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 when we 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 size to the appropriate microscope adapter.  We sell several types of T-mount adapters in magnifications of 2.5x, 2x, 1.6x and 1.38x.  For a full frame DSLR camera, the MM-SLR 2.5x adapter is the only one we sell that will not cause vignetting.  For the popular APS-C sensor size (sometimes called “cropped sensor”) cameras, any of our adapters will work, but we normally recommend getting the one with the widest field of view available for the microscope that is to be used.  This means if we have an MDSLR-xx 1.38x type adapter for your microscope, that’s the one we recommend (again, assuming an APS-C sensor camera).  However, for some microscopes, and particularly for those with ISO 38mm photoports, the Optem ST16-x 1.6x adapter would be the best choice (but also the most expensive).  Other considerations could be that the adapter needs to work with multiple types of microscopes, in which case our universal MM-SLR might be the best choice despite the smaller field of view that it captures.  The chart below shows the fields of view that each type of adapter will capture given a circular optical field of 1000 microns which is typical at 200x optical magnification.

FOV Chart for T-mounts

 

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, for instance, 750µ 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.