Two Sensors with a multiplication factor of 2.6x and 2.7x

I understand where your math is coming from; I can do the math in my sleep. The problem is that you said:

That does not happen. You are creating a semantic illusion. The lens is still 135mm and f/1.8; it doesn't "turn into" anything else, but it has an AOV like a 364.5mm lens would on a FF camera, and the context of a FF camera is the context that you mean for your "364.5". There is no other context in which the lens has any relationship to the number "364.5", so using that number means you are talking about what would give the same imaging on a FF camera, in which case it is like an f-number of 4.86; not 1.8. IOW, you are appealing to "equivalence" even though you are not using the word specifically, but only taking the part of it that you like.

Yes, the AF is dealing with "f/1.8" the same way it deals with native f/1.8 lenses designed for the body, but that 1.8 has nothing to do with the DOF or low diffraction that f/1.8 could give with a FF sensor with the same AOV.

I never mentioned DOF or opening equivalence.

Only those obsessed with the 24x36 format constantly bring back the equivalence term for the opening.
And you're misleading people with your obsession.

Do you think that photographers like me, who use APS-C, APS-H, MF, µ4/3 etc.…
uses these formats, always thinking of your equivalent for the 24x36mm format.

People like you learned the term equivalence 30 years ago and repeat it stupidly to give the impression that they are great connoisseurs.

I'm a little tired of it.
Maybe it's time to move on.

You still don't get it. YOU are the one who brought up equivalence, even without the word, because equivalence is implied by "364.5mm". Equivalence is the only context in which "364.5" has anything to do with your combo! Just applying the factor of 2.7x brings in equivalence! You did it, but you only took the part of equivalence that you like, the equivalent focal length. If anyone has shown an obsession with FF here, it was you.

At first, I thought the smiley at the end meant you were making an intentional funny, but the ensuing "conversation" shows it does not. The lens is a 135 / 1.8 no matter what sensor is behind it. But the effect of the lens, in terms of the visual properties of the resulting photo, is the same as the effect of a 364.5mm f/4.9 lens on FF.

Of course, the photos will not be exactly the same, just as the photos at 135mm f/1.8 on an R5 are not exactly the same as photos at 135mm f/1.8 on a Z6. But, in terms of why we care about the focal length and f-number, they'll be identical, just as 135mm f/1.8 on a J5 will identical to 364.5mm f/4.9 on FF in terms of why we care about the focal length and f-number.

If all you care about is framing, however, then no need to mention the f-number at all -- just give the focal length equivalence.

I'll ask you the same question

If I take a picture with my D810 FF and my 135mm at f/1.8
And I take a second photo with my D810 (Image area DX (24x16) 1.5x = 202.5mm still at f/1.8

At what aperture did I take my second photo? 😎

The answer is here in LR, see the info on captures.

135mm/1.8 = 75mm. 75mm no matter what sensor, and even with a TC or speedbooster, it would still be 75mm

The f-ratio is 1.8, and the focal length is 135, always. Mentioning "202mm" evokes equivalence, where the crop mode is like 202/2.7 on a FF. Other than that equivalence, "202" has nothing to do with your lens in crop mode.

You never get an image that is "like 202/1.8 on a FF", or 202/1.8 with any sensor size; you get what your pupil gives you, and you can't make it any larger than 75mm.

Actually, the answer is the following: you took both photos with a focal length of 135 mm and an aperture of 135 mm / 1.8 = 75 mm. The second photo was cropped to an equivalent focal length of 135 mm x 1.5 = 202.5 mm with the same aperture of 75 mm, yielding an equivalent f-number of 202.5 mm / 75 mm = 2.7. Thus, 135 mm f/1.8 cropped by a factor of 1.5x is equivalent to a photo taken at 202.5 mm f/2.7.

Specifically, photos of the same scene taken from the same position with the same exposure time, one at 135 mm f/1.8 cropped by a factor of 1.5x, the other at 202.5 mm f/2.7, both displayed at the same size, will have the same perspective, the same framing, the same DOF, the same noise. In fact, the main difference between the two will be resolution, since the cropped photo will be made with only 44% the number of pixels, and the differences in the sharpness of the Sigma 135 / 1.8 and whatever lens you're using at 200 mm f/2.8 probably isn't nearly as much as a factor as the difference in the pixel counts with regards to resolution. Bokeh, distortion, flare, etc., might look different, though.

And, in fact, this is extremely easy to verify. Take a photo of a scene at 135 mm f/1.8 and then take another photo of the same scene at 200 mm f/2.8 (where f/2.8 is easily "close enough" to f/2.7 for the purposes of this experiment) from the same position with the same exposure time (use whatever ISO settings give you the desired lightness). Crop the former to the same framing as the latter. Display both photos at the same size. Post them here, if you like. Tell us what you see and we'll tell you what we see.

