 |
|
Controlling Exosure on the Canon HV20 |
|
|
| By now you’ve probably heard about the Canon HV20; it’s the pocketcam that’s got the guerilla filmmaker world abuzz because it offers high-def 24p at an unprecedented price point: $1099 retail, with a street price that hovers down somewhere around $900. The HV20 is a marvel, it delivers an amazing amount of bang for the buck. But it’s not all things to all people; it is primarily a consumer palmcorder, not a professional tool. It offers a surprising amount of manual control, including manual white balance, manual audio gain control, and even a zebra display. It offers 24p for a filmlike look, and it even offers a 1/48 shutter speed (desirable to emulate the exposure and motion characteristics of movie film). It even offers a Cine exposure mode. But what it doesn’t offer – in fact, perhaps what its biggest shortcoming is – is true manual exposure control. The HV20 is, at heart, an auto-exposure point-and-shoot
camera. When dealing with exposure on a professional camera you
typically have many parameters that you can control manually. These
include: The HV20 has all of these, but it doesn’t offer manual control over all of them. In fact, you can usually only manually set either Shutter Speed or Aperture, and whichever of those you set, it will set the other three parameters as it sees fit. If you put the camera in Aperture Priority mode (also called “Av”), you can manually dial in the aperture you want to use, but the camera will decide what shutter speed and what level of neutral density filter to employ. This leaves you with the potential that the shutter speed may (and frequently will) change away from the desired 1/48th shutter speed. Or, you could select Shutter Priority mode (also called “Tv”) and force the shutter to always operate at 1/48th; the camera will then change the aperture and neutral density filter as it sees fit, and (worse) it can start putting automatic electronic gain into your image, without you even knowing! And when it adds gain, it can add a lot of gain, up to 27dB of gain. The result is a surprisingly noisy, grainy picture, not at all what you’d want when you set out to capture 24fps 1080 high-def footage! There are two main problems at work here. One is that the camera doesn’t let you control the multiple parameters of exposure, and the other is perhaps even worse: it normally doesn’t tell you what it’s doing. As you shoot, the only exposure feedback you get is to know the shutter speed (if you chose “Tv” mode) or the aperture (if you chose “Av” mode), but it will not tell you what both settings are. And it will never tell you what the electronic gain is set at. So – is all lost? Must we relegate the HV20 to the scrapheap of auto-only camcorders, with a mournful wail of “so close, but no cigar”? Well, not quite so fast. There’s lots of cigar here after all! We just have to take a few steps to work at it. The HV20 doesn’t let you see or manipulate your exact exposure settings, that’s true. But it does allow you to lock your exposure and adjust that exposure using a fixed scale. With some careful control over the parameters (and either a mini-SD memory card, or a bit of record-keeping) it’s possible to determine exactly what the camera is doing, and lock it into a valid exposure range while avoiding the dreaded electronic gain or shutter adjustments. The key to this is the EXP lock function, which is available by pressing the joystick in, and then scrolling down to EXP, then pressing the joystick “up”. This brings up a numerical scale, which can range as far as –11 to +11 (but which varies depending on the prevailing lighting conditions; you might end up with a scale that goes from 0 to +10, or from –10 to 0, for example). But the EXP lock is key; when you lock the exposure it does indeed stay locked. And as you adjust the EXP value up or down, it will adjust the iris, shutter speed, and gain – but it does it in a repeatable, predictable fashion. All we have to do is determine a specific circumstance that yields 1/48 shutter, no electronic gain, and a range of iris settings and we’ll be able to repeat those settings in the field. Now, there’s an easy way to do this, and a hard way. The easy way is to buy a mini-SD memory card and leave it in the HV20’s memory slot at all times. If you have a mini-SD card present (and ONLY if you have a mini-SD card present!) you can get the HV20 to tell you its current aperture and shutter speed, by partially pressing down the “PHOTO” button. This is incredibly handy, especially because it works while you have the EXP LOCK function activated. You can adjust the exposure, and check yourself by partially pressing the PHOTO button; this will let you know what your shutter and iris are at any given point. But what if you don’t have the mini-SD card (or, if you just like doing things the hard way?) Can you still lock the exposure exactly where you want it to be? Yes, and it’s easy once you know how, but it requires an absolutely controlled set of light entering the HV20’s lens. The easiest way to force this circumstance is to completely block the HV20’s lens so that no light enters the camera whatsoever. An external lens cap would be ideal for this purpose, for example. Start out by zooming out to 100% wide angle; you want it as wide as possible so you have a consistent iris to open up to (the further you zoom in, the smaller your maximum-open iris can be; start at maximum wide angle to get the fullest range of iris settings). If you set the camera in Shutter Priority (“Tv”) mode and set the shutter speed to 1/48, and then cap the lens (so that there’s no light entering the camera at all) the autoexposure system will force the camera to select a completely wide-open iris (f/1.8) and maximum electronic gain (27dB). So cap the lens, then lock the exposure, and you’ll see that you have an exposure range of –11 to +0. (Note, this is one of those cases where you don’t get the full –11 to +11 range). The reason is, at “0”, the HV20’s already maxxed out – it’s got the iris wide open and the gain pumped up to a startling 27dB. Finally we have a way to guarantee what our exposure is for all three parameters! But all is not well. Because each tick of the EXP wheel only takes the gain down by 1.5 dB per step, you can’t get all the way down to 0dB, the best you can do is get it down to 12dB of gain. Again, that’s useless. But what if we ignore Shutter Priority mode, and
instead use CINE exposure mode? The CINE mode is a fully automatic
exposure system (so it can dictate what the gain/shutter/iris all
are, you don’t control any of them) but in extreme low light
circumstances it relies more on shutter speed changes than on gain.
