Thursday, August 14, 2014

Tide pool...

Tide pool...  In Puerto Peñasco, Mexico.  Click to embiggen.  Beautiful, no?  That's a technically challenging photo, with such a huge range of brightness...


  1. Wow, very beautiful! I'm a complete moron with photography...why would the huge range of brightness make it challenging, exactly?

  2. Digital camera sensors (and old fashioned film) have a linear response to brightness, while our eyes have a logarithmic response. If that's new terminology, think of it this way: camera sensors, if you double the brightness of the light striking them, put out twice the signal – and similarly, if you halve the brightness, they put out half the signal. Our eyes don't work that way. If you double the brightness of the light striking our eyes, we perceive that as less than doubly bright, and with half the brightness, we perceive something more than half as bright. This logarithmic response gives our eyes an incredibly broad "dynamic range" – we can see differences in brightness in the same scene even when some parts of the scene are hundreds or thousands of times as bright as other parts.

    Camera sensors have a much narrower range, because of that linear response. In the photo above, all the important parts of the scene are within the range of the camera (because the exposure was correctly set) while everything else is either white or black (over or under exposed). If you were looking at that same scene in person, you wouldn't see the black parts of the photo above as black - you'd still be able to see shades of color in those areas. Similarly, only the sun and its immediate surrounds would be white - you'd see more shades of that blue sky.

    Getting that exposure right, so that the important bits are correctly exposed, can be very tricky. The camera's light meter may not be of much help to you, either, as the bright and dark areas will tend to throw it off. On the other hand, with a digital camera it costs nothing to take extra photos, so If I was trying for that shot I'd probably “bracket” the exposure (that is, take several shots with slightly different exposures) after using a spot meter to estimate the foreground exposure. To someone as ancient as I am, who grew up with film cameras, that almost feels like cheating :) Because each photo cost a non-negligible amount (both for the film and for the development of it), I and most photographers would go to considerable lengths to get the exposure “right” the first time. I recall spending a substantial amount of money on a handheld exposure meter, and on challenging shots taking 15 or 20 minutes to carefully check and recheck the exposure. With my digital camera today, I wouldn't bother with any of that – I'd just set it to automatic bracketing mode, where it can take up to 7 photos (each with slightly different exposure) on a single press of the shutter release. Cheating, that is :)