Exposure
Exposure is dependent on 4 MAJOR FACTORS
- ISO rating
- Subject lighting
- Subject reflectance (also called the “level of reflectance”)
- Your own personal preference
ISO Ratings
The ISO ratings in Digital SLRS originate from the concept of film sensitivity. Film with a high sensitivity to light has a high rating. Film with a low sensitivity to light has a low rating.
Before ISO there was:
DIN (Deutshes Institut fur Normung)
ASA (American Standards Association) – which ISO is based on.
From 1987 - ISO (Internal Standards Organisation)
ISO ratings range from 25 to 3200 depending on the camera (Canon 400D = 100 – 1600). A doubling of the rating makes the film (or sensor) twice as sensitive.
Eg: ISO 400 is twice as sensitive to light as ISO 200. Therefore, ISO 400 requires ½ the exposure as ISO 200.
Assume ISO 100 requires 125 at f.8, adjust the shutter speeds for ISO 800, 400, 100 and 50. *remember, 400 is MORE sensitive to light, therefore the shutter speed should be FASTER.
200 = 2x125 = 250th/sec
400 = 2x250 = 500th/sec
800 = 2 x 500 = 1000th/sec
50 = 125 ÷ 2 = 62.5 (round that down to nearest speed) = 60th/sec
Adjust Aperture for ISO 800, 400, 100 and 50
If 100 = f.8
200 = f.11
400 = f.16
800 = f.22
50 = f.5.6
Digital Cameras use a sensor to convert light to an electronic signal.
ISO ratings are used, and same exposure theory applies, the difference is that film has a FIXED sensitivity and can’t be changed once the film is in the camera, whereas the sensor sensitivity can be manipulated by amplifying the signal.
A HIGH ISO can lead to image degradation. In film it increases the image “grain”, with a sensor it increases the image “noise”.
USE THE LOWEST ISO IN A GIVEN SITUATION
Level of Illumination
A high level of illumination allows use of a faster shutter speed and a smaller aperture.
A low level of illumination needs a slower shutter speed and a WIDER aperture.
Subject Level of Reflectance
Camera uses a “Reflected Exposure Meter”
- Measures light reflected from the subject
- Calibrated to always render the subject as mid-tone.
- Auto or manual results in same rendering.
