhead consumed by the filesystem - the metadata used by the computer to organize the photos.

For example, one of my "8 GB" SDHC cards has a capacity of 7,960,788,992 bytes - 99.5% of 8,000,000,000 (8*10^9) bytes but only 92.6% of an 8,589,934,592 (8*2^30) bytes.