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!

Monday, 22 July 2013

Olympus OM 50mm f/1.8 on Micro Four Thirds

One of the strong points of the Micro Four Thirds (MFT) system is that, thanks to its short flange focal distance, you can mount lenses designed for almost any other camera system.

I believe you can get the best deals by buying Olympus OM lenses: There is no current camera supporting those lenses anymore, but they were produced in mass in the 80's and 90's. These are ingredients for a high supply, low demand, therefore low prices on auction websites.

This is especially true of the Olympus OM 50mm, f/1.8, that used to be a kit lens with many film Olympus cameras. Almost a year ago, I bought one on eBay, for 13.50 USD (+ 11 USD shipping). I mounted it on my Panasonic DMC-GX1, using a OM to MFT adapter (less than 10 USD).

I originally bought this lens to use it as part of a custom tilt-shift adapter, but realised that the 50mm focal length is usually too narrow, and purchased a Promaster 28mm f/2.8 for that purpose (OM mount as well).

This lens is really amazing (especially considered its price): It becomes a short telephoto lens on the MFT system (100mm full-frame equivalent), which gives you interesting constraints: you have to focus on details, or put some distance between you and your subject. The large aperture makes it particularly interesting in low-light conditions (museums, night markets, etc.). On the other hand, it does require ND filters in bright daylight, as you are hitting the maximum shutter speed of the camera (1/4000s for the GX1): a 3-stop ND filter, that is ND8 or 0.9 optical density, works perfectly for these situations. I actually never stop the aperture down: I would rather switch to another lens if I want more depth of field.

Focusing is not easy, especially without a viewfinder. MFT cameras provide a magnified view to help you focus, but, with a bit of practice, I'm able to get a reasonably good focus without using that mode, by moving the ring back and forth until I have a good idea of the best position.

The lens I got was in good condition, except for the aperture, that is a bit sluggish: you need to jiggle the aperture ring to get it back to f/1.8 if you stop it down. I could also see some oil on the aperture blades: probably the reason why the mechanism is not working as well as expected. But again, since I only use it at maximum aperture, this is not really a concern for me.

I used that lens for a number of night shots, and realised that the bokeh is not exactly as round and nice as it should be: there is some "dirt" on the left side of the disk (when held in landscape orientation). This does not show up clearly in most shots, but it looks quite silly when the same pattern repeats in different locations on the frame:
Each of the bokeh rings shows some black spots at the bottom: looking through the lens, I can see some oil marks.
The next post will show you how I managed to fix the problem, by opening up the lens.

In the mean time, I uploaded on Flickr a collection of photos taken with that lens: