.jpg)
Do you remember the principles behind how light meters work? Do you remember for what they are calibrated? Do you understand what you need to do if your scene is very bright or very dark? Here is a very short reminder of how basic light meters work.
First, we are fortunate to have "matrix" or other sophisticated metering built into our digital cameras. The manufacturers use thousands of known scenes to program the light meters to recognize certain histogram patterns from well exposed images and then adjust the exposure based upon those patterns. By the way, Photoshop does the same thing with their "auto" adjust for exposure, levels and contrast. But, we aren't talking about "matrix", sophisticated, computerized metering systems. We are talking here about basic light meter theory.
First, all light meters want to make the reflected or ambient light which they are reading a middle tone (think medium gray). That goes for spot meters, center-weighted meters or averaging meters. All want to take the light it is reading and change the brightness of the light to a medium tonality.
Second, light meters are calibrated to 12% reflectance. Nope, not 18%. That is a myth.
Third, if you are photographing a snow scene and use the meter reading, your snow will come out middle gray, not white. Remember all light meters want to turn everything into a medium tone. What do yo have to do to get the right exposure and make the snow white? You have to let in more light to lighten the snow or any white object. Typically, for snow, you should open up 2 stops or use two whole shutter speeds slower.
Fourth, if you are photographing a night scene of a dark forest with the full moon overhead, the meter will want to give a lot of extra exposure to lighten that black up to middle gray. Your scene will be way overexposed. Your moon will be a featureless white blob in the sky. You have to stop down or allow the shutter to open for a shorter period of time.
There are also what are called "incident" light meters. These don't measure the reflectance of light off your subject, but are turned toward the light source and measure light falling on the subject—ambient light. Typically, these are much more accurate than reflectance meters and don't need adjusting for the luminance of the subject, with a couple of rare exceptions. Why? Because the reflectance can be more or less than the actual brightness of the scene (examples above), and incident meters just measure the light directly falling on the subject and the reflectance is largely irrelevant.
All that being said, the meters today in our digital camera/computers are extraordinarily good. You almost never have to adjust exposure from what the meter shows, if you don't want to. Typically, the current crop of sensors have enough dynamic range that if your scene is slightly over or underexposed, it won't matter for all practicality. Additionally, the today's digital image files are so good that you can almost always adjust the exposure successfully in your image editing software.
But—that doesn't mean that you shouldn't know the fundamentals of photography. Light is fundamental in photography and you must understand it.
Just like knowing how to add, subtract, multiply and divide is fundamental, even though we use a calculator. It is essential that we understand the processes in play behind our computers with lenses we call cameras.
Thanks for looking. Enjoy!
Dennis Mook
Many of my images can be found at www.dennismook.com. Please pay it a visit. I add new images regularly. Thank you.
All content on this blog is © 2014 Dennis A. Mook. All Rights Reserved. Feel free to point to this blog from your website with full attribution. Permission may be granted for commercial use. Please contact Mr. Mook to discuss permission to reproduce the blog posts and/or images.
No comments:
Post a Comment