Tuesday, 16 April 2013

TAOP Part 4: Light

Time to make a start on this part as I have very little time in which to complete part 4.  The introduction to this section in the course manual covers light as "the raw material of photography" and then leads on to "The intensity of light".  Here there is a general discussion of measuring the brightness of light and easily translating it onto a scale which can be used practically to provide the appropriate exposure when taking a photograph.  There is also a brief discussion of sensor sensitivity which is measured in ISO numbers as well as histograms.  These latter two subjects I will expand on further later in this blog.

Measuring Brightness

At this point I reached out for my little-used camera manual to remind myself what metering patterns are available.  Being rather lazy, I generally use what Canon refer to as 'evaluative' metering and this does seem to work well in most circumstances, though when working with speedlights, whether in the home studio or out in the field, I use a hand held meter.  As I have been using hand held meters for some time, I thought that I'd cover those off first.

Hand held meters

First of all, on of the major advantages of using a hand held meter is that it becomes possible to take incident light readings.  Why is this important and when should an incident light reading be taken?  An incident light reading measures the amount of light falling on the subject and this gives an accurate reading to be used in determining exposure.  Such a reading is independent of the texture and colour of the subject, which means that it is not necessary to make adjustments to take those into account.  It is appropriate here to touch on how meters are calibrated, regardless of whether they are internal to a camera or hand-held.  The basic principle which is adopted in metering the brightness of a scene is to reduce it to an 18% reflectance grey card.  This has considerable significance where the subject is predominantly at one or another end of the scale, ie either black or white.  A black surface will cause the meter to calculate an exposure some times up to 2 stops higher than required and a white scene, such as snow goes the other way and causes up to 2 stops  under exposure, giving rise to grey snow scenes.  An incident light reading is independent of these factors so more likely to give you an accurate exposure.

There are practical considerations, however, and it may not be possible to take an incident light reading in most circumstances, so I use it primarily for studio work, whether by flash or other light sources, and also portrait and close up work outdoors.  My first hand held meter was a Weston Master V, which is currently not working, however in doing this research, I discovered a company which repairs these meters, so I may well do so.  What did it look like, well, somewhat like this:

The white plastic attachment seen in this photo is called an "invercone" and it fitted over the area where the selenium cell receptor was located.  This is shown in the photo below.  Using the invercone was how incident light was measured.  Reflected light measurements were made without this attachment, pointing the receptor directly at the subject.

The hand held meter which I currently use is an Sekonic Flashmate L-308S, which is shown below:

The digital display and multiple functions, make this a very versatile tool despite being one of the cheaper models.  In the picture above, the white area is a "lumisphere" which is used for taking incident light readings in the same way as the Weston.  The main advantage of this type of lightmeter is its ability to take readings of flash output.

For metering small areas, referred to as spot-metering, I recall using at college a Pentax Spotmeter V which looked like this:

A rather strange device which incorporated a viewfinder which you used to find the "spot" you wished to meter.

In-camera metering

Reading the manual for my camera, the Canon EOS 5D mark II, I find the following on a page describing the Metering Mode:

Not a wonderful scan on my part, but it illustrates the point.  There are 4 metering modes, all with specific attributes, well described in the illustration.  I have a tendency to use the default which is the evaluative mode, but this is not necessarily the most appropriate in all circumstances.  Having said that, it all depends on how much you get to know your camera.  In theory, partial and spot metering will provide the best exposure values where you are looking to apply your own compensation, for the scene.  Evaluative does many calculations across a number of zones and it could well be applying compensation, so adjusting further may not make much sense.

As the course text suggests, different scenes require different treatment and an understanding of how the metering works in the specific camera in use so that appropriate compensation, if any is required, can be applied at the time of taking the image.

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