Dumb-ish back focus question

[Douglas Underdahl]

A lot of users say they don't feel the marks on their Canon or Nikon or Leica, etc still lenses are accurate. Maybe that's true. If you want to find out, and you have a Red camera, then you have the perfect tool to do so - just compare tape and eye.

This means - set a chart say, 4 feet from the focal plane of your Red camera, if the lens you want to test has a mark for 4 feet. Do this with a tape measure.

Now, turn the focus ring on the lens this way, then that, so that it is no longer at 4 feet and you don't really know where it is. Focus the lens using the camera's monitor and the awesome zoom function. This you do, by eye.

Get it as sharp as possible, then look at the focus ring. What does it say? 4.3 feet? 6 feet? 3.9 feet? Do the test over and over again. Do you get the same result over and over again? Do you get one result focusing near to far, and another far to near?

If you eye focus during a shoot, then the fact that your lenses might have inaccurate focus rings, or not be collimated correctly - well it doesn't matter. But if you tape focus, asking your actors to hit their marks, then you rely on those lens marks to be dead on.

 

[Douglas Underdahl]

Some lenses might not have enough focus marks to really be usefull if you tape focus and ask your actors to hit marks. That is, the actor walks up to his close up from ten feet away to 2.3 feet away, and you twist the focus ring or the follow focus from ten to 2.3 feet. Your lens has a mark for ten feet, then 6 feet, then three feet, then 1.5 feet, and that's it. You can use your camera and some skinny Letraset type tape to add marks to your lens, by comparing tape and eye.

Or there may be a lot of play aka backlash aka slop in the focus ring, so that changing focus from near to far isn't the same as far to near. Oh dear.

This is where those high dollar cine lenses come in. You pay to have accurate focus scales with a lot of marks on them. But be warned, expensive lenses can have wrong marks, not enough marks, or backlash, and cine lense get old and warn, too. So if you tape focus, get out the charts and the tape measures and the Letraset and test, test, test, or your actors will have nice sharp ears and soft eyes.

 

[Douglas Underdahl]

Oh, and if your actors don't hit their marks, or you didn't put the marks down correctly, or their feet hit the marks but they lean in at a dramatic moment - well, ya better have an AC that can sense this and make adjusments, on the fly, during the take. It's tough. This is why it's nice to give them "a stop they can live with", that is, one with enough depth of field so that the actors can move a bit and not go crazy out of focus. This is one reason that a majority of Hollywood films are shot at or near T4.

 

[Terry VerHaar]

Doug - I think my expectations for still lenses come from the fact, as you noted in one of your comments above, that there just really aren't enough marks - and they are set into such a tiny "throw distance - that the they really aren't very helpful. How does one judge the precision between 5 feet and 10 feet when they are 1/4 inch apart. If your actor's mark is 6 feet, 4 inches away, I think you are pretty much better off getting a visual focus at the mark and marking it on your follow focus, no?

 

[Douglas Underdahl]

Right, you use the power of your Red with zoom focus function, and add a mark to your lens focus ring, or you add one to your your follow focus. But if the throw of the ring is too short, then the distances between the marks might be so tiny that it's just not possible to do a good job of pulling focus no matter how many marks you make. Also, if your focus ring has play in it, then if you pull focus from near to the three foot mark, the actual focus distance might be 2.8 feet, while if you pull focus from far to the three foot mark, the actual focus distance might be 3.2 feet. So lenses with lots of throw that are precise and have lots of marks are better and come at a premium. Jeez.