What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 58,581
Re: What the heck is an f-stop?

Paul Anderegg wrote:

Thanks for the reply Bob, the math formulas were informative. All broadcast video cameras rate their s/n ratio at 0db.

Where 0dB means not 'gained up'. The problem is that these terms are really based on old analog tech where the output from the vidicon was suitably amplified and formed the signal. It doesn't really describe what's happening nowadays. '0dB' is purely arbitrary, so far as the sensor is concerned - much the same as 'base ISO' in still terms.

My f13@2000 2/3" standard definition Panasonic is rated at 65db, and my f10@2000 2/3" high definition Panasonic is rated at 54db. Obviously those big SD pixels play a role in that!

Indeed, strictly speaking, SNR is bandwidth related, as sound engineers know. SD is about 1/4 MP per frame, while HD is about 2MP, that's 8 times the bandwidth at the same frame rate. 8 times the bandwidth, which just about gives the spec difference.

My problem with the current selection of new and used 1/3", 1/2", and 2/3" cameras, is there ability to obtain luminance in very low light.

In the end, this ability is down to (assuming exposure time limited by frame rate):

The SNR you can tolerate,

The f-number of the lens,

The scene luminance,

The efficiency of the sensor,

The size of the sensor.

Generally sensors of a generation have similar efficiency - so that can often be discounted.

I am a news shooter, so I rely on each 6db (1 stop) I can get my hands on to boost the picture. The Panasonic HPX600 referenced above, will only gain up to 18db, or roughly 3 stops above 0db gain. The JVC will gain up to 24db, which would be roughly 4 stops above absolute 0.

And of course, there is what the control that the manufacturer gives you.

Obviously, in a news environment, being able to click that one extra 6bd doubles your picture brightness, an can mean the difference between seeing the SWAT officer hiding behind a bush, and not. The JVC can go down to 1/4th sec shutter, the Panasonic only down to 1/15th.

I guess my main question would be the following. At 0db, with identical shutter speed, which camera would provide the most luminance at wide open (f1.4/f1.8) aperture?

If luminance is your only concern, then assuming that you have the same max aperture available, you will full luminance available is proportional to 12*(log2 F-stop) + gain-up above 0dB, and the sensor size doesn't come into it. The first term gives you luminance available above a reference of f/1 (arbitrary but easy to calculate). So the Panasonic gives you 12*(log2 10) + 18 = 40 + 18 = 58. I don't know the f-stop rating of the JVC. But if you're willing to blur it with the slow shutter the JVC is obviously giving you 2 extra stops on top.

Second question, at full telephoto, with the 1/3" at f1.4 and the 2/3" at f2.3, what would be the luminance difference between the two cameras?

Again, for luminance, regardless of noise, the sensor size is irrelevant, so your luminance difference will be down to the F-stop rating of the cameras and available gain as above and then the difference in f-stop of the lens, which is 1.3 stops (7.8 dB) between f/1.4 and f/2.3.

90% of what I shoot at night is shot wide open, and exposure is adjusted by gaining up.

Strictly, you can't adjust exposure by 'gaining up', gain isn't a part of the exposure, what you're changing is the luminance for the exposure you're using.

Night is night, you work with what you have in news. Thanks!


If it's quality as well as luminance, then you need to take the total light captured into account, which is the combination of sensor size and minimum f-stop lens available.

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