Exposure and Metering

Exposure

In general exposure determines how much light is in the image, you could correctly expose your subject, the background, both. Exposure is generally conditioned by four things; Shutter speed, Aperture, Film speed, and light value. This of course is assuming you are not using any altering filters such as an ND, or Circular Polarizer which causes a certain amount of additional stops of light to be lost. Before continuing it is important to understand that there is no truly correct exposure. There will always be something that will either be overexposed or underexposed. Only real way for something to be 100% perfect exposed is to have a scene all in the same tone, but that would be too boring.

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Understanding the four factors

Film speed or the sensor equivalent in digital is determined by an ISO number. The ISO is more or less an easy to understand guide of how sensitive film or the sensor is. In general the lower the sensitivity the less noise or grain, the higher the more noise or grain. The ISO values are usually setup in stops of light, for example allows in twice the amount of light as ISO 100, and so on. In terms of producing picture for use with this website its best to keep the ISO as low as possible.

Shutter speed determines the duration that light falls onto the film/sensor. Shutter is usually set with mainly two things in mind; the speed of the subject, and speed of yourself. Given that every other factor involved is ideal, you want a shutter speed that is fast enough to capture the subject sharply, but at the same time fast enough to counter camera shakes, if you are shooting handheld. It can be hard to expose a sports scene at 1/250th of a second, when the available light source is not sufficient enough to expose at that desired shutter speed. But reducing the shutter speed to compensate would also introduce motion blur thus creating a dilemma for you. So shutter speed is basically how long of duration the shutter exposes the sensor to light.

Aperture is in its numerical format (as f-stops) is how much stops of light the diaphragm is allowing in during exposure. On most cameras when you are previewing the image in the viewfinder, the aperture is at its widest. So estimating the exposure or depth of field can be inefficient from the viewfinder alone (some cameras have a DOF preview button on them to stop down the diaphragm to give you an idea what the scene will look light right before it opens the shutter blades). In the last paragraph it was mentioned that shutter is normally set to prevent blur. Aperture on the other hand is set normally with depth of field in mind. DOF is determined by the focal length, aperture, focusing distance, and circle of confusion. So keeping in mind that shutter speed plays no part in DOF, you can in some cases use shutter speed to compensate for an exposure index when you need to keep a certain DOF. Also another reason not to use aperture all the time for exposure adjustment is because most lens are usually at their sharpest when set to f/8 to f/11. So trying to set a shot at f/22 as a method of combating a really bright scene would not only cause more than desirable DOF, but also soften the image beyond desirable sharpness.

There are those of you that may have no problem understanding exactly what shutter speed is, but are already confused by exactly does the aperture value mean. Basically each aperture number is approximately half of the one before. So the increments would be, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32 and so on. The aperture value is determined by the focal length of the lens being divided by the opening; a 50mm lens would have an opening of 17mm at f/2.8, where as a 200mm lens would have en opening of 71mm at f/2.8. This means while both the 50mm and 200mm lens are allowing the same amount of light through the lens, the 200mm is going to have a shallower DOF due to the wider opening.

The available light source is especially important if you are trying to shoot a scene where flash may not be appropriate; after all photography is all about light. In a perfect world there would always be enough light to expose a shot. There are often times where having too much light can be a problem, especially directional light from a flash, or from direct sunlight. Direct lighting can not only cause harsh shadows in your subject, but can sometimes confuse reflective light meters such as built into the camera depending on the direction of the light. Also directional light might bring out undesirable shadow detail such as in a model’s face.

Using Shutter and Aperture together (manual shooting)

Now that we understand the use of shutter and aperture and so on, lets get a general understanding of the use of the two combined.

Let us say for example that you are taking a picture of a sports scene, and that the camera when you have 1/500 with an aperture of f/2.8 set is correctly exposed, but let say that we need more depth of field, that aperture of f/2.8 is too shallow, but how would we go about doing this while keeping the scene correctly exposed. Let say that to stops of light would be enough to give us the focus area we want, so we set the camera to f/5.6, but because we want to make sure we keep the exposure the same (the same amount of light), we would drop our shutter speed down to 1/250 to compensate for the 2 stops of light loss.

The same is true if you need to increase the shutter speed but keep the exposure correct. For example let say you have a shot at 1/60th of a second, with an aperture of f/5.6, but the shutter is too slow for the subject, what could be done is increasing the shutter to 1/125th of a second or so, and then opening the aperture to f/2.8. This would give the same (more or less) amount of exposure, but allow for higher shutter speed (although a more shallow depth of field as well).

About Exposure Compensation (EC +/-)

For the most part your camera will determine the correct exposure for you. When the adjustments made by the camera are not correct is when you would use Exposure compensation (EC). Basically what exposure compensation does, is when the camera determines that for example 1/250th of a second and an aperture of f/8 is the correct exposure, you may choose to alter this. If you were to tell the camera to perform a +1 exposure compensation, in shutter priority mode, the camera would automatically increase the aperture setting to allow in an extra stop of light above what the camera would have determined was correct exposure. So in the case of above the camera may decide to use f/5.6 instead of f/8. The same is true in aperture-priority mode, where the camera will change the shutter accordingly instead. A good example of when to use EV is in scenes where overall would be brighter or darker than the subject matter you want to capture. Take a snowy day for example, you are taking pictures of kids playing in the snow, but in almost every shot the kids show up as being darker than you intended. This could be easily adjusted by pushing the EV up +1 or +1.5 or so, causing the camera in automatic mode to make adjustments to the settings to allow an extra stop of light or so to be captured, thus making the kids correctly exposed with the snow a bit overexposed.

