Introduction: How to Repair a Zoom Telephoto Lens and Mount It on Your DSLR Camera
Telephoto lenses are expensive and if you have one intended for an older SLR camera, you may consider using it with with your DSLR. My experience is that this is doable but there are a few critical issues that you have to deal with.
Contemporary DSLR cameras are rather unfriendly to older lenses for different reasons such as the lens-sensor distance , the mounting mechanism or even the camera software.
It is important to mount the old lens on the camera at the precise distance without damaging the internal parts..
In this instructable I'll show you how this can be done by a specific example using common tools and materials.
I picked this used and damaged telephoto from a street market. It was a zoom 80-200mm F/4.5, ~400gr weight, with the JCPenney (an American multistore) brand on it and the sign "made in Japan" . Even at its own time it would be inferior compared to those made by Yashica, Nikon or Vivitar with similar specifications.
It was practically separated in two and full of dust, but the external lenses did not seem to have any scratches. Initially I intended to remove the lenses for other uses but looking at it more carefully I realized that the mechanical problems could be fixed.
The challenge was to repair it and mount it on my Olympus E-420.
Step 1: Preliminary Steps
Find out the camera-lens distance
- Attach the camera on a tripod and remove its own lens. Holding the telephoto with both hands in front of your camera with the setting of the telephoto at infinity, try to focus a very distant object. If you have a problem to keep the lens to the correct position use a paper inner tube but do not touch the internal parts of your camera.
- Rotate the focusing ring a few degrees and focus again. In the final construction it is better to set the infinite point a few degrees before the rotation limit. This will allow sharp focusing by leaving some space around the correct point.
- Focus on close objects. Try to find the minimum focusing distance available. In my case the distance measured was 1.7 meters at 200mm and 1.5 at 80mm.
A T-ring is the best type of adaptor for this work. It offers two possibilities for mounting a lens, either by using the 42M threading of the internal ring or by removing the ring and fitting the tube on the main connector directly. The one shown here is the Olympus Four-Thirds T-ring for DSLRs which has a bayonette type mounting.
Step 2: How a Zoom Telephoto Works
- A zooming system transforms a parallel beam to another parallel beam of different diameter depending on its position (afocal operation).
- A simple method to visualize this, is to think of the zooming system + the objective lens as a composite "objective". The effective focal distance decreases when the components come closer.
- Thus when we approach the zoom to the objective the focal distance decreases (ZOOM OUT) while when we retract the zooming system backwards, it increases (ZOOM IN). In the case of my lens the limits are 80-200mm.
- Contemporary lens systems do not have a sliding tube as this one , they work mostly by turning a ring and move the objective lens away from the zooming system. An independent lens system is used for focusing (see the next design in this page).
Step 3: Take It Apart ! (if Necessary)
Reasons for doing this:
- Repair mechanisms.
- It was exposed to dust for long and needs Internal cleaning.
- You want to see how it works.
- It is fun!
I do not recommend taking lenses apart in general. There are systems with 25 optical components , better not touch them. In this case the lens had about 5 optical components and the connecting screws where all 1.5mm and could be manipulated with medium difficulty.
How to do it:
No violence and avoid coffee. Just use your hands and appropriate screw drivers. Do not force any other kind of tools (knives, pliers, screwdrivers) on the lens body.
Start disassembling the most obvious external parts and proceed with care as far as it is needed.
In the case described here it was only necessary to separate the three lens systems in order to clean them. However following arguments (3) and (4) I went all the way down.
Do everything in a tray in order to avoid searching for 1.5mm screws on the floor.
Use gloves and fine tissue paper to handle the lenses.
Use toothpicks to align tiny holes on different tubes.
If you cannot put it back together don't panic, leave it for another time.
Step 4: Repair Work
I had to make new nylon washers for two delicate screws that serve as guides to the external focusing/zooming tube. These have dimensions 2.5mm external diameter and 1.5mm internal and thickness 1mm. Fortunately an ink pen filling has a similar size and it fitted nicely. The tolerance was ~0.2mm.
Some parts of the thin metal tubes were rough so they had to be sanded gently on a table with a 200 sand paper.
