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With Magic Lantern it's possible to increase the video bitrate up to a point your memory card can no longer keep up and Magic Lantern will notify your recording has stopped.

While i can understand why disabling the audio will increase the maximum CBR. Why does setting a higher ISO decrease the maximum CBR i can record on my card?

[Update 1]:

As suggested in the comments i created some samples to see when exactly ML would stop recording and the sweetspot seems to be at the limit of the SDHC ard, no suprises there. But looking at the samples of "CBR2.0" and "CBR2.0 [2nd run]" you could conclude the setup has a better chance of keeping up at lower ISOs.

CBR1.5

MVI_0099_ISO100.MOV 42819 kbit/s
MVI_0100_ISO160.MOV 41561 kbit/s
MVI_0101_ISO200.MOV 33533 kbit/s
MVI_0102_ISO320.MOV 22485 kbit/s
MVI_0103_ISO400.MOV 19161 kbit/s
MVI_0104_ISO640.MOV 20102 kbit/s
MVI_0105_ISO800.MOV 20927 kbit/s
MVI_0106_ISO1250.MOV 21614 kbit/s
MVI_0107_ISO1600.MOV 21090 kbit/s
MVI_0108_ISO2500.MOV 19227 kbit/s
MVI_0109_ISO3200.MOV 18146 kbit/s
MVI_0110_ISO6400.MOV 11778 kbit/s

CBR2.0

MVI_0137_ISO100.MOV 55846 kbit/s
MVI_0138_ISO160.MOV 62127 kbit/s
MVI_0139_ISO200.MOV 70309 kbit/s
MVI_0140_ISO320.MOV 86262 kbit/s
MVI_0141_ISO400.MOV 87295 kbit/s [ML stopped]
MVI_0142_ISO400.MOV 85249 kbit/s [ML stopped]
MVI_0143_ISO320.MOV 87745 kbit/s
MVI_0144_ISO400.MOV 85335 kbit/s [ML stopped]

CBR1.9

MVI_0145_ISO100.MOV 72905 kbit/s
MVI_0146_ISO160.MOV 82645 kbit/s
MVI_0147_ISO200.MOV 84003 kbit/s
MVI_0148_ISO320.MOV 83824 kbit/s
MVI_0151_ISO400.MOV 84654 kbit/s
MVI_0152_ISO640.MOV 84219 kbit/s
MVI_0153_ISO800.MOV 83637 kbit/s
MVI_0154_ISO1250.MOV 83835 kbit/s
MVI_0155_ISO1600.MOV 75655 kbit/s
MVI_0156_ISO2500.MOV 57014 kbit/s
MVI_0157_ISO3200.MOV 43319 kbit/s
MVI_0158_ISO6400.MOV 19803 kbit/s

CBR2.0 [2nd run]

MVI_0159_ISO100.MOV 78976 kbit/s
MVI_0160_ISO160.MOV 87553 kbit/s
MVI_0161_ISO200.MOV 86067 kbit/s [ML Stopped]
MVI_0162_ISO200.MOV 85187 kbit/s [ML Stopped]
MVI_0163_ISO160.MOV 87122 kbit/s
MVI_0164_ISO200.MOV 87559 kbit/s
MVI_0165_ISO200.MOV 84523 kbit/s [ML Stopped]
MVI_0166_ISO320.MOV 87753 kbit/s [ML Stopped]
MVI_0168_ISO320.MOV 87507 kbit/s [ML Stopped]
MVI_0169_ISO400.MOV 85972 kbit/s [ML Stopped]
MVI_0170_ISO640.MOV 85869 kbit/s [ML Stopped]
  • The limit of the SDHC card is at 85Mbit/s or 10Mbyte/s.
  • At ISO 800 and up the video gets very bright which might explain the drop in bitrate towards ISO 6400.
  • These samples where created using:
    • Canon Rebel T3i/EOS 600D
    • Kingston Micro SDHC 4GB Class 10
    • Magic Lantern v2.3.600D.102
    • Audio recording disabled

Sample: http://namsral.com/assets/MVI_0141_ISO400.MOV

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closed as off topic by dpollitt, Itai, Imre, John Cavan, Matt Grum Feb 1 '13 at 13:00

Questions on Photography Stack Exchange are expected to relate to photography within the scope defined by the community. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about reopening questions here.If this question can be reworded to fit the rules in the help center, please edit the question.

    
Are you saying that increasing ISO increases the bitrate when QScale (constant quality) is used, or does increasing the ISO decrease the maximum constant bitrate you can record on your card? –  Matt Grum Jan 29 '13 at 11:09
    
The latter, i've rephrased my question. –  namsral Jan 29 '13 at 15:16
1  
Slightly related to this question. –  Bart Arondson Jan 29 '13 at 16:11

2 Answers 2

I would say noise. As you increase the ISO you get more noise, and noisy images do not compress as well as clean ones. Noise increases the amount of information in the image, so that results in each frame being larger and taking up more bandwidth.

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How can i test this, record total darkness with high ISO? Total darkness should be easily compressed. –  namsral Jan 29 '13 at 15:17
    
Test this by taking samples at different ISO's up to the point it cuts off. Open each on your computer and see what the stream bit rate is (SUPER has a good detail viewer). If it is positively correlated with ISO you have an answer. –  Phil Jan 29 '13 at 16:55
    
This doesn't fully explain what the questioner is experiencing: Magic Lantern uses constant bitrate (unless in QScale mode, which the user manual advises: do not use) so increased noise in the image does not make each frame larger or increase the bandwidth. –  Matt Grum Jan 29 '13 at 17:04

I'm surprised by this as I would have though the card write speed is the slow link in the chain, and that shouldn't depend on any other settings!

High ISO images do contain more noise, which is more difficult to compress, but that should just result in lower quality, since the compressor aims for a constant bitrate. My guess is that the constant bitrate isn't quite constant, and the extra noise causes it to spike a little, and the spikes creep over the maximum write speed of the card.

You can test this theory by shooting a piece of paper at high ISO and setting the camera up to overexpose. This should yield images that are pure white, and thus contain no noise, despite the high ISO setting.

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