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Thread: Partial image selective control of shutter speed

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    Partial image selective control of shutter speed

    While on holidays recently I came up with a question regarding shutters on digital cameras.

    From the (brief) research I've done it appears that most digital SLRs utilise a mechanical shutter. Is technology not good enough at present for us to use electronics to control the on/off status of the CCD?

    The reason is ask is this. While on holidays I did some rapelling down a cliff that happened to be located next to a waterfall (I survived!). As I gained confidence I started jumping out from the rock face in arcs as I descended. It struck me that it would make a great photo of you could use a long shutter speed to capture the silky water of the waterfall, but to also be able to capture the instant in time at which I was jumping out away from the rock face.

    To use a short shutter speed would capture the person, but not the get the silky water and vice versa. (And I don't think using a flash would be feasible at the distances you would shooting from)

    It then struck me, if the the CCD on SLRs is digital, then would it be possible to develop a function wherbey the user could select on the CCD those parts of the image that they want short/long shutter speeds. This could be a broad selection (eg. bottom 2/3 of image at 2secs shutter speed, and top third at 1/2000...might be useful for early morning sunrise shots?) or you could go so far as to select a small circle of interest on the CCD that you want a specific shutter speed (eg. the waterfall).

    Thoughts?
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    Administrator ricktas's Avatar
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    That's what second sync flash can do!

    I think although in theory this is an interesting idea, doing it in practice would be a futile endeavor. First off, the process of selecting the area that you want the CCD to expose for longer or shorter, other than having a selection of geometric shapes (square, round, ovoid, etc) how would you deal with the area you wanted that was not geometric?

    What you are suggesting for a result can be achieved in post processing with layers, masks and motion blur filters and due to the non geometrical aspects of most things in nature, I think you would end up with a better result trying this in PP.
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    Regarding selection of an area, I wouldn't be surprised if in the near future we have touchscreen LCDs on our cameras. You could quite easily implement a system where you select points on the LCD and it joins the dots to create the boundary. This could be as simple as a straight line (ie. a dot on either side of the lcd) or multiple dots for more complex shapes. You could easily extrapolate this to include a graduated neutral density filter (ie. draw two lines and designate the gap between them as requiring a graduated drop off in light being processed). For those that like to travel into remote places with a minimum of gear (ie. flashes, filter kits etc), I could see this as being quite useful.

    I agree regarding the PPing, but generally it's better to get the lighting correct in camera, rather than trying to scrape information out of an image that is barely there (ie. blown highlights or shadows) or trying to create a 'look' that doesn't stand out as being fake/photoshopped.

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    Administrator ricktas's Avatar
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    But your LCD is small! how accurate is your selection on a touch screen that is 3 inches across going to be? I think your idea is an interesting one, but I think it is something that we are a few years of perfecting, if the camera manufacturers head that way at all. It would also mean a big shift in sensor tech. At present the entire sensor is activated and 'exposed' for the same amount of time. To create a chip to manage variable exposure across certain sectors of the capture would be an interesting endeavour. It would mean electronics would probably need to be created to deal with the variable electical circuit load, and sending that load to differing parts of the sensor depending on user selection. Quite a complex process to achieve.

    It would be a very interesting development, but one I am not sure the camera manufacturers might even consider attempting.

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    Member James T's Avatar
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    Not gonna happen. Way too fiddly. You'd need selective areas of different sensitivity as well (ISO) to control the exposure.

    The way to do it would be two shots, one long exposure, and one short. And the mask them together.

    Then, once you've done that, zoomed in at 200% in Photoshop.. imagine trying to get anything that looked less than horrendous by placing a few dots with your finger on a 3" LCD.

    Far quicker and simpler in the field to take two shots than faff about like that anyway I'd imagine.

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    Whilst this would be a neat gimmick to do in camera, how often would one use this feature? Even with such a feature, it would be almost impossible to setup to get that perfect shot with one attempt so you'd be shooting multiple times (with the person rappelling every time ) so why not use one of these 3 existing solutions (that only require the person to rappel once):

    1. flash to freeze the motion of the person;
    2. multiple exposure in camera (1 long + 1 short); or
    3. multiple shots and then mask in PS to get the desired effect (as suggested above) .
    Last edited by campo; 29-04-2010 at 10:01am.

