Perfect lens focus is vital if you want to achieve pin-sharp stars in astrophotos.
While there are plenty of factors to consider – accurate polar alignment, tracking and processing skills – mastering these won’t overcome an out-of-focus image.
This DIY microfocuser can be attached to a DSLR lens to help your focusing efforts.
The design fits a range of Canon lenses with outside diameters of 67–95mm, but should also be suitable for other makes and models.
Get help with this build via our DSLR microfocuser template (PDF)
Why use a microfocuser?
DSLRs are a popular option for deep-sky imaging for beginners.
Not only are there many low-budget models, but they’re also versatile for other types of photography, and we can swap lenses around for different focal lengths.
DSLRs commonly have an autofocus system, which works great in daylight but is not reliable for astrophotography, when the lack of available light reduce its effectiveness.
Precise manual focus is therefore an essential skill.
Aids can assist in identifying the optimum focus point on stars, such as zooming in using the DSLR’s ‘live view’ or adding a Bahtinov mask to the front of your lens.
Even with these aids, however, the manual focusing rings on DSLR lenses can be clumsy to use when homing in on the perfect point of focus.
Our microfocuser overcomes this by allowing very fine movements of a lens’s manual focus ring.
It also retains the focus position once achieved, so you can’t accidentally knock it when removing a Bahtinov mask or moving between targets.
Using the microfocuser
Our design consists of two plywood rings. One is clamped to a fixed part of the lens body using three nylon bolts – these are soft, so they won’t scratch the lens body.
The second ring is similarly attached to the rotatable focus ring. There are ‘lugs’ on each ring which hold the adjustment mechanism.
By turning the adjustment knob (switch to Manual focus first!), the focus ring is rotated towards or away from the fixed ring and focus is adjusted.
The M6 thread moves the ring 1mm after a whole turn, which means you would have to fully turn the knob about 475 times for one complete rotation of the focus ring!
This microfocuser can only move about 5° either way, so it’s important to first rough-focus your lens before tightening the nylon bolts. This means it only requires a small amount of final adjustment.
The build is quite straightforward, but the threaded inserts need generous holes in the plywood so they’re very lightly screwed in or even glued without splitting the plywood.
The slots on each metal tube may need to be enlarged too to allow the threaded adjuster to work freely – a bit of fine-tuning may be required.
Tools and equipment
- Drill, drill bits (12.5mm for tubes, 10.5mm for inserts, 6mm for slots in tubes)
- Coping saw, hacksaw, files
- A4 piece of 15mm good-quality plywood or similar
- 80mm length of M6 threaded rod
- M6-sized knob, nut, barrel nut and two washers
- Short length of metal tube with 10mm inside diameter (to suit barrel nut)
- Spring, 9mm diameter x 40mm length
- Six M6 threaded inserts, six M6 x 50mm nylon bolts
- Sandpaper, epoxy two-part glue, paint or varnish
Step-by-step
Step 1
Using the template found in the Bonus Content on our website, mark out the design on the plywood. Drill the holes for each metal tube. Cut the inside of each ring with a coping saw or by ‘chain drilling’ (to produce a continuous row of holes) and filing.
Step 2
Use a coping saw to cut the outside of each ring. Take care cutting around the two lugs for the metal tubes. Before removing the template, make sure to mark the three insert positions on each outer circumference for the nylon bolts. Sand everything smooth.
Step 3
Before using a hacksaw to cut two lengths of metal tube – one short (15mm) and one long (35mm) – drill 6mm holes about 7.5mm from one end of each tube length. Gently wiggle the drill while running, to turn the holes into slots. Remove any burrs with a file.
Step 4
Use a drill to make generous holes for the inserts. Normally these would be tight, but to avoid splitting the plywood make them at least 10.5mm diameter. Once completed, the plywood can be finished and smartened with a coat of paint or varnish.
Step 5
Drill a slotted hole through the side of the lug to align with the hole in the short metal tube. Add the threaded inserts and nylon bolts to each ring before gluing. We used epoxy glue to stick the metal tubes and inserts into their holes.
Step 6
Screw in the nylon clamping bolts and assemble the adjustment mechanism. Give it one last check; we had to do a little filing to get everything to align and move freely. Carefully fit to the camera lens, keeping the rings concentric and parallel.
If you complete this DIY project, let us know! Get in touch by emailing contactus@skyatnightmagazine.com
