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I do have to take pictures in a low light environment, where part of the assignment is to capture the naturally present light. (its a chill-lounge sauna) I could get higher ISO, and wider aperture, but I need to get as high quality as possible images with as much depth of field as needed.

My squestion is: Is it possible to take 2 shots, one with a long shutter speed to get the lighting and the color and whatsoever, and then an identical shot with low shutter speed and a flash, and combine those afterwards? sorf of like a frequency separation, but taking the color from one shot, and the structure from the other? Would the resulting image be any good?

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    For taking pictures with a long shutter speed a tripod was invented. Would it help you? Not clear why you need a second image. – Zenit May 30 '17 at 14:57
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    Is any element of the shot potentially in motion, i.e. people? If not, I see no need for anything other than a tripod either. – Tetsujin May 30 '17 at 16:02
  • yes, there is water in motion and a person might have some minor motion too.. with frequency separation it should not be a huge problem, given that the low frequency layer is blurred anyway.. – sharkyenergy May 30 '17 at 18:17
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    I do not understand the term frequency here, really, though there is a frequency issue (usually you might call it color temperature), as a flash will often not match the white balance of your regular long exposure shot. For example, with incandescent light the long shot will be yellow compared to the more blue flash. You can also do what you suggest by low ISO for deeper color, and high ISO for shorter exposure, and combine, but I still do not really see the combination solving the low light problem well if there's motion -- and if there's no motion, shoot long exposure. – Linwood May 30 '17 at 18:33
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    @Linwood "Frequency" as used here refers to the decomposition of an image into frequency components using Fourier analysis. tldr; higher frequency means smaller details. – junkyardsparkle May 31 '17 at 6:52
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I'm not one for doing your homework for you, but if you want big brownie points for the method you propose then there is an argument for exploring the Lab colour model (and space).

Pull your colour/ab channels from the long exposure and the luminance/L from the flash/artificially lit/high ISO source (mixed down into a black & white.) Even if there is movement in the low-ISO image you can get away with an awful lot of error before anyone will be able to see it.

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"Is it possible to take 2 shots, one with a long shutter speed [...] and then an identical shot with low shutter speed and a flash, and combine those afterwards?"

Yes it's possible. What you call frequency separation here would be better achieved with for example a bilateral filter. You can get some inspiration from this paper. Simply put, what it does is to take one shot with a flash and another without, deal with any shadow produced by the flash and use a bilateral filter to combine both images.

"Would the resulting image be any good?"

Good or mediocre will depend on your planned usage of the photo. The paper mentioned above shows some examples of what can be achieved.

Notes:

  • don't you may mean that the second shot with the flash is taken with a high(er) shutter speed rather than with "low shutter speed"? [edit: see comments below for discussion on shutter speed and shutter time]
  • based on one of your comment of an example use case saying you want to capture a girl in 2 shots, she is is unlikely to remain strictly still during the long exposure and between both shots, unless your workflow is well designed and you and your subject are well prepared.
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    Technically speaking, a "low" shutter speed is a shorter duration than a "high" shutter speed. It's a fractional denominator, much like an f-number. The higher the number, the smaller the fraction, and thus the shorter the exposure time. 1/1000 is a smaller number than 1/100, thus 1/1000 second is a "lower" shutter speed than 1/100 second. To avoid this confusion I usually will say "longer" or "shorter" Shutter time rather than high or low shutter speed. – Michael C Oct 3 '17 at 6:17
  • @MichaelClark i guess it can be natural to associate the adjective "low" or "high" to the speed of the shutter, low speed = slow to close the shutter = long exposure duration, high speed = fast = short duration. I agree it'd be better to talk about shutter time being short or long, since the shutter is parameterized as a function of time, not a speed. Good remark! – calocedrus Oct 3 '17 at 10:16
  • Yeah, it's very similar to saying "higher aperture." Does that mean a higher f-number, and thus a narrower aperture? Or a wider aperture that allows in more (higher amounts) of light but has a smaller f-number? You can find those who mean either one when they say "higher" or "lower." It's better, in my opinion, to use 'wider' and 'narrower' to avoid such confusion. – Michael C Oct 3 '17 at 18:54
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While it would work doing it with frequency separation I think its complicating things for no reason. Instead you can just shoot a color card without the flash and then with the flash inside the space and then use it as a reference for the entire photo.

Trying to do reverse frequency separation would have to involve a decent amount of fine tuning in the form of manual coloring afterwords. The only reason I would ever say it could help would be if you have a photo of the scene as like a jpg that was altered either color graded or black and white and you're trying to bring it back to normal. So you could restage it as close as possible for reference colors but would still have to manually color it in for the most part and fill in any missing items from the original shot to the new one.

Since you have the ability to shoot at the location trying to do some reverse frequency separation just seems complicated for no reason.

  • Hi ryan, thanks for your answer. Since my english isnt the best, I am afraid that I did not explain myself well enough, OR I did not understand your answer very well.. What I had in mind is this: Take one picture of a scene (for example a lady in a pool illuminated by a candlelight and a few low power led lights for mood) with a long exposure to capture the lighting. the girl will probably be unsharp because of motion blur and the water too. but the colors will be perfect. Now, take the same picutre again, but with low ISO and a few flashes. the flashes are placed where the lightsources are... – sharkyenergy May 31 '17 at 10:52
  • ...so they do not crated shadows that do not make sense with the original illumination. Colors wont be good, but the image will be crisp and sharp. Then, in PS do a normal frequency separation of the later image. now, add the first image with proper color on top and do again a frequency sep. on that one using the same settings. delete the topmost high frequency layer, and move the low frequency layer right below the high frequency of the crisp shot. That way i have the low frequency from the colored image, and the high frequency from the crisp image. why would it require some repainting? – sharkyenergy May 31 '17 at 10:56
  • @sharkyenergy it will work which is the first thing I said. I'm just not sure that its the best solution. But now that you're saying two completely different shots (going from a candlelight to a well lit room) it makes more sense. Just be careful to try and get them as close as possible in structure to minimize colors being out of place / bleeding. – RyanFromGDSE Jun 1 '17 at 10:01

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