The Objective lens system stayed on a threaded part of the main body and a silicon grease was necessary there. This was done at the final stage of assembly after cleaning and mounting the lenses.
Step 5: The Iris
If the iris is working then fine. In the case discussed here , the iris was in place and it is operational but it was obviously handled by a ring close to the camera which was missing. Although there was a way to do something similar I selected to immobilize the iris in the open position, by placing a plastic tube through an opening in the focusing lens compartment.
Step 6: Build a Proper Mounting for Your Camera
- The mounting combines the last component of the lens with the T-ring. These are connected by a brass inner ring using 2M and 3M screws. Since the part of the lens is thick enough (2.5mm) I selected to drill holes on thread them for 3M. The screws are placed from inside.
The internal diameters of these tubes are not equal , so I used 1mm steel collars to match them.
If one has access to a lathe this kind of modification can be done in a better way. I think an inner tube is still the best method to connect the two parts but one could eventually use the M42 thread on the T-ring.
The mounting shown here is rigid enough to support the weight of the telephoto (400gr). In fact I trust it more in terms of robustness than I trust the rest of the lens!
Step 7: Assembly and Focusing Range
Before the final assembly I mounted the lens to the camera a few times in order to adjust the correct distance for focusing. I had to shorten the lens part of the connector by precisly 1.3mm in order to get a focusing from 1.7m to infinity for the 200mm focal length. I also needed some space for fine focusing at the 200mm limit.
Before mounting, all parts were cleaned with alcohol and cotton. I used a special liquid and tissue paper for the lenses.
When placed on the camera, the measuring ring was adjusted in order to read the correct distance.
Step 8: Measure the Field Size and Estimate the Magnification
Measure the field size
- This can be easily done even indoors. Focus on a surface distant a few meters and shoot. Then measure with a tape the object distance and the length of the surface spanned in the photo. The lens I am describing has a field size of 5o x 3.7o
- If you prefer to think of the field size as a distance, in the case of 5o , the length spanned when you are focusing on an object at 1000m is 175m.
- Another simple method is to shoot the moon , which extends ~0.5o and then measure on the photo the relative size of the moon. This was done in the photos shown below. The moon was photographed a cloudy night, rain followed. I wanted to catch the seek and hide of the moon behind the clouds.
Estimate the magnification
- This term is not very precise for cameras. The "magnification" of a 200mm lens on a DSLR can be estimated taking into account two facts. (a) 1x magnification in a conventional SLR corresponds roughly to 50mm focal length and (b) the sensor length of the DSLR is about 1/2 the 35mm film. Therefore in this case we have 8x magnification.
Step 9: Estimate the Resolution Limit
What the resolution limit is
- Think of a car coming to your direction on the highway with the lights on. When it is far away both lights are merged in one. You need to now what the maximum distance is for the two lights to be separated by an optical instrument (e.g. your eyes). This depends mostly on the size of the objective lens and is usually expressed as an angle.
Estimate the resolution limit
- .My house in Athens is facing mount Ymittos on the top of which there is a group of telecommunication towers (see photo). The tallest one has two red lights 0.6m apart (measured with a telescope of known field size) while the distance to the tower is 7000m (measured from Google Earth). It is a perfect calibration target!
- The angle of 0.6/7000 radians expressed in arcsec is ~18arcsec. The size of one pixel in a 10Mpxl sensor when this lens is used, corresponds to 5 arcsec and this means that the distance between the lights spans 18/5=3.6 pixels, quite a small number.
- The theoretical resolution limit for such a lens (44mm diameter) is about 2.5 arcsec much smaller that what was estimated here. I don't think that this can be reached because there are other factors involved (complexity and quality of the lenses, atmospheric conditions) .
Step 10: What You Can Do With It
- In terms of magnification using this lens you have a viewing field through the camera as that through a set of 8x44 binoculars. Thus it is great for shooting distant objects. The moon is also an interesting target. Try also to shoot Jupiter with its satellites.
- Besides long distance a telephoto is ideal for medium distances e.g. portraits or nature, because of this nice blurring effect on the background. All the examples shown here were shot at 2-4m.
- Overall you can add one valuable piece of equipment in your photo bag at a minimal cost.
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