HSM vs SDM vs Screw Drive

Started Feb 4, 2014 | Discussions thread
chary zp Senior Member • Posts: 1,452
Re: HSM vs SDM vs Screw Drive

edspen wrote:

After playing the what focuses best game with my gear, I pondered this : All the mechanical linkages in screw drives (body motor, gears, couplings, lens couplings, lens gears, etc.) have their individual tolerances when made. These have to add up to focusing inaccuracy. Electrically coupled lenses have less mechanical play, so wouldn't they have more accurate focusing ability ? When the camera sensor shows you're in focus how close is it really ? And, the hunting that some lenses love to do, is that due to loose tolerances ?

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Ed

It doesn't have to add up to any focusing inaccuracy at all. Imagine a bicycle with a loose chain and the reverse-pedalling brake. I don't know what is the proper term for it, but with the "simple" bikes you do not have a "gear changer" but you pedal forward, you go forward. You stop pedalling and you go forward by inertia. You pedal backward and you brake the bike. Now...

The chain is loose. In perfectly tight chain you pedal forward and the wheel moves immediately. Thats sportsmen's bikes, like for bike football. No room for tolerances, no room for loose chain. On the other hand, an old lady in a village doesn't have top grade bike. So, with a loose chain you pedal a little and only the chain tightens, the wheel doesn't move, and when the chain is fully tightened, the wheel starts to move. Thus, motor, gears and coupling tolerances. You want to brake and the loose chain has to tighten up the other direction, so you pedal backwards and for a short while nothing happens but the chain tightens in the other direction. But after that the brake engages. Thus, focusing backwards and forwards, hunting there and back, looking for ideal focus. The tolerances just make the motor move a bit more than necessary until all the tolerances are tightened. And the motor only moves until the focusing circuit tells it to stop. So, in fact after the fist move, when the motor only tightens the tolerances, the focusing circuit still tells it to move since there is no change in detected contrast.

With my well set screw driven lenses I never had problem to focus precisely, provided the contrast was good and the light was good enough for the given aperture.

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