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Tutorials | Long Exposures with no Filters. Part 3, Back At Base


The processing for this when you get home is where the magic happens. It's pretty simple really, once you've loaded your images onto your computer and you've done your RAW processing for your frames (a good tip is to make the levels as similar as possible across all your frames) you need to get them into Photoshop (or whatever image editing software you use) as a stack - i.e. a single document with all the images in separate layers.


Note to Lightroom users

Sadly Lightroom can't handle layers (at least not without extra software) and this technique requires your editing software to layer your exposures and set the opacity of the individual layers.

That said, if you already have Photoshop then you can of course replace all mentions below of Bridge and substitute the alternative Lightroom commands.

You still can use Lightroom. Select all frames on the film strip, right-click and select open with Photoshop as layers. - Thanks to "Guest" for the heads up.


Loading The Frames Into Photoshop

Using Adobe Bridge

I use Adobe Bridge for this coz it has an easy peasy lemon squeazy menu item to do the stacking for you.

I select all the images I want and using the menu in Adobe Bridge, I go to: -

Tools | Photoshop | Load Files into Photoshop Layers

I know for a fact, this method will preserve the EXIF information although - I did some checking, it appears to use the last file you select. I'm not sure if the order of the files will affect it. As your exposures should be the same, it shouldn't matter too much.


Alternative Loading Without Bridge

You could also load the files individually into Photoshop as you would normally then from the menus go to: -

File | Scripts | Load Files into Stack

From there you can choose the files you already have open (if you loaded them already - press the Add Open Files button) or select the ones you want by pressing the browse button.

I don't use this method so I don't know if it preserves the EXIF information.

Whichever way you choose to stack the images go make a cup of coffee, especially if you have a lot of frames and are shooting RAW, as it can take a while to load and stack your frames.


Time To Apply The Magic Fairy Dust

Photoshop Layers Pallet

Once Photoshop has loaded your files into the layers it's time to do the magic.

Automatic magic fairy dust | An Image Averaging Script for Photoshop

In the introduction earlier I was talking about a sprinkle of magic fairy dust.

This dust in reality is a bit of a pain in the arse. It's lots of stacking images then setting opacities. It gets a bit technical, fiddly and repetitive. I quickly got bored of the processing involved and so I wrote a small Photoshop script which will automatically set the opacities and merge the layers for you - if you keep reading the tutorial this will hopefully make more sense.

Anyway, there's a page with install instructions and a download link here. Once you've got the script installed you can trigger the script either from File | Scripts |Image Averaging Layers v1.0.0or via an Photoshop Action you made to run it.

If you use this script then the rest of the tutorial below won't be of much use to you other than to let you know what the script is doing for you.


Photoshop CS6 Extended

If you have Photoshop CS6 Extended then you don't even need the script, you can use the inbuilt stack tools. More information here.

If you've installed the Image Averaging Photoshop script then you just need to trigger it and wait a few moments while it averages then optionally flattens the images for you.

Otherwise, read on MacDuff: -

Layer Opacity Table

Layer #   Opacity
20   5%
19   5.26%
18   5.55%
17   5.88%
16   6.25%
15   6.66%
14   7.14%
13   7.69%
12   8.33%
11   9.09%
10   10%
9   11.11%
8   12.5%
7   14.28%
6   16.66%
5   20%
4   25%
3   33.33%
2   50%
1   100%

The technique simply involves setting the opacity for each of the layers in a set order. The screen grab of the the Photoshop Layers Pallet shows 10 frames, each layer named with: -

1) A layer number (Layer 1 is ALWAYS at the bottom of your stack) I added this just for the tutorial
2) A file name (this is automatically added by Photoshop/Bridge when you use Load Files into Photoshop Layers)
3) The opacity I've set the layer too. Again, added just for the sake of the tutorial.

The magic is setting the correct opacity. Here's the formula for making the calculation: -

The Magic Formula
Image Averaging - Layer Opacity Formula
Where L is the number of layers below the current layer.

So in our screen shot of the Photoshop Layers Pallet we see I have selected layer 6 (counting from the bottom layer) so I will use that as an example in our formula.

  1. There are 5 layers below layer 6 so 5+1 = 6. Easy enough.
  2. Divide 1 by 6 we get 0.167
  3. Multiply this by 100 and we get 16.67

Photoshop can't do an opacity with a fraction of a percent so you can choose to round up or down, In this example I chose to round down to select an opacity of 16%

From everything I've read about this technique you really do have to get these opacities as close as possible. Because Photoshop can't do fractional opacities you're limited with your accuracy. As you can see from the Layer Opacity Table, some of the layers will have the same opacity - especially as the number of layers below goes up.

I believe the effect of this is to give or take undue bias of the layers with the wrong opacity - depending if it's rounded up or down. How visible this problem would be I'm not sure. I've done 20 frames using the numbers on the table and it's looked fine to me.

I've got an idea of how to correct for this by grouping layers and then setting the group opacity using the formula but replace the variable L which is supposed to be number of layers below with number of groups below.

So say we did 30 frames. Group them in to sets of 5 for example. Use the opacities for each of the 5 frames in each group using the Layer Opacity Levels table and then for each of the 6 groups, repeat the process.

However as my maths ability is so mind bogglingly poor I don't know if this would actually work mathematically speaking :)

Answers on a postcard please :)


Tutorials | Long Exposures with no Filters. Part 1, Introduction

Tutorials | Long Exposures with no Filters. Part 2, In The Field

Tutorials | Long Exposures with no Filters. Part 3, Back At Base

Tutorials | Long Exposures with no Filters. Part 4, Sample Images

Tutorials | Long Exposures with no Filters. Part 5, Links



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