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Old November 24th, 2013, 04:31 PM   #1
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AC90 Question: Does cam have 'aperture priority' mode?

Does the AC90 have aperture priority shooting? As in, can I lock the aperture at, say, 2.8, and have the camera ride everything else automatically?

Or does locking the aperture also lock the shutter? (Which is what my SDT750 does.)

Thanks.
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Old November 25th, 2013, 08:32 AM   #2
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Re: AC90 Question: Does cam have 'aperture priority' mode?

No, it locks the shutter as well. Also it deploys automatic variable ND internally at f/2.8 to avoid diffraction from stopping down. In other words, from f/2.8 to f/6.4 the iris is not actually closing, but adding ND. Past that it resumes closing. In daylight, the camera benefits from adding external ND, 0.6 is ideal. Then just let the cam choose the optimum combination of iris and shutter for sharpness, contrast and smooth motion. You don't get much creative control of dof with a tiny sensor camera like this by overriding the automatic exposure settings.
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Old November 26th, 2013, 05:21 PM   #3
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Re: AC90 Question: Does cam have 'aperture priority' mode?

That's extremely helpful (and apparently not in the 'Advanced' manual I downloaded.)

One workaround, I suppose, would be to always set my focus range with the lens wide open (the camera's locked, anyway), before toggling over to auto. At least then I wouldn't get any surprise OOF areas when the light shifts.

Of course, I could use the same technique with the x920, which for my purposes might be enough, at what right now is a startlingly low price. (I'd really like to stay with the 3MOS system.)

Any thoughts, in terms of image quality under good conditions, between the x920 and the AC90? Or are the AC90's optics simply leagues ahead? Thanks again.
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Old November 26th, 2013, 07:21 PM   #4
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Re: AC90 Question: Does cam have 'aperture priority' mode?

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Originally Posted by Kris
That's extremely helpful (and apparently not in the 'Advanced' manual I downloaded.)

One workaround, I suppose, would be to always set my focus range with the lens wide open (the camera's locked, anyway), before toggling over to auto. At least then I wouldn't get any surprise OOF areas when the light shifts.
Only because I've used this camera extensively, I'd recommend not adopting that approach. For one thing, ND filters are sometimes suspected of causing focus shifts, and you don't have complete control over the internal ND filter on the AC90. You will also find it's really hard to focus manually with the AC90 because the dof is so deep. Not only that, I'd avoid zooming in for focus and then pulling back. The non-parfocal characteristic of the servo lens makes that a problem. The best bet is to just lock your focus down at whatever zoom range you're at by switching focus from auto to manual just before recording, and if you're going to be zooming during the shot, leave the auto focus on. It's seldom off by very much, and very fast responding owing to the small sensor, deep dof.

Quote:
Of course, I could use the same technique with the x920, which for my purposes might be enough, at what right now is a startlingly low price. (I'd really like to stay with the 3MOS system.)

Any thoughts, in terms of image quality under good conditions, between the x920 and the AC90? Or are the AC90's optics simply leagues ahead? Thanks again.
I doubt that the optics on the AC90 are "leagues ahead" if at all. They might even be identical, but note the x920 is Leica branded and the AC90 makes no similar claim. The 3mos sensors are not the same however. How much difference that makes I can't say, not having used the x920. I suspect the AC90 is going to give you more creative control over the look with multiple gamma curves to choose from, and probably ship with a flatter, more gradable default gamma/color scheme.

The main point I'd emphasize about the AC90 that has generally worked best for me, is to use the 0.6 ND filter in daylight, and then choose AUTO, (not iA). The camera will choose the appropriate shutter and iris for the best exposure, sharpness and contrast sort of like "programmed automation" on a Canon, and to trust the auto-focus. The external 0.6 ND will slow it down just enough to make sure all your outdoor shots in strong light will retain smooth motion without resorting to stopping-down into the diffraction limited range. The strength of small format cameras like the AC90 is in the automation, made possible by the small sensor, the speed of point and shoot recording, fast and accurate white balance and focus, smooth response to exposure and changing light, getting the shot when there isn't much time to plan for it and having it return something usable.
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Old November 26th, 2013, 07:36 PM   #5
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Re: AC90 Question: Does cam have 'aperture priority' mode?

Also note, everything I've said is intended for shooting at 1080/60p, which transfers very well to YouTube, If you are looking for 24p, you should probably look elsewhere to a cinema camera.
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Old December 13th, 2013, 04:58 PM   #6
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Re: AC90 Question: Does cam have 'aperture priority' mode?

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Originally Posted by Tom Roper View Post
Also it deploys automatic variable ND internally at f/2.8 to avoid diffraction from stopping down. In other words, from f/2.8 to f/6.4 the iris is not actually closing, but adding ND. Past that it resumes closing.
Wow. I had no idea. I checked my consumer-grade camera to see if a similar mechanism is at play, and shot a few stills (here animated) of the thing in operation:


Kind of scary, but now that I know what's going on I'll know what to expect.

A bigger problem is that 4-blade diaphragm, which creates weird linear patterns in out-of-focus highlights...for me that's shiny foliage and bright sky peeking through treetops. Canon touts an 8-bladed iris in their 60p enabled G30, but its price hasn't budged.

Quote:
Originally Posted by Tom Roper View Post
You don't get much creative control of dof with a tiny sensor camera like this by overriding the automatic exposure settings.
I'm starting to understand that, And while the AC90's gamma adjustments would be useful, for my limited applications the x920 might be enough.
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