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Thread: Exposure time

  1. #1

    Default Exposure time

    My excuses in advance if this is a dumb question. And am hoping it makes sense.

    With 'film' cameras, the brightness of the image was related to the film speed (ISO), the exposure time and the aperture ('f' stop).
    My cheap 'Flip cam' records at 30fps. I assume its 'auto exposure' simply adjust the exposure time.Understandably, I get motion blur when a object moves fast. The blurring is equal throughout the period of the frame.

    But, when comparing similar footage with a Kodak Zi8 there seems to be a huge difference. For example, in MarkPeter's recent Windor castle clip, although blurs occur, the result is more like one perfect frame superimposed with a semi-transparent frame a few milliseconds later. That gives a sharper image.
    I had assumed this was because of Kodak's image stabilisation system which *might* be giving more value to the beginning of frame; then lessens throughout the frame time.

    Whilst it seems common for cams to record at 30 or 60fps; do proper (semi pro and better) cams offer controls over the exposure time? If so; what is the common range of speeds? With old style film cameras, exposure times of 1/1000th of a second were common. Is it just the case that electronic video cams don't commonly achieve those speeds?

  2. #2


    'semipro' cameras can film at 24,25,30,50 and 60fps. For filming in the UK you should be using 25fps to play back on PAL. USA is 30fps or 29.97fps.

    Im not great at this stuff, I just stick with what I know mostly but the blurred effect you've mentioned could also have something to do with wether your filming progressive or interlaced perhaps.

  3. #3


    Thank you for the reply but I am interested in knowing the length of time of frames. 30fps is only the rate at which the frames are recorded - not the duration that the frame capures.
    For example. If the exposure lasts 1ms (1/1000th) of a second; the images can still be recorded at 30fps. What does your manual say?

  4. #4


    I think the missing words are shutter speed. Yes some semi pro cameras will shoot between 12fps up to 50/60fps (UK/US) cameras like the Panasonic Varicam, HVX200 etc. So, if I am shooting fast motion I would shoot at 50fps. If I wanted the "film look" I would shoot at 24 or 25fps

    Most other cameras can shoot at different shutter speeds this can rang from 1/25 up to 1/2000. Some cameras have different speeds. One trick to help get a shallow depth of field is to use a fast shutter speed so you can then open your iris more.

    Now the interesting bit. Shooting at 25fps with a low shutter speed will give a natural blur to images (film like). BUT increasing the shutter speed will give that more "stuttery" but sharper images on fast moving objects (video like).

    The thing you described in Marc's video, I think is a "progressive" artifact as apposed to an "interlaced" artifact. Which is the same problem (coping with fast motion)but looks different due to the nature of the way the image is being produced.

    Hope this answers your question.

  5. #5


    Again, thank to MB and FM for the pointers. Ah! 'Shutter speed'. Drats, I had forgotton that term.
    The image below shows enlarged frames from the Windsor clip and a recent post by Cslinks. These demonstrate the effect rather better than I described. Is this an example of the difference between Progressive and Interlaced?
    The left shows 2 (or perhaps 3) superimposed images; but the right section (the player's right hand and the almost invisible drumstick) only shows a blur.

  6. #6


    These examples show what I was trying to explain about the slow and fast shutter speed. The blur on the drumming picture isn't an interlace artifact. That would be more like jagged horizontal lines like the example below.

  7. #7


    I don't want readers to think this is the be all and end all on this subject there are other factors with digital video cameras such as how the camera sensor and software work, post production methods etc..

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