Slow shutter speeds and 'sense up.'

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Slow shutter speeds and 'sense up.'

Postby Viewmaster » Sat Jan 14, 2012 12:53 am

Many of us film NBTV with modern digital cameras, and bad flicker is the generally the result as the camera is too
fast for our old slow NBTV frame rate of 12 1/2 per sec.

To avoid the dreaded flicker a slow shutter speed is required but
most cameras have a slowest speed in the region of 1/50 sec.

I have been trying to find out what the difference is between a 'slow shutter speed' and 'sense up' now built into some mini cameras.
(I may need something like this on my Nipkow Nipper. )

MY old Sony digital camera does have a slow shutter speed facility but no 'sense up.'

'Sense up' can be up to x250. Looking on the net seems to indicate that this refers to x250 exposure time.

I believe that 'sense up' is short for 'Sensitivity up' and the camera's image chip is given a longer exposure
to enable it to see in the dark...called Starlight cameras.
So I cannot see the difference betwee this and a slow shutter speed, if any, bearing in mind that digi cameras do not have a mechanical shutter as such.

Has anyone here had experience of this and can throw some light (!) on these 2 terms and the exact difference, if any, between them?


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Postby Steve Anderson » Sat Jan 14, 2012 1:16 am

That's interesting, I'm not familiar with that model or the phraseology that Sony use (bless 'em), but all digital still cameras I have seen in the last decade usually have shutter speeds beyond eight seconds.

When photographing the CRT NBTV stuff I've done I usually set the cameras for a two-second exposure (25 frames) and tweak the aperture and ISO speed to get a reasonable exposure. This of course requires a still picture on the screen.

So I have no direct answer to your problem, I just find it strange that a (presumably) recent camera has such a limited shutter range.

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Postby Viewmaster » Sat Jan 14, 2012 6:55 pm

Steve, we are at cross purposes and it is entirely my fault.
I should have put the word 'video' in front of the word 'camera' each time !
So my thread was about filming NBTV with a video digital camera not a stills digital camera.
Sorry about that important omission.

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For the want of a word the meaning was lost!" :)
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Postby Steve Anderson » Sat Jan 14, 2012 8:50 pm

Viewmaster wrote:...I should have put the word 'video' in front of the word 'camera' each time!


Ah! That explains it. With a field rate of 50Hz for PAL/625 there isn't much that can be done without some severe editing software and a lot of patience. 10 seconds comes out to 250 frames or 500 fields. No thank you.

Some video cameras do seem better than others, but I've not seen any that can reproduce exactly what our eyes perceive.

Graham Lewis's 50Hz NBTV frame rate converter could help (a lot possibly) but it would need synchronizing to the 625 video camera, or perhaps the other way around. Then one could argue that it's not 'real' NBTV anymore. No disrespect Graham.

I also made a presumption in that you wanted stills for the newsletter rather than a video file.

So I'm still at a loss..

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Postby Viewmaster » Sun Jan 15, 2012 12:38 am

The secret of good flicker free NBTV video filming seems to be a slow shutter speed. No special software or equipment required with it.

I mentioned this example on a previous thread, but here it is again. It uses a slow shutter speed video camera filming NBTV Nipkow.
The results are completely flicker free due probably to the use of the slow shutter as you can see, although the scanning direction of the NBTV is opposite to normal !

http://www.youtube.com/watch?v=59_-Lj8uSO4

It was this that led me on to 'sense up' video cameras and the difference between those and slow speed shutters, if any.
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Postby Lowtone » Sun Jan 15, 2012 1:26 am

I got a new VHS-C camcorder, it has shutter options, i should try it
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Postby M3DVQ » Sun Jan 15, 2012 4:06 am

As I mentioned in another thread, adding a "motion blur" effect can do a lot to solve the black band problem. Provided there isn't too much movement in the scene and the camera is steady this seems to work rather well. It's just a case of tweaking the amount of persistence until it averages two or three nbtv frames together.
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Postby gary » Sun Jan 15, 2012 9:58 am

I agree that increasing exposure time such that it "merges" 2 or more frames is at least a workaround for not being able to synchronise frame rate (whatever happened to that camera that purported to have an external sync Albert?).

It seems to be what I did here (attached) with a webcam - problem is I can't remember how I did it (actually I am pretty sure it was an old Avatar still camera in "motion mode" which had a very low frame rate - possibly compensated for by long exposure time?). Not perfect but better than the dreaded black band.
Attachments
44.avi
(2.88 MiB) Downloaded 637 times
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Postby Steve Anderson » Sun Jan 15, 2012 1:20 pm

A web-cam sounds entirely plausible as generally their frame rate is quite low. So out of curiosity I dug out mine (I never use it) and had a play with it.

Entering 'Camera Settings'>'Driver Settings'>'Advanced' allows you to alter the frame rate in quite fine increments. But not quite 1/12.5s, either 1/12s or 1/13s. Then control of gain etc to get a decent exposure. The camera is a Logitech of around six years old, no clue as to the model number.

Either of those two settings should be an improvement over 50 fields/sec. To get it exact (if that's possible with mechanics) either speed up or down the NBTV signal to the monitor slightly, or if there's a talented software guy around 'tweak' the driver(s) for the web-cam for 12.5Hz.

I would think that the 'black-banding' should be absent as (I assume) there's no need for the 625-like vertical blanking within web-cam output. And the output is already a file, in this case an AVI.

Just a few thoughts.

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Postby Viewmaster » Sun Jan 15, 2012 7:10 pm

gary wrote:I agree that increasing exposure time such that it "merges" 2 or more frames is at least a workaround for not being able to synchronise frame rate (whatever happened to that camera that purported to have an external sync Albert?).


Gary that little £5 camera I bought doesn't have external sync....shall I send it back to Hong Kong and ask for my money back? :wink:
It's exposre time is 1/50 or less, that is why I am looking into the
slow shutter/sense up possibilities on other video cameras.

This mini Sharp starlight video camera here may do the trick but it's more expensive ! :cry:
Has an OSM and 'sense up' adjustment from x2 up to x256.
'Sense up' here is also called 'Frame integral.'
The plot thickens !

http://www.ebay.co.uk/itm/200676818577? ... 1439.l2649


Mentioned by M3DVQ motion blur looks to be a long exposure but the effective aperture would be reduced to achieve correct exposure I suppose.

The camera in the link above doesn't seem to have motion blur
but has some interesting looking items such as 3D-DNR, Mirror image,
Shelter area, Point compensation etc etc.
Now the plot really thickens. :lol:
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Postby gary » Sun Jan 15, 2012 7:44 pm

I guess then it comes back to what "sens-up" really refers to and whether it'll do what we want it to do - it's more expensive but still quite affordable - I guess the main draw back is it only has a an AV (PAL/NTSC) type interface meaning an extra step in getting it onto the computer - not a huge problem I suppose.
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Postby M3DVQ » Mon Jan 16, 2012 8:43 am

I was meaning just film it at 50hz normally, black bands and all.

Once it's on the computer you apply the motion blur in post processing.
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