The problem is that you're talking about another subject (mimicry) or how to take identical photos with two different sensor formats.

You've bypassed (avoided) my first question, here's the second, is it possible to take a photograph at f/1.8 with the µ4/3 format?
If so, how?

No, that's not the problem at all. The problem is that you think a 135 / 1.8, when mounted in front of a 1" sensor (2.7x), becomes a 364.5 / 1.8 and that when a 135 / 1.8 is used on a FX sensor in [1.5x] crop mode, it becomes a 202.5 / 1.8. This is simply false. Completely and totally false.

But I didn't. I directly answered your question. Here's your question again:

At what aperture did I take my second photo?

And my answer was:

The second photo was cropped to an equivalent focal length of 135 mm x 1.5 = 202.5 mm with the same aperture of 75 mm, yielding an equivalent f-number of 202.5 mm / 75 mm = 2.7.

Too many words? I'll shorten it: you took the second photo with an aperture of 75 mm. That's it. That's the answer to your question. Now, what f-number did you take the photo at? That's probably what you meant. Well, if the focal length is 135 mm, which it is, then the f-number is 135 mm / 75 mm = 1.8, which it is. If the equivalent focal length after the crop is 202.5 mm, which it was, then the equivalent f-number was 202.5 mm / 75 mm = 2.7, which it is.

Yes, it is. Put a lens that can shoot f/1.8 on the camera and take the photo. There's another direct answer to your new question. Adding to that answer, if, for example, you put a 135 / 1.8 on an mFT camera, and take a photo wide open, then the focal length is 135 mm and the f-number is 1.8. The aperture is still 135 mm / 1.8 = 75 mm. So, if you want to convert to the equivalent settings for FF, then you apply the crop factor to both the focal length and f-number: 135 mm x 2 = 270 mm and 1.8 x 2 = 3.6. So, a 135 / 1.8 on an mFT camera is a 135 / 1.8. Full stop. However, if you want to know its equivalent in FF terms, it is equivalent to a 270 / 3.6 on FF.

Today, almost all photographs published on the Internet have been more or less cropped.
For you, this means that all the opening information is false.

And that the pose-meters built into cameras all give false information.
PhotoShop and Lightroom also give false information.

Did you notice where it said "Distance focale 135 mm"? That means the focal length is 135 mm. Did you notice where it said "Distance...(35 mm) 202 mm"? That means the FF equivalent focal length is 202 mm. Did you notice that it did not say "Exposition...(35 mm)"? That's because it didn't give the equivalent f-number. It only gave the actual f-number, which, of course, corresponds with the actual focal length.

Again, if you have a 135 / 1.8 lens, neither the focal length nor aperture, which are both physical attributes of the lens, change when mounted on sensors of different sizes. However, the equivalent focal length and f-number absolutely change as a function of the size of the sensor, and they both change in the same proportion.

And what do you answer for the published photos?

If I take a photo at, say, 135 mm f/1.8, crop it, publish it, and say the photo was taken at 135 mm f/1.8, then that information is not false. It is, however, incomplete, and, depending on the reason for publishing the EXIF, misleading.

However, if I take a photo at 135 mm f/1.8 on mFT, do not crop it, and present it as a photo taken at 135 mm f/1.8, then that is not false. But if I were to present it as a photo taken at 270 mm f/1.8, then that is false. Even if I present it as "equivalent to" 270 mm f/1.8 on FF, that is also false. But if I present it as equivalent to 270 mm f/3.6 on FF, then that is true. The reason is painfully obvious: a photo taken at 135 mm f/1.8 on mFT will look rather different than a photo taken at 270 mm f/1.8 on FF.

My turn to ask a question: what is the purpose of giving EXIF at all? For example, why do you want to say "270 mm f/1.8", "202.5 mm f/1.8", "364.5 mm f/1.8", etc., rather than what it is, 135 mm f/1.8, and what camera was used?

At 135/1.8, in your case.

Only the "202 mm" is fictitious. The "135 mm" and the "1.8" are correct, and would be, with any amount of cropping. For the fiction part to be more complete, it should give the 35mm-equivalent focal length and the 35mm-equivalent f-number:

Focal Length: 135mm (35mmEQ: 202mm)
F-number: 1.8 (35mmEQ: 2.7)

If the EXIF data can keep up with cropping, then it could be possible to alter the 35mmEQ parts to reflect actual sensor area still in the file. Maybe even a field that tells how the center of the crop sits relative to the center of the image circle:

Cropsize: 4.8mm x 3.6mm
Crop offset: 3.4mm, 4.1mm

You seemed to play at your convenience (as you wish) between “false” “totally false” and “incomplete information” and misleading exif. 😉

The only thing that is wrong or incomplete is wanting at all costs to indicate that the "depth of field" is not the same at equivalent aperture.
A recurring and unhealthy problem for some photographers!

