Open Photography Forums  
HOME FORUMS NEWS FAQ SEARCH

Go Back   Open Photography Forums > OPF Welcome Hall > Moving Pictures and Sound Recording

Moving Pictures and Sound Recording Still/Video Camera & Slide Show movies also Sound for such Production!

Reply
 
Thread Tools Display Modes
  #1  
Old September 1st, 2010, 09:54 AM
Doug Kerr Doug Kerr is offline
Senior Member
 
Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,558
Default Video camera sensitivity

It is very common for the "sensitivity" of dedicated video cameras to be stated in terms of the minimum scene illuminance (ambient lighting) under which they can deliver a "usable" image, and to state that illuminance in the SI unit, lux.

I thought it would be interesting to compare this with the terms in which we commonly think in the still camera world.

A common rating for dedicated video cameras is 0.5 lux. In APEX terms, this corresponds to Iv -7.1. This is said to be about the illuminance afforded by the light of the full moon under good conditions. My incident light exposure meter provides scale markings down to 0.125 lux, with good readability down to 0.5 lux.

Under the "standard exposure equation" with scene luminance known (often described as the "incident light metering" form of the equation), for a camera sensitivity of ISO 400, this scene luminance would call for a photographic exposure of EV -0.1 (about f/2.0 at 2 sec).

By way of comparison, an exposure of 1/30 sec with an aperture of f/2.8 is Ev 7.9.

Thus, it would seem, a dedicated video camera with a rated sensitivity of a scene illuminance of 0.5 lux would seem to be 8 stops more sensitive that a still camera at ISO 400, 1/30 sec, and f/2.8.

Lest my math have gone wrong, or my favorite exposure calculator, I had my light meter do the same reckoning, with the same result.

I am startled by this result.

To take a properly exposed still picture with a scene illuminance of 0.5 lux (assuming the normal range of scene item reflectance) would seem to require perhaps this setup:

ISO 3200, f/2.0, 1/2 second.

How can this be?

Best regards,

Doug

Last edited by Doug Kerr; September 1st, 2010 at 12:03 PM.
Reply With Quote
  #2  
Old September 1st, 2010, 10:30 AM
Asher Kelman Asher Kelman is offline
OPF Owner/Editor-in-Chief
 
Join Date: Apr 2006
Posts: 34,769
Default

Doug,

I have always wondered about this. Perhaps the frames are considered as a subunit of overlapping sliced sections of a cube so that data on each feature is accumulated.

Asher
__________________
Follow us on Twitter at @opfweb

Our purpose is getting to an impressive photograph. So we encourage browsing and then feedback. Consider a link to your galleries annotated, C&C welcomed. Images posted within OPF are assumed to be for Comment & Critique, unless otherwise designated.
Reply With Quote
  #3  
Old September 10th, 2010, 04:52 AM
Jeremy Waller Jeremy Waller is offline
Member
 
Join Date: Aug 2010
Location: Cockatoo Valley in South Australia
Posts: 58
Default

Hello Doug and Hello Asher,

I read the original post a couple of days ago typed a reply - session timed out and I lost the response so here I am again!!

First of all I find the original question most interesting.

The initial finding that a video camera is 8 stops (x 256) more sensitive than a still camera puzzling and prompts one to get answers to some basic questions (of the manufacturer) about the video camera itself.

How do the manufacturers define sensitivity ?

How is a video image described? ... Picturing a static scene and integrating a number of exposures? ... I don't know.

The video camera system "has it all over" the single frame cameras in that it can coherently integrate many images to reduce the effect of noise - now to do this will make an unfair comparison!!

Very simplistic noise analysis:

The video camera has 3 sensors. So the noise power (uncorrelated) in an image pixel is a weighted sum of the separate noise powers. But in a single sensor still camera each image pixel is the result of the combination of nearest neighbor sensels. This implies that there are more contributory noise sources in the case of a still camera.

The video cam is looking increasingly better in low light. But we are still not comparing apples with apples. To do that we really much more info from the manufacturers.

