Catch Up Blog

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 = 250^th/sec

400 = 2x250 = 500^th/sec

800 = 2 x 500 = 1000^th/sec

50 =  125 ÷ 2 = 62.5 (round that down to nearest speed) = 60^th/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.