In order to get a higher dynamic range you can blend multiple differently exposed images. The easiest way to do so is to average all of them. However, the resulting image usually suffers from low detail contrast and looks dull. On the other hand, we can preserve detail contrast while compressing overall contrast by using blurred masks, gradient enhanced masks or -even better- with the smart blurred masks.
The basic idea is to combine the images using only shadows and midtones. We create a blurred mask for each image with a curve mapping highlights to black and mid tones and shadows to white. The background image is of course not to be masked.
This mask should be blurred for smooth gradients and small details in order to prevent their contrast, but leave hard edges intact, where the contrast can be reduced savely. For this purpose the desaturated copy of the image is treated with the smart blur filter and then inverted. See Manually step by step HowTo for more information on this technique.
The details of the individual processing steps are contained in six Photoshop-actions. Two can be used in droplets or batch processing, two to add images manually, and two to change between the mask modes.
The simple 'Blur Mask' and 'Gradient Mask' actions are no longer part of the package. They did not yield very good results. For version 1.3 I refined the settings for the actions called 'Smart Blur big' and 'Smart Blur small' for big images that will be reduced in size later and for small images that won't be reduced. Version 1.3 actions are now compatible to photoshop CS 16 bit layers.
Many thanks to John from http://www.sandiego360.com who permitted me to use his images. The originals are available. They where shot to stitch a spherical panorama. On the following pages you find higher compressed but equally sized copies.
ResultsThis images cover the whole contrast a monitor can display. Be sure to have your monitor adjusted correctly. You should see a dark gray area with an even darker frame in the black furniture.
|The averaged result is dull and too bright for the outside scene.|
|The unadjusted smart blurred (small) version shows good contrast and exposure for the outside scene but the shadows are too bright.|
|The output of the 'Smart Blur big' action shows pixelated edges visible in the pictures on the wall or the sofa.|
|The layer opacity of the smart blurred version were adjusted individually to get a better result. From top to bottom (light to dark) the layers got this opacities: 20%, 30%, 15%, 50%, 50%, 50%, 50% (only the top three layers where adjusted).|
TutorialThe contrast blending actions can help you a lot, but you still have to find the right way of using them, since the default values sometimes don't work very good. I have written this tutorial to help you find a workflow which leads to satisfactory results.I assume you have first experiences with the actions and some images to work with. You can take the images above if you want, but I suggest you shoot your own. You'll learn easier by solving your own problems ;-)
|First open the darkest image in Photoshop. Then use the actions to apply the other images starting with the next lighter and ending with the lightest. You should end up with the darkest image as background and layers from the darker at the bottom to the lighter at the top. (Note that you can reorder layers in the layer palette by dragging them)|
|The resulting image is a bit dull and too bright for the interior.|
|Now start at the top switching layers off until the remaining scene appears to have good contrast. In our example switching off the top three layers make the outside scene look pretty good.|
|By switching the layers on again, we can see which layer is responsible for dulling the result. Here layer 5 makes it significantly duller.|
|Carefully reduce layer opacity until the result looks good again. Always try to optimize the brightest areas that are affected by this layer. Here the opacity is reduced to 15% to increase contrast for the objects on the balcony.|
|Proceed to the top layer by layer. Be careful not to dull previously better parts and not to make areas lighter than they should be in reality. However, you should try to use the maximum opacity that still gives a tolerable result. In the example layer 6 affects the interior shadows. It could be set to 30%.|
|If you set the layer
opacitiy too high, darker parts in the
image may become too light. Consider two areas from the scene, the walls and the shadows in the corners. If you set the layer opacity to a very high value, the shadows may become even brighter than the walls. In our example a value of 20% would be good.
In images where you have smooth gradients
from very dark to very light all layers can be adjusted to have the resulting
gradient smooth again.
|The result shows tolerable good contrast in all parts of the image. Dark areas stay dark and light ones are light but not over-exposed.|
© Erik Krause 2004