Note: theoretically, digital light sensors could equal or even exceed the dynamic range of film emulsions. However, most mainstream camera manufacturers have constrained this theoretical range to the much smaller middle ground, for optical/ image quality reasons. Also, the camera lens attenuates some of the UV wavelengths that would possibly contribute to dynamic range.
Background for this article:
I have been a photographer for over twenty five years.
About two years ago, I bought my first digital camera, a Canon Power Shot
A60 compact. This is an entry- level camera, but I bought it to see whether
or not digital imaging was for me, given my long background in film photography.
The experience that I had with this A60, and the growth that it triggered,
convinced me that it was a path I did want to follow. So last year, I purchased
a Canon 10D and some new lenses, and there was no looking back. In particular,
my recent three-week photographic journeys in Europe, going digital for the
first time on a long trip, enabled me to capture more, and better images than
I ever had before.
However, there is also a learning curve to be overcome with the switchover
to digital, espcially a digital SLR with as many complexities as the Canon
10D. One of the challenges on this recent trip was in dealing with the widely
varying image contrast properties on my travels, since I was in Scotland,
with its misty, soft light, and in Italy, with its much stronger and high-contrast
light. During the trip, and after returning to Seattle, I've come to understand
on a more personal basis much more about the strategies for responding to
different lighting situations. This article on contrast control is a distillation
of my experience.
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Controlling Contrast with Digital Cameras - An Overview Matching Camera Dynamic Range to Image Dynamic Range The dynamic range (the ability of the image chip to record variations of light intensity from bright to dark) of most consumer and prosumer level digital cameras may only be about five stops. Stops are a convenient measure of light intensity – a one-stop difference is a doubling of the light. Digital camera sensors come with CMOS and CCD chips as their main light-sensitive detectors, and although the CCD type has slightly more dynamic range, both have only about the range of a traditional slide film, such as Fuji Velvia or Kodachrome. In comparison, color print emulsions may have up to seven stops of dynamic range available. Black and white films, such as T-Max, Tri-X, or HP-5, offer up to ten stops of available dynamic range, and thus are able to capture a far broader scale of light intensity. Therefore, digital camera users need to be more clever and careful about exposure, since they have so much less usable range to work with. That is what this article attempts to explain, and to give you some insight into the choices you can make to achieve the very best results possible. One of the best features of digital cameras is their ability to provide a photographer with immediate feedback about the exposure. This direct information about how a scene will be recorded was formerly only available with Polaroid tests that developed in a minute. Most digital cameras do a decent job of controlling contrast automatically when lighting conditions are good. When lighting is diffused and even, such as on a lightly overcast day, “good” photos can be captured without much user intervention or manual settings. Likewise, when used in direct sun, digital cameras can deliver high-quality results, particularly of “typical” shots with people in the foreground, if the fill-flash from the on-board flash is used to light up facial shadows. However, when lighting is “untypical”, difficulties can arise. In conditions of very flat lighting, very high-contrast light, night-time shooting, and the like, digital camera exposures may be disappointing. Of course, experienced photographers know of many ways to use manual settings to improve the “automatic” results. Exposure compensation, fill flash, reflector cards and panels, and intelligent positioning are a few of the ways to compensate for difficult lighting conditions. Sometimes, it is possible to wait for better light, which may more favorable late in the afternoon. Good photographers often have mental lists of potential shots for certain weather or atmospheric conditions, and may be able to come back on an overcast day. The information that is available to guide you in optimizing exposure will depend on your digital camera model. Many cameras will show you a blinking “out of gamut” warning on the LCD viewer. This indicates that the area blinking will be beyond the dynamic range of the camera’s imaging chip. Some entry-level compact cameras may simply display the image you’ve just taken, and you have the difficult task of assessing whether the exposure is optimal. This can be very hard to do, especially if you are trying to examine the image on a sunny day! The ideal feed back tool, available on a variety of mid-priced digicams and digital SLRs, is the histogram display, combined with blinking warnings of blown-out highlights or underexposed shadows on the LCD image viewer. The techniques of intelligently using the dynamic range that you do have available can be divided into three areas: (click on a topic below, or just click NEXT to continue the article) 1: Contrast Control Overview |
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| Images available for purchase. Contact me via email at [email protected] for pricing information. All images Copyright 1992-2005 by Edwardo Aites. No reproduction or redistribution in any form whether digital or print without prior written consent. |
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