To finish, if I take a photograph at 1/125 f/1.8 with my camera, (information indicated by my light meter) no matter with which sensor and lens format
and/or that I subsequently decide to crop my photo as I wish before publication. The values 1/125 f/1.8 will always be valid

What bothers you is the incomplete and not false information! To indicate the behavior of the depth of field between different sensor formats and the f/-number.
An obsession that appeared a long time ago (over 25 years). Between the first digital 24x36 and the APS-C format of the time.
I think everything has been said on that.

For your question

It can be fun (see my smile on the original post) to see some advantages of using lenses designed for 24x36 and applying them to µ4/3 or 1" sensors etc…
Without always wanting to demonstrate this famous difference of the depth of field.

Nikon 1 S1 (2013) / Sigma 600mm mirror f/8 by Marc Aubry, sur Flickr

The Sigma Sigma 600mm mirror f/8 turns into 1620mm with the little Nikon 1 S1 :)

P.S.
Cute avatar 👍🏻

???

???

Now that is what I've been saying!!! I also said that it is good to include the model of camera used.

Quite the opposite, really. What bothers me is false and/or misleading information.

???

For sure -- I don't think anyone has said, or implied, anything different. What "we" are saying is that, for example, a Sigma 135 / 1.8 on a J5 is, in fact, a Sigma 135 / 1.8 on a J5. It is neither a 364.5 / 1.8 nor "equivalent to" a 364 / 1.8.

???

I'm not sure what you are trying to say about DOF. I am talking about accurately labeling the f-number. If we give the actual focal length, then we give the actual f-number. If we give the equivalent focal length, then we give the equivalent f-number. One would think this to be common sense, really.

If the smiley is tongue-in-cheek, that's fine. But if not, what is the mechanism by which the focal length of 600 mm, a physical aspect of the lens, changes to 1620 mm when the lens is mounted on an S1? Why not simply say that a 600 mm lens on an S1 is equivalent to a 1620 mm lens on FF? And if all you care about is field of view, then full stop right there. But if you care about aperture, then why not say that a 600 / 8 on a J1 is equivalent to a 1620 / 22 on FF? Because it sure as heck is not equivalent to a 1620 / 8 on FF!

With regards to the purpose of giving EXIF at all, I think the most useful information would be something like "Taken with the Sigma 135 / 1.8 on a Nikon J1 at 135 mm f/1.8 1/400 ISO 800". If you want, you could add the following to the EXIF: "(365 mm f/4.9 1/400 ISO 5800 FF equivalent)", but that addition certainly wouldn't be necessary.

Thanks! I think it's important that one's avatar catches one's "true self". 😬

Your aperture doesn't magically change, and neither does your focal length.

The obvious answer is "yes", Marc. Is your first language Canadian-French?

While searching for an actual µ4/3 f/0.9 lens I found this 29mm f/0.8 lens!

petapixel.com/2021/05/11/voigtlander-29mm-f-0-8-super-nokton-review-the-fastest-lens-in-the-world/

I have and use both a D500 as well as a D850. I also routinely crop shots taken with both of them. When pressed, I can figure out the equivalent focal length and then divide that by the aperture I used to determine the equivalent aperture. No one presses me though, in part because I never say or bother to figure out what the equivalents are, I'm just after the framing and DOF I have previsualized and when I'm happy with the results I sometimes share them as a photo with no further explanation offered (unless asked).

I hate the emphasis on the so-called "crop factor" in the lexicon of modern digital photography.

Especially when that factor for my DSLR is 1.7 - which it's manufacturer calls APS-C (nowhere near!).

I agree with you for once 😎
When the conversation goes around in circles, you need to know when to stop.

Tu mérites une étoile, pour avoir deviné 🎓

I don't even do frame/cropfactor math for focal-length-limited decisions; that is more useful at the wide end of the AOV range, IMO.

I find crop math to be a distraction. A longer "35mm-equivalent focal length" has no value except as a limitation. Focal-length-limited photography is best thought about, IMO, in a model of subject size and lighting, distance, and pupil size. Those are what determine the main analog qualities of the subject as projected on the sensor. The next tier of considerations is pixels-on-subject, and the usability of the angle of view, but the angle of view has nothing to do with subject quality, if it doesn't crop away any of the subject.

1) Subject size - subject distance - pupil size

2) Focal length divided by pixel pitch (preferring less aberration than necessary diffraction)

3) AOV - doesn't affect subject quality, but there may be a sweet spot for operation of the camera.