Registration. Currently on ebay is a depth micrometer for sale. It is billed as a good tool fo r measuring film registration. I don't think you need it. Registration is the de pth, for focussing, the inner surface of the ground glass screen is away from th e back of the camera. Obviously if you focus on the inner surface of the ground glass then you hope the film (when a holder is inserted) will be in the same pos ition. The myth is you need to be concerned about this and measure it. In realit y what you might wish to do is CHECK your registration. Here is my method. Take two good rulers (perhaps steel ones). Lay the long edge of one across the raised inner surfaces (if any) of your ground glass holder(the width of the ruler shou ld be held perpendicular). Now take the second ruler and genly lower it so that its bottom short edge sits on the inner surface of the ground glass. It should b e held flat to the first ruler. Now check where the top edge of the first ruler comes against the marks on the second. The measurement is not important - what i s, is that you note carefully where the first and second rulers meet. Check it a gain. Now take your dark slide and lay the first ruler across the flats of the i nner surface. You will need to have a sheet of film inserted and the slide remov ed!. Now place the second ruler flat against the first and lower the end until i t rests on the film surface - compare where the two rulers meet. Is it the same as your first reading, against the ground glass? Now you can check other film ho lders - Fuji Quickload or Polaroid. I bet your eye will be able to detect very s ubtle changes in depth (registration) a few micrometres! I have not noticed sign ificant differences (perhaps luckily) with my holders. Be careful if you use a F uji quickload - it has a pressure plate which you shouldn't depress - obviously. This method is in effect a vernier - using a fixed point to check against a mov ing one. What you do if you have substantial differences in registration is anot her matter! I agree that for large format film in situations when the film plane is inclined then film sag may be a real problem - hence Sinar's sticky film hol ders etc.