f/4=F/4 discussion

With apologies, your reply is much too convoluted for me to even consider a further response.
Edit: I think this thread has been contaminated enough already.

Ah -- well, hopefully Jim Kasson's much shorter reply did the trick!

No you don't. The focal length doesn't change with format so you don't get f_2/4 unless you are adding a negative diopter lens to double the focal length.
The equivalent focal length is changed by crop, this does NOTHING to the optics, it's just a guide for those used to one format to convert to another.

One has to do it for every affected parameter, too, otherwise, one gets a lot of implied magic with no substance to back it. Too many people thirsty for so-called "reach" try to take the "equivalent focal length" to the bank without also completing the equivalence for f-number, and without taking angular pixel resolution due to focal length and pixel density, into account. I run into wildlife photographers all the time who claim this magic with APS-C and m43 ILCs, and with 1" sensor superzooms; they quote the FF-equivalent focal length for their system, and the actual open f-ratio of the lens, and compare the combo to FF lenses that seem to be unnecessarily huge, by comparison. One of the most common claims is "600/4" with a Sony RX10, when in fact, "600mm" equivalence is with "f/10.8" equivalence. That's certainly a hair "better" than something like a 20MP Canon R6 with the RF600/11, but it does not come close to an R6 with an actual 600/4 lens.

What is that notation? I've never seen that before.

Subscripts, as in LaTeX.

Jim and you both deserve a comment.
Of course the magnification ratio differences due to various formats affect depth of field.
f/4.0 with my 21mm Super-Angulon-R and 250mm Telyt-R at the same shutter speed/ISO result in the exact same exposure in the same light with my DX and FX DSLRs.
Diffraction at f/4.0 with any lens is nearly impossible to measure - at least, I have good and not-so-good lenses and have not noticed it.
I've seen noise/grain from high digital/film ISO, but am not aware of how it could have anything to do with the aperture, all else being equal.

It's what causes the Otus 85 to not be as sharp on axis at f/4 as it is at f/2.8.

It does have something to do with format size, however.

I've had a 55 and 85mm f/1.4 Zeiss Otus for about 3 years, been shooting mostly Leica R lenses since 1969 - never had much occasion to fret over minor diffraction issues with any of them.

Then all else would not be equal (I think!).

I think the text of the OP is well written. It is hard to digest for beginners. But that may be in the nature of the subject itself. Especially that "yes and no" answer is not helping to gain trust in the understanding. But otherwise, I don't know if I could do better.

I have tried to explain "equivalence" in my blog here: photography-by-rg.blogspot.com/2023/01/equivalent-settings-for-different.html

Judge for yourself if that is better to read and understand. In case you find errors, I'd be glad to hear too.

Please see my edit above.

Exactly. That is why dedicated lightmeters never consider sensor or film size in order to give an exposure.

Broken link or some other problem: "Sorry, the page you were looking for in this blog does not exist."

I was just about to post the same thing. As portable light meters exist, we know f/4 remains f/4

If you are shooting film, it’s important to hit the film’s target exposure, no matter how big or small the frame size. With digital, there is no requirement that the same exposure be used for a small sensor, as for a large sensor. In fact, with digital, you can make a good case that with smaller sensors you generally should target higher exposures (up to the limit the sensor can handle).

The bottom line is that light per unit area (“exposure”) on the sensor is an implementation detail. If you care about implementation details (and with film you had to), then you care that f/4 gives the same exposure on any sensor size. If you are more concerned with the resulting image, then you know that f/4 on a small sensor yields different results than f/4 on a larger sensor.

“Equivalence” is easy to explain: At the same angle of view, same shutter speed, and same aperture diameter, you get essentially the same image.

By “essentially the same image” I mean same framing, same depth of field, same diffraction issues, same overall image noise and same motion blur. In other words, the resulting images will look pretty much the same. Obviously, there can be differences due to the quality of the lenses, the way the data was processed, etc.

Come on! How computes the aperture diameter and the angle view when he has a camera in the hand?