So I used photographic references of models I took myself for the Magnus Crucifix.
I didn't do any tracing at all. I simply taped the photo reference to my drawing table and "eyeballed" it to create a drawing. I wasn't trying to get a perfect facial likeness. I also got my model, who conveniently lives in my home, to pose from time to time for hands, fabric folds, etc. but those were just quick studies. I made numerous photocopies of my own figure studies for the Virgin, St. John, and Christ in many different sizes, and used these photocopies to work out my composition. Getting the figures the right size was something that couldn't be changed later on.
I then made a more finished final drawing which I photocopied on regular paper. I put graphite from a pencil on the back of the photocopy and taped it to the panel. Then I used a hard sharp pencil to very lightly transfer the drawing to the panel. Then I carefully patted down the graphite marks with a kneaded eraser to lighten them still more. Then I wadded up the photocopy. After the drawing was transferred, I first went over it and cleaned up the details, then made some changes here and there. Then I went over the drawing with ink. At that point, I was done with modern techniques.
I thought seriously about using pouncing to transfer the final drawing to the panel. I mean, I know how to do it, and I've done it before, and it works OK. I chose not to because pouncing is way messy, and I was afraid that my lack of expertise would result in poor reproduction of the details of the drawing on such a small panel. Even transferring with graphite loses a lot of detail.
Using photo reference to aid in painting a highly realistic Renaissance painting is a necessary concession to the modern pace of life. From time to time, you may make something by hand, or decorate it by hand, but some things just take too long. I'm about to turn 57, and if I was ever going to finish this piece, I had to use some modern techniques at the beginning.
As anyone out there read David Hockney's book Secret Knowledge? It is absolutely fascinating. He asserts that many European painters, from the time of Van Eyck on, used projection devices to achieve their near photographic results. Hockney is a painter himself, albeit a postmodern one, who uses photographic reference, and reading his book was like having a productive conversation with another working artist. He obviously knew what he was talking about. Of course, a lot of art historians have their panties in an absolute wad over Hockney's book. One of these days, I want to make a mirror projection device such as Hockney describes and draw some portraits using it. Maybe in silverpoint! On prepared paper! I have some in my studio!
In mundane life, I am also a working professional artist, and I also use photographic reference from time to time, and I know what it can and can't do. It can't draw the picture for you. You can't "trace" a photo and get a photographic result. All you can do is use the photo to indicate certain landmarks on the face and then let these landmarks help you draw an accurate likeness. Or you can "eyeball" the photo, and trying to work around photographic distortion, use it as you would a live model. When using photos for portraits, I will use the photo to show the line between the lips, the bottom of the nose, and the upper lids of the eyes, with a mark on the bottom of the chin and a general outline of the head. This, I can assure you, does not a portrait make. Photos are only a tool. You still have to learn how to draw.