With the advent of the new Retina Macbook Pros, the new Mac Pro almost here, and Dell announcing new so-called “4K” monitor models at affordable prices more Mac users are considering to get a 4K monitor. It promises a similar display sharpness as the native Retina Macbook Pro displays already offer.

This promise though has recently been discussed controversially (e.g. ATP) with keywords being Pixel Density, Screen Space, Retina Resolution, HiDPI Modes.

Here is my take.

HiDPI Modes vs. Retina Resolution

Targeted at their Retina displays Apple has introduced specific screen resolutions they call “HiDPI Modes”. Instead of rendering one pixel per point of a user-chosen screen resolution of x by y points these modes render 2 by 2 pixels per point. This naturally results in a much sharper display specifically when using screen resolutions below the physical screen resolution of a display. And this is the standard use case with Retina displays on Macbook Pros and probably 4K displays since they offer such high pixel densities you wouldn’t want to choose the physical screen resolution as your actual resolution.

There is one specific HiDPI mode referred to as the Retina resolution which is the screen resolution that is exactly half the resolution of the physical screen. I.e. since being HiDPI it renders exactly the amount of pixels of the physical screen. It is sometimes also referred to as the @2x resolution.

Some people argue that this clearly is the preferred resolution to choose from using a retina screen since it best “matches” its physical screen resolution (read: delivers the best sharpness). Although mathematically there may be some truth to it I have found that in practice it doesn’t matter. You will hardly be able to pick any other HiDPI mode OS X provides and call it out to provide less sharpness than the retina resolution. Apple has done a tremendous job on this.

The above being said and assuming HiDPI modes being sufficiently available for a 4K monitor you could argue that

• In terms of sharpness it is safe to pick higher than retina resolutions on a 4K monitor. Thus, you are not at all constrained to a 1920x1080 points screen space.
• Display sharpness then becomes solely a function of physical screen pixel density

HiDPI Modes vs. Performance

There is of course a performance penalty for using higher resolutions in general and it is the job of the graphics card and the data bus to handle that.

We’ve got high performance graphics cards and thunderbolt2 delivered with the above mentioned computer models but it seems that much awaited display modes like 2560x1440 HiDPI still surpass their capabilities. Specifically Thunderbolt2 doesn’t seem capable of transferring 2560x1440 HiDPI mode data at reasonable rates to a monitor (See joshjet’s tweet). And I doubt that HDMI would do any better (but would love to be proven wrong).

Pixel Density

Anyhow, If you want to judge roughly what display sharpness you will get from a hypothetical 27” 4K monitor (it has a pixel density of about 163 ppi) place your non-retina iPad mini at same distance and height in front of you as you place your regular monitor, hook up a keyboard to the iPad and start typing within your preferred editor.