HMC-150 / HMC-151E noise problems - read this

[Justyn]

this cam is like the HVX. You have to push it to get the most out of it. It loves light... I also don't use the Dynamic range..


I have an eye for what I know will resolve and what won't. For sure a blown out image is like armagedeon to this.. but something with a bit of blooming looks killer

 

[Kit_L]

Nice explanation, Kit_L, but I think you're wrong about the 18% reflectance and its relation to caucasian skin. The average reflectance of caucasian skin is not 18% - its higher. As far as I know the 18% reflectance is related to the zone system - the system that every cinematographer should be familiar with. Its aim is to allow for thinking about various elements of the frame in terms of exposure and setting the exposure (and film development) properly. The dynamic range has been split into 11 zones (0 to 10). Zone 10 corresponds to pure white, zone 0 represents black. Each zone differs from each other by one f-stop. This is where 18% is derived from - if zone 10 (pure white) is 100%, the lower zones will have the following reflectances (obtained by dividing by 1.4142 equivalent to stopping down by one f-stop):
zone 10 - 100%
zone 9 - 70.71%
zone 8 - 50%
zone 7 - 35.35%
zone 6 - 25%
zone 5 - 17.68% (rounded to 18% for convenience)
So, the 18% is the reflectance of the 5 (middle) zone (middle gray for b/w photography).

The caucasian skin reflectance is closer to 25%. As a result, in a properly exposed photograph the caucasian skin will fall into zone 6. Of course it is up to the cinematographer to decide what 'properly exposed' really means - you might want the caucasian skin to fall in zone 2 for backlight silhouette effect with some skin detail. The zone system gives you an easy hint on setting your f-stop for such effect (you will have to underexpose by 3 f-stops).

I once was told that 18% reflectance is the average reflectance of the world. Strange statement to me. If I lived in an average world I wouldn't like to be a cinematographer, I guess

Even though the zone system is of great use for videographers we should rely more on the vectroscope (ASA rating becomes fuzzy term for the video camera and setting the exposure with incident light meter is tricky - the meter is still great for setting light ratios, though).

Personally, I don't like using the 70-80% zebras for setting exposure. How they relate to the way the footage looks depends too much on the gamma curve. I prefer to use the 100-105% zebra which indicates the limit of my dynamic range. This way I can decide which areas of the frame will be clipped. If I don't need any detail on f.ex. bright window I clip - no problem to me at all. But I want don't want to loose the information of the essential elements of the frame. If I have this information I can always adjust the footage in post. I only use 70-80% zebra as a guideline for checking if I pump enough light to the scene so that the faces are properly lit. But I prefer relying on 105% zebra for setting the exposure... Just my 2 cents...

Trez, I agree completely re. the *real* average value of Caucasion skin; but I was quoting the Kodak line (their scientists in Rochester many years ago made that determination; the 18% grey card was one of the results). If you Google this, there are a number of sites that present this explanation. And I was trying to give a not-too-technical background for people trying to use the Zebra bars for exposure.

IMHO, there is no precise figure for skin (including the Zone system, which I am also familiar with); for example, in Australia now I have a dark tan on my face from working on the vegetable garden; in winter, my skin will reflect much more light. So, if I expose the skin correctly (to make both reflectance values the same), both will look very similar in brightness—and that is not what we would want (we want darker skin to look darker). This is another use of the 18% card: to get a reference point, from which we might open up or stop down a little to get the look we want.

And your point about gamma and its effects is definitely relevant too (as is black stretch, etc.) but the effective use of these modifications of the sensor's response depends crucially on a fundamental understanding of exposure, I believe.

Personally, I use a combination of the methods I described, relying most of all on a calibrated monitor. cheers, kl

 

[trez]

Trez, I agree completely re. the *real* average value of Caucasion skin; but I was quoting the Kodak line (their scientists in Rochester many years ago made that determination). If you Google this, there are a number of sites that present this explanation.

Are you sure they determined the reflectance of the Caucasian skin? I was sure they attempted to average the reflectance of 'typical photo' ('averaging the world's reflectance')

And I was trying to give a not-too-technical background for people trying to use the Zebra bars for exposure.

Sure, your explanation is very clear!

IMHO, there is no precise figure for skin (including the Zone system, which I am also familiar with); for example, in Australia now I have a dark tan on my face from working on the vegetable garden; in winter, my skin will reflect much more light.

