How do you focus using cross hairs? I use to focus using split screen in most cases.
You don't. You just have to focus directly on the screen without any other aids.
You focus like GLS say, and you make sure both your image and the cross hairs are sharp. That tells you the aerial image and the cross hairs are on the same plane.
If you have ever taken photos through a mircoscope with the dedicated microscopy screen, it is a clear screen with a cross hair with mm marks.Because the screen is clear (not frosted) it transmits the aerial image only, which is also a lot brighter - a regular frosted screen would be too dim to see or focus. The cross hairs are used for both scale and a focus aid. The aerial image can be above or below the screen and your eyes will focus naturally on them, so when both the cross hairs and the image is focused to your eyes, then they are on the same plane. However, because microscopes also have a very small aperture, the depth of field on the aerial image is great, which makes it hard to get the plane coincident. The standard technique is to move your head (eyes) from side to side, and if the aerial image is above (or below) the cross hairs, the parallax will cause them to appear to move differentially. When the cross hairs and image don't move wrt each other, you have achieved focus.
Prismatic screens like the Acute Matte break up the aerial image when they are not on the same plane (same as the split image and mircoprism aids do). And because they are transmitting the aerial image, they are very bright. However, when you are very close to focus, the shallow prisms (which are needed for the slower lenses) don't break up the image significantly, so the image looks sharp. Hence the complaint that the critical focus point is a little vague. For f/4 lenses, this has not been an issue as you are usually within the DoF, but it's very noticeable at f/2. Checking that the image and the cross hairs looks sharp to your eye is enough to get that last bit of critical focus (no need to move your head side to side).
Acute Matte D resolves this issue (I think by switching over to a lenticular screen, which is used by most modern SLRs - I don't own a "D" so I have not been able to check)
