Camera, Aerial Photography and the hunt begins

Posted: December 18, 2012 in General
Tags: ,

Currently we are using canon G10 for image acquisition. Obviously the decision of using it was good enough. Canon stopped manufacturing G10 even before we selected that camera. To make things (the Machine Vision code especially) better, I thought to change certain things. Now when the airframe is being restructured, I am thinking to change the camera. If the size, weight and obviously the budget allows I can think over to shift to a DSLR. Selecting an appropriate camera is important because if the acquired image is not good enough then obviously the image processing code will lead to false or no results.
So to select camera, I first short listed few cameras. I first prepared the list of cameras that may be given special attention because they have already been used for similar purpose like ours by some other people. Secondly I searched for all cameras that are supported by gphoto2 library (Canon G10 is one of them). Thousands of cameras were listed in this list. Then I started eliminating all point and shoot cameras. Obviously my aim was to select the best DSLR with in our budget that satisfies all our requirements. I already have been using a point and shoot camera (G10) that apparently satisfies our entire requirements. So now to make significant improvement in the image I need to consider the difference in a point and shoot and DSLR cameras. Moreover due to weight, economical and quality constrains I was made to focus only on Nikon and Canon cameras. This is how I shortlisted my long list of cameras. Obviously I knew what all parameters I am looking for within these cameras.

Cameras are framing systems which acquire a near-instantaneous “snapshot” of an area (A), of the surface. Camera systems are passive optical sensors that use a lens (B) (or system of lenses collectively referred to as the optics) to form an image at the focal plane (C), the plane at which an image is sharply defined.

For efficient processing I need a detailed image of the ground. The ground coverage of a photo depends on several factors, including the focal length of the lens, the platform altitude, and the format and size of the film. The focal length effectively controls the angular field of view of the lens and determines the area “seen” by the camera. The longer the focal length, the smaller the area covered on the ground, but with greater detail (i.e. Larger scale). Since the plane is moving with high speed, I need to find the optimum focal length so that I do not miss any land in continuous images and image still remains detailed as much possible. A zoom lens of focal 18-55mm with APS-C camera was found appropriate.

The area covered also depends on the altitude of the platform. At high altitudes, a camera will “see” a larger area on the ground than at lower altitudes, but with reduced detail (i.e. Smaller scale). In my case altitude may vary from 100 ft to 700 ft.
Also I need a fairly good shutter speed so that there is almost 10-50 percent land overlaps in my consecutive images. Using G10 now delivers one image in almost 3 seconds. Moreover I need very fast shutter speeds in order to combat the movement and vibrations of the aircraft. I can achieve these speeds with a combination of wide aperture and ISO. While there are many ways to set the camera, my suggestion is using Aperture Priority mode. Setting the aperture to one of the widest settings allows plenty of light into the camera. Depth-of-field is not an issue since most everything will be focused at infinity.
Depending on the brightness of the day, I start with the ISO at 100 or 200 and check the shutter speeds that are achieved with that aperture / ISO combination. If the shutter speed is below the range, I raise the ISO until it is in that range.

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