Mega Pixels vs Megabytes

Hi Folks!

I've been playing with a K110D refurb Chris and Nikki so kindly sent me from SRS - can't thank them enough for their patience and care.

I've been paying much more attention now, to the data attached to the shots, and can't figure something out.

So - here comes today's dumb question:

I take a variety of shots. They all say 3008 x 2000 Dimension (I assume this equals 6.1 megapixels) The shots are JPEG. (Haven't played with RAW yet).

But the amount of memory these shots take on my computer varies from less than 1 MB to over 2.5 MB.

Often the shots are very similar.

Any simple answer?

Love the new format - congrats!

The JPEG compression essentially takes similar areas of the image and remembers them once instead of multiple times. An over-simplification, but it means that an image that might be 18MB of space when opened could be as little as 2-4MB when compressed. When the file is opened it is reconstructed from the compressed state, and of course approximations and assumptions are made in this reconstruction. This is a lossy format, but good enough for most purposes.

Now the original (compressed) file can vary in size and that's because it could vary from a picture of a grey wall (lots of similar areas, huge compression possible) to a picture of a detailed map (lots of detail that has to be recorded).

If you look at your images there will be variations that account for the difference in file sizes.

Many Many thanks, John!

To expand on John's post a little, there are two (main) sorts of image compression. The first (lossless) compression will recognise a block of pixels with identical values, and only record the values once. It will then effectively say that every pixel in the block is the same. This takes less space than recording the same data multiple times. PEFs are lossless.

Lossy compression (eg: JPEGs) does the same, but it also says that a lot of information in the picture can't be seen by the human eye, so is unnecessary, and it discards it completely. This is great for creating small files, but it may discard data that you would prefer to keep.

If you have under-exposed your image, you will often be able to "re-expose" a PEF (Raw) file, but there may be nothing left to extract from a JPEG.

I believe that TIFF files (from Pentax/Samsung cameras) are not compressed at all, which is why they are much larger. They are not all exactly the same size because they also contain a compressed JPEG file.

G

Thanks George,


I seem to remember seeing TIFF8 and TIFF16 as options somewhere.

Something else to play with.

Seems as if everything is better than default JPEG?

Right now, I'm experimenting with AutoFocussing options, Metering options and possibly linking the two so as to meter only where I've autofocused.

Boy, I'm sure glad I started with the "simple" DLSR

TIFF is a very poorly defined standard with a number of variations.

you can have 8 or 16 bit TIFF (tiff8 and tiff16 as Mac mentioned)

you can also have various types of compression applied, bizzarely including JPG, so it's actually possible to have a TIFF which is compressed just like a JPG.

Most TIFF images are uncompressed as this is the most widely supported form.