Close Focus Lens Mod

Posted by Andrew on 17/06/2007 @ 05:25 +1000

If you've got one of these...

Cosina 28-300mm f/4-6.3

...and you'd like to halve your minimum focus range, to get an effect like this...

... then read all about modifying a Cosina 28-300mm lens for close-focus...

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I recently pulled apart my Cosina 28-300 to see if I could fix the focusing ring. Something was rattling around in there and stopping it from moving, and the lens is a bit too crap to pay someone to fix it, so I figured I either fix it myself or throw it.

I got the thing open and found a screw had fallen out of its hole and into the cog that turns the focusing ring - Sweet! Put the screw back where it should be, and everything turns again! I put the lens back together and took great joy in my lens which now refused to focus to infinity.

In fact, it refused to focus even close to infinity.. though oddly, it was happily focusing on my workspace - about half the previous minimum focusing distance! Whole thing opened up again, and I realised I'd misaligned a plastic stop which decides how far the focus moves in each direction. Fixed that, put everything back together, and it worked as it used to.

..but that close focus was awesome.. If the stop wasn't there I would get the normal old focus plus the fancy new focus..

So here, with pictures, are the steps I took to halve the minimum focusing distance of my lens.

In this picture you can see, apart from the UV filter that got misthreaded and now won't come off again, the rubber ring that allows you to get a good grip when manually focusing. Remove it. It just lifts straight off, it's not glued down or anything like that. Note that the Auto/Manual switch on the lens is in Manual position and will stay that way for this entire exercise.

Underneath the rubber ring is some kind of strange sticky tape. This is, in fact, a structural component of the lens. Once you remove it, you'll find that the objective movement is no longer coupled to the focusing ring movement. So don't lose/destroy the piece of tape!

With the tape off, the you should have an objective which turns freely, and a plastic collar with slips up and down. Unscrew the objective all the way, and it and the collar will come away cleanly from the rest of the lens.

Now, looking down into the lens, you'll see three small screws laid out around the inside of the focusing ring. Take them out, and don't lose them. Once they're out, another two pieces will happily come off the lens body. One of these is the focusing ring, and one is an inner guide rail for the whole front assembly, with a long silver rod hanging off it.

On the inside of the focusing ring are two plastic cubes. These stop the rotation of the ring - You'll notice they're about 90 degrees apart, and 90 degrees is how far the ring turns. COINCIDENCE? No.

Get rid of one. Specifically, the one located in the same place as the "METER/FEET" writing on the outside of the ring. You can remove it any way you feel is effective, but remember to get rid of all of it, and not to damage any other part of the ring. You can see here the remaining stop on the left and the scratched area where I've ground away the second stop on the right. Putting It Back Together ------------------------ Okay, here's the fun bit. Put the focusing ring onto the lens, making sure the feet/meters indicator numbers are in the vague area of the red focal length modifiers on the lens body.

Take the inner guide with the silver rod, and find the hole where the rod goes. You'll only be able to see it if you have the lens at its shortest length. In order to get your rod to fit onto the lens' rod, turn yours until the shapes match. It's reasonable important that you get this all back together without damaging it, as this rod is what transfers the movement from the slow, weak, noisy AF motor to the crappy plastic focus ring.

Once the rods are aligned, the inner guide should slide right down to sit comfortably inside the focus ring. If it doesn't sit perfectly flat, jiggle the whole thing around gently and try turning the focus ring slightly. You're trying to make a gear's teeth slide into place. Note that even when perfectly flat, the focus ring still won't turn very freely until you've put the three screws back into the inner guide piece. Don't do them up too tightly! It didn't take you much effort to get them out, and you shouldn't use much effort to put them in. If you destroy a plastic thread then the screw will come out and all sorts of problems will be caused.

To put the objective assembly back on, first put the loose plastic collar over the smallest part of the objective's casing. Remember that the ridges point away from the glass, and don't forget to line up your little knobbly bits on the inside, like in the picture..

Screw the objective down towards the lens body, but stop before it feels tight. At this stage, your lens is basically back together. All that remains is to align the focus again.

Turn your focusing ring (just the ring, not the whole objective!) around until it's on the infinity mark .. ie it's hit a stopper. Now point your camera out the window and find something really really far away to point at. I've used Kopiec Kościuszki, but if you prefer you can use some smaller kopiec, or even Mariacki.

Extend the zoom fully, to 300mm, and focus manually by turning the objective (not the focusing ring) with the shutter button half-pressed. When the AF system says you've focused on that very very far object, you've basically got infinity. Well, as infinity as you're likely to get with this lens.

At this point, without moving any of the parts (don't want to bugger up the alignment!) reapply that structural sticky tape that you carefully placed somewhere earlier. Make sure it's well stuck down all the way around, and check your infinity focus again.

If everything's okay, you can put your rubber ring back on over the top of the sticky tape, and switch the lens back into AF mode. It takes about 3 seconds from end to end, so a whole AF process can take up to 6 seconds. This is now the slowest AF lens on the planet.. but at f/6.3 and the crappy little motor it's got, I've been using this one on manual only for quite a while now anyway, so it's hardly any loss.

You'll also notice that at minimum focus, holes open up between the objective and the lens body that weren't there before. This means that dirt and sand and ice cream and anything else you can name will go into those holes and get things dirty in there. Mind you, they weren't very well closed up before anyway, and at least now you know how to pull the thing apart to clean it..

The Results:

Here is a portrait of my friend Bob. In the first photo I caught him playing merrily in the garden with a zoom of 300mm at a distance of 1.9m - The old focusing minimum. In the second shot, he's in a surprisingly similar pose, this time also at 300mm, but only 50cm away from the camera. His face now fills the frame and we can see his lovely smile!

This actually turns the lens into what is technically a "macro" - It will do 1:3ish magnification with a pretty serious working distance - Great for big bloody spiders!