HMC40 and low light performance

[droliff]

Hi all,

I recently purchased an HMC40 and am mostly quite pleased with it thus far.
I did a lot of research before investing and did hear that the 40 does not perform real well in low light conditions.
Now that I've had the camera out on a few gigs, i can see that the 'low light' performance is a bit of an issue in some cases (available light in some venues).
Is the poor performance in low light due to the cameras 1/4" capture chip size?
Is the HMC150, which has 1/3" chips, a better performer in less than ideal light conditions?

Thanks for any input.

Dennis

Dennis Roliff Photography

[GrahamH]

Yes and yes.

http://www.dvxuser.com/V6/showthread.php?t=182865

Sensitivity -- okay, so here's the bad news. We knew it wasn't going to be nearly as sensitive as the HMC150; when you cram 4x as many pixels onto a smaller chip, you can expect a drop in sensitivity. Mathematically, I would have guessed about 3 stops' less sensitive. The pixels are 4x as many, meaning 1/4 the size, meaning 1/4 as sensitive (2 stops), but also the chips are smaller (1/4" instead of 1/3") making the pixels even smaller, so... yeah, 3 stops. When I observed the two cameras side by side, yeah, looked about 3 stops different. When I shot a grayscale chart, lighting for best response, with the HMC40 at f/2.8, the HMC150 needed to be at f/8 to deliver the same brightness level. So... yep. 3 stops. The HMC150 is about 3 stops faster than the HMC40. Which, when rating the HMC150 at 500 ISO, that puts the HMC40 at about 64 ISO. Not exactly a speed king. Plenty fine for outdoors, but indoors you're going to either need lots of light, or lots of gain. Fortunately the HMC40 provides bucketloads of gain, up to +34dB! That's nearly four stops' worth of gain. That gets you your sensitivity back, at the expense of noise.

[v3rlon]

The short answer: yes to both questions.

Think of the sensor like a chess board with each sqare representing one pixel. For each square, a given amount of light is available. Just to keep it simple, we'll call that one light unit.

So each of 64 squares on our standard sized chess board is getting one unit of light. If we used a smaller chess board, then each sqaure would get less light because the area of the sqare in part multiplied by the length of time the square is exposed determines how much light you get.

Also, if you take a board that is the same size as our original board, and double the number of squares on it, then each square will get less light because they will be smaller than the sqaures on the original. Again, thinking of the chess board, the only way to add squares without making the board bigger is to make the squares smaller. The HMC40 has high resolution sensors (lots more squares than needed for 1080p so it can take 10MP stills) that are small (1/4") so its lowlight performance is lackluster.

So, for a given technology, smaller sensors tend to have worse lowlight performance.

The only caveat there is "for a given technology." In the same way that they can make computer chips use less and less power, they can also make them use less and less light as they find ways to tweak the process. So, cameras 5 years from now will not compare evenly to cameras of today. They will reach a point where laws of physics get in the way, but we still have some room to work with yet.

Does that all make sense?

[Robert M Wright]

Obviously, the 150 is going to outperform the 40 in dim lighting, but the difference is mitigated a bit by cleaner gain with the 40. The HMC40 is the first video camera I've ever handled that I'm comfortable cranking the gain up to 12dB.

[Bic Parker]

Hi all,

I recently purchased an HMC40 and am mostly quite pleased with it thus far.
I did a lot of research before investing and did hear that the 40 does not perform real well in low light conditions.
Now that I've had the camera out on a few gigs, i can see that the 'low light' performance is a bit of an issue in some cases (available light in some venues).
Is the poor performance in low light due to the cameras 1/4" capture chip size?
Is the HMC150, which has 1/3" chips, a better performer in less than ideal light conditions?

Thanks for any input.

Dennis

Dennis Roliff Photography

The size of the chip is irrelevant. But the size of the pixels sort of has a bit to do with the light sensitivity, but only compared with similar chips. The actual answer is a bit more involved.

So, first thing, comparing a CMOS chip (in the HMC40) to a CCD chip (in the HMC150) is not a very good comparison because of how the electronics are designed. Essentially, because of the onboard electronics in the CMOS chip, you are going to inherently have more electronic noise, which will, in turn reduce the signal to noise ratio and reduce the overall light sensitivity compared to a CCD chip with similar pixel dimensions. A CCD can be more sensitive than a CMOS and even the comparative pixel dimensions may be irrelevant within a range.

The pixel size is important for many reasons, but in the case of light sensitivity, it is a factor in the well depth of a pixel (i.e., how many photons can a pixel potentially read at a given point in time). A larger pixel can have a potentially larger well depth (emphasis on "potentially").

