Be honest, how many knew what this symbol is for?

John from PA wrote in post #19524705
Rather interesting, my first SLR was a Konica Autoreflex; specifically the initial model released in 1965 that gave the user the ability to shoot full frame or half-frame 35mm. It had the mark and used the nomenclature "focal plane mark". That was 58 years ago!

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I loved those old cameras.
Depth of focus scale. Useful when I needed it

It's a sensor plane mark. It's used to set the sensor at the location where the primary image is formed in the microscope. I imagine it is used to locate the sensor on a telescope as well.

that is the location of either the film plane or the sensor face. for extreme macro work if I remember correctly.

I have used that symbol to compare mirrorless to EF mount bodies to show to others when they tout "all that extra bulk of the ef/rf adapter" stuff, I can show that the only bulk is the difference is related to the differences in the focal planes on the cameras. Using EF lenses on mirrorless means about 3 oz of bulk for the adapter, and about 20mm or so of added length to the end of the lens. Not all that bulky really...

TeamSpeed wrote in post #19525714
I have used that symbol to compare mirrorless to EF mount bodies to show to others when they tout "all that extra bulk of the ef/rf adapter" stuff, I can show that the only bulk is the difference is related to the differences in the focal planes on the cameras. Using EF lenses on mirrorless means about 3 oz of bulk for the adapter, and about 20mm or so of added length to the end of the lens. Not all that bulky really...

"Bulk" is a relative thing, though. On a longer lens, such as a telephoto, I barely notice the adapter. However, on a small lens, such as the EF 40mm f/2.8 pancake, the adapter approximately doubles the length of the lens, making it longer than the RF 50mm f/1.8 -- and consequently, rather silly.

There is only about a 20mm addition in length for all lenses due to the difference in focal plane placement.

This means that the end of the lens to your eye is about a constant of 20mm more on mirrorless with adapter than the equivalent EF DSLR. Your lens holding hand will be extended a bit more to make up that 20mm, which is less than what would be needed to cover the distance of the entire adapter.

Many people just look at that adapter and think how much longer their lens is when attached, but don’t consider the body depth in the equation as compared to the DSLR.

TeamSpeed wrote in post #19525898
There is only about a 20mm addition in length for all lenses due to the difference in focal plane placement.

This means that the end of the lens to your eye is about a constant of 20mm more on mirrorless with adapter than the equivalent EF DSLR. Your lens holding hand will be extended a bit more to make up that 20mm, which is less than what would be needed to cover the distance of the entire adapter.

Many people just look at that adapter and think how much longer their lens is when attached, but don’t consider the body depth in the equation as compared to the DSLR.

In practice, it doesn't make me any difference. I could have bought the lens like that, and it would have been like that, and I would have taken pictures with it anyway.

Yes. Many don't know because it is next to useless.

How many have to calibrate lens on close distances?
Or take close up photos by the ruler, not AF.

Speaking from first hand experience on rangefinder lenses calibration and taking close up photos with Minox.

I knew what it was from the MF/film days. My question has always been, what good is it? I’ve never seen a lens with a focus scale that was accurate or fine enough to just “dial in” the distance and get good focus. Maybe on a view camera or with TS lenses, but not the 35mm SLR lenses I’ve ever used.