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Old Mar 12, 2016, 8:45 AM   #1
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Default Combining Layers in GIMP to simulate ND filters

Was watching an interesting video by Tony Northrup in which he explains why you don't need filters https://youtu.be/YcZkCnPs45s and describes a method of combining multiple short exposures to create a single image with the same effect as an ND filter.

Not having anything recent I went back to a series of shots I did for a timelapse last summer.

Here's an individual frame from the series

Longlake Sunset by ramcewan, on Flickr

And here's a combination of 49 images where each image is set to 5% opacity and then stacked and combined to yield a result with the same water, sky and boat blurring effect one would get from a long exposure with an ND Filter.

ND Filter Simulation by ramcewan, on Flickr


I also created this image by combining 4 shots and instead of making them a set opacity and merging them I used some of the other options like darken only and lighten only to bring through only portions of certain layers over a single frame resulting in something a bit glitchy but sharper than the full blown 49 image combo.

Summer Flashback by ramcewan, on Flickr

I'm going to play some more and try combining some milky way images to see if I can get more luminosity with less noise.
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Old Mar 12, 2016, 1:50 PM   #2
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Interesting. Is the free GIMP software pretty easy to use?
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Old Mar 12, 2016, 2:51 PM   #3
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Interesting. Is the free GIMP software pretty easy to use?
Not really but there are YouTube tutorials. Photoshop is much easier and automates the process
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Old Mar 13, 2016, 8:56 AM   #4
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Interesting. Is the free GIMP software pretty easy to use?
A couple more notes about the ND Filter simulation;

If it wasn't clear I had shot the 49 frames from a tripod using the timelapse feature of the E-M1 which allows you to tell it a set number of frames you want to take and how far apart. Shooting was conducted in aperture priority f5.6 and manual ISO of 200, shutter was set by the camera as was WB. With the frames all having same aperture, ISO and position they were easy to stack. If they had been off alignment would be needed and that would be automated in Photoshop, in GIMP there is some feature called register images but I haven't used it yet and can tell already it's not going to be intuitive.

Since I was using a set of sequential frames from timelapse, 49 in total, spanning a time taken from 6:57pm through 7:00pm, the effect has the movement of a 3 minute exposure.

Combing in GIMP requires you to determine what opacity to use for the number of frames. You can also mask layers so that only sections are brought in.

In photoshop from what I've seen ( I don't have it ) you can simply ask it to combine layers with handy pre-sets, in this case combine with mean (average) was used by Tony in the video I linked to.

I've seen video of people combine sunset images of city scapes with after night images where the lights are on using PS to basically light the buildings but otherwise not alter the sunset image in a few clicks.

In GIMP this can probably be done but it won't have an intuitive guided tool for it, instead you'll have to research and try a few different methods, basically GIMP has the abilities of PS but not the ease of use and built in automation.
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Last edited by ramcewan; Mar 13, 2016 at 8:59 AM.
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Old Mar 13, 2016, 9:39 AM   #5
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Very interesting, if not complicated process. The graduated neutral density filter in ACR will even up an exposure in about 10 seconds with no masking needed since the program is doing that seemlessly in the background, maybe 20 seconds if you count the extra 10 it takes to clean up/brush out the affect where you don't want it below the horizon, but it is part of a program that costs $9.95 plus tax a month.

Adobe Camera Raw is so feature-laiden at this point, I do very little in Photoshop itself anymore unless I am creating collages, removing some annoying object within a complex area of a file or utilizing Neat Image noise reduction at higher ISO's.

Last edited by Greg Chappell; Mar 13, 2016 at 9:50 AM.
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Old Mar 13, 2016, 1:32 PM   #6
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Very interesting, if not complicated process. The graduated neutral density filter in ACR will even up an exposure in about 10 seconds with no masking needed since the program is doing that seemlessly in the background, maybe 20 seconds if you count the extra 10 it takes to clean up/brush out the affect where you don't want it below the horizon, but it is part of a program that costs $9.95 plus tax a month.

Adobe Camera Raw is so feature-laiden at this point, I do very little in Photoshop itself anymore unless I am creating collages, removing some annoying object within a complex area of a file or utilizing Neat Image noise reduction at higher ISO's.
just a point of clarification, in this I'm simulating a ND filter like the big stopper, not a graduated neutral density filter which is easily applied in lightroom something I bought before they turned it into a monthly subscription. Unfortunately light room does not have the ability to combine multiple images as individual layers.
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Old Mar 13, 2016, 2:23 PM   #7
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Way too complicated for me. I'm just getting comfortable with Photoshop Elements 10 and haven't even tackled layers yet.
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Old Mar 14, 2016, 6:25 AM   #8
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just a point of clarification, in this I'm simulating a ND filter like the big stopper, not a graduated neutral density filter which is easily applied in lightroom something I bought before they turned it into a monthly subscription. Unfortunately light room does not have the ability to combine multiple images as individual layers.
Using multiple gradient filters in one image achieves the look easier since you are using just one image. Sometimes I'll work down from the top with one gradient, then up from the bottom with another, brushing out the areas I don't want included in the effect. No chance of something changing in subsequent images being added.

Too wide of a tonal range can defeat the process for sure, but it needs to be a really, really wide dynamic range before I have to resort to layering in Photoshop. This was a multiple gradient. I maxed out the highlight recovery in the sky using the basic sliders, then used a gradient mask in ACR, not to turn down exposure but just turn down highlights in the sky so I would not wind up with annoying all-white, blown areas in the clouds you see in so many landscapes. Used a second gradient to raise the exposure in the foreground. I don't know how many stops of dynamic range this image equals, but it was close to about as many as one can get out of one E-M1 file.

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Old Mar 14, 2016, 7:49 AM   #9
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Using multiple gradient filters in one image achieves the look easier since you are using just one image. Sometimes I'll work down from the top with one gradient, then up from the bottom with another, brushing out the areas I don't want included in the effect. No chance of something changing in subsequent images being added.

Too wide of a tonal range can defeat the process for sure, but it needs to be a really, really wide dynamic range before I have to resort to layering in Photoshop. This was a multiple gradient. I maxed out the highlight recovery in the sky using the basic sliders, then used a gradient mask in ACR, not to turn down exposure but just turn down highlights in the sky so I would not wind up with annoying all-white, blown areas in the clouds you see in so many landscapes. Used a second gradient to raise the exposure in the foreground. I don't know how many stops of dynamic range this image equals, but it was close to about as many as one can get out of one E-M1 file.


That's a beautiful image and certainly I follow your points about the multiple gradients, lightroom has them and they are really helpful at balancing sky and foreground as you've shown.

However in the image combining 49 frames shot sequentially the effect I am going for the effects of a long exposure; blurred water, blurred clouds showing movement and blurring of any foreground objects in motion.

I don't believe you can achieve this using gradient filters. Only through layering multiple aligned images taken over a time frame or by doing a long exposure can you achieve this result.

ND Filter Simulation by ramcewan, on Flickr
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Old Mar 14, 2016, 9:00 AM   #10
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That's definitely one way to achieve the softening affect. You could try using the Lightroom clarity slider on one file in a negative amount to simulate an overall softening like what you've done here, brushing the effect out on the land where there's no movement and keeping the boat sharp on the softened water. Using multiple frames, you can see the movement in the upper clouds in the banding/ray-like projections. Negative clarity would just produce a softening affect with none of the banding. I can see liking it either way and preferring one over the other.
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