Tuesday, September 7, 2010

Week 2 - Correcting Lens Distortion In PS

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The purpose of this blog is to:
  • Discuss and evaluate individual photos taken by Gerald Catalano, co-owner of Catalavro Concepts, Inc.
  • Offer one another critical information on techniques, camera settings, lens settings, lighting settings and whatever pertinent information went into getting the shot.
  • Learn from each other to improve our beloved craft.
  • Replies with photos and technical details are welcomed and in many cases, preferred. 
Week 2: Correcting Lens Distortion in PS
Sept. 5th 2010 - Sept. 12th, 2010
Legends of St. Johns Luxury Condominiums - St. Augustine, FL 

Last week we explored the use of an ultrawide lens to photograph a landscape scene. In that shot, the horizon line appeared straight when shot at approximately the same distance and exact same focal length as the image above - but with significantly different results. Distortions may not be as apparent in the example shot of the sunrise from last week because there isn't anything in the foreground to accurately offer us a sense of depth (no foreground objects or vertical lines), but when ultrawides are used on subjects that contain prominent vertical lines either in the subject or foreground such as walls on a building, the distortions become significantly more obvious. Today we are going to analyze an image that I shot using the Tamron 10-24mm 1:3.5-4.5 ultrawide lens - the same lens I used to photograph the sunrise shot from last week.

What is lens distortion? A simple and crude explanation of lens distortion is that it is the skewing or "distorting" of lines in an image caused by the curvature of the glass on the lens. This is primarily caused by the building and camera lens being on two different angles. Basically, the wider the focal length, the more "bowed" or "skewed" the outer edges of the subject become. While I can get uber-scientific with you as to why it occurs, my goal today is to teach how to correct it in photo editing software, specifically in PS (and PSE). My experience with ultrawide lenses has taught me some neat tips and tricks to minimize these effects, but there are times, such as in the example image above (or shooting architecture in general), when there is no getting around this!

If you take another quick look at the image above, it is a building shot from approximately 12ft. at a 10mm focal length. While the exposure is ideal and the colors are rendered well, the image still doesn't quite look right, does it? Do you notice that the walls are tilting inward at the top? This type of distortion is known as a keystone distortion and makes the building appear as if it is leaning backwards. While ultrawide lenses allow us to photograph subjects at very wide angles - there is a price to pay for such an allowance in the form of lens distortion, of which there are many forms and degrees. Today we will be dealing only with keystone distortion. While I have found many artistic uses for an effect such as this, shooting architecture for a keepsake print or for a client who wants an accurate representation of the subject would definitely make this image highly unacceptable and would need to be corrected. Luckily there is such a solution in photo editing software.

I use PS (Adobe Photoshop) CS4 and LR3 (Adobe Lightroom) primarily for these edits but there are many other programs to perform these tasks that work equally well. Today, we will only be learning the method as it appears in PS CS4. Let's go...

 Figure 1
The Lens Correction filter (choose Filter > Distort > Lens Correction) lets you address barrel and pincushion distortion, chromatic aberration, vignetting, and perspective errors. This opens up a screen that allows you to make adjustments.

 Figure 2
By using the slide adjustments found on the right side (specifically the Remove Distortion slider), you can make the necessary enhancements to your images to correct keystone distortion. Since each camera lens presents its own levels of distortion, there is no single setting that will correct keystoning, however, there are charts available online for a variety of lenses that will offer you a "starting point" for commonly used settings on commonly affected subjects. By sliding the remove distortion slider over +4.00,  I was able to remove the distortion from the photo. I used the grid lines available to me in PS to ensure that I was correctly aligning the vertical references.

 Figure 3

This correction does have a drawback, however, in that you lose a part of your image. To correct this, you must enlarge the photo OR crop that part of the image away. I tend to save my images as RAW files for future edits, prints or web applications, however, if you prefer to save them as jpegs, I suggest doing so at a common size used for printing, such as 5x7 or 8x10. This will allow you to print your images later without having to reopen and resize the images over and over. Since cameras do not natively shoot or save to these sizes, we must do so manually.

 Figure 4
Open a new file (choose File > New or Ctrl-N on Windows or Cmd-N on Mac) and set the size and resolution of the image to the preferred size in which the file will be saved.

