twrama 1841_august_2.. - AMA WA

PHOTOGRAPHY
Make the histogram a habit
by Denis Glennon
have just returned from Namibia where I spent the last
I five weeks, three of which were spent leading a group of 10
Australian photographers on a self-drive photo safari and two, on
a private photography expedition into some of the most remote
territory on the planet – into the far north-west of Namibia, right
up to the Angolan border.
During the private expedition, I travelled for five days without
driving on even a gravel road or sighting another vehicle or human
being. The landscape here is “mars-like” and the nomadic Himba
tribes roaming these parts are truly inimitable. It is a distinct
privilege to photograph such austere, natural beauty and rarity of
culture. Being in such isolation and driving for up to seven hours
each day, all in dried-up river beds, off-road tracks and mountain
passes, allowed me time to think about the question most
frequently asked by the photographers in the group I led during
the previous three weeks.
The choice was easy. It was to help them understand, interpret
and make better use of the histogram on their DSLR cameras.
This was prompted by the behaviour of the three professional
photographers on the photo safari. Each of us automatically looked
at the LCD immediately after taking a shot. Most photographers
in the group thought we were looking at the image on the LCD.
When we explained to them we did not bother looking at the
image and instead it was the histogram that commanded our
attention, it opened several discussions and a whole new world for
many of them.
The histogram
The histogram is the key to interpreting your digital image. A
simple analogy will help you understand. The 10X4 mosaic in
Fig. 1 contains 40 coloured square tiles which are sorted by colour
and stacked accordingly in Fig. 2. The higher the pile, the more
tiles of that colour are in the mosaic. Think of Fig. 2 as a basic
histogram representing the colour distribution of the square tiles
in the mosaic.
Fig. 1: Mosaic of tiles.
A digital image is basically a mosaic of square tiles or
‘pixels’ which are tiny. Instead of sorting them by colour, like
we did manually in the mosaic of tiles, the computer in your
camera sorts the pixels into levels of brightness, and stacks
them accordingly, as shown in Fig. 3. For the purposes of
explanation, think of each ‘bar’ in Fig. 3 as being one pixel
wide, stacked by side, with no space between them. The height
of each vertical ‘bar’ tells you how many pixels there are for
that particular level of brightness, from the dark shadows on
the left, through the midtones in the middle, to the bright
highlights on the right.
On your camera LCD, you will not see a series of individual
‘bars’ having different shades, as in Fig. 3. Instead, a total of
256 of these ‘bars’ are shown bunched together, all in black, as
in Fig. 4. The left hand side is given a value of 0 (black) and
the right hand side a value of 255 (white), with 254 grey levels
in between. Fig. 4 is a nice example of a ‘good’ histogram from
a correctly-exposed photo, where plenty of details have been
captured in the shadows, midtones and highlights.
Fig. 3: Pixels stacked by level
of brightness.
Fig. 2 : Tiles stacked by colour.
Fig. 4: Histogram as seen on
camera.
60 MEDICUS August