Improve your digital photography – Exposure

If you are a newcomer to digital photography, you may be unsure of where to start to get the most out of your camera. This is the first post in the series that is written to try and help you to move away from your camera’s auto setting, and aims to let you take more creative control of your DSLR. The first topic to be covered will be exposure. This is the first in a series of posts to help you understand the basics of using your DSLR camera. They won’t replace your camera’s manual, but will try to cover enough of the basics, or at least point you in the right direction to get you started on your journey to becoming a more creative photographer. 

Other topics to be covered in future posts include:

Focusing.

Camera shooting modes

Image capture.

White balance.

 

The Exposure Triangle.

 

What is meant by this term?

A correctly exposed image depends on several factors. By “correctly exposed” I mean an image that is neither too dark, or too light. This is achieved by setting the camera’s Aperture, Shutter speed, and ISO.  These three things together determine how much light hits the image sensor in your camera / smartphone. These three elements are interlinked, and need to be adjusted carefully to produce the image exposure that you want to achieve.

When starting out with your camera, you probably set the mode dial on it to auto, and started taking your pictures. While this will generally produce images that are O.K. They may not be exactly what you were expecting. The reason for this is that your camera is making all the decisions about the exposure of the shot.

Like I said, this will probably give an OK picture, but to allow you to take more creative control, you need to get your camera off auto. For this the modes like Aperture priority (usually A or Av on mode dial), or Shutter priority (usually T (time) or Tv on mode dial) can be a good way to start. With these modes, you are responsible for setting either aperture, or shutter speed, with the camera taking care of the rest.

You can set the ISO either to auto, where the camera will choose what it believes to be the best ISO setting. Or set a specific ISO for the scene yourself. So in Aperture priority, with ISO set to 100, for example, all your camera is doing is setting the shutter speed. These modes allow you mode creativity, whilst having less to worry about when you are starting out with your new camera.

ISO value

This relates to how sensitive your camera’s sensor is to light. The ISO sequence is generally 100 – 200 – 400 – 800 etc. Each step is double the previous, and doubles the amount of light reaching the image sensor. A problem with the higher ISO values, is that the image will show more “graining” or noise. With the improvements in camera technology over the last few years, this is not as big a problem as it once was.

Below are two versions of the same image. As you can see in the images, the exposure is approximately the same for each image, however, one is shot at ISO 100, the second shot at ISO 1600. The area indicated in each image has been cropped, and is below the main image. The one at ISO 1600 has significantly more graining than the image shot at ISO 100.

 

Exposure with the ISO set to 100
Image at ISO 100

cropped-100-small

Same exposure but now with the ISO set to 1600
Image at ISO 1600

cropped-1600-small

As can be seen there is quite a lot of noise (grain) in the ISO 1600 image. You should always try to keep the ISO as low as possible to produce the best image quality you can. In low light situations this may not be possible.

Aperture setting

If you liken the aperture of a lens to the iris of the eye, the wider the opening, the more light reaches the camera’s image sensor. Aperture size is measured in F-stops (or stop), the smaller the number of the F-stop, the wider the aperture opens. Opening the aperture by a full stop (smaller F-stop number), doubles the amount of light to the sensor. Each lens will have it’s own aperture range. 

The aperture also controls the Depth of Field (DOF) of the image. DOF is the amount of image that is in focus from front to back of the image. DOF is also affected by the focal point, and how close you are to the subject, and also by the lens used to capture the image.

In the image below, focal point was the front edge of the plate. You can see that as the aperture becomes smaller (larger f-number) the DOF of the image increases. The shutter speed was altered to keep the exposure constant for each image, so as aperture becomes smaller (letting in less light) the shutter speed becomes slower to let in more light.

How aperture size affects DOF

Shutter speed

The speed that the shutter opens and closes affects the amount of light to the image sensor. The faster the shutter speed, the less light allowed to the sensor, and vice versa. Shutter speed can vary from several seconds (maximum regular shutter speed 30s) down to mere thousandths of a second.

Each increase in shutter speed lets in half the light of the previous shutter speed. So increasing from 1/60th to 1/125th of a second, The shutter now opens and closes twice as fast at 1/125, so only half the light can reach the image sensor. 

The shutter speed also controls how much subject movement shown in an image. The faster the shutter speed used, the more any action in a scene will be frozen. The slower the shutter speed, the more motion blur is imparted into the subject in an action shot.

By using what you have learned about the exposure triangle, you will be able to accurately shoot the scene. by using more creative use of shutter speed, aperture, and ISO you stand a better chance of capturing the image you want to.

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