(Disclaimer before my terrible wall of text below.. sorry everyone. Anyway, I check my images on my Phone (AMOLED) and Surface Tablet (High-end glossy IPS) and they look identical to my desktop monitor, just nicer, crispier. But effectively, look the same, which I take as great comfort)
I am glad if my clients watch my renders on Phones and Tablets, because nowadays, they feature absolutely stunning displays compared to even the most high-end monitors. There are multiple reasons for that:
1) At worst, they feature high-quality (2000:1 contrast) IPS panels with glossy screen (Older iPhones).
At best, they feature stunningly high quality ( million or rather..infinity:1 contrast) OLED panels with glossy screen (Every high-end Android and latest iPhones).
2) High brightness (They effectively go to sustained 800 NITs... that is bonkers) compared to low-brightness of desktop monitors (Sustained brightness max 250-300 NITs, but we usually use them around sRGB standard of 125)
3) Crispy details of up to 600 PPI, compared to average 140 PPI for 4k 32" display. Phones can look twice as good as high-end glossy magazine.
4) Contrary to popular opinion, they often feature perfect calibration... but by default show incorrect color gamut (often called "Vivid" mode). But all popular brands (iPhones, Samsungs, Huawei, etc..) feature standard sRGB mode for natural colors in unamanaged applications/environments.
But even if your clients use wide gamut (whether managed like DCI-P3 mode in iPhone, or just random something like Vivid in Samsung/Huawei), they will just see your image like punchier, over-saturated version. It will not look uglier. Quite contrary.. most people enjoy that carnival look, which is why it's default. "Nice colors !!"
So with that said... we can conclude that in most situations, the phones actually show your work more accurately. Often time... lot more accurately. In small amount of cases, the client is at fault with wrong device setup.
Now we can use bit of stupid pseudo-reverse engineering to try to pinpoint where comes the difference reason in typical situation :-)
A) Phones show the image too dark ---> While some phones feature drastic power savings and default to low brightness in "auto-mode brightness", which can look quite dark in outdoor setting, I presume these clients aren't total simpletons and watch the images inside, where even drastic power saving still means above 200 NITs. Which would be still brightness then how common desktop is setup.
So rather, most people use too high brightness or too dark environment. Worst, many people use both (Bright displays..and work in pitch black room at night), which is complete opposite to how people consume visual content on cellphones (during daylight in bright room).
Solution: Set your Display to 125 Nits, for most displays, that is 40-50 perc. brightness. Work in moderately lit room. Obviously not in direct sunlight... but the bullshit with dark room has to end. That makes sense for IMAX cinema post-production where the content will be consumed in dark theatres.
B) Phones show the image too contrasty ----> Unfortunately, phones show contrast much more accurately than even the best monitors on market. Since most Phones have OLED screens, their contrast is absolute. The blacks are true blacks, whites are nicely bright. That cannnot be said for desktop monitors. We have few types of panels for desktop:
- TN - Only used for cheap gaming and office monitors (Sub 250 Euros). The static contrast is often as low as 1:500.
- IPS - Used for all professional displays, feature static contrast from 1:700 (cheaper sub 700 displays) through 1:1000 (most common) to 1:1300 for the most expensive, high-tech screens (costing few thousands).
- VA - Used for most of gaming displays, all curved screens. While VA can have contrast as high as 1:8000 when used in TVs, desktop monitor VAs only have contrast of 1500 to 2000. So they are barely above IPS, not enough to offset their absolutely abysmall watching angles.
One OLED LG monitor is coming to market this year and there will be FALD and Mini-LED IPS displays (They feature array of LEDs to increase dynamic contrast mostly during HDR displaying. The static contrast is still only 1:1000 and these displays will cost 3l to 5k Euro. Very expensive. Apple XDR Pro Display also belongs into this group with its 384 LED zones behind its 32" 6K panel).
Solution: - Use correct brightness (125 NITs, or 40-50perc.) and correct contrast ( default for sRGB mode that your monitor should have).
- Use moderate ambient lighting (daylight). Don't work in dark room. If you do, set your monitor as low brightness as you can.. but really, don't. Don't imitate cinema post-production & color grading, it's different beast.
- Use bias lighting to increase the perceived contrast on your desktop monitor. Because the contrast on most desktop monitors is too low, it's best when you can perceive it as high as possible and bias lighting (something behind your display) can do this really well. It's also much easier and healthier for your eyes.
I've only touched the main hardware reasons, because color calibration and compression quite frankly, don't make great image into terrible image. They just make it slightly wrong temperature, saturation and clarity. But if the image appears dark and contrasty, Facebook's compression algorithm is not at fault.