How to modify 8x21 binoculars like these to focus to one metre

These binoculars are made in the Far East, and may carry a name such as Tasco.They might be 20 or 30 GBP in Camera Shops, but are often seen new at carboot sales or as Special Offers in magazines etc for 10 pounds or so.

There is a similar 10x25 version for about the same price, but it's larger than 'pocket size' without offering any great advantage.

The focusing mechanism is accessible, and works by moving the objective [front] lenses; the further forward, the closer the focus. The cross-piece that moves the lenses engages with a peg screwed into each lens carrier. For some reason there is always, as far as I know, a choice of 4 holes to put each peg in.

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Deconstruction:


A couple of screwdrivers are needed to take the things apart.First unscrew the plugs carrying the cord, if present. This exposes the grub screws holding the 2 halves together. Work first on the left half, with the non-adjustable eyepiece.
The grub screw will be done up pretty tight... use a screwdriver that fits well, to avoid damaging the slot. Make sure you don't lose the 2 ball bearings. If the focus mechanism is far forward, you should be able to twist the left monocular away from the centre-piece without having to unscrew the peg. Remove the collet, if that's the right name for it!
Try focusing the monocular on infinity by moving the peg. If you can go 'beyond infinity', there's a chance you can start by moving the peg to a different hole. So now's the time to see which of the 4 holes the peg is in...you should be able to see this by looking into the lens with the light behind you. If not, remove the rubber lens protector, unscrew the peg and use the screwdriver to push the lens carrier forward, noting which hole was used. If it wasn't the hole nearest the eyepiece, rotate the carrier to line that hole up with the slot, push the carrier in and fit the peg. Don't over-tighten...the carrier is only plastic. Can you still focus on infinity? Decide if it matters.

Typically the first hole, nearest the eyepiece, allows focus from 1.8 m to 'almost infinity'.
The second hole gives 2.7 m to 'beyond infinity'.
You could obviously make a new hole for the peg such that max focus distance is precisely infinity. It's easy with a 2mm drill and a 2.5mm tap, so long as you do exactly the same to the other carrier.
Locating a new hole such that min focus is at one metre will only result in a focal range of a couple of metres or so. The idea at this point is to allow the peg more travel by lengthening its slot.

Increasing slot size

The slot can be extended forward till it's about 10.5mm long before the mechanism is compromised. At this point the two 2mm guide rods that stabilize the moving cross-piece come out of their tunnels. With the slot that size expect focusing to one metre.
From the 'before' and 'after' pics you'll see the brilliant job done by using the wrong tool, an electric drill with 2 to 3mm bit used as a sort of rotating file. Smooth the edges with screwdriver or similar. The important thing is to keep aluminium fragments away from the eyepiece... cut a cardboard disk to fit and drop it down the tube, then add some newspaper .... keep tube upright while working on it... clean out afterwards without removing all the grease too.
Refit lens carrier, screw peg into chosen hole and test focus in monocular mode.

If the lens hits the rubber lens protector at close focus, refit the rubber inside out, or discard it.

Now do all that again with the other half, comparing new slot lengths to get them equal.

It's quite possible to reassemble with the pegs in place, as they were when you took it apart. If you decide to fit the pegs after assembly, take care you use the same hole with each eyepiece.

I have a pair of these 8x21s with the slot increased to 12.7mm, giving min focus of 80cm. This is only possible after fitting longer 2mm guide rods and extending the holes they run in. Bicycle spoke is fine for the rods. Trouble is there's not much overlap between fields of view at 80cm... it's like using 2 monoculars.

Disadvantages of these modified bins

The focusing relies on pushing or pulling the objective lenses in parallel, but it doesn't always achieve that due to play in the mechanism. That gets worse at close focus. Expect to tweak adjustable eyepiece more than usual.

Advantage:

They are so cheap... put the money you save towards a better camera!

Comments,disagreement,offers of money,etc.