But there are other factors go hand in hand to define the overall light sensitivity. The combined factors are the chip's characteristics in pixel well depth, signal to noise ratio, dark current, and quantum efficiency. These factors together will give an accurate picture of how well the chip (and ultimately, the camera) does in low light.

I did the math in another post, estimating the pixel size (for image scale reasons, not light sensitivity) for the HMC40 sensors, since I couldn't find any published information (I put the math here again at the bottom of this post). It basically works out that each pixel is about 3.5 microns across (that may be the average dimension, since they could be rectangular instead of square). That is not just small, that is tiny for imaging chips (ultra sensitive chips may have pixels as large as 25-40 microns, by comparison).

Now 3.5 microns, though, isn't completely bad since each color has a pixel. So think of 3.5x3 = 10.5 microns, which means that you have a moderate light gathering potential. It is probably somewhat less than the light gathering potential of a 10.5 micron pixel, though, probably closer to a 9.5 micron pixel.

The kicker, though, that brings this back is the CMOS architecture. This essentially means that dark current is higher (that is basically the inherent electronic noise in the pixel when the cap is on), the quantum efficiency is lower (this is a percentage of photons detected to those received), and the signal to noise ratio is lower than an identical pixel on a CCD chip.

When you kick up the gain on a camera, you are going to be boosting the overall data from the chip, which is going to include noise, which why noise appears. Some of the more sophisticated designs these days, mathematically eliminate some of the noise (actually it is a statistical elimination from the tails of the histogram curves, usually) with the increase in gain, which is what I suspect is going on here with the HMC40 (which is really cool, by the way).

Here is the estimated calculation, by the way, of the pixel size in the HMC40:

Each sensor has ~3.05 million pixels on a ~1/4.1" square (I love converting metric to english and back!). So 1,747 (the square root of 3.05 million rounded up for conservative measure) divided into 6.1722 mm gives you ~0.0035mm (3.5 microns), which sounds about right for a chip that size with that many pixels (interestingly enough, this also gives a ballpark explanation of the well depth of the pixels, which goes to in part, not including dark current and quantum efficiency, its light sensitivity).

[GrahamH]

The size of the chip is irrelevant.

.... I did the math in another post, estimating the pixel size ... It basically works out that each pixel is about 3.5 microns across ... that is not just small, that is tiny.

.... 3.5 microns ... sounds about right for a chip that size

Interesting post - thanks for that. I have to say, though, that your second two points above contradict your first.

[Barry_Green]

Not really. The individual pixel size is far more important than the chip size. The chip size does restrict how big each pixel can be, of course; the more real estate you have to work with (for a given pixel count) the more surface area can be allocated to each pixel.

I think Bic's summation is pretty excellent, with the only real issue being the math on the chip size. Because a 1/4" chip is nowhere near 1/4" in size, in any dimension, and it isn't anywhere near to being 1/4" square. We don't know the exact dimensions of the HMC40's chip, but it's more likely to be about 3.7mm x 2.1mm (for 16x9). I think the HMC40's chip is actually 3:2 because of its still capabilities, but I don't know what the dimensions would be for that aspect ratio.

[GrahamH]

Not really. The individual pixel size is far more important than the chip size. The chip size does restrict how big each pixel can be, of course

Fine. I don't want to get into an argument about choice of words. Chip size being 'less important' sure, but I don't agree it's "irrelevant".

[BobDiaz]

This is an interesting side test. I took a video shot at 1920x1080/30p and reduced it down to 640x480. In this test, 9 pixels combine to make 1 pixel. In theory, this should help to reduce the impact of noise in the video.

No question that it helps, but it's subjective as to how much more you can push things. Some may feel that +18 dB should be the upper limit, others may find that +24 dB still holds together very well, and a few may be willing to push to +30 dB.

In order to avoid any extra compression noise getting in the way, I compressed this video at a very high bit rate, thus the large size (352MB). You can download the fie and judge for yourself what the upper limit should be when converting to SD resolution.

You can download the video here and tell me what you think:

http://vimeo.com/9991641


Bob Diaz

[Barry_Green]

Fine. I don't want to get into an argument about choice of words. Chip size being 'less important' sure, but I don't agree it's "irrelevant".

Well, I guess the point is -- if someone says "my camera has an S35 sensor, so it has better low-light performance than your 1/3" camera" -- that isn't necessarily true. So that's what I think he was getting at. You can have a 1/3" camera that's way better in low light than a "full frame" camera, depending on how they build it.

I guess in other words, it's not a determining factor. But, all other things being equal, the larger chip does provide the *potential* for better low light performance, depending on how the engineers utilize the additional space.