For people wanting to make footage that looks like film, changing
the shutter speed is really bad – perhaps worse than adding
gain – so initially this doesn’t sound like such a good
idea. However, the shutter speed changes the exposure so much more
than the 1.5dB of gain does, what ends up happening is that we get
more exposure change over a shorter range, and this is good. So,
if you cap the lens in CINE mode (using both HDV 24P recording and
the CINE exposure mode), and then lock exposure, you’ll be
presented with a range from +3 to –11. The actual settings
the camera chooses look like this: Now, as you look at that table you’ll see that the iris stays constant, it’s always at f/1.8. The gain varies, there’s gain from –5 on up to +3. But the interesting one is the shutter. In almost every table entry the shutter is slower than 1/48, meaning that it’s almost compeletely uninteresting for those of us wanting to make filmlike footage. But look at the entry for –11: there it is! There’s the one we want. It has 1/48 shutter, a known iris value, and no gain! Finally we have a known, locked exposure that delivers the characteristics we want from the camera: no gain, with a fixed 1/48 shutter, using the CINE exposure system. This is exactly what we want. Or is it? The problem is, the camera’s locked at wide open iris, and there’s no range left for you to adjust that. How will you compensate for exposure? I guess you could go out and buy a dozen neutral density filters, but that seems really clumsy. Sure would be nice if we could lock the exposure such that we have more of a free range of exposure settings rather than having only the “-11” setting useful to us… Is that possible? Sure, but it’ll take some work on your part. What you have to do is have a fixed level of light coming into the camera’s lens, a completely repeatable source of light that fully fills the frame so that the camera will always set its autoexposure to exactly the same settings every time it sees that. My solution was to put an all-white picture on my cell phone, so I can bring up that white picture anywhere in the field. I then cover the HV20’s lens with the all-white picture, and my cell phone becomes a white light source that’s not too bright, but bright enough that the HV20’s exposure will be inherently stopped down some. When doing that, here’s the range of exposures I get (again, this is using CINE exposure, with the HV20’s lens at 100% wide angle, and filling the HV20’s lens with an all-white light source being emitted from a Nokia N93 cell phone): EXP Shutter Iris Gain Now how does that strike your fancy? We have a fully usable range of exposures all the way from –11 up to +6, all with a fixed 1/48 shutter and a set 0dB of gain. Using the “cell phone trick”, I know that I’m safe setting my EXP anywhere from +6 down to –11 and that there won’t be any artificial gain added in the picture, and my exposure time will always be 1/48th of a second. What happens if I go higher than +6 (i.e., +7 to +11)? In those cases the shutter speed starts changing, going as slow as 1/24, so you don’t really want to do that. Notice anything curious about that chart though? Like, at the bottom – why do the numbers stay the same for –7 all the way to –11? Is the exposure changing when I change the EXP dial from –7 to –11? The answer is yes, it changes quite a bit, but the parameters don’t change. So what’s actually happening? The camera has what appears to be a rotating variable-density neutral density filter that it brings into play when the light gets too bright. If you watch deep in the lens as you change that EXP setting, you can see an ND filter rotating behind the iris. It’s only a guess, but I’m guessing that the neutral density filter in the HV20 is not a fixed filter, but that it’s a wheel that gets progressively darker so the HV20 can select the amount of ND it wants to use fairly seamlessly. So yes, the exposure is changing down at the bottom end of the scale, but it’s not due to iris or shutter changes, it’s actually a rotating neutral density filter that’s shifting to stronger and stronger positions. Okay, one other question: what’s with the repeating settings for +1 down to –6? Assuming the exposure is changing, why does the iris repeat? It’s not due to the neutral density filter (that doesn’t come into play until about –8) so what’s happening? Since the exposure’s actually changing (we can see that –4 is darker than –3, even though the iris still reads 4.0 in both cases) then what’s happening? The iris is actually changing, but I believe that the camera’s numbering system doesn’t have a way to accurately reflect what the true iris is for those settings. I would suspect that the “true” iris for +0 would be f/3.1, the “true” iris for –2 would be f/3.7, the “true iris for –4 would be f/4.4, and the “true” iris for –6 would be f/5.2. The exposure does change, but