HV20 Latitude and exposure details
 

I just did a relatively crude test of the HV20's latitude, and for fun superimposed it on another latitude chart relating to much higher-end HD cameras (originating here http://www.cinematography.net/hdcamtests/HDcurves.htm). Here's my version.

Note that this was done in using still images saved to minisd with these settings:

24p HDV
Cine mode
whitebalance daylight
sharpness -1
contrast -1
color gain +1

The test shot was bracketed over 16 stops, and is subject to any inaccuracies in the exposure selection system in the hv20. It is assumed that each click of +-1 exposure is 1/4 stop. Measurements were taken every other click, aka every .5 stops. I expect it is best to allow for +-0.5 stops error (which is conservative as long as the exposure meter in the camera is consistent). Fstop and shutter readings were checked at each measurement so exposure steps appear to be consistent. There are 10 +-1 exposure steps (probably 2.5stops), however, that are not accounted for, they may be gain. The sequence of aperture and shutter changes used to get the various exposures are something I posted on in another forum: http://www.dvinfo.net/conf/showpost....0&postcount=21 (some parts need correction)

Noah:

So what does it all mean ??

...what model is the Canon camera with the cyan line?

mike:
I assume it is the XL-H1

chris:
What does it mean? maybe nothing? But if my methods are comparable with those used to create the original graph, it might mean the HV20 set up in the right way could be comparable in terms of latitude to a much fancier camera. I was pretty impressed with the curve for the camera, it is fairly linear (relative to exponentially increasing exposure values) and the slope is not as steep as it could be, covering a fair range of exposure stops (thats a good thing). That says nothing about noise or iso equivalent ratings. Those will probably be the next thing to test.

Any input as to how this might be used to determine how many stops of latitude the HV20 has?

Awesome! I have an HV20 on order and am going to test it with a Stouffer chart - look forward to seeing how my results match yours.

Bruce Allen
www.boacinema.com

I'm no expert but it looks like something like 7.5 stops +-0.5 stop depending on how noise looks at the bottom. I think the curve should probably be shifted to the left since it certainly doesnt have 5 stops overexposure latitude and definitely has more than 2.5 under (who determines 0 exposure anyway? does it really matter after its all been graded?). What is important is the distance between the last low value and first high value. I just put the curve where it fit based on my starting and ending data. Also note this is a graph for all the other cameras when they are rated at ISO 320, which i am sure the HV20 was not rated at 320 for this test, but most of those cameras are supposed to be able to do 320 and still get a pretty optimal image, so i wouldnt necessary consider that too much to their detriment.

thanks for doing that test noah!

now if we can only get someone to use the imatest software to get something like (scroll down to "Dynamic Range Analysis):
http://www.imaging-resource.com/PROD...D40IMATEST.HTM

this all depends of course on noise levels. there's a good discussion here:

http://www.normankoren.com/digital_tonality.html

any DSLR tech people have that software?

well the way the hv20 data is presented, it means that the $950 hv20 has better lattitude than every motion picture digital camera except the arri d-20. lol.

okay so probably these datapoints were taken using uncalibrated noise criteria, but i think this is an important comparison: the technology to produce non-stratosphericaly-priced sensors and accompanying electronics for cinema-level capture is JUST ABOUT here.

There is a russian review HV20 vs HV10: http://www.videozona.ru/video_tests/hv20/

It contains Imatest comparision - measurement of dynamic range, resolution MTF50, color accuracy, ...

A "fun" mashine translated version of the article: http://babelfish.altavista.com/babel...ests%2Fhv20%2F

wow. tha'ts probably the single most information-full link i've seen posted on this forum. if you go to the end there is this one with more details:

http://babelfish.altavista.com/babel...s%2fImatest%2f

so a summary of the dynamic range result in f-stops vs. image quality is:
9.57 (low), 9.36 (med), 8.64(med-high), 6.13 (high)

which looks pretty good. it matches up to the graph noah sent which i read as showing a best case of a bit over 9-stops.

according to the other link i sent: http://www.imaging-resource.com/PROD...D40IMATEST.HTM it's about the same dynamic range as the canon eos 400 dslr. i couldn't dig up his settings on the test though. can anyone who can read russian tell me what they were? cinemode & contrast minus 1, i believe gives you the greatest range.

also look at this:

color accuracy with lots of light: http://www.videozona.ru/video_tests/...colorerror.png

color accuracy in low-light:
http://www.videozona.ru/video_tests/...colorerror.png

ouch. that agrees with the desaturated look i see when it gets dark.

sorry for my ignorance but can someone explain why lowering the contrast increases dynamic range? So the best option is cine with contrat -1 and brightness at 0?

Thanks

my explanation would be its because part of the problem in dynamic range in digital cameras naitively see light variations linearly but light increases exponentially. now if the data came straight from the sensor in the 8-bits (0-255 whole number values), it would be impossible for it to show more than 7-8 stops of light because the brightest 1-stop range of light in the image would be stored in the values 127-255 (half the possible values), the next 63-127, and so on down to 0-1 (6 stops later). so if there were any image data darker than that, it would get rounded to 0 or 1, making it pretty much unrecoverable. fortunately the data from a light sensor usually comes in in 10 or 12 or 14 bit form (allowing 1024, 4096, or 16384 possible values) so if we assume a 12bit image sensor, all the infinite dark stops of light in image that would have all been recorded into the range 0-1 and essentially crushed is now being recorded into 0-15 which allows for several stops of light in that dark detail to be differentiated, giving 4 more potential stops of light at 7-15, 3-7, 1-3 and 0-1. this data is then processed into 8bit (0-255) data to appear more like the curve i posted above and preserve that dark detail. lowering the contrast changes that processing to try to pull more data out of the bright and dark ends (where detail is normally lost)

This is a very informative thread! Thanks!

i hope my post makes sense. anyway, also, as I understand it, brightness just affects autoexposure. I would probably put brightness at -1 to avoid clipping highlights, but in general i wouldnt use autoexposure much so it wouldnt make much difference. It won't affect latitude.