DYNAMIC RANGE EXERCISE- MEASURING RANGE

MEASURING THE CAMERA RANGE OF F STOPS

• set at TTL meter to spot
• measure across the photographic scene
• measure white highlight and darkest shade
• make note of each area and its f stop 
• in Photoshop make notes of the pixel value
• RED, GREEN,BLUE channel readings
• calculate your cameras dynamic range.

set up

I used my backgarden as there was areas of brightness and darkness

on the day I shot this image.

I set the camera to auto white balance and then set to read the scene

both in shutter speeds and by f stops.

First I set the camera to an middle shutter speed of 1/60 sec and then

spot read the scene

I have maped out the readings in both f stops and shutter speeds

HIGHLIGHTS

The bright white sheet which was the brightest object also was
placed in the spot of bright sunlight.

At shutter speed of 1/60 sec the average brightest point read
at F25, ( F29 on the brightest point) through the spot TTL
 camera meter.

I then moved down to the bright door front that was not
a full white surface.

The f stop was one stop lower at f22.

MID TONES

The sun was not fully in the sky as this shot was taken about
10 am . This meant the sun was front right of this garden.
This meant that the intensity of light was reduced on the left
side of the door. I therefore measured the light on the back
wall as f8. This was considerably less than that of the white
front.

SHADOW

As mentioned before the light fell onto the right so the
inside of the covered bench had little light falling onto it.
The front of the bench was reading at f6.5 but this
reduced to the lowest point of f4.2.

the left of this was in complete darkness and the was registering
to low on the TTL, This suggest that with my lens of 4.5 aperture
it could not go lower than this.

BIT DEPTHS

In grey scale images each pixel is represented by some numbers of bits.

Photoshop uses both 8 and 16 bits to represent each pixel.

In 8 bit pixel there can be up to 256 possible tonal ranges.
black is represented by (0) and white is represented by (255).

Colour is defined by the mixture of RED, GREEN, AND ,BLUE
channels.

I have mapped out the tones of light and dark in bit( pixel ) readings

This graphic display shows the colour tones from light to
dark. and is reflective of the histogram from light to dark.

I re shot the image for both the light and dark extremes to
show the effect on there opposite tones

1/125sec

The exposure setting which is to show the detail in the shadow
clearly shows in the raw convertor the lost data from the over
exposed white card.

1/800 sec

The exposure setting for the card clearly shows that the detail is
lost in the shade.

Conclusion

Based on the readings from light and dark my camera has a dynamic range
which starts and f 4.2  and increases to f 29 which is roughly 12 stops
which covers most of the dynamic range.
This exercise shows that exposure setting in digital images is not only
about the capture of light but also the capture of pixel data
and it is important to ensure that you capture all tones from
pixel value from 0 to 255 . It is also important to remember that
data lower than o and higher than 255 will be lost and so unlikely
to be recovered in photoshop. I now shot using the histogram function
on the back of the camera and so can see what pixel data is avail.
The highlight clipping function also shows that overexposed areas of
the image.