Originally posted by BrianR Shutter speed and apertures are compromises. Equivalence can show you what settings you would have needed to get the same photo on a different format, and it can also show you if you've been given more (or less) room to pick your compromises. It gives you a reference point for the same image, it's not forcing you to shoot this way. For example, a landscape shooter with aps-c and iso100, f/11, 1/30s could use FF with iso 200, f/16, 1/30s. Or, if the scene is static and they have a tripod, iso 100, f/16, 1/15s and you'll get the same DoF and subject movement (since there isn't any in the first place). There's no free lunch (as your tripod has to be stable @1/15s), but some of them are pretty cheap and just as tasty.
If you want all your comparisons between formats to have identical shutter speeds and DoF, that's up to you. But we often have some wiggle room in the settings, and knowing whether a different format opens up room for improvement can be handy. Likewise, figuring out what compromises you have to make to get a noise advantage on FF can be handy (how much DoF do I have to sacrifice? How much blur will a longer shutter cause? Can I now justify some high powered broncolors
)?
But, you can make exactly the same compromises on APS_c, except of shooting wide open. That's what confuses people, this constant "you can get an advantage with 36mm on every shot". No you can't. You can get an advantage on 36mm where you are shooting wide open and narrower DoF either doesn't matter or is what you are going for.
It is so funny, 36mm protagonists get technical, until it's pointed out that equivalence really means, and then they get all fuzzy and start talking wiggle room as if there is more wiggle room on one than the other. Please stop. If you shoot 100 ISO, ƒ5.6 APS-c and 200 ISO ƒ8 36mm, there is the same wiggle room on both, until you are shooting wide open on APS-c. Then you get one stop more "wiggle room" on 36mm.
FF propagandists are always trying to fudge this point, to make it sound like on every image, 36mm gives you more. 36mm gives you more, in specific situations, it's not across the board. And in some situations 36mm gives you considerably less. So it's always a trade off. If you want the extra whatever that 36mm gives you, you have to give up the extra APS-c gives you. There is no free 36mm lunch.
That's why many in my field carry both 36mm and APS-c cameras when they go out, that way, they get the best of both. And there are enough advantages to both, to make that worth while.
I don't blame folks for the FF line of thought, it has been supported by many different photographers and websites, and it's proponents have been relentless, the problem with it is, most of it isn't true.
Anyone who uses the term "FF" is a suspect in my mind. FF is not a technical term, and anyone who uses has a fuzzy mind. When you start defining things in terms of technical sizes of the senosors, it's pretty easy to pick out what laid of hogwash there is that has been published. When you see the claims for 36x24 instead of the magical sounding Full Frame, you see what a bunch of hogwash most of it is. It's a format with many advantages based on convenient size etc. but there's nothing magical that gives it better IQ and noise etc. than everything else. If you cut a bunch of different size sensors out of the same wafer, there's absolutely nothing magical about the 36x24 sensor.
Especially once it sinks in, every time you increase sensor size, you reduce DoF for the same focal length lens. No exceptions, no getting around it.