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Lately I have been tinkering on several lenses for various types of SLRs. Perhaps my wanderings and meanderings in the mysterious innards of these fascinating contraptions might be of interest to a few.
So in this spirit, I will wander and meander in the telling of what I have learned.
It is amazing how a lens that is approximately the same size and shape and does the same functions as its other brand sibling can be so different internally. After all, the lens of a focal plane SLR only focuss, sets aperture and sometimes has a switching mechanism for auto/manual stopping of the aperture. And a few of them zoom. I mean...how many ways can you do these few things!? Obviously MANY different ways...
It is not my intention to recite what I did to any particular lens. So my meanderings will not be cookbook style instructions: unscrew nameplate ring with rubber sink stopper then use a lens spanner to remove the front element that allows access to three screws that...
What I will attempt to do is set down a general roadmap of directions and point out a few potholes I ran into along my trip. First, what does a lens do and how does it do it?
To be very astute, a camera lens attaches to a camera...via a lens mount. And nearly every brand camera has its own proprietary mount setup. Disregarding the new electronic mounts that I have no interest in dealing with on this journey, most mounts have two levers that stick out. These two levers (or pins) mate with mechanisms inside the body to tell the bodys meter what aperture the lens is set to and the other to automatically stopdown the lens during exposure. The stopdown lever always sticks out but sometimes the aperture set lever is recessed.
Some of the older lenses only have one pin. In this case, stopdown metering is to be used and often an auto/manual switch will be on the lens or body. It might even just be a button to be pressed and held until the meter reading indicates what aperture is appropriate for the given shutter speed.
And to be even more astute, a lens can be focused. It does this by moving just the front elements further or closer from the camera. Contrary to ones first thought, when the lens is in its shortest position it is at infinity focus. And the longer the lens becomes the closer it is focusing. Before I start any lens tinkering Ive learned to record the overall length from mount to lens front when set to infinity. I have been known to insanely unscrew focusing helixs. I measure this with a thousandths measuring caliper. Accuracy here is vital. A visual sighting with a wooden ruler wont cut it.
Zooming is like focusing, only the measurement is taken zoomed all the way in and at infinity and all the way out at infinity.
Why infinity? The short answer...my caliper is only 6 inches long and infinity is shorter. On some zooms and teles my caliper gets an inferiority complex when trying to measure at close focus. It just aint big enough...
Continuing my astuteness, there are three ways to get into a lens. From the front element, the lens mount and the outer periphery (where focus/aperture rings are). Usually a complete teardown requires all three.
Baring impact damage the reason you are wanting inside is to clean oil off the aperture blades because they are moving slowly or are stuck completely. Generally these blades are buried deep, deep inside.
Ones first instinct is to start disassembling the mount. After all this is where the levers stick out that move the blades and the blades are clearly closer to the mount than the front. Fight this instinct... Most aperture housings Ive removed come out from the front.
First remove the nameplate ring. If you are lucky it will have tiny spanner slots. If unlucky youll need to remove it by friction. Find a similar sized/shaped piece of something and put rubberized cloth between the nameplate and tool unless the tool itself is rubber. Table leg coasters, sink stoppers, anything with a soft, high friction rubber surface may be used directly without the rubberized cloth.
If the filter ring is dented...even a little... the nameplate ring may not be able to be removed. Straightening this ring is difficult but possible. Dont harm the threads!
Sometimes, like on some Kiron lenses, there is a tiny setscrew on the outer periphery of the filter ring. After loosening the setscrew a couple turns, the whole outer ring unscrews from the front element.
Usually the front element must be removed next. Spanner time. Actually removing front, rear and inner element groups before going deep is a good idea. Screwdrivers can slip. Just about anything can be fixed on a home workbench except scratched glass. Sure scratches can be polished out with toothpaste or glass lapping compound but the original lens shape will be changed. There goes optical quality...
Carefully wrap each glass piece in soft tissue. Make a note of lens curvature direction and location. Usually obvious, but note taking is a vital tinkering tool. Use it often!
Under the nameplate ring there is usually three screws that allows the whole lens front to be removed. On Canon FL lenses there are a series of brass retaining rings. Dont get these mixed up! They are very similar in size. Line up all the parts and screws as you remove them.
On one lens the nameplate was so tight the thin spanner slots stripped. If this happens, two holes will need to be drilled in the ring to allow the use of a needle point spanner.
Often the aperture housing is held into the body by screws accessed through the front, but even more often the housing is held by radial screws/setscrews that are located under the aperture/focus ring. The aperture ring usually has setscrews that hold it in place. Caution! There is often a detent ball that will drop out when the ring is loosened.
Sometimes the mount must be removed to get access to these setscrews. Under the mount is the linkage that connects the rotating aperture cams to the pivoting stopdown levers. Sometimes these get bent and the aperture stops working. If the stopdown lever feels too free, check these linkages before going into the lenses front..
Caution! Dont remove the screws and cover from Canon FL mounts stopdown mechanism. Lots of little steel balls will fall out. Also dont remove the 4 screws (2 about a half inch apart and then two more 180 degrees opposite) on the 50mm f/1.4 (and probably others). These hold the focusing guides. Youll lose infinity focus accuracy if you separate the focusing helicoid and start it back in the wrong screw lead. This is why I make the initial caliper measurement!
Anything you disassemble and discover didnt need to be, reassemble immediately. Youll thank yourself later!
When doing a cleaning oil off aperture blades job, clean oil residue off all inner surfaces. You dont want to have to go back in there in 6 months! Dont lubricate the blades with any liquid. Run the blades dry. If they are a bit tight some graphite may be massaged on their surfaces. But having to do this is rare. The only lubricant usually needed in a lens is the focusing helix damping grease. And it rarely needs replenishing. Only if the focusing is real stiff (dried out grease) and all the dried grease had to be cleaned off. In that case relube with a thick moly grease. I always try to rehydrate the grease with Rem-oil (gun oil) first. A drop every 90 degrees, then cycling several times. Then letting the lens set for a couple hours before adding another drop. Do not over lubricate or lubricate too quickly. It takes time for the oil to be absorbed by the grease. Better stiff focus than oily aperture blades.
The glass should be one of the last parts reassembled. Brush particles off and blow before cleaning with alcohol or Windex. Cut pieces of Kleenex into small rectangles and wipe lightly in circles. Wish I could tell you the secret of not trapping dust inside lenses while reinstalling the elements. Only advice I can give is to blow out the inside with canned air while holding the element in white cotton darkroom gloves. Screw in the element immediately after blowing. Dont blow on a stopped down aperture, blade damage can result. Btw, dont blow with your mouth! Even if you manage not to spit, condensation can form and dust will stick.
Ok, maybe I will give some cookbook directions...
Hanimex 35mm f/2.8 K-mount: example of mount access to aperture.
Kiron 28mm f/2.8 Canon FL mount: access via the front.
Takumar 200mm f/4 M42screw mount: perimeter access.
See? Wasnt that simple? I should tell you what I went through discovering this simplicity. I would, but it is too embarrassing.