SOLVED External Monitor Display Scaling/Zoom/Resolution Issue

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I've recently purchased a new 27" display to go alongside my existing iMac and 21" monitor. However, the display is significantly enlarged, as can be seen in the screenshot provided below. I've highlighted the difference in size between the menu bar and the Chrome tab (dragged across each display to ensure consistency).

I've tried adjusting the resolution, changing cables and ports, resetting the SMC and NVRAM - both suggestions from Apple - and nothing I've tried thus far has worked.

It also appears to me that the iMac is treating the 27" display as a 21.5" display and upscaling to fit the monitor? Hence the sizes on the Alignment tab. The deadzone between both monitors - again as seen on the Alignment tab - also exists when in reality there should be no deadzone as both displays are the same size?

Any suggestions would be greatly appreciated. Feel free to ask any relevant questions.

A link to a demonstration of the issue: (Image too large to post)
 

Cory Cooper

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Hello and welcome.

That looks normal to me. When you have two displays that are different physical sizes - 21.5" and 27", and they are running the same resolution 1920x1080, items will appear "larger" on the 27" because the screen is physically larger displaying the same number of pixels (resolution). The 27" display on the right at 5120x2880 is not running at the same resolution as the 27" 1920x1080 display, so things will appear smaller, even though the display is the same 27" physical size.

It is pretty much impossible to run three display that have different physical diagonal sizes and pixel densities, and have the images on the screen look exactly the same, as the physical pixels are actually different sizes. Older 27" non-4K/5K iMac displays have a much smaller pixel density than the Retina 4K/5K displays.

PPI - Pixels Per Inch
21.5" 1920x1080 ~ 102 ppi
27" 1920x1080 ~ 82 ppi
27" 2560x1440 ~ 108 ppi
5K 5120x2880 ~ 218 ppi

Hope that helps,

C
 
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Hello Corey, and many thanks for responding.

I feared beforehand it would be a case of differing resolutions causing the issue, but couldn't be certain as the 21" monitor displays things correctly, although I guess this is due to it's significantly smaller size.

Having spoken to Apple before posting here, I also expected it to something that they'd have flagged up; after all, I provided them with the same information and diagram as done here.

I guess I'd better get the monitor returned another look for a more compatible replacement.

Many thanks,
Stephen.
 

Cory Cooper

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No worries.

Just so you know, it's almost impossible to match an external display to the built-in panel in your iMac, as they are custom display to Apple specifications. Also, there are 4K/5K Retina display on more recent iMac models, with somewhat custom ppi specifications.

Let us know what you look at and if you have additional questions.

C
 
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Don't know if you'll be able to answer; but what causes the 21.5" monitor to display applications at the correct size, as this is 102ppi to the iMac's 218ppi? Is this just because of the naturally smaller size?

My next plan of attack was to purchase this monitor, in hopes that larger resolution and ppi would at least mean it's more accurate than the current one.

Thanks once again,
Stephen.
 

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