What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 56,014
Re: What the heck is an f-stop?

Paul Anderegg wrote:

I am may be in the market for a broadcast video camera soon, and am having some issues understanding the relationship between sensor size and f-stops.

For those not familiar with broadcast video cameras, the sensitivity is rated as f-stop at 2000 lux necessary to produce a certain video signal level (brightness).. The first generation of CCD video cameras, 1990, were rated at f5.6 @ 2000. The next generation, around 1993, was rated at f8, then f11 hit around 1996. The best ever standard definition cameras were the Panasonics, which reached f13 @ 2000 around 2004. These ratings are for the 0db (no gain up) settings of a video camera.

So fast forward to today, and you've got mostly f10 @ 2000 HD gear, with a few more expensive f11 and maybe 2 or 3 at f12. Which brings me to my inquiry. I am trying to figure out which of the following two cameras would be best suited for night and low light use. A JVC GY-HM890 (f11@2000 1/3" chips) with a constant f1.4 lens, or a Panasonic HPX600 (f12@2000 2/3" chip) with an f1.8/wide-f2.3/tele lens.

With the same 18x series lens, the JVC would be f1.4 at full telephoto, where the Panasonic would be at f2.3. My confusion with this results from not understanding how f-stops relate to sensor size. Is an f11@2000 rating for a 1/3" the same as an f11@2000 rating for a 2/3", or is that aperture just some random number once you start playing around with sensor size? Short of putting two cameras side by side in the same light, there seems to be no technical info about this on the internet that I can locate with google.

Although this is a video inquiry, it would seem that the PHOTOGRAPHY world would be better equipped to technically answer this question. Also, in the video world, Canon and Fujinon sell the same series lenses in 2/3", 1/2", and 1/3" versions, so I am assuming they are the same glass. Not sure why the 1/3" versions all have constant apertures, perhaps the small sensor just takes advantage of the larger glass elements designed for the larger sensors? Thanks in advance.

Paul Anderegg

OK, let's go through this a bit at a time.

'f-stop' in its absolute sense means the ratio of the focal length to the aperture. So, if the focal length of the lens if 16 times the aperture, then the f-stop is 16. The f-stop, along withe the amount of light coming from the subject, controls the density (amount of light per unit area) at the focal plane. Conventionally aperture controls are marked in f-stops, rather than absolute aperture sizes (although in film and video often 'T-stop' is preferred, which is f-stop adjusted for the light loss within the lens). Thereon, thing get a bit more complex.

By and large, equipment users like to work with quite simple rules, rather than going back to the basics of how things work. The simple rules evolve quite differently according to the use case, so they are different in stills, to video, to cinema camera to broadcast camera.

Looking at the broadcast camera rule what you have is a simple rule defining 'sensitivity' 'brightness' and 'f-stop', but what's behind the rule is not so simple. Canon has a nice paper on their C300 camera here:


That defines the f-stop rating as follows:

The traditional broadcast video specification for lens-camera sensitivity is measured by the lens aperture setting in F-stop required to achieve 100 IRE units of Luma when the system is imaging a reference white chart having 89.9% reflectance under 2000 Lux of 3200 degree Kelvin illumination (with Camera Master Gain set to 0 dB, Gamma switched off, and all image enhancement switch off). The amount of electronic noise in the image is another important issue closely allied with this sensitivity specification because of its bearing on dynamic range. Under these conditions, the EOS C300 camera has a reference sensitivity rating of F-10 and a Luma signal to noise ratio of 54 dB.

As Canon points out, it's important to specify the SNR, because in principle any sensor illumination can produce any output luma signal you like, so long as you'll tolerate the noise.

So, to bring that back to your question. The problem with a smaller sensor is that it has less area to intercept the light, so at the same f-stop, the image will be more noisy. Or to put it another way, for equal efficiency sensors, a smaller sensor will require a smaller f-stop (bigger aperture) to achieve the same SNR. So, all else being equal, reducing the sensor size will increase the noise. You don't know whether different manufacturers F stop ratings take this into account without looking at the small print and seeing if they specify the SNR which the F stop rating applies to - quite likely they haven't, often the consumer is expected to know that a smaller sensor will give more noise at the same F stop.

If they are not, then a fair assumption is that the F stop rating (to output the same noise) is in proportion to the linear ratio between the two sensors. So, taking your example of F-10 rated cameras with 1/3 and 2/3 inch chips. In this case, they'll both produce the same brightness, but the noise won't be the same. The linear ratio between 1/3 and 2/3 is 11:6 or 1.8, so to find the F stop on the 1/3 sensor that will produce the same noise as f/10 on the 2/3 sensor, we multiply by 1.8, or f/18.

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