The Recent F/stop Controversy

Started 1 month ago | Discussions thread
fferreres Senior Member • Posts: 1,550
Re: Really?

LoneTree1 wrote:

fferreres wrote:

Michael Fryd wrote:

Aaron801 wrote:

...

I think that I get what you're saying, that the amount of light that a sensor receives is totally different depending on what size it is and therefore the size of the opening of the lens in front of it. Of course f-stop is not the size of the opening but is a ratio... You need to do some math to figure out what the "equivalent aperture" is between different formats... but I'm OK with that.

Actually, at the same angle of view the "equivalent aperture" is the one with the same diameter. In other words it really is the same aperture; no math needed.

Actually, it's not. Not only because I am picky, but because lenses are not required to have circular apertures at all.

What is the diameter of the square?

What is the diameter of a triangle?

What is the diameter of a pentagon?

What is the diameter of a circle?

For of these questions have no answer. Did you know there are other more exotic apertures, some for example with dots? What about mirror lenses (usually fixed aperture). They don't even have a center.

You only need to do the math if you normalize everything to light per unit area (which is a great idea when shooting film).

Actually, no. I don't use film and would still have trouble normalizing a triangle and its newfound diameter. Or have to thing in terms of the amount of light as something related to each individual sensor size.

Area is area. Doesn't matter if its a triangle or a circle, it can be calculated and thus the amount of light coming through is known. There are some trade-offs. Diffraction (if the aperture is small enough) can become a real problem with non-circular apertures. The "spikes" produced when a common six-sided aperture is stopped down are an example. Funny thing though; most automatic apertures today are not fixed, they don't stop down to f/4.0. It could just as easily be f/3.456 or whatever had been called for to properly expose the image. To the camera, a modern lens has clickless apertures.

Except that 1900 lenses with apertures started clickless, and modern Zeiss gems in production today still carry clicks in camera models. Area is area but at least f-stop in metric agnostic and doesn’t pretend to convey a triangle is circle. But I agree with what you say too

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