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Focus adjustment of a zoom lens: Kiron 35-135mm f/3.5-4.5
Zooms must be adjusted for infinity sharpness at both extremes of their zoom range. The wide angle adjustment is made by moving the rear element assembly closer or farther from the film plane, while the telephoto adjustment is made by moving the front element assembly. It is often necessary to readjust (perhaps several times) until infinity is sharp in both positions. If focus is sharp in both extremes the middle ranges will be sharp automatically.
On this Kiron, turning the macro ring moves only the rear element assembly so it is used for the wide angle adjustment. The macro ring is secured to the macro helix by three setscrews (blue arrow). Turning the focus ring moves only the front element assembly so it’ll be used for the telephoto adjustment. Pull back the rubber hand grip as shown in the picture to expose the six setscrews (yellow arrow) securing the focus ring to the front element helix.
Sliding the front of the lens in and out (zooming) moves all elements.
The zoom/depth of field cylinder moves with the macro helix and is retained to it by three setscrews (green arrow). So if the macro ring must be repositioned in the course of wide angle adjustment, this cylinder must also be repositioned. But repositioning it can wait until all adjustments are finalized. It serves no mechanical function and its position does not affect focus.
Mount the lens on a camera body and either sight a very distant, high contrast object or use the technique of backsighting described in the test equipment section of this website. See Collimator article.
Now that you have a bit of theory the adjustment procedure becomes obvious.
First adjust the widest angle zoom position. Verify the macro ring is in the zero detented position, then zoom to the 35mm position and turn the focus ring to infinity. During all adjustments the focus ring must be against the infinity stop. The distant object should be sharp. If not, turn the macro ring until it is sharply in focus (red double arrow near camera body). If the macro ring must be turned beyond the zero stop to achieve sharpness, loosen the three setscrews (blue arrow) on the ring until the ring freewheels. Then back it up a ways and snug up one setscrew. Turn the macro ring until focus is sharp. After verifying that the telephoto focus is also sharp, loosen the snugged setscrew and return the macro ring to its detented zero position.
To adjust the telephoto infinity focus, zoom out fully to the 135mm position. The distant object should be sharp. If not, loosen the six setscrews (yellow arrow) and turn the outer ring (red double arrow near front element, not the focus ring, it stays against the infinity stop) until focus is sharp. Snug one setscrew and recheck wide angle sharpness.
When both wide angle and telephoto focus is sharp, retighten all setscrews and reposition the zoom/depth of field cylinder so the line coincides with the infinity mark.