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Wednesday, 4 May 2016

Exposure Control in Real-World Cameras

3ds Max emulates the real-world cameras to help you create physically accurate lighting and render realistic looking images. In order to create accurate lighting, you need to understand the terms shutter speed and aperture. They are used to control the amount of light in the scene as well as the focus effects such as depth-of-field effect.

In optics, an aperture is a hole that allow you to control the amount of light passing through the lens of a camera. It controls the cone angle of a group of rays that come to a focus in the image plane. The term f-stop [sometimes also referred to as called focal ratio, f-ratio, f-stop, or relative aperture] is used for the quantitative measure of lens speed. The widest apertures have f-stops with the smallest numbers. The maximum aperture available is dependent on the lens you are using. The standard f-stop values are: f1.8, f2.8, f4, f5.6, f8, f11, and f16, refer to Figure F1.

Exposure Control in Real-World Cameras
Figure F1

The aperture can also be used for controlling the depth-of-field effect. This effect is generated by focusing on a fixed point [called focal plane] in a scene. The area around the plane remains in focus whereas the rest is blurred. You can get higher blurring using wider apertures [smaller f-stop values]. If the aperture is very small, the depth of field will be large whereas if the aperture is large, the depth of field will be small.

Note: An iris diaphragm is used to control the opening of the lens.

Shutter Speed
The shutter speed or exposure time is the length of time the shutter of the camera is open when taking a photograph. In other words, the shutter speed controls the length of a time a film sensor is exposed. The shutter speed is measured in seconds, mostly in fractions of seconds, for example, 1/512. Bigger the denominator, faster the speed. 

Tip: To know more about shutter speed, visit the following link: http://digital-photography-school.com/shutter-speed.

When shutter speed is fast, a small amount of light travels and visa-versa. Generally, you are required to adjust both the aperture and shutter speed to ensure that correct amount of light travels through. Slow shutter speeds allow more light into sensor and are used for low light or night photography. The faster shutter speed helps to freeze the motion.

Below are some possibilities:

On a sunny bright day, you will use a faster shutter speed [1/250s] and small aperture [f/11] to compensate for the bright outdoor light.
On a cloudy day, reduce the speed so that more light hits the sensor [1/100s, f11].
If the scene contains a fast moving object, use the faster shutter speed [to prevent blurring]. You are required to open the aperture to let in more light [1/500s, f/2.8].

ISO is the level of sensitivity of the camera to the available light. It is measured in numbers. A lower number represents lower sensitivity to the available light whereas higher numbers mean more sensitivity. As the ISO increases, the grain or noise in the image also increases. The examples of ISO are: 100, 200, 400, 800, and 1600.

How Aperture, Shutter Speed, and ISO Work Together
Here's what happens when you click the shutter release button:

1. You point you camera to the subject and press the shutter button.
2. The subject enters into the camera in form of light.
3. The light passed through various optical elements and then goes through the lens.
4. Light hits the aperture curtain.
5. Shutter opens for some milliseconds.
6. Light hits the sensor for the specified time [shutter speed]
7. Sensor gathers the light based on a pre-defined sensitivity [ISO].
8. Shutter closed and light is blocked from reaching the camera sensor.

As you can see the aperture, shutter speed, and ISO work together to create exposure for the images. When you are lighting your scene in 3ds Max, you are required to experiment with different numbers to obtain correct lighting condition.

Exposure Controls
3ds Max provides different controls for adjusting the output levels and color range for a scene. The process for adjusting the levels is known as tone mapping. This process compensates for the limited dynamic range of the computer display in comparison to the human eye. The exposure controls adjust the colors so that they are close to the dynamic range of the human eye. 

You can adjust the exposure using the options available in the Exposure Control Rollout. The path to access this rollout is as follows: Rendering menu > Exposure Control.

The exposure controls included with 3ds Max are: Automatic Exposure Control, Linear Exposure Control, Logarithmic Exposure Control, mr Photographic Exposure Control, and Pseudo Color Exposure Control. However, the mental ray renderer supports only the Logarithmic, mr Photographic, and Pseudo Color exposure controls.

You can use the Automatic Exposure Control for preparing the first draft of the rendering. It is also useful in rendering still images. If the primary light source in your scene are standard lights instead of photometric lights, use the Logarithmic Exposure Control.  If you are dealing with scenes that have the moving camera, use the Logarithmic Exposure Control. Use the mr Photographic Exposure Control to render high dynamic range images using mental ray.

Figure 2 shows the Exposure Control rollout. You can use the drop-down available in this rollout to select the exposure control that you want to use. The Active check box when checked, the selected exposure control from the drop-down is used by 3ds Max. If you want to affect the scene background and environment maps, select the Process Background and Environment Maps check box. To preview the render of the scene with the selected exposure control applied, click Render Preview. The preview thumbnail appears above the button, see Figure F2.
Exposure Control in Real-World Cameras
Figure F2


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