What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 59,128
Re: What the heck is an f-stop?

Paul Anderegg wrote:

My main confusion here is with similar f10@2000 sensitivity ratings, but with the dissimilar maximum f-stop ratings.

I'm presuming that you mean minimum f-stop rating of the lens, that is the maximum aperture available from the lens. Remember f-stop goes down as aperture goes up.

Is it as simple as if both the 1/3" and 2/3" are 100IRE at 2000 lux at f11, then the 1/3" sensor will produce a brighter picture at f1.4 than the 2/3" at f1.8 if both are set to their lowest and default 0db gain setting, because it can "open wider"? My limited understanding of the math involved would seem to indicate that I am confused here.

If they both have the same brightness at f/11, then they will have the same brightness at any f-stop, and if one has a smaller f-stop it will be brighter (or, the other way, give the same brightness in lower light). So if they are both 100IRE @ 2000, f/11and one has an f/1.4 lens while the other has an f/1.8 lens, then the f/1.4 lens will give more brightness in the same light, or will give 100 IRE in lower light. That might be the way to think of it. If 2000 lux gives 100 IRE at f/11, and f/1.8 gives 5.25 stops (37.5 times) more light than f/11, then f/1.8 will give 100 IRE at 2000/37.5 = 53.3 lux. By the same token f/1.4 gives 6 stops (64 x) more light than f/11, so f/1.4 will give 100 IRE at 2000/64 = 31.3 lux. So the smaller sensor with lower f-stop lens will give you 100 IRE in lower light, but the quality will be lower, since the sensor isn't receiving as much light.

Noise in the video world is simply accepted at the price level you are able to compete in. For cameras with a fixed 0-18db gain/ISO range, you've got the lowest setting, the highest value, and a choice in between available in a three position flip switch. When everyone is shooting a scene at 18db, noise becomes irrelevant, and the main priority is to actually be able to discern detail in the darker areas, with the limited amount of on camera lighting you have available.

Remember that the SNR is ultimately determined by the amount of light that the sensor collects. Remebering that the linear ration between 1/3" and 2/3" is 1.8, which gives an area ratio of 3.24 or 1.7 stops. So in the example above, you gain 0.7 stop going from f/1.4 to f/1.8 and lose 1.7 stops due to the smaller sensor. So in the end the smaller sensor is losing you a stop in terms of total light. If the question is, what is the lowest light for a given quality (discerning detail in the darker areas) the you are better off with the 2/3" f/1.8 combination gained up 0.7 stop above the 1/3" f/1.4 combination.

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