Exercise 4 – Bit Depth and Colour Modes

1) Download the file ‘Img 1 – Exercise 4’ from eLearn and place it on the desktop. (Remember to delete this file from the desktop when you have completed this exercise).

2) How big is an A4 page in cm? 29.7 x 21

3) Open the file ‘Img 1 – Ex 4 - Granger-Rainbow.jpg’ in Photoshop :-

4) Select ‘Image -> Image Size …’ and ‘Image -> Mode’ and answer the following questions :–

Image size in cm? 29.63 x 20.32

Pixel Dimension (W x H)? 3500 x 2400

Image size in megapixels? 3500 x 2400 / 100 000 = 8.5

Resolution of the image(ppi)? 300

Image size in megabytes? 24 m

Colour Mode? RGB colour mode


Bit depth 8/bit channel

Hint: Use the Status line to display the image size – use the number on the left.

5) Use the ‘Image -> Mode’ menu to select the following modes / bit depths and fill in the table for each case listed. (Note: Use ‘Undo’ or the History list after each case to return to the original image state) :-
Colour Mode Bit Depth per channel Total Bit depth No. of available colours
RGB 8 24 16.8 mil 24
RGB 16 46 2x8x10(14) 4.81
CMYK 8 32 4.3 mil 32
GS 8 8 256 8
Bitmap # 1 1 2 1
Indexed Colour – 256 colours N/A N/A 256 8