15 April 2010

Do not trust your camera's metering

Many of us who use digital cameras started by using Auto mode. In this mode, we simply have to frame a picture and take the shot. The camera takes care of everything else. Then as time goes on, when we are more serious and when we want to control how our pictures will end up we moved on from auto. To take a picture, the camera controls the aperture, shutter speed and ISO values based on the lighting conditions. The exposure of a photographed is determined by these 3 values. You can control these three setting to get the most out of your pictures based on various conditions.

Most of us will use A mode known as Aperture mode as it gives us the control over the depth of field. Although it only applies to DSLRs users as compact cameras do not give much depth of field control. The smaller the value of the aperture, the shallower the depth of field is and vice versa. I will demonstrate more in detail about this later. In this mode, the camera takes care of the shutter speed. ISO values can be changed manually, if not it can be set to auto.

Note: A rule of thumb, there are two ways to get the shallow depth of field:
a) Use a lower aperture value
b) Use a longer focal length lens

The S mode known as shutter speed or Tv mode known as Time Value in Canon camera bodies on the other hand gives us the control of the camera's shutter speed. We want fast shutter speed to freeze action, like in sports or a live performance. Pictures will end up blurry, due to motion, if shutter speed is not fast enough. So make sure your aperture value is low and ISO and its optimum value. The higher the shutter value, the better it is to freeze motion. In this mode, the camera takes care of the aperture value. Again, ISO values can be changed manually here, if not it can be set to auto.

Note: A rule of thumb, for night shots, if you do not want motion blur, make sure your shutter speed is not lower than your focal length that you are using unless your camera setup has image stabiliser be it in the lens or in the camera body.

Not many people use the M mode known as the manual mode because we have to adjust everything ourselves. Any changes in light conditions, we will have to resort to change our camera settings as well to accommodate for the light changes. However, this mode is useful in studio conditions as the lighting conditions are determined by us, hence we want full manual control to get the best results.

Ok here's what this post is about. Many of us rely on the camera's metering system to decide the exposure of an image captured. Our camera's metering system is not perfect. If a picture is underexposed, we can compensate it by adding the Exposure Compensation Value (EV) and vice versa. So how do we know if the exposure is right? There are two ways and no, its NOT by using your eye to judge the LCD screen.

a) We can use the histogram is your camera has it. Its a good practice to use the histogram to ensure good exposure. Some cameras offer live histogram, meaning that the histogram is shown in real time, before shutter is pressed which is a definite plus. Although the histogram may look complicated it is very easy to use it. Here's how:

i) An underexposed image will look like this:

The histogram will look like this:

ii) An overexposed image will look like this:

The histogram will look like this:

iii) A good exposure image will look like this:

The histogram will look like this:

Just make sure that there is not peak at the left and the right side of the histogram and try to center the peak to the center and you will have a perfect exposed image.

b) There is an alternative and faster way for those who happens to be using film or digital cameras that do not have a histogram to begin with. This only works during the day though. This calls for the Sunny F/16 rule. The rule states that in a sunny scene, and I mean sunny not hazy or cloudy, at the setting of ISO 100 and and at the shutter speed of 1/125, perfect aperture is achieved at F/16.

It doesn't have to be at this setting. You can achieve the same thing with any setting: For example: at F/8. Since you overexpose by two stops, (F8-F16 is a two stop difference-read below on how to count stops) compensate the exposure with the value of the shutter speed by underexposing it by 2 stops with 1/400 (1/100 -1/400 is a two stops difference). Don't believe me? Go try it but makes sure its real sunny.

So what happens if its not sunny, well, overexpose it by one stop if there is sun but hazy, two stops if it is cloudy, and three stops if it is overcast. Usually we adjust the shutter speed unless it is at night because it has the least effect on our images. Changing the aperture changes the depth of field, so be careful where you focus.

Note: This is how you count stops:
a) For aperture, the common values are as follow: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. The values are all of only one stop difference; 1.4-2 is one stop, 8-11 is one stop, even 16-22 is considered one stop. The easiest way to remember is when the value is multiplied by two, it means its two stops.

b) For shutter speed, the common values are as follow: 1/1, 1/2, 1/4, 1/8, 1/15, 1/30, 1,60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000. These are all at a stop increment. Just remember to multiple the value by two and you get one stop.

c) For ISO values, the common values are as follow: 100, 200, 400, 800, 1600, 3200, 6400. These are all at a stop increment. Just remember to multiple the value by two and you get one stop.

Most cameras now come with more precised adjustments of 1/3 of a stop. So, to make your lives easier just remember to just count the number of clicks of the dial. Three clicks of the camera dial, be it aperture, shutter speed or ISO means only one stop.

Hope this information helps everybody to get better exposed pictures...


The Red Gummy Bear said...

itu u punya rumah ke? haha

n i didn't know u have a blog! it is interesting though.

u punya jiran kat rumah 28 ;)

S.C. Boey said...

yoh, jiran... yeas my house... gummy bear? nice... lol... my blog is about photography and my journey haha...

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