Checking Camera Exposure

Use MCC to check for over/under exposure of calibration target images

We ship our cameras with default exposure (shutter speed, ISO) that is typically best to leave at the defaults in most cases. The default exposure is set based on a good exposure when capturing images outside during a full sun day. A good exposure means the calibration target panels are neither overexposed (too bright) or underexposed (too dark).

If you are capturing images in brighter lighting environments, which is rare, you can decrease the exposure (make the images darker). It is best to only adjust shutter speed and leave ISO alone to reduce adding additional noise. To make the images darker you would increase the denominator of the shutter speed. So if shutter speed is 1/S, increase the S value to make the images darker. For example, if it is currently 1/500s, then to make the images darker change to 1/1000s.

If you are capturing images in a darker lighting environment, which is common when inside a building or outside during a cloudy day, you can increase the exposure (make the images brighter). It is best to only adjust shutter speed and leave ISO alone to reduce adding additional noise. To make the images brighter you would decrease the denominator of the shutter speed. So if shutter speed is 1/S, decrease the S value to make the images darker. For example, if it is currently 1/500s, then to make the images brighter change to 1/250s.

You should be careful though not to adjust the exposure such that the panels become over or under exposed. You can now use MCC to check your exposure and MCC will tell you whether it thinks they will be over or under exposed after reflectance calibration. If MCC warns you your images are over/under exposed it will not stop you from processing them further, so keep that in mind and pay attention to the log output.

An example workflow for checking the exposure is described below. This is recommended for all users looking to get the best data they can from their MAPIR cameras.

Checking Camera Exposure in MCC

Start by placing our calibration target package such that the ambient light reflects off of it similar to how you will be capturing the non-target images. Typically this means just placing it on the ground, but you should make the best choice based on your own particular imaging needs.

Start with the default exposure and capture an image at that shutter speed. For example, let's assume our default is 1/250s. Then capture an image that is brighter by decreasing the shutter speed, in our example that would be 1/125s. Then capture an image that is darker by increasing the shutter speed, in our example that would be 1/500s.

Arrange the images in a folder like this:

You can see how image 002 (1/125s) is brighter than image 004 (1/250s), and 006 (1/500s) is darker.

We can easily see the change in exposure of the JPG, but the RAW will be converted to TIFF and always be much darker than the JPG. So you can't use the JPG as an exposure gauge if you plan to use the RAW pixel data. It is best to test in MCC like we are discussing now.

MCC will not process images of varying exposure, so you need to put the image you want to test into its own folder, let us name that folder "test":

If you are processing RAW+JPG then place both the RAW and JPG you want to evaluate into the test folder. If you are only capturing JPG images, then place the single JPG in the test folder.

Now choose the test folder as your input folder in MCC:

For this exposure evaluation we only need to run the quicker Analysis step, so copy the test image into the test folder and press the Analyze button. Do this for each image you want to test the exposure.

Here are the results for our 3 images:

As you can see in the MCC log output above our image 002 (1/125s) is too bright (overexposed) and our other images are usable because it did not warn us that they were over or under exposed.

You would then set the camera's shutter speed to either 1/250s (004) or 1/500s (006), not 1/125s (002).

Assuming the ambient light doesn't change substantially you can now safely use the 1/250s or 1/500s shutter speeds to capture the rest of your images.

If you test multiple exposures and find multiple shutter speeds that will work, like we did in our example (1/250s and 1/500s), it is typically recommended to choose a usable exposure that is darker (1/500s), since it is more common you want the results from the pixels in the brighter regions rather than the darkest ones (shadows).

Here is what the white panel of the overexposed image 002 looks like after calibration:

Notice the excessive amount of red pixels in the white target, as well as the excessive green pixels in the light gray target. These pixels are likely overexposed and will adversely affect the calibration formulas used to calibrate the images. We recommend you pay attention to the warnings in MCC and ask us any questions you may have so your data is correctly captured. It does not take long to check the exposure as shown above to improve your results.