What the heck is an f-stop?

Started Apr 25, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 59,118
Re: What the heck is an f-stop?

Paul Anderegg wrote:

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.

Just to make sure there is no misunderstanding here.........the lenses in question, be they the f1.4, or f1.8, are specific to the sensor size mount, and not interchangeable. I will give you three links of the "same" lens (would appear) in different mounts, so you can give your take on if they are the same "glass". This identical lens series thing has been around as long as CCD video cameras have been.

Fujinon 18x7.6BERM 2/3"

Fujinon 18x5.5BERM 1/2"

Fujinon 18x4.2BERM 1/3"

The constant aperture of the 1/3" model still baffles me.


That's some serious outlay on lenses. The 1/3" one isn't constant aperture, it's constant f-number, the aperture will change as it zooms to keep the f-number constant. This occurs because the zoom lens is a prime lens with an 'afocal converter' on the front, and the 'aperture' is the size of the aperture stop viewed through the front, so the zoom converter changes the apparent size as it zooms. In the larger format lenses they didn't make it quite big enough, so it forms the restriction on aperture size at lower f-numbers and longer focal lengths.

I would hazard a guess that front of the lens (afocal converter) is the same, linked to a different back half to give the appropriate FL range for the format.

However, given the cost of the lenses, I'd be going with the 2/3" and the extra 1 stop of light on the sensor when the difference in the f-number has worked itself out.

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