Is sketching or drawing someone else's photograph a copyright violation?

One needs to clarify the usage and intent....

If you merely look at a photograph and draw based on what you see, there's rarely any infringement. Using reference is exceptionally common and almost mandatory at times. I mean, there's no way I can travel to Giza to personally look at the pyramids in order to draw them. There's no way I could travel the south pole to view penguins in their natural habitat. [Extreme example: There's no way I can cut open a cadaver to view the internal organs of a human.] I would need to use photographs as reference..... etc.

However, if one uses a photograph as a tracing basis - meaning you use some automated or manual method to directly mimic the object(s) within the photograph, then yes it is often infringement or at a minimum derivative work (2). This includes things like painting over a photograph in Photoshop. Or using Illustrator to place the photograph as a template for drawing on top of, etc.. A vector recreation of a raster image is derivative at best and most often is an infringement.

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Typically using a photograph as reference won't yield any direct representative copy of that photograph and if one is using a photo as reference there's rarely a desire to replicate the photo as close as possible. But even if drawing by hand, if you are so adept that you mimic a photo exactly, that too may be infringement.

There are often some major legal battles surrounding photos as reference. One such case can be read about here. And as explained, it really depends upon how a photograph is used and the resulting artwork created.

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Ultimately, what is or is not an infringement can only be definitively determined by viewing the original photograph and the subsequent artwork created. It's all art, not science so there's no "formula" or "acceptable limit". So until actual work is viewed everything is merely speculation.

Be aware... you'll often see or hear people state "as long as you change it by X%, it's okay". This is entirely false. There's no such claus or loophole in any copyright law.

In most instances... if you question whether or not you are infringing up the rights of someone else, you probably already know the answer.