Metering

One thing to keep in mind is it usually takes a few years to get light metering down, trust me I haven’t quite tackled it just yet. This response is probably going to be the exposure section of that other article just into some more fine depth.

Manually Metering In-Camera

If you are shooting manual mode on most DSLR or even SLR cameras for that matter, will have a bar inside the viewfinder, when pointing your camera at a subject thru the lens, half depressing ( some cameras have the light metering always going ) the shutter to get a reading, on my rebel for example you’ll see a bar that goes from -2 to +2, as I change my shutter and/or aperture value the needle on the bar will move based on how exposed the image is, with center being correct exposed. The TTL Metering systems in most cameras are reflective metering (more on that in this reply). As far as the camera goes not really any place to input an “EV” number. But let’s get an idea what EV is and how it pertains to shutter/aperture.

EV (Exposure Value)

EV stands for exposure value; it’s somewhat a combination of shutter, aperture, and ISO film speed. The number after the EV is literally the stops of light.


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So if a light meter gives you a reading of EV-8, then you can use any combination of shutter speeds and aperture long as the Exposure value adds up to 8. This is assuming the light meter you are using already has Film speed value and Light value calculated into the equation to give you back out the Exposure Value, or Shutter/Aperture combination. But with this it means that if you a setting of 1/125 @ f/8, which totals 13 EV, you could essentially do 1/8th @ f/32 because the EV would total up to 13 stops of light and thus would be the same level of exposure.

Light Meters

For the most part the in-camera metering would be all you’d need for a TTL or reflective metering. But there are times when having a separate handheld light meter will give much more accurate results.

Reflective Metering

Reflective metering is more or less the most common type of light metering cameras will perform built in. In a nutshell reflective metering will meter the light being reflected off the subject. This can be problematic when the subject matter is not exactly within an 18% gray area. A subject that is bright will give off more reflective light than a dark subject so the light meter could be fooled. It’s the same with kids playing in snow, where the camera ends up metering off the snow as the extreme. Of course you can do reflective metering by center-weighted, evaluative, or spot. Spot will help you get accuracy on a pin-point area of the subject pointed to, but again one area of the subject may not be within reasonable range of the rest of the subject’s tones. When you take a reflective reading you are pointing the light meter right at the subject.

Incident Metering

Incident metering is considered more accurate than reflective, and is not fooled by situations such as strong backlighting and such. Incident meters take readings from the light that falls onto the subject; basically you would reverse the meter, with a diffuser cap on it, and meter the light from the direction the camera is pointing from. In this way it gets the exact amount of light that is hitting the subject, and meters off of that. If I were holding a light meter to a subject directly in front of me, I would be pointing the light meter at myself in incident mode. Incident is easier to take because it doesn’t have to find an 18% gray area of your reflected subject that would be considered exposed.

Using the Histogram

Histograms are rather useful bits of information that most Digital SLR or even digital cameras will have available to you. You can use the histogram to get an idea of exposure after the shot had been taken. Here are some examples what to look for.


Above you’ll note a histogram with most of its bars to the far left. Keeping in mind, info on the left is shadows as pitch black on the very left, and to the right are your highlights. The above histogram shows an image that is underexposed, not only because a majority of its data is past the middle to the left side, but because some of the data actually went past the left edge of the histogram (information that cannot be recovered).


This one shows an over-exposed image. Note how the bars on the right went past the right of the screen, thus ‘blowing out’ the highlights. In most cases if your image is underexposed you can recover some information to some degree. If you blow out the highlights, or clip them, this information is lost permanently; there is no recovering from overexposure.


Now this one is ok, has some shadow detail, most of its other details are in general around the second half and such. Around the middle is what most people would call correctly exposed. The key is to make sure that data doesn’t go past the left or the right side.


Now this is what I would consider the ideal histogram, if you are shooting in raw mode. The reason being is that with sensor technology, light is captured in stops. The first couple stops contain the most data. As a result if you can get the image exposed as close to the right of the histogram without overexposing the image, you’ll have a lot more details to work with in Photoshop and such, but also you’ll have less noise than you normally would when normally trying to brighten a ‘correctly-exposed’ image. More information pertaining to Signal to Noise Ratio, and exposing to the right can be found here.

Alternate method of metering

There are some cases where you either do not have access, nor can afford a dedicated light meter. In which case if you do not want to rely on the reflective metering of your camera you can turn to an alternate method that makes your in-camera metering behave like an incident meter. What you would do is use an 18% gray card. 18% gray in a black and white scale is what reflective metering would consider correctly exposed. As a result when you place a gray card in front of the camera and set the exposure you are metering off the light reflecting off the card, but in a way you are doing the same thing incident meters do. Although unconfirmed I have read in places where some cameras meter towards 12% gray more so than 18% so there may be some differences in the way in-camera meters expose the card.