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    Can someone explain the flash and camera settings to do this, I am a bit confused how you would set this up, I have little experience with flash, and to be honest am still trying to decide which one I will buy to learn with.

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    Bally,

    Not sure what gear you're using but if you use Nikon, flick through the camera manual for details about flash modes and what they can do and if you use Nikon and have a Nikon speedlight, the booklet that comes with the flash units has pretty pics too :-)

    Anyways, "slow sync" mode is your friend for this stuff it allows a slow shutter speed to capture background light whilst still firing the flash to expose the main subject properly. You can also have front curtain or rear curtain sync which basically means the flash fires the moment you press the shutter (front) or just before the shutter closes (rear) - best to google it to see various examples of the effects you can create with front/rear.

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    What you describe there Stoogest is certainly possible to do, but considering the processing power required on the camera, I think at this early stage in the Digital Camera cycle, not really going to happen.
    (but I do think that variable exposure across different parts of the sensor is one feature we'll end up seeing. That kind of technology or feature basically makes graduated filter requirements redundant, and HDR images would end up looking a lot more natural.

    The problem tho is that shutter speed has to remain constant, as shutter speed is one of the three most important aspects of SLR type photography.. so the shutter has to remain constant over the entire face of the sensor in many situations.
    The way it could be implemented is that various parts of the sensor could be electronically 'gaited' to protect them from over exposure, and leave the darker areas to expose for a longer period. I suppose an electronic version of HDR at the sensor level.

    The D70 and other cameras with that sensor(I don't know if other manufacturers do this tho) actually have 'electronic' shutters as well as the standard mechanical(focal plane) shutter.
    The way this one works is that the mechanical shutter is rated to only something like 1/90s and the rest of the shutter is electronically controlled by using the sensor as switch(as P&S cameras do.. they don't have an actual mechanical shutter.. purely sensor based electronic shutters).

    This can be both a good thing and bad thing, depending on your point of view.
    D70 has a 1/500s flash sync speed because of the hybrid elector/mechanical shutter system(which apparently is a good thing), but then again the shutter speed is also limited to only 1/4000(where a proper mechanical shutter can be rated to 1/8000s or even 1/16000s too.
    Apparently the flash sync goodness is also a bad thing too, but only in that you can't use a shutter speed(FP) of any higher than 1/500s at all, as you can with powerful external lighting and more advanced cameras(where the power of the lighting system becomes the shutter speed).
    More advanced cameras can be used at shutter speeds above their rated flash sync limits such as 1/8000s but you need powerful external lighting to compensate due to the lower GN. With the D70 1/500 is the absolute limit.

    The other problem that the D70 had(in particular) was sensor blooming at very high shutter speeds and bright light sources(such as the sun). What basically happens is that even though you shoot with a 1/4000s shutter speed(if shooting into the sun, as an example) the sensor creates something like a hot spot in the image with a blooming blowout around the light source. This is due the limitations of using the sensor as the shutter(exact technical details I dunno about), but needless to say that it can sometimes be an issue in rare cases.

    The other problem is that CCD's sensor work by capturing the light and reading the light level and converting it into an electronic signal over the entire surface of the sensor. That makes it hard to electronically limit a particular area very hard as the sensor readout is over the whole sensor.
    CMOS works differently, where the readout and conversion to an electronic signal happens on an individual pixel basis. So a CMOS sensor would be the better technology to do variable exposure levels over the frame.

    You'd have to assume the limitation in implementing this kind of feature is more to do with the CPU processing ability available in the camera than actual sensor technology.

    But, I for one would love a camera that allows you the ability to use a virtual graduated filter of various strengths at the sensor level(ie. during the time of exposure), rather than stuffing about with real filters in front of the lens, or HDR's after the fact.

    lets just hope that the technology isn't too far off.
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