The Recent F/stop Controversy

Started 1 month ago | Discussions thread
Lee Jay Forum Pro • Posts: 54,462
Re: Really?

Aaron801 wrote:

Lee Jay wrote:

Aaron801 wrote:

f-stop is just the standard definition... has been for years. No reason I can see to change it now. Not sure if any of this talk has anything to do with light transmission (or t-stop) but I get that this can be a bit different than the f-stop value. it's not typically a very large difference though and is really only important when you're shooting film/vide, which is why those lenses are rated in t-stops... so it seems that it's all good, right?

Well, the basic idea is this:

In the film era, ISO was hard to change and post-processing was usually pretty limited, so you're always stuck trying to get light-per-unit-of-area multiplied by time (f-stop * shutter speed) to be just right. Plus most people (but not all) shot just one format (35mm for example).

However, in digital none of those three assumptions are true - we can change ISO easily, we can post process like crazy, and many, even most people shoot more than one format (full-frame + 1" compact + 1/2.3" cell phone, for example).

Given that fact, and the fact that image quality is not controlled by light-per-unit-area but rather by total light captured (and DOF is actually controlled by the same thing minus the effect of shutter speed), it makes more sense for some people to think in terms of aperture diameter rather than in f-stop (ratio of focal length to aperture) terms. This keeps things independent of ISO and format.

I have no issues with f-stop either (it's just math) but there is a logical reason to think in another way.

By the way, astro folks have been doing this more-or-less since the dawn of telescopes. Telescopes are almost always sold in terms of aperture diameter rather than focal length, f-stop and format size.

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.

Right, or you can just say "50mm of aperture" (or 5mm or 200mm, etc.) does the same thing on all formats, assuming you keep angle-of-view the same.

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Lee Jay

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