Blurry text when hooking up LG 4K 27UD88 to 2013 iMac

[wayne905]

I'm hoping you guys/gals can help me.

I work from home and I used to hook my 2013 iMac up to an Apple Thunderbolt Display. Never had an issue with it. Both the iMac and the 2nd display were set at 2560 x 1440 without issue.

A few weeks ago, I moved into a smaller house (downsized) and now I want to use my LG 27UD88 4k Monitor as double duty - both as a second monitor to my iMac during the week, and on weekends as my 2nd monitor to my 2018 MacBook Pro - thus eliminating the need for my Apple Thunderbolt Display. When I hook my 4k Monitor up to my MBP it displays 2560x1440 in retina beautifully (scaled). I love it. However when I hook it up to my 2013 iMac and set the resolution to 2560x1440, it says low resolution and it's indeed low res. The text is quite blurry and much worse than my 10 year old Apple Thunderbolt Display.

I've tried the following cables to fix this:
- Mini DP to HDMI (passive cable)
- Mini DP to HDMI (active adapter)
- Mini DP to Display Port

All of the above give me the same fuzzy text.

I thought maybe the 4K monitor doesn't like 2560x1440 but it looks perfect when I hook it up to my MBP.

I know my iMac is older (2013) and I don't need it to output retina on the LG monitor if it's not capable of doing so - I just want it to at least have crisper fonts (at least the same level of clarity as my Thunderbolt Apple Display).

Like I mentioned, I've been trying different cables/adapters to fix this problem, but no luck so far. My only other option which I haven't tried yet is a Mini Display Port to USB C cable to see if that fixes it. But given the track record of all the other cables I've tried, I'm doubtful.

Any thoughts or suggestions would be very much appreciated. Thanks!

 

[konqerror]

It's working as designed.

Your iMac is outputing true 2560x1440 to a 4K monitor. The monitor has to scale it up. 3840/2560=1.5, so there will always be blurry. It has to come up with 1 pixel every two, so every other pixel is an interpolation.

For your Retina-compatible MacBook Pro, the Mac is drawing the screen at 2560*2=5120. It is then scaling 5120 down to 3840 (4/3), so it is discarding information. Along with the lower scaling rate, the higher DPI makes it much less noticeable.

I think the best thing to try is to run the non-Retina computer at an integer ratio of the screen resolution, i.e. 1920x1080.

 

[wayne905]

Thanks for your quick reply konqerror. How you've described it makes sense.

I meant to mention that by default my iMac wants to use 1920 x 1080 as it's resolution - and when it does it looks beautiful and all retina-like. I was just hoping to use the same resolution as my iMac - so that documents open at teh same size on either monitor. So it sounds like if that's what I want then I have to live with the blurriness.

Thanks again.