Some image processing tips
Posted 03 December 2002 - 01:15 AM
I was asked to write down what i'm doing to improve my photos after i posted the "optimized" version of my scorpionfish last month. I'm not a PS pro but some of what i've learned about adjusting and correcting uw-photos might be useful for others too.
Yesterday i added a new section to my website with links to
Levels and Level Adjustment Basics (theory only)
Manual and Automatic Level Adjustment (adjusting and correcting levels for 6 photos)
How to correct too much depth of field (thats the one with the scorpionfish)
Posted 03 December 2002 - 10:11 AM
Auto Contrast simply moves the black and white points in until they hit "popular" pixel data. This feature does NOT correct color casts, only "washed out" or under exposed shots.
Auto Levels does the above, but it does it for each of the three channels. BUT! The problem is that underwater shots can have a good deal of blue in them which skews the results. If you're pointing your camera at a reef with a variety of colors, you should be okay. If you shoot a blue fish in front of blue water, Photoshop will go too far in correcting it with Auto Levels.
Auto Color in Photoshop 7 does what Auto Levels does, but it also shifts the gamma point in an effort to get neutral colors back to neutral. (I'll need to chat with an engineer to find out if it simply does it based on the number of pixels occupying a specific spot in the curve, or if they actually do testing of composite pixel values to reach neutral grey.)
In short, "Auto" anything isn't going to be quite as accurate as manual methods.
Sr. Evangelist > Adobe Systems Inc.
Posted 03 December 2002 - 10:17 AM
Thanks for not mentioning your mentor!!!
i should have mentioned you and Jeff Farris in the one with the scorpionfish.
On the other hand i'm "manipulating" my photos for quite some time now (several years), thats why i forgot to give credits.
Posted 03 December 2002 - 10:37 AM
as far as i understand the difference between AutoLevels and AutoColor is that AutoLevels is using another threshold value for the number of existing pixels to consider, i.e. looking at the histograms of an AutoLeveled photo shows that there are existing pixels outside the new level values.
AutoColor sets the new values exactly to the point, where existing pixels start.
This makes a big difference for many photos where Red is strong in the shadows but very weak in the midtones and highlights, which happens in many uw-photos.
Since PS7 introduced AutoColor, i'm using it right after the dive for photos that i want to burn on CDs for fellow divers. It's the fastest way to "optimize" them and works very well.
Posted 05 December 2002 - 02:17 PM
AutoColor sets the new values exactly to the point, where existing pixels start."
The difference between the two has to do with the gamma point.
Note that, if you're in the Levels dialog box, there is an "Auto" button. If you go into the "Options" within the levels dialog box, you'll see a bunch of... well... options. Each of the options listed are the "ingredients" for the three auto functions.
Again, no substitute for a human eye (and for having a pretty good knowledge about how channels work and what they contain), but it's pretty good.
Sr. Evangelist, Adobe Systems Inc.
Posted 27 December 2002 - 09:31 AM
thank you for the great tips. I have a scorpionfish foto with the same problems I am going to try your methods!!