F-Number in Electro-Optics

F-Number. What is it?

In Optics, the F-Number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens’ focal length to the diameter of the entrance pupil. This means that the smaller the numerical F-number Value, the larger the entrance diameter and more light passes through to the image intensifier tube.

It is easy to understand that a lens with a big aperture diameter (or opening) allows for more light, or infrared radiation in the case of thermal imaging, to go through it. Consequently, more infrared radiation will reach the detector, meaning the detector will react more to the incoming radiation (as there is more of it to go around). Under the same circumstances, with the same detector, a thermal camera will produce much crispier and clearer image when the device is equipped with a lens with a large inner diameter or low F-number.

F-number is directly responsible for such parameters of an electrooptical device as sensitivity and operating distance.

F-number. The Formula.

The diagram presented below schematically shows four different situations when the focal length (FL) of the front lens is the same but entrance pupil diameter (EPD) is different. As a result, F-numbers for these cases vary significantly. The espression for calculating the F-number is simple:

Low F-Number Vs. High F-Number: Visual Comparison.

Take a look at the lens' inner diameter. Lenses with greater inner diameter (but the same focal length) have lower F-number.

Now you know what F-Number is, how to calculate it and what are “good” and “bad” F-numbers. You are armed with information to compare night vision and thermal imaging products based on their visual appearance. You can ask suppliers to provide you with a front objective lens' focal length and entrance pupil diameter. Using the above formula you can easily calculate the real F-number and thus determine the quality of optics yourself. Manufacturers that provide quality lenses, will not have anything to hide. 

Typical

F-Numbers

Relative Light

Gathering Ability

f/ 1.0 4 Times More
f/ 1.4 2 Times More
f/ 2.0 1

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