Cinematic focusing: 35mm DOF adapter how-to

DOF adapter revision 4

Following up from my earlier post on 35mm DOF adapters, here’s the way I’ve built mine. Note that this is revision four, which uses much better materials, including a Thorlabs 2″ lens tube (my earlier designs were crudely made out of cardboard, file dividers and lots of electrical tape!)

This design borrows heavily from Richard Mellor’s.

Although the DOF adapter is considerable in length; it’s not heavy. I’ve never had any problems using it on my camera; a consumer Panasonic NV-GS300 1/4″ 3 CCD with a 37mm thread. It seems to hold up quite well without any strain.

Materials

The following items are from Thorlabs.

1 x 2″ stackable lens tube for 2″/50mm optics (PN: SM2L20)
2 x retaining rings for stackable lens tube (PN: SM2RR)
1 x 50mm ground glass (1500 grit) (PN: DG20-1500)
1 x 50mm plano convex lens (PN: LA1401)

The following can be obtained from eBay vendors. Otherwise, I’d suggest UK-based Harrison Cameras, they seem to have a decent collection of step-up rings and filters.

1 x step-up ring 37mm-46mm (if required)
1 x step-up ring 46mm-52mm (if required)
1 x 52mm 10x macro close up lens
1 x 52mm ND filter (optional)
1 x Nikon HR2 rubber screw on lense hood (optional)
1 x reverse macro adapter for Nikon lenses (recommended)
1 x Nikon f1.8 SLR lens (f1.4 suggested, but they’re usually pricier) (52mm thread size)

Some electrical tape
Strong epoxy bond

Assembly

Thanks to the Thorlabs tube, assembly is a matter of attaching the parts in the right order. The tube itself has a 52mm thread on each end. The only tricky part is the Nikon lens. It can get a bit messy, but with a reverse macro adapter, attaching the SLR lens to the lens tube is simplified.

The completed adapter has parts in the following order:

37mm-46mm step-up -> 46mm-52mm step-up -> macro close up lens -> lens tube (inside lens tube: ground glass -> plano convex lens, both held by retaining rings on each end) -> reverse macro adapter -> SLR Lens -> ND filter -> lens hood

1. Place a retaining ring inside the lens tube, followed by the ground glass (ground side to face towards SLR lens), and right after it place the plano convex lens (convex side facing towards SLR lens). Screw in another retaining ring to secure the assembly.

2. If you’re using a reverse macro adapter, bond it to your SLR lens. The threaded side of the reverse macro adapter should then attach without much hassle to the lens tube.

3. Point the assembly at a far away object that would need infinity focus. Then make sure the lens is set to infinity. The image reflected on the ground glass end of the assembly should be in sharp focus. If not, you’ll have to shift the ground glass and plano convex in the tube farther or nearer to the SLR lens. This will require some trial and error (I would suggest attaching only one retaining ring at first, and then holding up the lens with the tube to try to find the right distance. It’s clumsy, but will save you some time instead of fiddling with both retaining rings.)

4. Attach the macro close up lens and required step-up rings to the lens tube (to the side which has the ground glass.)

5. Attach the ND filter followed by the lens hood to the SLR lens.

6. Attach completed assembly to camera.

7. Set camera to manual focus, and focus it on the ground glass. The best way to do this is to again, have the lens on infinity focus at a far away object. The image on the ground glass will be in focus, but you’ll have to adjust the camera’s focus on the ground glass. This is the last time you’ll need to do this, as from now on you’ll be focusing using your SLR lens. Welcome to the world of shallow depth of field selective focusing!

8. You might want to use electrical tape around the part where the tube meets the reverse macro adapter and the SLR lens to prevent any possible light leakage.

You can download test footage here (7.35mb.)

By no means is this design perfect. I still have a diffuser lying around that I’ve yet to see the benefits of using. By far the best improvement came with the 10x macro lens and the plano convex lens, which has considerably lessened the vignetting seen on my earlier attempts.

Be aware that the Thorlabs ground glass is not the best out there. Mine has imperfections, and is not consistent throughout. I haven’t tried other vendors, but I’d suggest you ask around and look at the message boards to see what others have used.