Sunday, January 11, 2009

DSLR Photography lessons - Lesson 1.

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Hi Pentaxian friends.

I posted the article below in March of 2008. I am posting it again because this year I will post a complete series of DSLR lessons. I figured I should start here.

Too often, we forget that new Digital Single Lens Reflex (DSLR) cameras are sold every day to someone with no experience with DSLR cameras at all. Whoever we are and however good we are, we started somewhere. .

This post will not make you an expert in photography alone. Like any other skill we learn, practice is what makes us excel in any of our undertakings. There are many books on photo techniques on the market that you can get to further learn the craft or art of photography. For years, Kodak has regularly publish updated books targeted to beginners all the way on up to the professional photographers. Of course, lots of information is also available on the internet.

Photography, is about light. It’s about light reaching a media, film or digital sensor, and turning the results into an image that we can see, on a computer screen for example, or print on paper.

Other than the lighting conditions of a scene, there are three major items that directly affect how the light reaches the recording media. The Aperture (lens opening), the shutter speed (the speed at which the shutter opens and closes) and the media sensitivity also known as ISO number (the amount of light recording on the media for a given exposure). Too much light reaching the media, and the image will be washed out. Too little light reaching the media, and the image will be too dark. This is to say that the balance between these three items must be just right. Technically speaking, a large aperture will let a lot of light in, and therefore the shutter speed and/or the recording media will need to be adjusted accordingly. Let’s assume for a moment that the sensitivity is set at ISO 200 and cannot be changed. We now have to balance the aperture and shutter speed to get the correct amount of light in. An aperture of f/4 at a shutter speed of 1/125th of a second would give the same results as an aperture of f/5.6 at a shutter speed of 1/60th of a second. In other words, if you increase the aperture by one F-stop and decrease the shutter speed by one shutter step, the same amount of light would would reach the film or sensor. The difference between the two is the composition. Since we are dealing with digital single lens reflex (DSLR) cameras, from now on lets use the word “sensor” for media.

Aperture scale explained
The aperture, being the lens diaghphram opening, lets more or less light pass through the lens. The f-number (aperture opening) is proportional to the ratio between the lens focal length and aperture diameter, which is proportional to the square root of the aperture area. Big analogy but what does all that means? Well, lenses are usually marked with the F-numbers ranging from the largest aperture to the smallest aperture. For example, a typical lens could have an aperture range of f/2.8 to f/16. The lens would be marked as follow : f/ 16, f/11, f/8, f/5.6, f/4, f/2.8. In this example, the largest aperture would be f/2.8 while the smallest would be f/16. Have you noticed something a little surprising here? The larger the number is, the smaller the aperture is. You need to remember that. Furthermore, each (f-stop) number to the right lets twice the amount of light in as the (f-stop) number to its left and each (f-stop) number to the left lets half the light in as the (f-stop) number to its right. For example, f/4 lets twice as much light in as f/5.6 but only one half the light of f/2.8, and so on. One unit of increment in aperture is called a stop.

Numbers to the right are letting twice as much light in.
Numbers to the left are letting 1/2 (or twice as less) light in.

Shutter Speed scale explained
In photography, shutter speed is the length of time the shutter takes to open and close. The total exposure is proportional to the duration of lighr reaching the imaage sensor. Similarly to the aperture, a standardized 2:1 scale was adopted for shutter speed so that opening one aperture f-stop and reducing the shutter speed by one step resulted in the identical exposure. The agreed standards for shutter speeds are typically 1 sec,1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, 1/500 ses, 1/1000 sec, etc. A shutter speed of 1/125 sec lets twice as much light in as a shutter speed of 1/250 sec, but half the light of a shutter speed of 1/60 sec. For example, combining aperture and shutter speed, a shutter speed of 1/125 sec with an aperture of f/16 is equivalent to a shutter speed of 1/250 sec and an aperture of f/8. Alternatively, a shutter speed of 1/125 sec with an aperture of f/16 is also equivalent to a shutter speed of 1/60 sec with an aperture of f/22. Remember that one unit of increment in shutter speed is called a step.

Numbers to the left are letting twice as much light in. Numbers to the right are letting 1/2 (or twice as less) light in.

ISO Value scale explained

Similarly to the aperture and shutter speed, the ISO linear scale, which corresponds to the older ASA scale, is 2:1. Doubling the speed of a film implies doubling the numeric value that designates the film speed. Here again, a film rated at 200 ASA or ISO 200 will absorb half of the light of a 400 ASA or ISO 400 film, but twice as much light of a 100 ASA or ISO 100 film. In the digital world, the sensitivity defines ISO speed in terms of the amount of light needed to achieve a certain quality in the sense of a per-pixel signal-to-noise ratio. The image sensors in digital cameras can be adjusted, or can have their outputs adjusted, in sensitivity to function with metering at any given comparative ISO setting. This is usually done by simply amplifying the output of the image sensor, which increases image noise, sometimes beyond the level that the ISO standard says is acceptable. Just as with photographic film, greater sensitivity comes with some loss of image quality, visible as image noise. What does that mean for you? The lower the ISO value on your digital camera, the less noise you will have and therefore, your image will appear clearer. ISO 200 is twice as sensitive to light as ISO 100 but half of ISO 400.

Next lesson, we will be Combining all three elements.

Thanks for reading,
Yvon Bourque
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