Why does Apple offer full "native resolution" at all if it functionally locks buyers out of it?

You seem to assume that when the Mac is running in Retina modes, that it isn't using the resolution of the display.

I'm currently running a 27" 4K monitor in Retina "looks like 2560x1440" mode. This involves three resolutions:

• The System Settings > Displays, or "UI looks like", resolution is 2560x1440 pixels.
• The Mac draws on a canvas that has 2x as many pixels in each direction – a 5120x2880 pixel (5K) canvas.
• The Mac downscales the 5K image to 4K to get the picture that it sends to the monitor.

This is not the same as "2560x1440 (low resolution)" mode, where the canvas only has 2560x1440 pixels. (That mode is hidden away in the "Show all resolutions" view because most people have little use for it.)

If go into Displays Settings and select Retina "looks like 3008x1692" mode, the Mac draws on a 6K canvas before downscaling things to 4K for the display.

Ed-Finnerty wrote:

I appreciate that, but I don't know what that means in the context of my Air. All I see is 1470x956 set as "default," but when I select the advertised native resolution of 2560x1664, the UI elements are unusably small.

Applications have traditionally sized things in terms of pixels – and assumed that more pixels = more workspace. So physical sizes of text and objects have varied depending on the PPI density of the display.

• 24" 1920x1080 monitor has about 91.8 PPI.
• 27" 2560x1440 monitor has about 108.8 PPI.

So text would be visibly smaller on the second monitor than on the first, and the human sitting behind the screen would be expected to adjust.

A 13" M2 MacBook Air has a screen with a pixel density of about 224 PPI. So something that is 100 pixels wide on that screen will be physically a lot smaller than something that is 100 pixels wide on the others.

If you selected Retina "like 1280x832" mode, text and objects would be sized "as if" you had a 112 PPI screen – that is, to say, they'd be about the same physical size as on a 27" 2560x1440 monitor. Most applications would draw on a (2x1280) x (2x832) – or 2560x1664 – pixel canvas whose resolution exactly matched that of the LCD panel. So that mode would be a good match both for the screen's native resolution and for your eyes.

The default squeezes out a bit more "workspace" at the expense of somewhat small text and object sizes.

Ed-Finnerty wrote:

I understand all of that, and I don't want to be talking past one another. I'm also not turning on my beloved Air. It's a great machine.

What I don't understand is why my Air cannot do what my other computers do - allow me to scale up the UI elements like nav bars and fonts to be a usable size at the highest supported resolution, So I can maximize my benefit from the full resolution available on the display. I can see a digital photo at 4K res, and still navigate and read in the UI effectively. Only my Apple machines differ in this way, and I'm trying to understand why.

Backwards compatibility.

You cannot change APIs and expect applications that have no understanding of the new APIs to use them. This is why I believe that Windows users suffered great pain with applications in the early days of high-PPI displays. Just having some "global scaling" control (like the one in Windows that you seem to want in macOS) is no good if none of the applications use it, and control over whether to use it is in the hands of the application vendors.

Apple's approach placed the burden on new applications to show that they understood about high-PPI displays – by identifying themselves as Retina-aware applications to the OS, which allowed them to use new APIs to make high-resolution drawing requests to the operating system. A legacy application that did not know about the API changes would think the Displays Resolution was the real one. Because the operating system controls access to hardware, being able to tell legacy requests apart from new ones meant that it could transparently adjust legacy ones. "Draw a line from (100,50) to (200,50)" (in terms of the Displays Preferences "resolution") might become "Draw a line from (200,100) to (400,100)" (in terms of what the OS actually does with the request) – and so on.

If you run your 13" M2 MacBook Air's screen in Retina "like 1280x832" mode,

• The Displays Settings or "UI looks like" resolution will be 1280x832. Retina-aware applications (just about all, these days) will size text and objects "as if" you had a 1280x832 pixel screen.
• The drawing canvas will have 2560x1664 pixel resolution. macOS will use this extra resolution to draw letter shapes more accurately, and applications which fill in photo areas will have the opportunity to fill them using high-resolution bitmaps that are 2x as wide and 2x as tall as what the "UI looks like" resolution suggests.
• This drawing canvas will map perfectly to the actual 2560x1664 pixel resolution of the LCD panel.

Bottom line: In that mode, the MacBook Air will be using the full resolution of the screen, and it will be displaying text and objects at a size that is readable. Don't get hung up on the idea that the Mac must do things in exactly the same way that Windows does them, just because Windows does them that way.

I could be mistaken, but I believe that in the early days of high-PPI displays, many Windows programs ignored the Windows scaling factor, and would display unusably small output. On Macs, Retina-aware applications identified them as such. So if the operating system got a drawing request from an application that was not Retina-aware, it could assume that the application thought the Displays Preferences resolution was the real one. The OS was then in a position to transparently adjust the legacy application's drawing requests so that the legacy application drew stuff at the right size, and in the right place, even if it wasn't able to provide more detail.

Basically, when it came to high-PPI displays, Apple paid more attention to backwards compatibility than Microsoft did.

Ed-Finnerty wrote:

That's a plausible reason for a default behavior matching my experience. I just don't understand why they can't allow someone to select the advertised maximum resolution and retain a usable interface.

Commercial book printing equipment has much higher resolution than any display, even a Retina one. So a book publisher could print their books in all 2-point and 3-point type, the better to use the printing press's "maximum resolution". Yet publishers consistently choose to use much larger typefaces. Could this have something to do with the fact that human vision is sensitive to physical size?

There is an inherent tradeoff between the physical size of a display, the physical size of displayed text & objects, and "workspace" (or the amount of "stuff" you can cram onto the screen at one time). This is true whether you are talking about Retina scaling modes or about a global scaling factor. The idea that more pixels should always be used for more workspace simply doesn't work for high-PPI displays.

Why even offer that resolution for the built in display at all in that case?

If it wasn't in the list, we'd probably be getting posts on these forums asking why it wasn't there.

But as to "why" Apple includes it or excludes it, you'd have to ask them.

Ed-Finnerty wrote:

What I don't understand is why my Air cannot … allow me to … see a digital photo at 4K res

The 13" M2 MacBook Air's screen has only about 51% as many pixels as a UHD 4K display.

(2560x1664)

------------- = approximately 0.51, or 51%

(3840x2160)

You can display a 4K picture on the built-in screen, but your software will need to downsample the resolution of the picture to get the picture on there.

Apple does what it does for reasons known to them. We can not speak for Apple here as we are volunteer users like you are.