Moonrise & Aurora

Moonrise & Aurora

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Why don't we get hot pixels while using live view mode, but do get then on a long exposure? I read somewhere that they appear due to the sensor being activated for too much time, but then, live view should also show them.

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1 Answer

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On a long exposure charge is allowed to accumulate on the sensor over a period of time, whereas in live view the sensor is discharged for every frame (so thirty times a second or so). Some hot pixels result from slow charge leakage which only occurs during a long exposure, hence you wouldn't expect to see these in live view.

Also liveview subsamples the sensor so you dont see every pixel.

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+1 - Learn something new every day. Didn't realize LV subsampled. Upon thinking about it, it would really have to - just didn't realize. – rfusca♦ Jul 25 '11 at 15:05
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Question is - does it subsample when viewing 10x zoom in?? – ysap Jul 25 '11 at 15:12
@ysap As far as I know there's always some form of sampling/interpolation going on - you never get a 1:1 correspondence between sensor and LCD pixels. 10x magnification is the same size regardless of how many megapixels you have so there must be some scaling going on! – Matt Grum Jul 25 '11 at 15:30
Ok, running some numbers, I believe the LCD screen on a 5D is 640 by 480. It's marketed as 920,000 "dots" but they count the red/green/blue subpixels as separate dots, thus 640 x 480 x 3 = 921,600 the sensor resolution is about 5616 x 3744 (give or take some masked pixels) so at 10x magnification you're looking a patch of sensels measuring 562 x 374 so it would be possible to show the sensor pixels 1:1, however given the sharpness, and the existence of cameras with less than 21MP, I think the camera subsamples, then upsamples the live view data! – Matt Grum Jul 25 '11 at 15:53
@MattGrum - That makes sense given what I see when using 10x LV for astro. – rfusca♦ Jul 25 '11 at 17:31
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