Everyone is a photographer these days, and it’s no surprise when we’re carrying around cameras in our pockets with the capability to take photos at a quality unimaginable even 10 years ago. But capturing high quality images with a high-speed camera is significantly more nuanced than taking a photo for your Instagram account! Here’s some information to help you get started:
The Exposure Triangle
Before addressing considerations particular to high-speed camera usage, you should be familiar with some basics of photography. There are three controls that work in balance with one another to create a properly exposed, high quality image.
Aperture
This refers to the size of the opening of the lens of the camera (sometimes referred to as the “iris” or “f/stop”). When the iris is opened to allow more light to reach the sensor, the Depth of Field becomes more shallow. Many television shows and movies use this to create the dramatic effect of a character in the background out of focus, while the character closer to the camera is crisp and clear. When you are able to close the f/stop of the lens to reduce the amount of light required to provide proper illumination, the Depth of Field can become significantly greater. For scientific imaging, this impacts a subject being able to remain in focus as it moves toward and away from the camera during an experiment.
Shutter Speed
This setting controls the amount of time that the shutter allows light through to the camera’s sensor. A faster shutter speed, with shorter exposure time, freezes fast-moving objects in space, but it also requires more illumination. Longer shutter speeds increase the light to the sensor but can result in blurred images on objects that move through the Field of View of the camera.
The image on the left was shot with a 198 microsecond exposure time and shows motion blur as the trap is sprung. As the exposure time is shortened in the images from left to right, we steadily see a reduction in image blur. The final image on the right was captured with a 10 microsecond exposure time to eliminate the motion blur of the fast-moving object. You will also see that in order to capture images at the shorter exposure times, we needed to adjust the f/stop of the lens to allow more light to reach the sensor.
ISO
ISO values increase and decrease the sensitivity to light for the sensor in your camera. This can be adjusted to balance against your needs for Depth of Field and/or shutter speeds, but “boosting” sensitivity by adjusting Gain, Gamma, Brightness and Contrast settings can reduce the picture quality by adding unwanted “noise” or “artifacts” into the images.
In order to achieve a properly exposed video, you use these three settings in balance with one another. Proper understanding and use of aperture, exposure time and ISO will result in optimum images for any application.
Other Important Terms
Resolution
The resolution of a high-speed camera refers to the number of active pixels being imaged. Higher resolution doesn’t necessarily mean “higher quality” when it comes to using a high-speed camera for many applications. The key is to make the most of the pixels that are being imaged by using the appropriate optics that fill the available resolution with the subject that is of interest.
The first two images here were shot with a 60mm lens from the same Working Distance. (The area of interest is the center of the object.) The first image is captured at full 1024 x 1024 resolution and the second is captured at 640×640 resolution. By not changing the lens or position of the camera, we are simply reducing the Field of View when using a lower resolution. The last image shows 640×640 resolution and is captured with a 100mm lens to focus specifically on the area of interest.
Frame Rate
The frame rate of a high speed camera is the number of images taken per second. Standard video is taken at 24 frames per second, and slow-motion video taken with high-speed cameras can capture as many as millions of frames per second! With current high speed camera technology, the sensor in the camera is able to achieve a particular frame rate at a given resolution. In order to capture images at higher frame rates, the user will reduce the number of active pixels being imaged and be able to set the camera to higher frame rates.
The images above were captured at different frame rates. The top row of images display the first frame of the sequence after the camera began capturing images. As you can see, the position of the arm on the mouse trap is in the same position at T0 in the top row of images, regardless of the frame rate. The bottom row of pictures show this same event at (29) frames after the trigger frame (T0) at the different frame rates.
Monochrome v. Color
All high-speed cameras are initially built with a monochrome image sensor. In order to obtain color images, a Bayer color filter is physically applied to the sensor and an algorithm is used to “create” a color image by assigning Red, Green and Blue values to each pixel.
In applying the Bayer filter to the light sensor, the light that reaches the pixels on the sensor is reduced. This means that more light is required for the same level of exposure in your photos with a color camera and amplifies the trade-offs of the exposure triangle.
The images above were taken with the same f/stop of 16. As you can see on the right, the color camera has less light sensitivity than the monochrome version due to the Bayer color filter that is used to create color images on a high speed camera.
The images here illustrate the same illumination levels of a monochrome vs. color camera – the f/stop of the lens on the monochrome camera is set to f/11 while the f/stop of the lens on the color camera is set to f/8.
The best way to select a high speed camera for your applications is by requesting an on-site demonstration of the camera to see the image quality, sensitivity and User Interface for yourself.
These are only the very basic concepts for successful image capturing, but they’re a good start to help you select a high-speed camera system that’s best for your applications.