AC90: Why is 24p so bad with this camera?

M

MathewM

Active member
I don't normally shoot in 24p, but I like the option to. However, upon testing the 24p mode on this camera, I'm under the impression something isn't quite right. Under all indoor lighting conditions I've noticed severe smearing and trails when the camera is either zoomed or quickly panned. Say what you will about the need for more careful shooting 24p, my dvx100 never exhibited these issues. Strangely 30p looks just fine, so there must be something going on with the internal processing. Unfortunately, I've put over two hours on the camera so I can't send it back to B&H. I'm concerned that if I send the camera to panasonic that it will be awhile before I hear anything and then the answer will be "the camera appears to be functioning within specs..."
 
Unfortunately, there doesn't seem to be any answers. It would be nice if a professional from Panasonic could chime in. I think I'm going to contact B&H to see if they will accept the return without a restocking fee.
 
Again, there should be no trails or smearing. Can you post a camera-original MTS file so we can at least see if it's something that's happening in your post workflow?
 
Well I think I've deduced it to the gain being applied or a combination of gain and avchd compression. The smearing and trails are not there at 0db and they gradually get worse as I increase the gain. I even see it a bit in 60p, but not nearly as much. I finally tested it outside on a bright day and there are no trails or smearing. Image looks great. If possible it would be nice if other ac90 owners could do similar testing under low lighting. It's really visible in low light, zoomed in on a single object with a flat background. Do some quick pans back and forth and you'll see it. I'll try to post an image or two later.
 
I uploaded a test: http://www.youtube.com/watch?v=BgBA6mEhZ2s

There is very noticeable ghosting with 24p and high gain. It does seem to confirm MAR10s experience as well. This was the first time I've shot with the AC90 at 24p so I have not seen this particular artifact before. I shoot exclusively at 60p and have never seen this even when using high gain.
 
Any thoughts on those samples above Barry? Sorry for calling you out. Just curious as to your opinions on this.
 
Well, that looks like about exactly what I'd expect... nothing to do with 24p, but everything to do with using 30dB of gain and the artifact being a leftover from the noise reduction the camera does to get a decent picture. The AC90 does an incredible job of noise reduction, such that its 30dB footage looks like about 6dB from some other cameras. But there's a price to be paid for that, and that's the potential of this type of ghosting, which is a common artifact from noise reduction.

That's why you don't see it in daylight, because if there's no grain, there's no noise reduction going on. The higher you crank up the gain, the more noise reduction the camera employs, and the more likely you'll see some artifacts.

It's a simple tradeoff, really -- if they hadn't employed the noise reduction, you wouldn't get the ghosting, but you'd get massive swarms of grain. Try an HMC40 on +24dB and you'll know what I mean. So they did it that way; on this camera they tried another way, shrinking the noise aggressively. It results in surprisingly clean images at high gain levels, but ... yep, the tradeoff is the noise-reduction ghosting. Not visible under adequately-lit scenes, not visible in high frame rates, but on the very slowest frame rate (24p) each image is held on the screen longer, so it makes the afterimage that much more noticeable.

I can truthfully say I've never seen this effect before, but my mantra has always been that a video camera feeds on light; starve it of light and you won't like what you see, feed it with light and it will perform magnificently. I don't like to use a lot of gain whenever it can be avoided. I do understand that some people don't have the ability to switch to a more-suitable camera for the purpose, and must make do with the one they've got, and in a very low light situation, the AC90 is not ideal for that. It has a base sensitivity of only about 40 ISO.

So, we're left with the eternal conundrum: we all want it fast, cheap, and good; the manufacturers respond to us by saying "pick any two." So you can have great low light performance and no artifacts (choose a 2/3" camera), or you can have great sensitivity and no motion artifacts (just get a gnat's swarm of grain all over your footage) or you can have great sensitivity and clean images (you just might get some after-image effects). The HMC40 and HMC150 chose option 2, the AC90 chose option 3.

I normally go for option 4 whenever possible: turn on a light. It solves all problems. I know that's not always a viable option for events shooters, sometimes you just absolutely cannot add light, but ... fight for it. Adding a simple light would solve all the grain and motion artifact issues entirely...

There's no magic bullet, no one-camera-to-rule-them-all, and especially not at the price point. That's why it's important to know your camera and know how it performs under your intended usage circumstances. In bright light conditions the AC90 is extraordinarily good. In very dark scenarios, it is obviously going to suffer; it actually performs a lot better than I had anticipated, due to the very effective noise reduction, but this ghosting seems to be the other shoe that drops.
 
I agree that there isn't a perfect camera especially at this price point. The AC90 is actually pretty close to being a great camera except for those 1/4.7" chips. I knew this going in though. Oh well, when I start making more money again doing video I can then upgrade to something better.
 
Mathew said:
I agree that there isn't a perfect camera especially at this price point. The AC90 is actually pretty close to being a great camera except for those 1/4.7" chips. I knew this going in though. Oh well, when I start making more money again doing video I can then upgrade to something better.
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Exactly!

It's not perfect. It seems that so far, there is no perfect camera regardless of price. This camera seems to offer quite a lot for the price though. I look forward to getting mine very soon.
 
R Hudson said:
Exactly!

It's not perfect. It seems that so far, there is no perfect camera regardless of price. This camera seems to offer quite a lot for the price though. I look forward to getting mine very soon.
Click to expand...

I think there are some pretty nice offerings now in the $3500-4,000 range. I just can't afford to spend more than $2K which leaves me either the AC90 or the Canon XA10. I chose the AC90 because of the better user controls and that I have half a dozen DVX batteries laying around. I would love to have seen an HMC upgrade in the $2500 range though.
 
I just wanted to add that the AC90 handles blue channel noise very well. My DVX cams and the Canon Vixia are very noisy in the blue channels when gain is applied.
 
In case this discourages anyone from buying this camera, I would like to add that the ghosting only happens in a very specific set of circumstances which do not normally occur in regular shooting. I've shot around 30 hours so far with the AC90 and have never seen this in "real world" use.
 
The samples linked to show crazy camera movement as well... if that's what you need to do to show the problem, then I wouldn't worry too much. High gain and high noise reduction coupled with too-fast camera movement is asking for trouble.
 
Agreed. And, hey, it's good to know what the limits are, so you know what you can and can't do. But for any reasonable shooting scenario, the AC90 excels at its price point. It would not be my first choice for filming "COPS" at night without a camera light, but with adequate lighting it's a great camera and performs far better than one would expect given its small chip size and affordable price point.
 
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