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I plan to shoot an object (a banana peel to be precise) and then replace the background with another image (1). What would be the most effective approach given the following conditions:

  • Canon 550D
  • Sigma 18-55mm f/2.8-4.5
  • Maximum camera to subject distance: 1.5m
  • Distance from object to wall: around 0.5m
  • No flash, but continuous household lighting (2)
  • I have a black cardboard backdrop of 1x0.6m. It's not a requirement to use it.
  • The background should be able to be replaced automatically. There will be a lot of photos (it will be a stop-motion), so I can't manually mask every single one.

Footnotes:
(1): The full story for those interested, (I think) this is not relevant to the question: I want to make a stop-motion featuring a banana peel as a person. I can place the peel in different positions and hold it with transparent thin fishing lines. However, I would also like to animate the background. Doing this simultaneously with the banana animation is tedious. My plan is to do a stop-motion of the background separately and then replace the banana background through some keying process.

(2): I do have an off-camera flash, but no way of syncing it. If I hook it up to my DSLR it would probably fry it as it is an old 300V flash. Household lights range from 10 to 150W.

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4 Answers 4

up vote 2 down vote accepted

Here are all the ways I know of for removing the background (in order of my preference):

  1. White Background

    This is done by using a white-ish background and lighting the background about 3 stops brighter than the subjects (exact lighting depending on your camera).

    There's no way you can do this with household lights but a flash aimed at the wall behind the subject does this easily, you can do it by shooting your subject with a very slow shutter speed and using your flash's test button or you can get a $40 flash from ebay (plus about $10 for adapters and cables to sync it off-camera).

    Note: you will want to get the subject as far from the background as possible to minimize light bouncing from the background hitting the subject.

  2. Chroma-key

    Use a solid color background (most commonly green), make sure the background is lighted evenly and that there are no shadows falling on the background.

    You can sort of do it with household lights but here it is even more important to not have light from the background hitting the subject (because it will cause a green color cast) so you will need some distance between the background and subject.

  3. Black background

    This is done by simply placing the lights very close to the subject and letting the light falloff turn the background black, the more powerful the light the easier it is to get a black background.

    Here distance to background is also important but you can manage without it if the light is powerful and very close (but a powerful light very close will create very dramatic hard light - so if you want even soft lighting this is not for you).

  4. Masking in software

    If you just carefully paint the mark for each photo you don't care what the original background is - and you don't need any special lights, however, this is obviously very tedious.

    You can make an almost-white or almost-black background, use automatic selection and then just refine the mask to save some time.

Whatever you do all of those will confuse the camera's auto mode, don't forget to meter for the subject and use manual mode and manual white balance - and to take test shots and watch them on the computer before starting with the stop motion animation.

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When you extract the banana from your original background, you'll tend to include some background pixels. If the background is white you'll have white pixels. If you then place that on a light background, those white pixels won't show much, but against a darker background, they will stand out.

So as you're shooting the banana, if you can try to use a plain background that is similar in brightness and colour as the final background you'll be using, you will save yourself having to spend a lot of time refining the extraction. You can do a fairly rough extraction and the pixels will blend in with the animated background.

In other words you can use a contrasting background (white, black, green) to make the masking easier, but unless your masking is really good, those contrasting pixels will work against you unless you plan ahead and match them with the final background you'll be using.

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You have an overly constrained problem space. The easy way to do this is to have a couple of off-camera flashes, strobes, etc. and use a white wall or piece of white seamless.

Alternatively, you could use a green-screen (paint, wall, screen), which is what all the video folks do.

See if you can borrow or rent a couple of flashes.

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Thank you for your contribution. I can see why the white wall would make replacing the background easier, but I don't understand where I need the flashes for. How do they help me with my problem? –  Bart Arondson May 26 '13 at 22:43
2  
@BartArondson - Household lights pulse with 50 Hz AC current. It looks to us humans like a steady flow of light, but to your camera it is dim-bright-dim-bright-dim... 50 times a second. –  Esa Paulasto May 26 '13 at 23:04
    
You don't specifically need flashes, but you need to control the light on both your subject and the background. Any lighting setup that lets you do this will be fine. I suggested flashes because even the cheapest ones have variable power output, and because they are easy to rent. –  Pat Farrell May 27 '13 at 1:37
    
@esa, only some lights pulse. Incandescent lights, carbon arc, and some others use the AC mains power to heat something, and it can't cool down enough during the off cycle to matter. Florescent, LED and other lights do flicker and it can have major impact. –  Pat Farrell May 27 '13 at 1:39

Any constant shade background that is high contrast (and doesn't use the same color) as your object will work for trying to do an automatic removal. The best quality is often still accomplished by manually extracting the object with matte or selection painting in post.

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