Pre-focusing on the A7s

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deltoidjohn

Veteran
Hi all

Just a quick question about auto focusing on the A7s with the 28-70 kit lens:

I'd like to get an A7s to accompany my FS700, and one of the big uses will be when filming in boats and on the surface (because it's much cheaper than buying an FS700 housing).

I'll likely get one of the Meikon housings which have a fixed port and control for zoom but not for focus. So to get any kind of focus control, I'll need to use push-AF of some sort. Obviously the touchscreen is inaccessible so I can't use that - so what other options does the A7s have to preset/lock AF? Does a half shutter press do the trick? Or is there a dedicated push-AF button? I'd rather not resort entirely to continuous AF as it's never predictable, especially when a water droplet splashes onto the lens, so I just need to know if there is a way to set and lock single-AF prior to recording?
 
I am not sure about the A7S, but with the A7RII, it depends on a few things as the focus modes are limited in manual movie mode. If you shoot video in the other modes, such as Aperture Priority (for example), you can choose a mode like DMF and half-pressing the shutter button will auto-focus for you (and only when you press it).

On a side note, the Sony cameras are extremely programmable. So many buttons (I believe 10-12 in total on the A7RII) that can be set to many commands/features.

I don't know if your housing will affect all of the functionality of the A7S, but a very useful tip is to set one of your buttons to toggle auto/manual spot focus.

I have my finger on the button and constantly use it when reframing something or zooming. I press it, auto-focus quickly nails focus depending where I have the box (usually in the center), and I press the button again to turn it off quickly. This allows you to shoot in manual movie mode and works better than using the shutter button..

I think the A7S would be very similar as other Sony cameras are.
 
You can set the A7S to manual focus and press the AF/MF to do a temporal auto focus kinda like the push auto on the FS700.

the A7S doesn't have a touch screen btw..,
 
Thanks for the info. From what I've read the housing I'm looking at has access to all buttons on the camera body, as well as one geared knob to fit the zoom ring on the 28-70.

Your workaround sounds like something I could work with, especially as it'll be used outdoors in bright light with small apertures and deep DOF, so I'll often be able to set focus once and forget about it.
 
Anybody got experience with the 28-70 lens on the A7s at all? Given the nature of what I'm shooting, I'm thinking of maybe just setting the lens to the hyperfocal point before closing the housing up. Of course this will only work if (1) The lens will remember what focal length it was set to if the camera is switched off and back on again, and (2) the lens will hold focus reasonably well when zooming (doesn't have to be parfocal, because If I set the hyperfocul distance for 70mm @ f/16, zooming out should bring more things into focus. The important thing is if I zoom out and then back in, will it return to the exact same focal distance).

FYI I'm shooting surfers from a PWC, with a subject distance ranging from about 30m away to 5m away - which seems to fall pretty well within the hyperfocal range of a 70mm lens @ f/16, from the tables I've referenced.
 
Don't know about the 28-70 but if I remember correctly, the 18-105 reverts to the widest setting after camera is shut down.
 
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