Thursday, 25 July 2013

Bokeh-fixing: Opening and cleaning an Olympus OM 50mm f/1.8

In the previous post, I talked about the Olympus OM 50mm f/1.8, and how I find it interesting to use on my Micro Four Thirds camera (Panasonic DMC-GX1), along with some sample shots.

When taking some night shots, with an object close in focus, I could see obvious defects in the bokeh, that is, the round circle of light coming from a distant, out-of-focus, light.

The next image is taken by pointing at a spot light about 200m away, but setting the focus at its closest position (0.45m). This gives a large bokeh:
Spot light is about 200m away, focus set at 0.45m, aperture f/1.8. ISO 1600, 1/40s. The bokeh dimension is about 1000x1000 pixels, that is a little more than a fifth of the width of the image.
Clearly, something is wrong here: there are some black dots and strange reflections on the left side of the bokeh circle.

By looking inside the lens, I can see something that looks like oil drops, apparently not far from the back, maybe behind the outermost lens. I'm wondering if it comes from the aperture mechanism, since it's slow and has obvious oil marks on it, but I can't tell for sure.

I looked up online, and some people on dpreview forums advise that it may not be worth the fuss trying to open it up, and it would be easier to buy a new one, considered the price. On the other hand, it is such a cheap lens (~25USD with shipping) that it would not be a disaster if I broke it. Looking further, I found some diagrams on Olympus Dementia, but even if you can figure out which exact model of lens you have (Olympus made multiple fairly different versions over the years), it still does not tell you how to open it.

Anyway, since my problem looked like to be at the back, and since there are 3 obvious screw there, I decided to start on that side:

It comes out easy. The lever to unlock the lens from the mount falls down (left on the picture below), but it isn't very tricky to find out how to put it back:

Then, a big part of the aperture mechanism comes out easily. This mechanism contains a spring that opens the aperture to the maximum. When the aperture lever is pressed (right of the picture), the spring is extended, and the lens stops down to the desired setting on the aperture ring. I took out the whole thing, taking care of keeping all the elements together. The lever falls out, but it's easy to figure out how to put it in again:

Then I'm left with this, and nothing obvious to remove. I want to remove the metal ring at the top, as it looks like there is oil right behind the glass that it is holding. It is screwed to the bottom part, but hard to remove. I notice some glue near the joint, so I scratch it off with a box cutter:

And after this, I managed to open it up, using a soft cloth to give me more grip and avoid damaging the lens (you can see some scratch on the screw thread, that's where the glue was):

The top glass is now free, and the easiest is to remove it by gravity: invert the lens, hold it in a soft cloth so that the glass does not fall down too hard, and shake it a bit.

No oil on that lens, but, luckily, I could spot it on the lens just below. I did not want to introduce any liquid in the lens, so I removed it the best I could, possibly smudging around instead of properly removing it, actually. A more proper way would have been to find the way to take out that glass, but, well, that would have required significantly more work.

After getting convinced that most of it was removed (or at least evened out...), I reassembled it, and took the same picture. Notice the improvement!
Left: before, Right: after. There is still a slight smudge on the right, but it is noticeably better.
And my 13.5 USD 50mm f/1.8 recovers it's original beautiful bokeh!