Sure there's variation. But the Caucasian skin reflectance factor is said to be quite reliable for setting exposure as compared to other things you might find in the real world (grey card do not belong to the real world in this context . Many books on cinematography say you this is just about the only thing you can use for setting exposure if you only have reflected light meter (no incident light meter)...
Wish I had vegetable garden and lived in Australia, by the way... Here people's skin reflectance is more like 90% during winter (good proof there's variation

Personally, I use a combination of the methods I described, relying most of all on a calibrated monitor. cheers, kl

Sure the monitor is good. But if you don't want to carry it the grey card and the vectroscope do the job pretty well too...
I still prefer to use the 105% zebra for most shots, though - quick and easy and I can adjust the footage in post when needed. The vectroscope is also useful - for checking whether the entire dynamic range is used wisely - sometimes looking at it reveals that I have to clip more to get enough shadow detail I need for a shot... Or pump more light

 

[Mark Von Lanken]

Unfortunately not available in 24p/25p on my HMC151

Hi trez,

I know, it's a real bummer that it's only available in 60i and 60p.

 

[KeithAndrews.TV]

I'm very surprised at how much interest there is in using zebra bars in general. Like many tools the camera operator has at their disposal, zebra bars are certainly one of them, but I personally find them annoying on screen. Maybe use them here and there just to check exposure, but having a built in waveform monitor in a camera at this price point really makes zebra bars that much less attactive. I understand zebra bars are more discreet on screen while you are shooting an event perhaps, but they are also covering detail in the scene that you may need to analyze while shooting.

I have always suggested that the operator calibrate their monitor, or onboard lcd, to bars generated by the camera and then use your eyes to judge the scene. WYSIWYG. This has worked very well for me during my five years as a news videographer looking through a black and white viewfinder on a DVCPRO rig, and for the past 10 years I have worked professionally in the video production industry shooting commercials and corporate videos.

In conclusion, zebra bars, pointless, definitely not, but absolutely necessary, well I guess that depends on what you are comfortable with. Remember, when in doubt, expose for the subject, and that's a big, fat period.

 

[Barry_Green]

Problem is, using just the LCD to judge exposure is quite perilous. These cameras' LCD panels have such a narrow angle of view that if you rotate the LCD 10 degrees off-axis, the rendered image will look incredibly different.

If you're used to using a calibrated CRT viewfinder on a DVCPRO rig, then yeah, your method works great. But if you try it with a 150, you may find that viewing the LCD at even slightly off axis will have a dramatic effect on the image. Try putting up the color bars, and then rotating the LCD's angle some, and look at how different it can be.

Waveform on the outboard LCD, and viewing through the viewfinder, could work. I think the zebras (especially the 105 zebras) are unobtrusive and give that extra measure of security in knowing that you're getting the exposure you want.

 

[Kit_L]

when in doubt, expose for the subject, and that's a big, fat period.

Absolutely, and spoken like a news pro, where getting the shot is everything; actually, to paraphrase "Red" Sanders, the only thing in news/current affairs, where I worked for years too. An audience will forgive a lot, visually speaking, if the subject is well exposed, the audio is good, and the content is there.

In the context of a general discussion re. exposure, though, I feel that Zebra bars can be handy (and that was the OP's concern). I would never suggest having them on all the time (which could be annoying, as you suggested), but can be useful in setting up, and when the light changes. cheers.

 

[trez]

I'm very surprised at how much interest there is in using zebra bars in general. Like many tools the camera operator has at their disposal, zebra bars are certainly one of them, but I personally find them annoying on screen. Maybe use them here and there just to check exposure, but having a built in waveform monitor in a camera at this price point really makes zebra bars that much less attactive.
I understand zebra bars are more discreet on screen while you are shooting an event perhaps, but they are also covering detail in the scene that you may need to analyze while shooting.

Well, this is not the case with 105% zebra. Setting the exposure is actually making this annoying zebra disappear (almost). If the zebra pattern covering parts of the LCD is annoying, so is the built-in waveform monitor - it covers entire frame. And you can switch the zebra off just as you can wfm once your exposure is set. Kit_L pointed that zebras can be very handy when the light changes - you immediately see something bad happens and can adjust your exposure so that your footage is not ruined. You can shoot while having zebra on, but not with wfm on. Sure wfm provides you with more information - it's almost like having zebras at all IRE levels at once. But it has its disadvantages, too.