Saturday, 9 June 2012

Light meter.

The weather has been great this past week and not one to pass up such a rare opportunity I have moved my office to the end of the garden where it is shaded  by some silver birch trees. If it was not for the shade I would not be able to see the computer screen.

As you know from previous posts I have become quite smitten with my Zero 6x9 deluxe camera. It's lack of through the lens metering! Your right no lens, I have had to revert to using a hand held one. NO! I'm not going to take another camera with TTL with me. Why load myself down, believe it or not even with a tripod it is lighter than my standard kit. Besides I don't need the light readings to be that spot on especially when needing to account for reciprocity.

While writing this I'm joined by a baby starling. It looks like the one I saved from the jaws of the cat the other day. The bird seems happy for me to be here as he/she walks up to and around the chair I'm sitting in looking for local delicacies. It appears I'm the last thing it needs to worry about.

Two types of hand held light meter
I have two types of meter to choose from they are tried and trusted run of the mill units. The oldest one uses a selenium cell which is a type of photovoltaic / solar cell. When the front of the meter is exposed to the light it produces a small current which moves a needle that indicates how much light there is. This type of meter is batteryless but one of the down sides is in low light situation it requires a certain light level to read accurately.

The other light meter is a CdS unit or Cadmium Sulfide cell that has a greater low light sensitivity. This light meter works by regulating the energy in the battery rather than producing a current as in the former. With this type of meter it can suffer from drifting this is where the meter has been shown a bright light temporarily making it blind. The cell has a memory and can fool you into thinking the level of light has not changed. It is a good idea to let the meter read the scene for several seconds to make sure it has read it properly.

Note the starling casting a
critical eye over the
Todays meters do not suffer from this they use silicon blue or gallium arsenide photocell coupled to a memory chip  so are unlikely to have a time lag in changing light conditions.

My light meters are general purpose, they read reflective light from a fairly large area of the subject which is about 30 degrees. They round this reading to an average of 18% ( a mid grey) no matter how dark or bright the scene is. This is where the zone system ( See Ansel Adams The Negative for more info) comes into its own or a system of your own to allow for it. I have my own method that works well with a little bit of intuition thrown in. Well! Most of the time.

Since writing this my CdS light meter has passed away. It will be missed.