Photography Basics – Exposure and Histograms

histogramIn my previous post: Photography 101 – Aperture, Shutter Speed & ISO we explored the basic camera controls, their relationship to each other and their use in both everyday photography and creative photography.

Today I want to look in greater depth at how this relates to the captured image in terms of Exposure and the Histogram.

Exposure

In simple terms exposure is the amount of light that is allowed to fall on the sensor (or film) when you take a photograph and modern cameras will have a built in exposure meter (or light meter) that will determine the light values of a scene and attempt to return a result that will give a suitable mid tone which equates to 18% grey.

Why 18% grey? Simply, this is the correct value for caucasion skin tones under normal lighting conditions and is considered the standard that meters are set to.

So with our camera set to auto the meter will evaluate the scene and attempt to set the aperture and shutter speed to give an average exposure of 18% grey, which generally will give a reasonable image. The problem with this approach is that we do not always want to photograph average subjects; we may wish to photograph non-Caucasian skin tones, snow and beach scenes, night scenes, high key and low key scenes.

In these situation the light meter will attempt to reproduce the scene to 18% grey which will result in under exposed snow / sand or blacks will look grey or muddy, in these cases we need to make a judgement of what is the most important part of the image to expose for and trick the meter into making the correct settings.

Exposure Meters

Exposure meters fall broadly into two categories; built in (part of the camera) or hand held and can feature several different modes.

Center Weighted Average Metering

In this system 60-80% of the exposure is biased towards the central portion of the viewfinder and gives less importance to the exposure towards the edges. This will generally return acceptable results with portraits and general scenes as it will be less influenced by brighter skies etc.

Average Metering

The exposure is calculated by averaging out the entire scene.

Spot Metering

In this mode metering is based on 1-5% of the viewfinder, typically the very center, variations will enable the photographer to select from different areas of the viewfinder or may offer multi-spot metering, where a series of targets can be selected.

Spot metering is ideal for subjects with high contrast such as back-lit subjects, photographs of the moon or actors / musicians against a dark or light background.

Multi Zone Metering

Also known as ESP, Matrix, evaluative or honeycomb metering. Here, multiple zones are metered and used to determine exposure across the scene, different bias will be given to different areas and this will in some cases be linked to the camera auto-focus to bias the point of focus.

So far we have discussed various metering systems and how they attempt to make the correct exposure, some of their drawbacks and basics of operation, all of which leaves two questions; how do we know if the exposure is right? If it’s wrong how do we correct it?

Histograms

The histogram is a graph showing the distribution of exposure levels across an image and can normally be found on most image editing software and can normally be enabled to appear on the rear LCD screen of most cameras.

Values to the left of the screen represent blacks and dark tones, to the right light to white tones and the graph will normally show a peak where the exposure is concentrated. If this peak is towards the left, the image will be dark and shadow detail may be lost, if fully to the right the image will be very bright and highlights may burn out. In the majority of situations we would want to expose so that the peak is just to the right of center as shadows can be recovered easier than highlights in post production.

If we need to adjust the exposure when using any of the camera auto modes e.g. program, auto, aperture or shutter priority, then we need to trick the meter by using the exposure compensation control to manually increase or decrease the exposure, use exposure bracketing to take a series of exposures (normally at 0, +1Ev, -1Ev) or use Exposure Lock (AEL) to meter an important part of the scene which is off center before re composing.

Correct use of the Histogram allows us to better understand where the light distribution is within the image and will lead to better captures, with experience and use of the histogram function you will learn to understand how light interacts with the elements of the image and how to better set the camera controls before taking a shot.

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Photography Basics – Aperture, Shutter Speed & ISO

1/6th sec @ f22

1/6th sec @ f22

Welcome to the first in a series of Photography tips aimed at beginners to photography and photographers who have a basic understanding but with to improve their creativity.

I decided to write this as a result of comments online that many DSLR owners used their camera as a fancy point and shoot, when I asked around it was apparent that some people who are new to photography may not have an in-depth knowledge of their camera controls or the basics of photography and that a few simple tutorials may help them to achieve their potential.

I’ll start with the basics as they have the most impact on what we all do in general and also have the most creative impact on an image, that is Aperture, Shutter Speed and ISO.

Aperture

The aperture determines how much light passes through the lens, it is basically a diaphragm that can let in varying amounts of light, similar to the iris in your eye. Fully open and it will let in more light, reducing the aperture size will let in less light, but that is not all the aperture does it also affects the Depth of Field (Dof) in the final image.

DoF is the amount of the image that is in focus on the focal plane, this is basically the point where your sensor (or film) is placed. With the aperture wide open the DoF will be shallow, the portions of the image closer and further away than the main subject will be out of focus and blurred, with the aperture stopped down more of the image will be in focus.

