Split-field filters - technical advice?

[Steven Carver]

Hi there. I'm new to the forum, new to filmmaking. I have a bit of a technical question about using close-up and split-field filters.

 

What I'd like to know is, is there some formula to work out how close-up filters of different diopters (+1, +2, etc) affect focus?

 

I have a reflex Bolex 16mm. Because it's usually quite hard to focus through the viewfinder, I tend to measure the distance to the subject using a tape measure and set the focus accordingly. With a split-field filter, where one half of the lens is covered with a close-up filter (giving you two planes of focus: a near subject and a far subject) the focus ring would be set to the distance of the far away subject (in the half of the frame not covered by the filter). What distance do I then have to be to the near subject (seen through the filter) for both far and near subjects to be in focus?

 

So, for example: far subject is 20ft away, and is in the half of the frame with no filter. The near subject is in the other half of the frame, seen through a close-up/split-field filter with dioptre +1. How close should the near subject be for it to be in focus, with the focus ring set to 20ft going through a +1 dioptre close-up filter? Is there some formula I can apply?

 

Phew! If any of this makes sense I'd be really grateful for any help anybody could offer. Thanks!

 

P.S. Let me know if you want an example and I'll post an image of what I'm talking about. Ta.

[Nick G Smith]

The information you need is on Carey Duffy's South London Filters Website: http://www.camerafilters.co.uk/page60.html

 

Nick

[Tom Jensen]

If I remember, I used split-diopters on primes and diopters on zooms. The split-dioptors usually need a line somewhere in the frame to mask the focus shift.

[Steven Carver]

The split-dioptors usually need a line somewhere in the frame to mask the focus shift.

 

The focus shift's what it's all about, man!

 

Thanks for the link.

[Tom Jensen]

The focus shift's what it's all about, man!

 

Thanks for the link.

 

Well, it's a split-diopter. When you put it on a lens the part of the lens that gets covered by the diopter is going to be focused on one plane and the part that isn't covered will be focused on something else. The line in the diopter is the dividing line. If the line is on an object you will see it.

[Chris Keth]

There is a handy rule that can help you choose which diopter to use and lessen the trial and error. For + diopters, they will move infinity focus to the reciprocal of the filter value in meters. This means that when the lens is focused to the infinity mark with a +1 diopter, the image will be focused at 1 meter. If it's a +3 diopter, the image will be focused at 1/3 meter. All marked distances will also be closer, of course.

 

If I need to calculate exact focus distances with diopters, I just use tables.