Photog Chris's Blog

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Basics – Exposure

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There are three parts to Exposure, if your Catholic you can call it the “Holly Trinity of Exposure”, if you are a cook call it “Mirepoix” 🙂 Okay, so again there are three parts to Exposure:

    ISO
    Aperture
    Time


ISO

This is the sensitivity of the camera’s sensor. In the days of film this was the sensitivity of the film. The great thing about digital is that you can change the ISO on the fly while with film you would have to change the film. The ISO number is suppose to be equal to film. Early on this wasn’t always the case but today ISO 200 on your dSLR will be very close to ISO 200 film.

Base ISO for Canon is 100. If you are outside in bright daylight ISO 100 is ideal. If there are some clouds you might want to use ISO 200, very cloudy ISO 400. In doors ISO 800 or 1600. Notice a pattern?

The scale for ISO numbers double as you go up

100, 200, 400, 800, 1600, 3200, 6400 etc…

Notice the each number doubles as it goes up the scale. The sensitivity to light doubles also. So let’s say your outside with a perfect exposure at ISO 100, if you changed only the ISO to 200 the exposure would be twice as bright or +1 stop. More on that latter.

So ISO 100 is base, going up the scale doubles the sensitivity each level but there is a drawback, Noise. In the days of film is was grain. High speed film is grainy, high speed digital is noisy. With most modern dSLRs the noise is not that bad. With my Canon XSi (450D) the top ISO 1600 is okay, a bit noisy but not bad.

Aperture

First let’s get the technical out of the way. The Aperture is a ratio between the distance of the focal point of the lens and sensor of the camera and the size of the hole letting light to the sensor. You do not need to know that. What you do need top know is the Aperture is a hole, it is big or small, wide or narrow and the larger the number the smaller the hole.

Aperture Scale in Full Stops
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32

I used that word again, “Stops”. For each stop in the scale you lose half the light. f4 is twice as bright as f5.6, the aperture hole of f4 is twice as big as f5.6.

So, why not shoot with the largest aperture the lens can at all times? Well, lens are not alway their best when wide open. Stepping down a few stops will sharpen a lens, reduce CA (chromatic aberrations) and other distortions. There are always trade offs. Most lenses are at their sharpest around f8.

But wait, there is more on the aperture, the depth of field (DOF). DOF is the area that is in focus at a given aperture and distance. Let look at the classic 50mm f1.8.

At f1.8 and a subject 1m away the DOF will be 2.7cm or f1.8 at 3 feet the DOF equals 1 inch. At f8 and 1m DOF = 12cm or nearly 5 inches. So, if you were taking a close forward facing portrait at f1.8 the eyes and nose may be in focus but the rest of the face would be out of focus. Changing to f8 and the rest of the face comes into focus.

One last thing about aperture, it is best to us apertures f16 and bigger. With most dSLRs going smaller than f16 bring on Diffraction, a form of distortion that softens the shot. The more mega pixels you have crammed onto a chip the more diffraction becomes a problem. So let me be clear f16 and smaller (f22, f32, etc…) will start softening  the image.

Time (TV)

You camera is capable of shutter speeds between 30 seconds and 1/4000ths of a second (some even faster). The faster the shutter the less shake and motion blur there will be.

First lets look at camera shake. When you are shooting hand-held there is always some camera shake but the amount has a lot to do with length of the lens along with the speed of the shutter. The classic formula is

1 / Focal length = time

For example if shooting at 50mm then the slowest speed should be 1/50th of a second. At 200mm the 1/200th of a second. To be safe this should be your base number, try and shoot at faster speeds.

Some cameras and lenses have IS, VR, OS… whatever they call it, Image Stabilization, Vibration Reduction, Optic Stabilization…. With current Canon lenses you can get up to three Stops of stabilization, other systems are most likely the same. What does this mean? Well, if you are shooting with a 200mm lens the normal 1/200 base could be shot at 1/30 of a second!

Motion blur. If you are shooting fast-moving subjects like sports, birds, children, fast cars, etc… You should be shooting at speeds of 1/250 or faster. For really fast things like splashes and water drops try going up to 1/4000th.

In Closing

Okay that is all three so let’s talk about Exposures and Stops. You need to understand that changing one part of the three means you must change a second one to keep the exposure the same. Lets look at an example

The first picture is at ISO 1600, f5.6, TV 1/80 while the second is ISO 400, f2.8, TV 1/80. The difference? The on second one the sensor is 1/4 as sensitive and the aperture is 4 times bigger, time is the same. Also the notice the first one has more noise while the image is sharper and more of it is in focus. Click on each image to see a larger one.

ISO 1600, f 5.6, TV 1/80th

ISO 400, f 2.8, TV 1/80th

I know this is a lot of information and I might not have been very clear. I would suggest a book “Understanding Exposure” by Bryan Peterson.

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Written by photogchris

January 2, 2010 at 12:56 am

Posted in Basics, Tutorials

Tagged with , , ,

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