Technical Standards and Assumptions
This App assumes that the baseline reading for a camera light meter from a standard Kodak 18% gray card in broad daylight is f/16 and 1/125 at ISO 100; this assumes there are no clouds and no shadows on the card. Although this is a standard which as been used for many years, manufacturers may set the camera’s metering system anywhere between 1/125 and 1/60 at f/16 with ISO 100 under the same conditions. Because of this, it will be necessary to adjust the calculations provided by this App accordingly to determine whether or not a particular camera is exposing correctly. Below are F/ stops given in full, half, and third stop increments to help you make the calculation (this article assumes basic knowledge of camera mechanics).
Full Stop Increments
f/2 , f/2.8, f/4, f/5.6, f/8, f/11, f/16 , f/22
Half stop increments
f/2, f/2.4, f/2.8, f/3.3, f/4, f/4.8, f/5.6, f/6.7, f/8, f/9.5, f/11, f/13, f/16, f/19, f/22
Third stop increments
f/2, f/2.2, f/2.5, f/2.8, f/3.2, f/3.5, f/4, f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16, f/18, f/20, f/22
This App offers readings in all the increments given above.
Is it necessary to use an 18% gray card to check the accuracy of a camera meter? Although it is preferred, this level of accuracy is often not necessary for most day-to-day shooting environments even at the professional level. When you need a gray card most is when you believe your metering system is malfunctioning and in those moments your gray card is likely in your studio keeping all your other forgotten equipment company. Enter the Northern blue sky which is approximately 18% middle gray in value (obviously not in color). The density of the Northern blue sky in the middle of the day is not likely to change within our lifetime, hence using it as a de-facto gray card will suffice for most purposes. If you do have a gray card please use it rather than the sky.
How accurate does your camera meter need to be? This depends on your particular needs. Fundamentally, the meter reading does not have to be as accurate as most photographers would have you believe. With the advent of digital photography the requirements for accuracy have become more forgiving. With color transparency film the latitude was approximately plus or minus 1/4 stop. With color negative film, approximately plus or minus one stop. With digital photography that latitude is now about plus or minus 2 stops.
So why bother calibrating your camera? Because the more accurate the exposure the less post processing needed to produce a great looking print.