I'm going to start with non-photo issues first though. The best way to paint real life is to go out and paint it, no photo reference and certainly no sunglasses. This photo of the two frogs are of the same exact frogs, same lighting, same camera, and same camera settings. The only difference is I set different sunglasses in front of them. The differences can be subtle but if you look closely at the red frog you can see how the different lenses affect the way the red shows up in photographs and how the white backgrounds are greatly effected by the type and colour of lens.
You definitely don't want to be wearing sunnies while painting people.
Getting back to photo references and how you shouldn't copy them. Last week I used the word plagiarize and I want to make it clear that it was a metaphor, or analogy, or whatever non-literal way of taking things there is. No, you shouldn't use someone else's photograph without their permission but that's not what I'm talking about. I'm talking about don't take a photo then go back to your studio and copy every thing in the photo. You could also look at it this way. The camera and the photos printed from them are going to lie to you so copying it is going to perpetuate a lie. It's like retelling gossip. Aside from your computer screens not being calibrated to show the correct colours, or just different from someone else's, you're most likely using settings on your camera that give you the wrong info to begin with. If you have your camera set to auto then you're getting misinformation. HOWEVER, if you're simply using the photo for reference, like you should, an auto setting is perfect. Knowing that you're taking a photo that is giving you false information should put you in the mindset to make adjustments to your painting. If you still don't believe me about your camera giving you false info please take the time to check out this article about how auto white balance is probably screwing up your photos. If you don't feel like reading scroll down to the pictures. There are three photos of the same exact sky but using different auto white balance settings. Again, if you're only using a photo for reference, and not to copy exactly what's there, it shouldn't matter what setting it's in because you, as an artist, can paint the sky whatever colour you want. Don't chain yourself to the photo. Don't say I have to paint the sky pink because that's what colour it is in the picture, or you have to paint in all 2,567 tree branches or blades of grass because they're in the photo.
The last thing I'll touch on here are polarized lenses. The photo here came from this article explaining how polarized lenses work and how they're not always a good idea. Hopefully the other photos already gave you a good enough reason to not paint plein air while wearing sunglasses but if they didn't add this one to the pile. The other thing about polarized lenses are that you may be using one on your camera and it's also skewing your reference photos. Polarized lenses do a really great job of removing glare and making colours vibrant, but they also remove a lot of information. Then there's the polarized windows in your car too. A lot of new cars have tempered windows for the rear and passenger windows. During the tempering process something called stress birefringence happens. The tempering is done so if the window breaks it doesn't break in sharp pieces. The effect of the process is basically having two polarized lenses for windows. So keep that in mind if you snap photos through passenger windows.
In no way am I saying you should abandon your camera and photos and only paint from real life. We all know that sometimes that's just not possible. What I am saying is, when you grab that photo to paint from don't copy it. All of this information I'm throwing at you is simply to show you how your photo references aren't true to life to begin with so why bother copying them?