Relation Between Iris And F-Stop

In regards to the class today, I just want to make sure I understand correctly.

Am I correct in saying that the F-Stop and the Iris are interchangable terms? (they are the same thing?) When one refers to an F-Stop, its the ratio of how "open" or "closed" the iris is right?


The aperture is the hole that allows light to hit the sensor. The iris is the mechanism that opens or closes and thereby allows more or less light through the apeture. The F-stop is the metric that allows us to quantify exactly how open the iris is, which tells us how much light can pass through the aperture through to the sensor. The higher the F-stop number, the more open the iris is, and the wider the aperture opening.

Iris is the engine, and F-stop is the RPMs. One is a term for measuring the performance of the other.

IMHO, They are related but distinct terms though in some cases are used (almost) interchangeably.

Iris describes the adjustable mechanisim which controls the opening (aperature) of the focal tube, limiting the light delivered to the focal plane.

The term F-stop though can be used two different ways, and this is often the reason for confusion:

  1. In the photography world, f-stop can mean the points at which an aperture (iris) can be adjusted to. Each of the clicks or 'stops' is labeled with the f-stop value to which it corresponds.
  2. It can be shorthand for f-stop number, which in the general sense, means ratio of length to width of the focal tube.

Definition 1 is almost interchangeable with an iris, but you are using definition 2. So in your mind at least just think f-stop number, if it is referring to the ratio of length to width.

As the iris is closed the f-stop number increases.

Finally to be clear, as far as definition 1 one goes, an f-stop needs an iris to exist, since it refers to a mechanical adjustment to the iris itself. But in your case, definition 2, f-stop numbers exist regardless of whether an adjustable iris exists or not, since a focal tube does not need one.

Hopefully this was not more confusing. (Don't stare at the gif too long, it hyptnotizes.)

Ari and Rukmini,

I think you both have helped clarify, I think parts of my confusion is coming from thinking from a photography stand point rather than survillience.

So as sort of a follow up, I was confused on how there is a single f-stop number when looking at spec sheets for cameras, and I couldnt wrap my head around on how the camera is able to adjust the amount of light entering if the f-stop could only be a single number. I think I understand correctly now that the f-stop number does not necessarily mean there is an adjustable iris correct? And I assume that is where the shutter rate comes into play, since the iris cannot open or close on say a mini dome camera, the camera just adjusts shutter rate to compensate for the different light levels then?

Let me know if Im close now or if I am way out to lunch...

"I was confused on how there is a single f-stop number when looking at spec sheets for cameras"

That's because manufacturers always list the best case scenario....

Andy, yes, if the iris is fixed or manual, the camera has to adjust shutter speed to compensate for varying light levels.

Andy, yes, if the iris is fixed or manual, the camera has to adjust shutter speed to compensate for varying light levels.

In low light at least, wouldn't you say that preferring slow shutter over high gain is not a common technique in video surveillance. Indeed, almost every camera manufacturer defaults to allowing gain to automatically max out in low light but many manufacturers restrict how slow the shutter can get. ;)

There's only so much image quality improvement you can get out of increasing the gain. So, manufacturers will quickly max the gain in low light but if it's really dark, slowing the shutter is a much more powerful option, because it allows in a lot more light. Of course, it also creates ghosting / blur which is a problem (Camera Slow Shutter / Ghosting Tested).

FYI, when I said, "wouldn't you say"',

Preferring slow shutter over high gain is not a common technique in video surveillance. Indeed, almost every camera manufacturer defaults to allowing gain to automatically max out in low light but many manufacturers restrict how slow the shutter can get.

I meant it because you had already said it, here. But you would not be tricked :(

I think I understand correctly now that the f-stop number does not necessarily mean there is an adjustable iris correct?

yes

And I assume that is where the shutter rate comes into play, since the iris cannot open or close on say a mini dome camera, the camera just adjusts shutter rate to compensate for the different light levels then?

Yes and no and maybe.

The camera has many options to automatically compensate for different light levels, (not all are available in all cameras).

  1. Increase/decrease gain.
  2. Increase/decrease shutter speed.
  3. Increase/decrease aperature size. (Auto-iris)
  4. Change to B/W mode from Color or vice-versa
  5. Increase/decrease IR light levels
  6. others i dont know