So last time we looked at time lapse of the wheel and now we will take a look at long exposure.
A long exposure is when you are allowing the sensor to be exposed to light for an extended period of time. Anything moving like cars driving down the street with the lights on will become blurry… At least that’s what we want. Normally when you take a photo it can be a fraction of a second in a well lit room. But if you turn the lights off to get the same photo you would need to increase the time the sensor is recording and thus anything moving will blur, or if you use the same settings as the lit room picture, your picture will simply be black.
In DSLR photography we have 3 settings we can control to adjust the amount of light reaching the sensor.
- ISO: Is an old standard from the days of film photography which is how the film will chemically react to the light. These days its all digital but the principle is the same. In digital terms its how the sensor will react to the light. The lower the ISO (ie ISO 100) the more light is needed to make a clear exposure and also the cleaner the picture. The higher the ISO (ISO 6400) much less light is needed but then also the grainier the picture has become.
- Shutter Speed: Is how long the shutter curtain is open exposing the sensor to light. The quicker the shutter speed the more still the picture will be. But the slower the shutter speed is you start to get blurry images and you will likely need a tripod.
- Aperture: Is controlling how much light is coming into the camera, in a normal camera lens this is normally thorough an iris. The larger the opening of the aperture the more light is received
That turned into a little photography 101! To finish, these three setting used together can create different effects in photograph. If you have a quick shutter speed you can adjust the ISO and the Aperture to make your picture expose correctly look great (depending on your lighting arrangement).. There is a wonderful book written by Bryan Peterson called Understanding Exposure I recommend if you want to learn more about exposure and photography.
Now how does this work with the Rasepberry Pi! Well 1st we need to consider what we have on the camera and understand its limitations. We don’t have an aperture to control its fixed at f2.3. The best ISO we can get from the camera board is ISO 100. Also the Shutter Speed wont go any slower than 6 seconds as its limited by the firmware.
So we want to make a long exposure. We want to take it at night to get those trailing lights! Then we will need the following things.
- Configured Raspberry Pi with Raspbain (updated)
- Camera board
- Tripod (or something you can stick the camera too so its not moving)
- Connection to the pi..
- If you are working through a window you need something black to surround the camera to stop reflections)
In my example (below) I had set up the camera on a tiny tripod I had and put it against the window. I then used the following command to take the picture.
raspistill -hf -vf -ss 2000000 -ISO 100 -sh 50 -br 50 -sa -75 -o wheel.jpg
A quick brake down of all the options are :
- -hf -vf : I had my camera upside down so these settings are for horizontal and vertical flip.
- -ss 2000000 : shutter speed at 2000000 nano seconds or 2 seconds.
- -ISO 100 : because we have exposing the sensor for 2 seconds we need to have the lowest ISO number so that the quality is crystal clear and that the light does not blow out. If we used a higher ISO number with this shutter setting the picture would be brighter.
- -sh 50 : Sharpness (optional) controlling the sharpness of the finished image.
- -br 50 : Brightness (also optional) controlling the brightness of the finished image.
- -sa -75: Saturation (again optional) controlling if the image is color bright or washed out.
- -o wheel.jpg: output with the file name.
Here is the image I manged to get
In this image example using ISO 100 and Shutter speed as 2 seconds we can use this as a reference to what would happen if we started to fuss with the options.
- If we just simply change the Shutter speed to 4 seconds the lights in the image would become brighter but the light trails would get longer as we are recording more of the movement.
- If we put it to 6 seconds then the lights would loose all color and would turn white!
- If we changed the image to 1 second the trials would be shorter the image would get darker.
- Returning the Shutter speed to 2 seconds if we made the ISO 200 the image would become a little more grainy and brighter but the light trails would remain the same as the image above.
- If we set the ISO to ISO 800 the image quality would be really grainy and the image would be almost white. But the light trails again would be the same as the 2 second Shutter speed is always the same.
Best thing to do is sit down next to your pi and play around with the options until you get what you feel is right.
I hope this helps in working out how to make a long exposure and if you have the time/money please do check out Bryan Petersons Understanding Exposure its a really great read for those starting out in photography
Please do feel free to comment with your examples of Raspberry Pi Powered Long Exposures!