Q on long exposure photos; how the photo is recorded- the process

Discussion in 'Digital Photography' started by LERsince1991, Jan 21, 2012.

  1. LERsince1991 macrumors 65816

    Jul 24, 2008

    I've done long exposure photos before and know how to do them etc... But I have a question about how the photo is recorded and the process the camera uses.

    Lets try to explain my thoughts;
    So your taking a long exposure and someone walks across the photo with a black Tshirt on, then followed by another person with a white tshirt on. As these are opposite colours would they cancel each other out a bit? The same could be said for all colours.

    The reason I'm asking is because I'm trying to show movement through a space as accurate as possible so if colours are being cancelled out it will affect results?

  2. TheReef, Jan 21, 2012
    Last edited: Jan 21, 2012

    TheReef macrumors 68000


    Sep 30, 2007
    NSW, Australia.
    The colours won't cancel each other out to cause the person to become invisible, the pixels will blend/average, theoretically to become a shade of gray - which way the average is biased depends on the light intensity of the subjects (and a black shirt will absorb a lot of light).

    A glowing (literally) green shirt will have more prominence over a dim red shirt when they cross paths in the exposure.
  3. LERsince1991 thread starter macrumors 65816

    Jul 24, 2008
    Thanks, I wanted to make sure people were cancelling each other out.
    That makes more sense to average them, so what I'm doing might work ok!

    I want the image to show ONLY the movement of people.
    So the steps I was thinking could be;

    1 - Mount camera on tripod in manual mode and take a photo every 30 seconds or so until I've got a range of images. Stack all the images into photoshop, convert to smart object, change the stack mode to (mean) I think it was, this will get rid of everything that moves basically giving me a photo with no one in at all.

    2 - At the same time of taking the previous photos I will then take a very long exposure photo in manual, bulb mode either linked to my macbook with EOS utility, or with an external trigger. This will make a long exposure of the time which will show the scene and everyones movement through it.

    3 - I will then load the blank scene created from step one and the long exposure created in step 2 into photoshop and set the image blend mode to difference. This will then cancel out the blank scene from the long exposure photo and create an image just showing the movement through the area! :)

    I think this will work pretty well but want to confirm people agree as it will take a lot of work - actually involving mounting my camera between 2 trees above a busy highstreet to get a near plan view etc...

    ... you don't want to know what I will then do with this image :) (generate a 3d model/form using scripting with 3ds max or rhino, then design into it for architecture)

  4. snberk103 macrumors 603

    Oct 22, 2007
    An Island in the Salish Sea
    I agree with TheReef. Also, generally speaking, in a long exposure (any exposure actually, but it's not usually an issue with normally short exposures) an object has to appear in the same spot for about 1/2 the exposure to become visible. More or less, depending on the tonal value of the object and background.

    So, in a long exposure of say several minutes, people walking through the scene won't register at all. If you are taking a photo of a busy sidewalk, if the time you can see the cement is vs someone in the scene is half or more of the time, then you will likely barely see a blur at all.

    I suspect this may be for a school assignment? If it's for a big mark, then you are going to need to experiment with much smaller data sets to get it right.... deadlines not withstanding. It's an interesting idea, and experimenting on a less ambitious time scale may show some compelling visual results that we armchair coaches can't predict. So, go and do some experimenting and be open to changing the assignment based on the early results.
  5. LERsince1991 thread starter macrumors 65816

    Jul 24, 2008
    Its for my 3rd year project we've been working on for 3 months and will have another 4 months or so on it.

    I think I'll mount my camera on tripod in my window and experiment a bit to see how well it would work :)

    Thanks for the help,
    I'll let you know if it works out :)
    I've looked into lots of different ways of doing it and if this works it'll certainly be the easiest. Other methods involve motion tracking in after effects from a video, 3d motion tracking and video frames to layers and smart object - stack mode - range. A LOT of effort for these alternatives though!


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