I'm working on a project to create, essentially, a servo-powered zoom lens, using a (D)SLR zoom, a zoom ring gear, and a stepper motor. After mounting the zoom ring gear, I'll attach a pinion gear to the stepper motor shaft to control the motion of the zoom ring.
ENG/cine-style servo lenses that do this by design usually feature:
- Parfocal optics. This means that the lens holds focus through the entire zoom range; a videographer can zoom into a subject, focus, and then zoom back out without needing to adjust focus. Most modern (D)SLR lenses aren't parfocal (they're varifocal) because they don't need to be (autofocus and still photography obviate the need to hold focus), and that makes them simpler, cheaper, and lighter.
- Linear zoom throw. As you move the zoom ring on many (D)SLR lenses, the field of view doesn't change linearly. Through some focal length ranges, the field of view changes at a different rate than it does through others. A linear zoom throw allows for smooth, consistent zooming that doesn't draw attention to itself.
- Constant aperture. The lens doesn't necessarily need to have a fixed aperture, but it needs to hold a constant aperture through the zoom range so that the image doesn't darken while zooming in.
- High price tag. This is mainly why I'm working on an alternative solution, though I also prefer the additional control that I'll have over a stepper motor in experimental shoots.
With that in mind, I want to find a (D)SLR lens to act as a budget alternative to a servo lens. It would need to have:
- parfocal optics
- linear zoom throw
- constant aperture
- full frame 35mm coverage
- a mount that can be adapted to Sony E-Mount (on an a7S) - fortunately, most mounts can
- the longer the zoom range, the better (ideally, at least a 2×)
- affordable price
I define "affordable" as somewhere in the ballpark of $300-1800 for a new or good used copy. Below that, it's probably not of very good quality; much above that, and I might as well just buy this and call it a day:
Weight, brand and speed aren't as important to me, as long as it's not of poor quality. I also don't need auto aperture or focus.
Does such a lens exist, or is this a pipe dream?
I've found various lists of (D)SLR lenses that are supposedly parfocal, but I don't know if they meet or approach these other criteria; I can (and will) research everything but the linearity of the zoom throw, since that's not at all a commonly-listed specification. I guess I'm mainly interested to know if anyone has experience with any lens(es) that they know to be parfocal, and can confirm that they have a linear zoom throw.