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Exposure Mode and Exposure Time

Tudor -

Exposure Time

Cameras are complicated things. The cameras on most DJI aircraft (not the X5) are a bit simpler in that they have a fixed aperture. This means that the "brightness" of the image being taken is primarily controlled by adjusting the camera's exposure time. The exposure time (or integration time as it is referred to at the sensor level) is the amount of time the camera's sensor is collecting (integrating) light. The shorter the exposure time, the dimmer the image. If you are looking at a really bright scene like a sunny snowy day, you will need a very short exposure time. If you are indoors or outdoors near dusk a much longer exposure time will be needed. Long exposure times are fine as long as everything is still but we are talking about an aircraft here and nothing is still...

ISO

Basically, don't touch the ISO unless you know what you are doing. Some people like to adjust the ISO (or gain as it is referred to at the sensor level) in order to be able to use shorter exposure times than the available light would usually allow for. Since the ISO setting (gain) doesn't really mean the same thing as it does in a film (what's that?) setting it is really just amplifying the noise in the sensor. Essentially, you are telling the camera to go ahead and make the image grainier looking so you can continue to use your desired exposure time despite the camera's better judgement.

Film used to do this trick with bigger/more sensitive cells for the different ISO rated products. You can't make your sensor's pixels bigger or more sensitive but you can make numbers coming out of them bigger by multiplying them. The problem is, multiplying very very small numbers rarely turns out well due to digital electronics and noise in the analog to digital converters in the sensors.

See? Cameras are complicated things.


Ground Smear

Ground smear is the distance that is traveled while the exposure time is happening.

If your exposure time is 1/1000 (1 millisecond or .001 seconds) and you are traveling at 15 m/s, your ground smear will be 1.5 cm (multiply the .001 by the ground speed of 15 m/s to get .015 m or 1.5 cm). 1.5 cm of ground smear is not bad, especially if your GSD (shown on the altitude selection slider in Map Pilot) is 4 cm. 

If you are traveling at that same 15 m/s but it is darker outside and the exposure time is set to 1/50 (20 milliseconds or .02 seconds), your ground smear will be 30 cm!!! Yikes! That's a foot! And unless you are flying higher than is legal in most places, your images are going to look like junk.

The general rule of thumb is to keep your ground smear to less than your GSD if possible.


Automatic vs Shutter Priority vs Fixed (Manual) Exposure

As of version 1.3.2 of Map Pilot, we have added two new exposure modes: Shutter Priority and Manual.

The early versions of Map Pilot used Automatic Exposure control which basically looked at the images coming into the camera and made a call as to whether or not the image looked good. If it was getting a bit dark, the exposure time is increased a bit and it will be reevaluated after the next frame to see if it was enough. It the image is getting too bright, the exposure time is shortened up to prevent the image from saturating (whiting out due to too much light) the sensor. The ability to automatically adjust the exposure based on the scene contents and variations in available light are good for mapping imagery to ensure evenness of the overall brightness of the created map. Automatic Exposure is the best option when there is plenty of light to work with. Automatic Exposure's downside is that it will allow the exposure time get so long that the ground smear is overtaking the GSD and causing significant blurring. 

With winter setting in for the Northern Hemisphere, we had a lot of users complaining of blurred imagery and attempted to use DJI's Shutter Priority Exposure mode in version 1.3.1 to try to address the problem. Shutter Priority mode uses a fixed exposure time and adjusts the ISO accordingly try to keep the image from getting too dark or saturated while maintaining an exposure time that hopefully minimizes blurring. It quickly became apparent to us that the exposure time that was adopted by the camera for use with the Shutter Priority mode was not always something that could be trusted. For versions 1.3.2 and up, we are leaving it up to the user to define what the proper exposure time set point should be. The default value of 1/320 is a good value that corresponds to a reasonable amount of ground smear while still letting the ISO adjustments do their thing. Advanced users can adjust from there based on conditions.

While we were adding modes, we figured we would add a Manual Exposure mode as well. We view this mode as an experts only setting. A few users have requested it but we think that its applicability is limited to super special use cases where there is perhaps a lot of reflective surfaces that would trigger strange behaviors with the other modes. This setting will allow the camera to let certain areas saturate or go dark without sending the exposure into wild oscillations. The ISO value is fixed at ISO 100 for the manual exposure setting.

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Comments

  • Avatar
    klaus mzfly

    Hi there,

    I have a question regarding the exposure mode setting.
    I want to play around with the settings, but I don’t find were.
    I can’t find it in the setting area as described in your article and in the flight window of the app the camera control button is always grey, so I can’t use it either.
    I have the app Version 2.3.0 and a DJI Mavic with newest SW on it.

    Besides this I really love the app und the service you provide for aerial mapping. 5 stars.

    Thanks for your help in advance
    Klaus

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