Figure 5

A new dialogue box will appear with several options for size, resolution, and color mode. While I am not going to go into too much detail about all of these options, I can tell you that these settings will highly depend upon the final use of the image. Since images for non-professional printing are generally referred to in inches, make sure the unit of measurement is in inches and not in pica or pixels. You may change these using the drop down selection boxes. Pica and pixels are used for professional printing and web applications, respectively and will cause you to be quite disappointed when your 8x10 image created in pixels is the size of a flea.

If the final images is for print enlargement, the resolution should be set to 300dpi. If it is for a website application or to upload to a share/social networking site for viewing, then 72dpi is acceptable and in most cases perferable since this will dictate the overall size of the file. A dpi setting means "Dot Per Inch" and directly affects the overall quality of the image (more is better in most cases for print). In general, high quality prints requires larger file sizes and web applications require less. Besides, uploading huge files to Fb, Flicker etc.can be a long process...

The color mode is important also: if you are printing this image on your home printer or local drug store lab printer, select RGB (in most cases - although many larger format home printers are now CMYK also - so if you have more than 4 cartridges of ink in your printer, select CMYK), however, if the image is being printed at an offset printer (for brochures or business cards) select CMYK. Check with your local printer first to see what they suggest as the results can be quite different.

The highlighted part of Figure 4 is for a file name. I tend to rename my files when I save them for easier reference later on (I generally use the original file name with an  _REDO at the end [ e.g: DSC1??_REDO]) but you can place whatever you want there, so long as it's something you can remember later or better yet, organize within your computer!

Figure 6

Now that my 8x10 file is open, I am ready to resize my picture.

Figure 7
The way I do it is to go back to the original file that we corrected and Ctrl-A to select the entire image, type Ctrl-C to copy that image and then go back to the new 8x10 file we just created and hit Ctrl-V to paste that image into the new file. It will always paste it dead center, which is great as we'll see later. Also, if you notice, the image does not take up the whole frame; why? Because we resized that image to be 8x10 even though it isn't...yet

Figure 8

Click Ctrl-T to transform the image (so you can control the image and affect its size). You will notice a box surrounding your image which has little points at each corner and in the center on each vertical and horizontal line. These are used to  "handle" the image. If you grab one of these points and drag your mouse in any one direction, you will be able to move that reference point without affecting the others. This is NOT ideal for image resizing, as it will create a skewed image. Try it...just hit Ctrl-Z afterward to undo it.

The best way to resize the image evenly is to hold the Shift button while you grab the points at the corners. Hit Ctrl-T again (if its not already) and go to the lower right hand corner (if you cannot see that corner, hold the ALT button while using the mouse scroll wheel and you can zoom in and out accordingly). Typically, I drag the lower right down to the lower right and the upper left up towards the upper left while maintaining the image in the center. I will stretch the image until I can no longer see any of the parts that were cut away during the lens correction steps. If you need to re-frame the image after stretching it, just simply grab the center of the image and move it until you are happy with the results. For even better results, go to (choose View > Rulers) and use the center of the image as a guide to properly place your images.
Note: Holding the Shift button is NOT required in PS Elements!

That's it...let's see what we have as a final image.

  • What tips or tricks for correcting keystoning have you used before?
  • Do you have any preferences for resizing that were not covered here?
While shooting with ultra-wide lenses, be aware that barrel distortion and keystoning are more than likely to occur at the widest focal lengths. As well, vignetting is quite possible also...especially with the use of a circular polarizer!

  • Try to keep ultra-wide lenses on a horizontal plain with the subject- refrain from pitching up or down to get a shot if possible.
  • When shooting straight vertical lines (e.g: architecture), always shoot a lot closer to the subject than you may have intended. Wide lenses are not really meant to be used to squish everything into a frame, but more so to capture entire foregrounds while also capturing the background settings. So move CLOSE!
  • Try to shoot low for low subjects and higher up for elevated subjects.
  • Don't shoot people with ultra-wides - unless you are looking for that distorted effect.
Next Week:  Review of the EyeFi Pro SD card -a wireless card that allows you to wirelessly move images directly from the card in the camera to your computer over a wireless network, ad-hoc connection or ATT wireless hotspot as well as Geotag your images using non-GPS triangulation.

As always, I welcome constructive feedback and great suggestions for improving these techniques...happy shooting!

    1 comment:

    1. Gerald! My head is going to explode! Lol. Thanks for the great info. Keep 'em coming!