Hello nice people. I am absolutely new to film making but nevertheless I have a question. I want to follow-focus a falling marble, that falls down 1 meter in about 300 ms.
One of the topics that arose while investigating how to do that is how lenses respond when the focus ring is turned step by step by a motor. I investigated a Sigma 24mm 1.8. The stepper motor I used initially did 200 steps for a full revolution. It took 107 steps to focus from closest focusing distance to infinity. Please have a look at the image I attached. The curve can be well described mathematically with a growth curve.
Now my question is this: Would you think that other lenses have similar looking curves, maybe a little flatter or so? Or: Did you ever recognize a lens as responding maybe with a "weird" curve with sort of tales or so?
Or even better: Is there any documentation anywhere out there about this topic?
Here are my first tests:
Thanks! Martin
This looks pretty typical of a helicoid response curve.
Linear motion of the objective lens leads to the focus plane moving logarithmically. When you think about it that's the only way to get a lens to focus out to infinity without spinning infinitely many times.
Some lenses use non-linier helicoids to reduce the steepness of the far end but the most effective systems use a wire guide or cam in track system.
Cookes Cam system gives a great deal of detail at the far end while having a smooth transition to close focus.
For more detail about how lenses actually work its probably best to find some books on the matter. I only know a small amount (enough to get by) mostly from lectures and conversations with Les at cooke or Art from arri at trade shows.
That response made me feel like "Haha focus motor goes brrrr". So my knowledge is probably way less then yours.
I know of the logarithmic way focus is moved but the helicoid response curve and non-linier helicoids had me confused. Out of curiosity could you explain further? Don't be afraid to explain it like I'm five.
Hey thanks for your replies! If it is as Chris says, then building the formulas for highspeed-follow focus for falling objects should always be easy: if one is done, the next one will be very similar.
In this video at about 2:08 this seems to be as Chris says, right?
https://www.youtube.com/watch?v=q1n2DR6H7mk&t=120s
Gene there is talking more about the general construction of the lens and how the total "throw" of the lenses is the same. 300 or so degrees rotation from close focus to infinity. Due to how every focal length is optically different the focus marks wont line up between different focal lengths and often even between the same focal length from a different batch/set/manufacturer.
This video from John at Filmaker IQ covers a lot of the detail about optics without it getting too complicated.
In his model, instead of the objective lens being moved by hand, imagine its attached to a screw. As you rotate the screw the lens moves up and down. With a traditional 'linear' screw you'll get a logarithmic response curse (John shows the maths in the video). If you somehow stretch out that screw a bit at one end and squash it at the other then as you turn the screw at a consistent rate the movement of the objective lens would speed up or slow down, somewhat smoothing out that response curve.
I hope this is making sense?
If we are talking about lenses without fancy tech behind focus scale to flatten the far end, than it's possible to calculate barrel position for given focus.
closeFocus/focusDistanceThis formula gives you value from 0 (INF) to 1 (closeFocus).