twrama 1841_august_2.. - AMA WA

PHOTOGRAPHY
Notice there are no are no ‘bars’ running vertically at either the
left or right hand side of the histogram in Fig. 4 – something you
wish to avoid in the majority of your photos.
‘Bars’ climbing vertically up the left hand side of the histogram
(like in Fig. 5) tells you that you have underexposed the image
and you did not capture details in the dark shadow areas. ‘Bars’
climbing vertically up the right hand side of the histogram (like
in Fig. 6) shows you have overexposed the image and did not
captured details in the bright areas.
you are wasting fully half of the available encoding ability of your
camera. The simple lesson is to bias your exposures so that the
histogram is snugged up to the right hand side, but not to the
point where you have some of the ‘bars’ climbing vertically up. If
the latter happens you have blown the highlights in your images
and these can never be ‘recaptured’. Using the ‘flashing alert’ or
‘blinking’ function on your camera will help avoid this happening.
To move the histogram to the right, simply let in more light
to the sensor, by opening the aperture further, or increasing
the time the aperture is open or increasing the ISO level.
Fig. 8 shows a ‘centred’ histogram and the same one pushed
to the right but not allowing it to climb vertically up the right
hand side.
Fig. 5: Histogram of an underexposed
image.
Fig. 6: Histogram of an overexposed
image.
If your histogram has verticals ‘bars’ running up both the left
and right sides, it means you has lost, or ‘clipped’ details in both the
shadows and highlights. This happens when the range of light levels
in the scene, from the darkest to the brightest, is more than the sensor
in your camera can cope with. There will be too much contrast in
what you are trying to photograph, to capture in one shot.
Fig 8: Histogram showing the principle of ‘shooting to the right’.
Use the histogram to help you shoot to the right
Fig. 7 shows a histogram box without the ‘bars’ and somewhat
arbitrarily divided into five equal zones or tones. Intuitively you
would think your camera’s sensor would be capable of capturing
and recording the same amount of digital data (or tones) from
each zone in any scene.
Fig. 7: Histogram divided into five zones.
Alas, this is not the way it works. The way it really works is
that the ‘Very Light’ zone captures fully half or 50 per cent of the
digital data (tones) available in whatever you are photographing.
The next ‘Light’ zone captures 50 per cent of what the ‘Very
Light’ zone has already captured. The ‘Medium’ zone captures 50
per cent of what the ‘Light’ zone has already captured, and so on
as we move towards the last ‘Very Dark’ zone. This means the first
three zones alone capture 87.5 per cent of the digital data (tones)
you can possibly capture in any photograph. The 85 per cent is
made up of 50 per cent (‘very light’ zone), plus 50 per cent of 50
per cent or 25 per cent (‘light’ zone), plus 50 per cent of 25 per
cent or 12.5 per cent (‘medium’ zone).
The message is clear. If you are not using the right-hand side of
the histogram for capturing and recording much of your images,
Fig. 9: Actual histogram straight from camera.
Fig. 9 shows the actual histogram for the Water Art photo
(pictured). This histogram is ‘straight from camera’ and is
the third image I took of the water breaking on the rock. By
interpreting the histograms for the two earlier shots, I increased
the exposure until I maximised the digital data gathered, without
blowing out the highlights. I have given myself the best chance to
work this image further in Lightroom or Photoshop, if I choose to
do so.
Almost all prosumer cameras and all professional cameras
allow you to view the histogram on the camera’s LCD so
that you can adjust the exposure and take the shot again, if
necessary.
Visit your camera’s manual to switch this function on so that
it automatically appears when you look at the LCD after taking
a shot. Soon you will not be using the LCD only for deleting
the poor and out-of-focus images. Concentrating on the
histogram will ensure you have much fewer images to delete.
August MEDICUS 61