Metering with AE lock

I'm sorry if this is posted somewhere. I could not find it. I see metering using objects such as the sky or foreground. What is the purpose for this and how would it best be used? I have checked with many manauls I have and they are all very vague. Thank you for your help.

Frank's thread here is a good place to start with an explanation for this: https://photography-on-the.net/forum/showthre​ad.php?t=89123

Basically, you want to meter off of something that is close to neutral gray. The sky and the grass are sometimes good to use. But you need to train your eyes to recognize when they are close in density to neutral gray.

Sageg wrote in post #8299551
Basically, you want to meter off of something that is close to neutral gray. The sky and the grass are sometimes good to use. But you need to train your eyes to recognize when they are close in density to neutral gray.

Or.....meter off something that is not medium grey [medium, not 'neutral'; neutral grey can be any grey from black to white] but which you know the difference between it and medium grey and then dial in the appropriate exposure compensation (EC). An example: meter off a white shirt and then add +2 EC.