Kind regards,

Jeremy.
Reply With Quote
  #4  
Old September 11th, 2010, 11:08 AM
Asher Kelman Asher Kelman is offline
OPF Owner/Editor-in-Chief
 
Join Date: Apr 2006
Posts: 34,769
Default

I hope you guys can get more information on this. It is important as to where one might best spend limited funds in getting videos in low light. One needs both increased sensitivity but also protection against blowout of the bight areas. For me that's the relatively low lit stage for music or ballet performance with lights from above giving harsh specular highlights on the forehead and face, while eye sockets are relatively dark.

Asher
__________________
Follow us on Twitter at @opfweb

Our purpose is getting to an impressive photograph. So we encourage browsing and then feedback. Consider a link to your galleries annotated, C&C welcomed. Images posted within OPF are assumed to be for Comment & Critique, unless otherwise designated.
Reply With Quote
  #5  
Old September 11th, 2010, 06:19 PM
Jeremy Waller Jeremy Waller is offline
Member
 
Join Date: Aug 2010
Location: Cockatoo Valley in South Australia
Posts: 58
Default

Hi Asher,

I'll take an " action item " and have a closer look at this issue of low light and see how it applies to our situation.

Interestingly there is a a huge body of knowledge "out there" on the use of video cameras being used to make a still image, this is the specialist field of " Lucky Imaging ". It gets its name from the statement, " You are lucky if you get a good image !"

Please see:

http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/

For any interested readers I'll refer you to some very sophisticated and free (there are some very talented and generous people "out there") software named REGISTAX :

( http://www.astronomie.be/registax/ )

Regards,

Jeremy.

Last edited by Jeremy Waller; September 11th, 2010 at 06:23 PM. Reason: correcting the english
Reply With Quote
  #6  
Old September 11th, 2010, 07:38 PM
Asher Kelman Asher Kelman is offline
OPF Owner/Editor-in-Chief
 
Join Date: Apr 2006
Posts: 34,769
Default

Jeremy,

This goes along with my idea that videos are really cubes of image data where the same thing is imaged over and over again with some movements needing alignment and various translations being made of angular movements. Essentially, I imagine a video as not a series of still images but rather a series of overlapping stacks of image data. So there's incredible potential for increasing resolution and decreasing noise.

but is that anything to do with how images are processed for video?

Asher
__________________
Follow us on Twitter at @opfweb

Our purpose is getting to an impressive photograph. So we encourage browsing and then feedback. Consider a link to your galleries annotated, C&C welcomed. Images posted within OPF are assumed to be for Comment & Critique, unless otherwise designated.
Reply With Quote
  #7  
Old September 12th, 2010, 02:02 AM
Jeremy Waller Jeremy Waller is offline
Member
 
Join Date: Aug 2010
Location: Cockatoo Valley in South Australia
Posts: 58
Default

Hi Asher,

Re: So there's incredible potential for increasing resolution and decreasing noise.

Answer: Definitely !

When one talks about stacking that's what the eye/brain combo does. Once, I remember looking at a video image thinking how good it looked but a single frame looked so noisy that it was disappointing.

I was looking at some of the newer Sony (v.expensive) and Canon (somewhat cheaper) video cams and the advertising blurb showed very very good low light performance but they did not tell the whole story this is where I would like to dig deeper.

BTW. I take issue with the way the marketing departments of the manufacturers twist terminology to make things sound so much better.

Cheers,

Jeremy

PS

Your mail box is full - does not accept PM's

JRW.
Reply With Quote
  #8  
Old September 12th, 2010, 06:55 AM
Doug Kerr Doug Kerr is offline
Senior Member
 
Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,558
Default

Hi, Jeremy,

Quote:
Originally Posted by Jeremy Waller View Post
Once, I remember looking at a video image thinking how good it looked but a single frame looked so noisy that it was disappointing.
Indeed, a very important point, and one that may in fact be pivotal to the issue in this thread.

