FIELD OF VIEW (FOV)

     When dealing with digital camera images, a common question is, "What's the magnification of the captured image?"  People are often frustrated with me when I tell them that the digital image's magnification is so variable as to be almost meaningless.  This is because of variability of computer screen sizes and resolution settings, print sizes, etc.  What is meaningful and unchanging is the Field of View (FOV).  Our goal is normally to maximize the FOV that the camera captures, but with no vignetting.  This means matching the camera's sensor (CCD or CMOS) size to the appropriate microscope adapter.  C-Mount adapters typically come in 1x, 0.67x or 0.63x, and 0.5x.  Our MM adapters work a little differently because we are less concerned about the consumer camera's sensor size and more concerned about how the relay lenses in our adapters match the zoom lenses attached to the cameras.  The principle remains the same, however:  capture the largest rectangle possible from the microscope's circular FOV without vignetting.  The chart below illustrates this concept:

In this chart the Canon PowerShot S5iS with our MM99-58 adapter allows a field of view that seems as if it would cause vignetting, but in fact it did not.  That indicates that the field presented to the eyepieces is actually a bit larger than the 10x/20mm eyepieces showed.  So, we could use 10x/22mm eyepieces in that microscope.  The "Dedicated Microscope" cameras like the Jenoptik ProgRes, Lumenera, Moticams, etc, all gave a similar FOV when matched with the correct C-mount adapter.  The "Consumer" cameras gave us more control over FOV by using their optical zoom lenses - we zoomed in to the point where there was no vignetting and stopped, but could have zoomed in more as desired.  The Canon EOS DSLR camera with standard SLR adapters (like our MM-SLR) has the narrowest FOV, but with our new widefield MDSLR adapter it gave an even wider FOV than the dedicated microscope cameras with C-mounts.  You can see that the sensor formats for the dedicated microscope cameras are normally 4:3, the DLSR cameras are normally 3:2, and the HD camcorder format is 16:9, so while you get a wide horizontal view, the vertical dimension is limited.

The FOV is a measurable constant.  If knowing the magnification of an image is important to your application, measure the FOV that your camera is capturing by taking an image of a stage micrometer scale, being careful to keep all settings the same.   If you know your camera captures 700µ horizontally on a certain microscope with a certain objective, you can use that information to determine the magnification any time you need to know.  If you print  the image in a 4"x5" format and again in an 8"x10" format, the magnifications are different, but the FOV is still the same.  

     Electron microscopes have used this concept for years.  They allow a "micron bar" to be placed on the image (like the 100um scale bar on the image above).  The micron bar is of known size, so can be used to determine the image magnification - just like a legend on a map.  Some of the dedicated microscope cameras include software for adding these micron bars, or any imaging software like i-Solution will have that feature, too.

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