IP Camera Learning blog

HOWTO: Configure the exposure on a Mobotix M24M camera

March 8th, 2010 by James Drinkwater

Mobotix cameras provide some of the best quality images in the industry. However, setting up each camera to automatically produce the best possible images in changing light conditions can be tricky. This article looks at the exposure settings on the Mobotix M24M and provides advice on adjusting settings to get the most from your camera.

Digital and mechanical exposure

On conventional IP cameras, image exposure is controlled by a device called an iris. The iris opens and closes mechanically to control the amount of light which hits the image sensor.

Instead of controlling the amount of light for each image, Mobotix cameras alter the length of time each image is taken for. This is called the exposure time and essentially shares the same amount of light over a longer or shorter time.

Shortening the exposure time makes images darker as less light is available for each frame whereas longer times make brighter images. However, longer exposure times mean that objects can move during the time it takes for each frame, causing blurring.

The camera we are using for this article is a Mobotix M24M-SEC-D22, although the setup procedure is identical for almost all Mobotix cameras. We will assume that your camera is accessible across your network and that you have full administrator access.

Exposure basics

We often get asked what the best settings are for exposure. Unfortunately this is something we really can’t provide. No two locations are the same and each requires different exposure settings. Don’t be tempted to copy the settings from another camera as it will not provide the best results.

To look at your current exposure settings, log into your camera and click the “Setup Menu” button at the top-left of the page. From here click the “Exposure Settings” link.

This page contains all the options which control how the camera will react to changing light conditions. Click the “More” button at the bottom right of the screen to make sure that you see all the settings available to you. There are a few options here which should be explained.

Backlight Correction

Backlight correction helps to provide detail in objects with strong light sources behind them.

Exposure mode

With Mobotix cameras you can zoom into the camera image to only view a small section of it. The Exposure mode setting tells the camera whether you want the exposure to expose the whole image (Full Image Area) or just the section you are monitoring (Visible Image Area).

Exposure Windows

Exposure windows tell the camera what part of the image you want to base the exposure calculations on. Essentially, the camera will alter the exposure time to properly expose the selected windows. There are predefined exposure windows held within the camera for specific applications, such as Centre, Right & Left and Bottom. Alternatively they can be set manually.

Exposure weighting

This tells the camera how important the exposure windows are compared to the rest of the image. 100% would tell the camera to only concentrate on the data inside the windows, whereas 0% would ignore the windows and only concentrate on the overall image.

Night Improvement

This increases the signal gain, amplifying image data in low-light environments to provide brighter footage. However, this also increases image noise.

Exposure time

As stated earlier, the longer the exposure time, the more objects in view will blur as they move. These settings allow you to set a maximum and minimum value for the exposure time.

Setting the exposure

To see how the exposure settings work, you first need to see the exposure windows being used. To do this, change the “Show Windows” option to “On” and click “Set”. You should see a green box on the camera image showing the area being used for its exposure calculations.

You can select different options in the “Exposure Windows” drop-down and see how the brightness of the image changes with each setting. Selecting “Custom Windows”, will open a new box on the page which lets you create user-defined exposure windows. Readers who have already set up motion detection in a Mobotix camera will recognise the setup process as being similar.

In order to create an exposure window, it is best to have the main camera view in the background with the Exposure Settings page in another window in the foreground.

On the main camera view at the top left of the area you wish to select, hold down the shift key on your keyboard and left-click the mouse. A yellow dot should appear. Now, move to the bottom left corner of where you want your window to be and left-click the mouse. A yellow-edged box should now appear between the two points you selected.

In the Exposure Settings window, click the “Add Rectangle” button on the right-hand side of the page. The yellow box on the main page will now disappear and a new line will appear in the “Custom Windows” box. An example of this is shown below.

0,300,300,200,200,in

The first number is the lens that the exposure window is set on and is used for multi-lens cameras such as the M12 or D12. For anyone using a single lens camera such as the M24, this is always 0. The next two numbers are the X and Y positions of the bottom corner of the box (distance up and across from the bottom-left corner of the image). The following two numbers are how wide and tall the box is, in this case 200×200. Lastly, the “in” tells the camera to include this as an exposure window. Having “out” would tell the camera to ignore all data in this box from the exposure program.

Once you change any settings you have to click the “Set” button for the settings to take effect and also store the settings into permanent memory. From the main camera page, click on the “Admin Menu” button and then click the “Store” link at the bottom of the page. Click the “Store” button to save the settings permanently.

Exposure Examples

Basic Examples

For any view, you have to set windows over the areas that must expose correctly. For example, if your image has a walkway in the foreground and a bright sky behind, you would place a box over the walkway so that the people in view are correctly exposed. It doesn’t matter if the sky is over-exposed so long as you have definition of the people on the walkway.

Conversely, if you have bright and dark areas where you want to have both correctly exposed , for example a yard in the foreground with the sky behind, place two boxes of equal size onto the image, one over the brightest part of the sky and one over the darkest part of the yard. The camera will balance the exposure between both and will allow you to see detail in each.

If the difference between the light and dark areas is too great you will not get definition on either. In this case you can either choose to view only a single area or try altering the camera position.

If you feel that the images are too bright or dark, you can either alter the brightness setting in the “Exposure Settings” page or you can adjust the sizes of the boxes you just entered. Making one box slightly larger or smaller will adjust the ratio between the two and adjust the exposure accordingly.

Horizon shots

We have had instances in the past where we have had a camera pointed at the horizon, often with the sun directly in view. When trying to correctly expose both the land and the sky, we have found that the following configuration works well.

The two smaller boxes combined are roughly the same size as the larger box in a similar manner to the example above. The light of the sky and the darker ground allow the camera to expose so that both are visible.

License plates at night

Quite regularly, customers ask us how to configure Mobotix cameras to pick up license plates in low-light conditions, for example a haulage firm wanting to record the license plates of trucks coming into a yard. Instead of trying to adjust the exposure based on each truck as it comes in, try installing a sample license plate in view of the camera, illuminated by the same lighting that the trucks. Once the exposure is set to the sample plate, the plates on each truck should exposed correctly too.

Tips

Check in different lighting conditions

The reason for setting exposure windows is so that the camera can adjust automatically to changing light conditions. Make sure to check your exposure settings later on when the lighting has changed. There is no point in having a perfectly exposed image at only a single point during the day.

If it ain’t broke…

Don’t try to over-complicate things. If the standard exposure settings work well for you, leave them as they are. It will save a lot of time over trying to use a user-defined setup which may or may not be just as good.

Control the scene

If you find it hard to correctly expose the camera, and rather than spending hours trying to find the optimal settings, it may be simpler to move the camera itself. For example, moving a camera higher and pointing it down can assist with problems with excess sunlight. Instead of recording license plates on the front of a car, where headlights can dazzle the camera view, move the camera and capture the rear plate. Controlling the scene initially when installing the camera can save a lot of time later on.

You should now have enough information to begin configuring the exposure on Mobotix cameras. If you have any other comments or suggestions please let us know below.