The location of the aperture control on your camera will vary but it will be used to adjust the f-stop or f-number, this number is the ratio of focal length to effective aperture diameter, but a smaller f-number (f3.5) equates to a wider opening and more light / less DoF, a larger number (f16) equates to a smaller opening and greater depth of field.

The actual range of f numbers will vary dependent on the lens in use and you may notice that a zoom lens will have two maximum f numbers i.e. f3.5 – f5.6 which relates to either end of the zoom, finally each change in f-number is one stop or Exposure Value (Ev) in light (remember this it will come up again later).

Shutter Speed

The shutter determines how much light hits the film or sensor, or more accurately, the length of time the sensor or film is exposed to the light entering the camera. On the old wooden plate cameras this was as simple as a wood or metal slide that the photographer would pull out from the film holder and replace after a matter of minutes had passed, as film became more sensitive, the shutter became more advanced offering higher speeds and has now evolved to a series of mechanical blinds in front of the sensor (or film) which open for a fraction of a second to allow light to hit the sensor.

Speeds can range from several seconds to a maximum of several 1000’s of a second i.e 12 s – 1/8000. Shutter speeds also traditionally increase in the following form 1/125, 1/250. 1/500 etc. each increase or decrease in shutter speed related to 1 Ev in light (I said it would come up again).

Again shutter speed has another use, a high shutter speed can be used to freeze motion, that’s great right? We all want the subject to be sharp, right? Well yes, and no. There may be times when you want to convey motion, how do you do this with a subject (or medium) that is static, well by introducing a degree of blur.

Sports photographers will often use a slower shutter speed(1/60th) and pan the camera with the action, when done right this will give a sharp subject but the background will be blurred, a lens can be zoomed during a long exposure to give a creative blur or the motion of a waterfall can be conveyed by turning the appearance of the water into a smooth, silky fog by using exposures longer than 30 seconds.

ISO

ISO is traditionally a rating of how sensitive film is to light, different films would have a different rating i.e. ISO 50, ISO 100, ISO 200, ISO 400 the lower the number (ISO 50) the less grain the film has so it produces a sharper image but is less sensitive to light so needs brighter conditions or a tripod, the higher the number (ISO 400 or even ISO 3200) gives a much more grainy image with possible colour shifts  but will allow you to shoot in lower light.

For example taking ISO 100 as standard; an exposure of f8 @ 1/250 at ISO 100 equates to f8 @ 1/125 at ISO 50 or f8 @ 1/4000 at ISO 3200.

So has the advent of digital cameras made ISO irrelevant? No of course not.

Your DSLR will still have an ISO dial, it will vary between ISO 100 to ISO 3200 on average, and guess what? Each step up or down in ISO equates to 1 Ev (you were paying attention to that – right?).

So what does this mean? Well, in general the Lower the ISO the better. A digital sensor works by converting the light that hits its surface into a digital signal, increasing the ISO increases the sensitivity of the sensor by increasing the electric charge across it, whilst this will allow the camera to work in darker environments it will also introduce noise (or grain) into the image, which can result in a degeneration of image quality. Sometimes, however you may need to sacrifice a little bit of image sharpness to get the shot, indoors at a concert or sporting event the lighting may not be bright enough without increasing the ISO, or you may want to use grain creatively.

The Exposure Triangle

Now, if you remember what I wrote earlier there is a common point: each increase or decrease of Aperture, Shutter Speed and ISO increases or decreases the Ev by one stop, therefore a combination of these values will give a correctly exposed photo but with varying effects. If you want just a small part of the image in focus a wide aperture will achieve that but will require a high speed, if you want more of the image in focus use a smaller aperture but you will need a slower speed, this is the basics of making a manual exposure (that funny M symbol on the camera settings).

Most Cameras will have at least a program (auto) mode, Aperture Priority mode and Shutter Priority Mode. Program will do a decent job at getting a well exposed image, but use the other modes and play. Taking a portrait? Open the aperture a bit (to f3.5 for example) and throw the background out of focus. Shooting a landscape? Close the aperture down more (to f11 or f16) and pack the frame with front to back sharpness. At a sports event? Set the camera to Shutter Priority and use a high speed (1/500th ) to freeze motion or use a slower Shutter Speed and deliberately add blur to emphasise movement.

Photography, as in life has rules; rules to get the right exposure, to freeze motion, to obtain the most pleasing composition. I would urge everyone to learn the rules and learn when to apply them for best effect, in this way the technical aspects of photography becomes second nature and will allow the photographer to concentrate on the creative aspect of photography. But just as importantly, or more importantly, knowing the rules means that when you deliberately break them, you know why you have done so, you will have done so for a creative reason, but you will also know how to reproduce the effect when you need to.