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MERGING BRACKETED IMAGES - THE BASICS

Camera sensor limitations

The human brain can interpret light signals thousands of times more efficiently than a camera sensor or film allowing us to see the whole spectrum of dark to light at the same time. For example - if I look out of a window on a sunny day I will see the sunshine and colours but I will also see the window frame and walls. A camera sensor can only "see" or in other words be exposed correctly, for either the outside view or the inside. The tonal range is too great for the sensor or film to cope with all at once. To achieve a final image "as I saw it" requires some creativity in taking images and software to merge the images.

Why merge images?

Taking more than one image then merging preserves all the digital information from the correctly exposed areas and so the final image is of the highest quality possible when printed or projected.

A correct exposure means that minimal or no changes to Levels are needed. If Levels are substantially altered then digital information is discarded or stretched resulting in a "posterised", degraded image that has lost subtle details and colours - a histogram that looks like a comb with broken teeth shows where the digital information has been lost.

Merged images can be made from two versions of one RAW image. However, a set of bracketed images arguably preserves more quality. Bracketed images can usually even be done on a point and shoot compact, unlike RAW's.

Tips

The camera should be on a tripod so the camera does not move between shots. Use the timer or remote control as pressing the shutter release button can move the camera. If you have no tripod or handy wall to sit the camera on but have a steady hand and there are only tiny differences between images it may be possible to align them - see below.

Merging Images

To start with just use two images that have an easy merge line eg. unbroken hills, shoreline. Don't start with trees, branches or anything with a complex outline.

1 Getting the images into Photoshop Elements Edit workspace (for full Photoshop there is no separate Edit workspace and the menus have slightly different wording) - click on File_Open and navigate to the first image, click on the image then click Open and it will open in the Edit workspace.

To get the second image in, again go to File_Open as before, find the image, click on it to highlight and click Open and the image will appear. This second image now has to be put into the project so to speak. So go to Select_All (this brings up the "marching ants" showing the whole image has been selected) then click Edit_Copy. The image is now stored in the PC memory so click on the X at the top right of this image to close it down. Now click Edit_Paste which retrieves the second image from the PC memory and it shows now in the workspace. In the Layers palette you can now see the two images as separate layers. If you click the eye icon on and off you will see the lower layer.

(If you have a third bracketed image to use then Open, Copy and Paste as for the second image but make this layer invisible for now by clicking off the eye icon. Merge layers one at a time. These instructions assume two images).

2 Using a Layer Mask to merge the images This method is for full Photoshop or Elements 9 and 10 (for earlier editions of Elements see below). Make sure Layer 1 is active by clicking on it. Click on the Add Layer Mask icon at the bottom of the Layers palette (or Layer_Layer Mask_Reveal All). A white rectangle appears next to the image which the rectangle is locked to, this is the mask - the double line around the mask means it is active.

Select the Brush tool (eg. 200 pixels, soft edge), opacity 100%, and make sure the Foreground Colour Box is black. Now start brushing the parts of the image that you want to remove which will reveal the underneath image parts that you want to keep. Lower the opacity and zoom in as you get nearer the join in the images. If you make a mistake or want to make subtle, localised changes then change the Foreground Colour Box to white which paints white back into the mask - ie. using white paints back the pixels. When you have completed the merging go to Layer_Flatten Image and save.

A bit about understanding Layer Masks The default colour for a layer mask is white (you can use a black mask but don't go there for now). Think of a white mask as hiding anything below the image the mask is locked to. Think of the black Brush tool as punching through the mask to reveal the equivalent part of the image below. Think of changing back to a white Brush tool as repairing the hole to restore the top image.

The beauty of a layer mask is you can swap from black to white any number of times to make changes and using opacity you can control the strength of what is revealed from below. Unlike the Eraser tool method as described below the pixels are not destroyed when removed and can be restored. As with everything brush and opacity control improves with practise.

Troubleshooting Layer Masks If you have been clicking through layers then want to do more work on a particular mask and find yourself literally paint black or white over the image you have forgotten to click in the mask rectangle to activate it. Just go back in the history palette to remove the black/white blobs.

If the Foreground and Background Colour Boxes are not actually black and white - click once on whichever box is not right and this brings up the Colour Picker window. Click on the blackest or whitest areas as appropriate and the colour will automatically be put in the colour box.

3 Using the Eraser Tool to merge the images Elements 8 and earlier do not have Layer Masks. Instead make sure the top layer is active and choose the Eraser tool - soft edge brush approx 200 pixels. Start with a lower opacity - say 50% and start to erase the areas you don't want and to reveal below the areas of the image you do want to keep. Use frequent clicks of the mouse because the only way to undo mistakes is to click back in the history palette. Likewise start on lower opacities because you can always erase more by raising the opacity - if you go in at 100% opacity you might remove too much especially near the joins between the images. When the merge is completed go to Layer_Flatten Image and save.

Tip - use the square bracket keys to alter the brush size - this is less fiddly than typing in the size or using the slider. Pressing the shift key then the square brackets alters the brush smoothness.

4 Aligning Hand Held Images If you were not able to use a tripod. If you try to merge two hand held images it is likely that, however hard you tried there was some camera movement and the images will not overlap exactly when copied and pasted. If the mis-match is small try this.

Open the Photomerge Panorama window and as with panos browse for the images and bring them into the Photomerge Panorama window. Now untick the box called Blend Images Together, then click Open. The images will be aligned only and you can then use the Layer Mask or Eraser method of merging as above. There will be small darkened edge areas that will need to be cropped - these are the parts lost to allow for the alignment. This did not work where there was a marked amount of camera movement.

Janet D - November 2011

This article can be downloaded HERE as a Printable Word Document.