the display doesn’t, so my guess is that the internal numbering system is just “rounding off” the display to the nearest whole stop for those settings. Curiously it does have the half-stop increments listed to display the changes from +2 to +6, so I don’t understand why it wouldn’t have the half-stop increments for +1 down to –6. So now we’ve arrived at a usable solution that gives us a wide range of exposure possibilities, from wide-open 0dB at 1/48 on down to f/5.6 with even some neutral density filter kicking in. It’s still not perfect, you might find that even at –11 a bright daylight scene will still be overexposed. You could compensate for that by using external neutral density filters if you want to be sure. Or, you could just let CINE exposure try to select a proper exposure. I’ve found that CINE really tries hard to stick with 1/48 shutter speed and zero gain. It’ll bring the iris down to f/8 and the neutral density filters up to full strength in bright conditions. You’re PROBABLY okay to just use auto exposure in bright exteriors, and you can use the “cell phone trick” or the “lens cap trick” to set a defined 0dB & 1/48th exposure indoors. For those who want to experiment on their own, how did I find this data? It’s true that the camera won’t tell you what the iris or the shutter is while shooting, and that’s really unfortunate. However, it will tell you both of them when you play the footage back. Turn on the Data Code display in playback, and the lower right corner of the screen will show you the iris and shutter speed that you shot at. So I recorded each of the settings on the EXP scale, from –11 to +11, and in playback I was able to use the Data Code to determine what the shutter and iris were. The gain, however, is a different story. The HV20 just plain won’t display the gain information; if all you have access to is an HV20, there’s not any easy way to find out what the gain was (if any) in your clips. However, if you have access to a Sony HDV camcorder (or perhaps a Canon XHA1 would do this as well?) you can play the footage back in the larger camcorder and turn on its data code display, and the bigger camcorder will actually display what the gain was. I was able to map out the gain values by playing the footage back in a Sony FX1. Do you really need to do this though? Probably not; I mean, what’s important to us is that there be no gain, right? You can know when you’ve crossed the threshold from 0dB gain into where the camera starts adding gain rather simply; as you play back the footage and watch the shutter and iris display, if you see the iris wide open (i.e., f/1.8) and then the image gets brighter, but the shutter speed doesn’t change, then you know that you’ve just seen some gain added into the picture. If the shutter changes (from 1/48 to 1/30, for example) that will make the picture brighter. If the iris changes (from f/2.0 to f/1.8, for example) that will also make the picture brighter. But if neither of those changed, and the picture got brighter anyway? That means the gain kicked in. If your iris is at f/1.8, and your shutter at 1/48, and then the picture gets brighter, the only way that happens is with gain, so you’ll know you’ve just exceeded the threshold of useful exposure range. The reason you need to know how to calculate these ranges on your own is because the “cell phone trick” only works for my particular phone; yours may be brighter or dimmer, and that will change the results in the chart for your particular circumstances. The “lens cap trick” will work for all HV20s regardless, but the “cell phone trick” will only work for the same cell phones. If you want to employ the “cell phone trick” for your own HV20, you’ll have to put a white picture on your cell phone screen and calculate the graph like I did, to make it relevant for your particular phone. An ideal accessory item would be a lighted lens cap, something that would cap the lens and deliver a repeatable (and appropriate) amount of light. I can simulate that with the “cell phone trick,” but it’d be more professional and repeatable if we could find a fixed light source to use (and, if it was a tad brighter, we could use the full EXP range instead of my current system that allows only up to +6). So in summary: yes you can control the HV20’s exposure manually, and dial it in to get exactly the exposure parameters you want. You can force it to 1/48th shutter and 0dB of gain in the CINE exposure mode. The easiest way is to simply install a mini-SD memory card, and use the PHOTO button to track what your exposure is. If you can get the shutter at 1/48 and the iris at f/2.0 or larger (i.e., 2.4, 2.8, 3.2, etc) then you’re assuring yourself of no gain, and cine-like exposure characteristics. If you don’t have access to a mini-SD card, you can still get there, you just have to go through the steps I outlined above. Happy shooting!
|