You shouldn't disagree because we're saying the same thing.
I know. Did you not read my post?
I know what you mean but as I said I disagree with saying that DoF doesn't exist before you output and view and I also disagree with saying that it's all about magnification... for many people much of the time, and before you jump on that I'll explain further at the end of this post.
But for now, Yes I know the theory and I know the practice but I do think that posts on forums confuse the issue and to me saying stuff like DoF doesn't exist until you display and view the image is just misleading. Of course it exists and what you'll see at whatever image size and viewing distance you choose is set by your choice of gear and settings and we should with experience be able to predict what we're going to get even when using a different format. All we need to know is the crop factor in relation to something that we're familiar with and once we know that it should be easy to predict the results. For example when switching from MFT to FF and back again as I do the results DoF wise are never a surprise to me.
When I read this stuff on line and much less often talk to people about it I find the confusion easy to understand as some people haven't used different format sizes or spent time comparing the results or are perhaps just confused by the IMO overly confusing and convoluted discussions on the internet. This really shouldn't be hard or confusing and IMO it's just made so unnecessarily by over technical discussion.
And on to my problem with saying it's all about magnification. Yes, you are right but in the real world I don't think it's that much of an issue most of the time. For example suppose I walk out the door with my FF camera and my x2 crop camera and take two pictures of the same subject from the same point as follows...
Shot one - FF camera with a 50mm lens at f8 and at a reasonable ISO setting that isn't going to affect the image quality.
Shot two - x2 crop camera with a 25mm lens at f4, ditto the reasonable ISO.
Substitute whatever focal lengths and apertures you want, just apply the crop factor.
If I then display the pictures on the sort of computer, tablet, phone or TV that people usually have about the house or print the pictures out at about the same size, not mega big but lets say at any size up to maybe A4 (portrait or landscape, I don't care and I don't care about one being 3:2 and the other being 4:3 either) or even a bit bigger and then look at the pictures normally or even quite closely no one is going to look at the pictures and say "It's obvious which is the shot from the x2 crop camera as the extra magnification has made the DoF and the CoC and general IQ differences easy to spot." That's just not going to happen. Only on the internet in discussions about format sizes and DoF
IMO you're only going to see obvious differences if you print really big or start excessively pixel peeping at very high magnification.
In the real world (and this is just my opinion) if you take the crop factor into consideration and adjust your settings accordingly there will be a sharpness advantage to the FF shot if you look closely enough but DoF wise and CoC wise I just don't believe that anything will jump out at you. The character of the bokeh and the shape of the aperture blades might but these are characteristics of the lens and not a result of the format size or anything to do with DoF or magnification in this context. And in the real world I've never heard anyone talking about circles of confusion and when explaining this to anyone I wouldn't even stress it
Yes, magnification matters but keep within anything like the normal and reasonable boundaries that normal people keep mostly between and magnification might not be an issue at all.
IMO talking about magnification in anything other than very simple terms is just confusing and IMO there's usually no need to even mention COC. All that's needed is to point out the simple concept that enlarging a picture more degrades quality if viewing from the same distance and as smaller formats need to be magnified more image degradation will occur sooner.
I went through all this years ago with film and later with digital I compared APS-C to FF and then MFT to both and it was an expensive process but worth doing as once I'd done it I decided none of it was a real world issue for me.
The only time I've had to think about display size, viewing distance and visible detail was years ago when producing BIG pictures for display on stage and obviously people at both the front and the back of the audience needed to be able to see enough detail but other than that I've never ever found that I needed to think about this stuff and especially about circles of confusion. This stuff is IMO irrelevant to most people most of the time
whole image wise at least, cropping and displaying big is another thing and should prompt more thought.
Anyway, sorry for the intrusion