14 ms·

The mention that TV cameras are Parfocal is really key: https://en.wikipedia.org/wiki/Parfocal_lens https://en.wikipedia.org/wiki/Parfocal_lens Which means tha

by zdw 4y ago

The mention that TV cameras are Parfocal is really key: https://en.wikipedia.org/wiki/Parfocal_lens https://en.wikipedia.org/wiki/Parfocal_lens Which means that you can zoom with it without having to refocus, which is not the case with less expensive lenses.

Retr0id 4y ago
Possibly silly question: with digital controls, why can't any lens become parfocal? Can't the camera/lens just do the math to figure out how to adjust the focus to accommodate any zoom level change, in tandem with the zoom change itself?
- charrondev 4y ago
  It seems feasible. I’ve noticed for example my Canon R5 autofocus in videos can pretty consistently hold focus while zooming. One thing to keep in mind though is in big production sets they are not using automatic focus at all, and cinematic lenses don’t tend to have focusing motors. (Also a focusing motor for a 100lb lens would likely be very noisy).
  - dannyw 4y ago
    Yep, Canon R-series is parfocal through clever software.
- chabad360 4y ago
  In theory it could, but that would require the lens to store the parameters for that function(which I don't believe existing lenses do). And additionally it would require a lens with a focus motor and a zoom motor (or even just feedback, which most don't have).
- kayfox 4y ago
  Most modern box lenses do exactly this, servos in the lens move the elements around to achieve the desired zoom and focus. Older box lenses use a convoluted set of gears to achieve the same thing. Some handheld ENG lenses for shoulder carried cameras get close to parfocal by making the lens elements travel in a complex way, but its not as perfect as the box lenses get.
- Gordonjcp 4y ago
  That's actually how a lot of cameras work, particularly smaller consumer and prosumer ones with integrated lenses. The zoom position and focus position is worked out by moving the front and back elements of the lens, with a bit of adjustment so they track together properly. I actually have a nice old Sony V1E where the back positioner has failed (or become disconnected, possibly) so it never really looks right, and it would be too expensive to repair. It works great as a DV playback deck, which seems like a bit of a waste I guess, but repairing it would be about the same price as buying a working one.
- kuschku 4y ago
  They often are! Even the relatively cheap (500$) kit lens of the FS5 uses exactly that to stay parfocal and prevent focus breathing. It's not perfect — there's always a lag of about 6 frames during fast zooms — but it's "good enough" for most situations (which means it's not ready for broadcast).
ekianjo 4y ago
Sony does autofocus at the sensor level.
- anotherboffin 4y ago
  Last I heard they did OIS (stabilization) at the sensor level, not AF. Would be curious if you had a source.
  - ekianjo 4y ago
    you are correct, I meant stabilization.

by zdw 4y ago

Retr0id 4y ago

Possibly silly question: with digital controls, why can't any lens become parfocal? Can't the camera/lens just do the math to figure out how to adjust the focus to accommodate any zoom level change, in tandem with the zoom change itself?

charrondev 4y ago
It seems feasible. I’ve noticed for example my Canon R5 autofocus in videos can pretty consistently hold focus while zooming. One thing to keep in mind though is in big production sets they are not using automatic focus at all, and cinematic lenses don’t tend to have focusing motors. (Also a focusing motor for a 100lb lens would likely be very noisy).
- dannyw 4y ago
  Yep, Canon R-series is parfocal through clever software.
chabad360 4y ago
In theory it could, but that would require the lens to store the parameters for that function(which I don't believe existing lenses do). And additionally it would require a lens with a focus motor and a zoom motor (or even just feedback, which most don't have).
kayfox 4y ago
Most modern box lenses do exactly this, servos in the lens move the elements around to achieve the desired zoom and focus. Older box lenses use a convoluted set of gears to achieve the same thing. Some handheld ENG lenses for shoulder carried cameras get close to parfocal by making the lens elements travel in a complex way, but its not as perfect as the box lenses get.
Gordonjcp 4y ago
That's actually how a lot of cameras work, particularly smaller consumer and prosumer ones with integrated lenses. The zoom position and focus position is worked out by moving the front and back elements of the lens, with a bit of adjustment so they track together properly. I actually have a nice old Sony V1E where the back positioner has failed (or become disconnected, possibly) so it never really looks right, and it would be too expensive to repair. It works great as a DV playback deck, which seems like a bit of a waste I guess, but repairing it would be about the same price as buying a working one.
kuschku 4y ago
They often are! Even the relatively cheap (500$) kit lens of the FS5 uses exactly that to stay parfocal and prevent focus breathing. It's not perfect — there's always a lag of about 6 frames during fast zooms — but it's "good enough" for most situations (which means it's not ready for broadcast).

ekianjo 4y ago

Sony does autofocus at the sensor level.

anotherboffin 4y ago
Last I heard they did OIS (stabilization) at the sensor level, not AF. Would be curious if you had a source.
- ekianjo 4y ago
  you are correct, I meant stabilization.