Best regards,

Doug
Reply With Quote
  #9  
Old September 13th, 2010, 04:06 AM
Jeremy Waller Jeremy Waller is offline
Member
 
Join Date: Aug 2010
Location: Cockatoo Valley in South Australia
Posts: 58
Default

Hi Doug,

Yes its got back to the problem of signal to noise ratio with the added complication of many many frames being used to constitute an image. Your original post got me intrigued and prompted me to do some "fact finding". As my mother (bless her heart, 88 and going strong!) would say "He's got a bee in his bonnet".

I'll take a close look at the subject of video images and how we perceive them etc etc. etc.
Then try to marry this with image quality.

I'll keep you posted.

Jeremy.
Reply With Quote
  #10  
Old March 15th, 2011, 10:42 AM
Jerome Marot Jerome Marot is offline
Senior Member
 
Join Date: Jan 2011
Location: Munich, Germany.
Posts: 3,789
Default

The reason why video cameras boast abilities to film in almost complete darkness is mainly commercial. At the minimum illumination levels, the image is usually very noisy and almost devoid of colors.

However, the basic sensitivity of video cameras is usually relatively high, because:
-they have relatively big pixels, since they have few of them on relatively large chips (*)
-the 3 chips models lose less light than a bayer array
-their lenses are usually pretty fast (f/1.6 is common)

Sensitivity levels are not directly comparable, because they depend on the gamma curve (which can be adjusted on the better video cameras), but one can try to simply compare readings between a DSLR and a video camera. I tried to do that with a Sony AX2000, and I find a sensitivity of about 640 ISO at 0 dB. A quick search on the net showed that other people found similar values for similar cameras, for example the Canon XH-A1 is reported at 340 ISO at 0dB. For simplification, I will take ISO 400 as a representative sensitivity at 0dB for video cameras.

Video cameras have a function called "gain", which doubles the apparent sensitivity every times the gain is increased by 3 dB. So we have:

GAIN - ISO
0 - 400
3 - 800
6 - 1600
9 - 3200
12 - 6400
15 - 12800
18 - 25600
21- 51200 (maximum for the AX2000)

At 21 dB, the image is still usable, the colors are still visible, but the resolution is visibly compromised by noise reduction. If one needs better images, it is better to limit oneself to, say, 9 or 12 dB gain.

Small consumer models may go to even higher gain values, typically 24 dB (102400 ISO), and silently reduce the shutter speed to 1/25 instead of 1/50s (1/30 vs 1/60 in NTSC), effectively doubling another time the apparent sensitivity if one does not check the shutter speed.

Now, if we had a camera with 102400 ISO, f/1.6 and 1/25s, we could indeed take pictures in very dark places indeed. Just as with a camera with 3200 ISO, f/2.0 and 1s, which is about what was found in the first message of this thread (we have a factor of 2, which is basically the accuracy of the various calculations).



(*) On large chips: please consider that DSLRs discard typically 4 lines out of 5, effectively reducing the active size of their chips by the same factor. A 2/3" video camera thus has the same active surface as a four thirds sensor, but typically a maximum of 2 millions pixels per chip (HD).
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
The blast of an HD camera and waterproof too! Asher Kelman Breaking News 6 August 6th, 2010 12:49 PM
Metering, ISO sensitivity, and stuff Doug Kerr Imaging Technology: Theory, Alternatives, Practice and Advances. 2 December 28th, 2009 05:58 PM
News: The Canon 7D is big news! Fast accurate focus and low light capability! Asher Kelman Breaking News 10 September 1st, 2009 10:09 AM
News: Camera that auto stitches and auto builds from multiple shots! Asher Kelman Breaking News 1 March 3rd, 2009 07:30 AM
The 50 D is here...where's the 5dMk2???? Kathy Rappaport Breaking News 30 August 28th, 2008 09:40 AM


All times are GMT -7. The time now is 10:41 AM.


Posting images or text grants license to OPF, yet of such remain with its creator. Still, all assembled discussion 2006-2017 Asher Kelman (all rights reserved) Posts with new theme or unusual image might be moved/copied to a new thread!