focusing

Sir
I am just learning large format focusing. On my 4x5 using a 150mm lens it has been easy to focus. I focus on my main subject placing it in the middle of the screen, tilt the bottom of the film plane toward the lens board to clear up the bottom half of the screen and then barely tweak the lens board and everything is in focus. My 4x5 is axis tilt on the film plane and the lens board has bottom tilt. I have used the same basic technique using a 360 mm lens on the 4x5. However when I use the 360mm lens on my 8x10 the same technique does not work as smoothly and I have to do a lot of focusing to get a correct screen. Yet when I go to a 240mm G-claron the focusing is much easier. My 8x10 is axis tilt on the front and back. What is going on? Does my near point change by going to a longer lens? Are shorter lens easier to focus? Yet why does the 360mm focus easily on my 4x5 and not my 8x10? Any thoughts would be greatly appreciated.
Jerry Cunningham

"Are shorter lens easier to focus?" Yes.

"Yet why does the 360mm focus easily on my 4x5 and not my 8x10?" When the 360 is used on 4x5 , there is a smaller area in which to get in focus, 20 square inches, or approx 150mm diagonally, versus 80 square inches, or approx 300 mm diagonally on the 8x10.

Others will be able to chime in and give you a more detailed explanation.

DG

It sounds like you may be getting confused by the differences between base and axis tilt, along with the differences in DOF with longer lenses, Jerry. With base tilt on the lens board, changes in distance between the lens and the GG are more dramatic, and must be compensated for. With axis tilt, that's less of an issue. Front and rear tilt also behave differently. Once you think about what is actually going on, I suspect the confusion will disipate.