CHAPTER 11 Digital Microscope Cameras 69
Optimum Resolution or Optimum Field
of View (FOV)
The eyepiece is designed to visualize the full area of the intermediate image
presented
by the optical system of the microscope. The size of this intermediate
image area depends on the microscope: which is to say, it can vary from 18 mm
in diagonal up to 23 or 25 mm. A digital camera on a microscope looks directly
into the intermediate image.
If the camera you are considering has a small sensor and you use a camera adapter
of 1.0, you will get quite a small FOV. To maximize the percentage of FOV you
capture or to enlarge the FOV, you could use a demagnifying adaptor of e.g. 0.5×
or 0.63×. It projects a smaller version of the sensor area of the camera so the sensor
can see a larger field. But this comes at the expense of the resolution as more
information is projected into a single pixel and you will need to recalculate the
pixel size to make sure you still match your camera to the resolution of your imaging
system. If you want to have optimal resolution and optimal FOV, you need
larger chip sizes with smaller pixels and more of them.
Collecting More Photons with the Right
Camera Adapter
Different types of experiments may require different types of microscope
cameras – but you already knew that! And there is another tiny but very important
thing to decide on. Choosing the right camera adapter for your widefield
fluorescence microscope can increase sensitivity and enlarge the field of view at
the same time.
A camera adapter is an optomechanical element that connects the microscope
camera with the microscope’s body. The simplest version of the adapter is just a
hollow, threaded metal tube. Such an adapter does not change the magnification
of the microscope system. The magnification factor of this adapter is therefore
1× and the adapter is referred to as a 1.0× adapter. On the contrary, a 0.5× adapter
includes a lens system that reduces the effective magnification of the microscope
system by twofold. This in turn reduces the size of the image on the camera
sensor. The effective size of the image pixels therefore increases by twofold and
the area of each image pixel increases by fourfold. Each of the pixels now collects
four times more light on average, resulting in a significantly increased SNR (Figure
11.1). The effect of the 0.5× camera adapter is similar to 2×2 sensor binning.