Building Blocks – Gaussian Blur to Create Shallow Depth of Field (A GIMP Tutorial)…


While creating a tutorial for an AB Friday Forum, I came to realize that it would be so much easier to have a library of ‘building blocks’ that I could refer to rather than explain each step every time I used it in an image.  Hence the beginning of a series I have called Building Blocks.

I wanted to separate this series from my Workflow Series because these building blocks will not necessarily be used every time on every image.  But rather, may only be employed on a few images that need something a little extra.  Or be in the steps required to get a certain look.


150223_NAT_008-gimp-bg-blur60-mask-tut

Today, I’m going to focus on blur.  See how I did that?  Haha.  But seriously, I’m going to show you how you can add some blur to your images to give the appearance of shallow depth of field.

wilber You will need:

  • A photo editor, I used GIMP*
  • Your imagination

Shallow v. Deep Depth of Field

The way I was taught to remember the difference is to think of a swimming pool.  The shallow end is a smaller number than the deep end.  So if your aperture was set to f/1.4 then your depth of field will be shallow, giving you a blurrier background.  Alternatively, if you set your aperture to f/22 then your depth of field will be deep and everything will be in focus (Thanks Syl Arena).

pool

Hmmm… In my drawing of a swimming pool the water is rather choppy, what are those people doing in there?  Perhaps a lively game of Marco Polo?  Whatever, it’s supposed to be a drawing of a pool. 😛

Gaussian Blur

We want to simulate the blurred effect to force the viewer’s eye to our subject and away from the distracting background.  I’m going to use Gaussian Blur because I have more control over just how much blur I will need, which will vary from image to image.  The size of your image and/or how much blur is already in your image will greatly affect how much blur you will need.

First let’s pick a photo, how about this one…
150223_NAT_008-gimp-bg-no-v

Duplicate that layer duplicate-layer, then go to Filters->Blur->Gaussian Blur.  After experimenting with the Blur Radius value, I decided on 60 px for my 5202×3465 px image…
g.blur-options

If you don’t like the amount of blur and want to redo it, simply Ctrl-Z and try again.

Here’s what my image looked like with the Blur Radius set to 60px…

150223_NAT_008-gimp-bg-blur-tut

Image Size: 5202×3465 px … Blur Radius: 60 px

Don’t worry we’ll fix it in a minute.  What you are looking for is the amount of blur you want in the background, ignore the subject right now.

I tend to go subtle, but feel free to apply more blur if you want. Actually, let’s test it out… I’ll apply 20px to a 500x333px version of this image and we’ll do this on both of them…

150223_NAT_008-gimp-bg-blur20LR-tut

Image Size: 500×333 px … Blur Radius: 20 px

Mask w/ Radial Gradient

We are going to use a mask to remove the blur from the image where we don’t want it.  Rather than painting it and it looking obvious, we are going to use a radial gradient.

With the blurred layer selected, go to Layer->Mask->Add Layer Mask… and select White (full opacity)white-maskTo set the Foreground (FG) and the Background (BG) colors back to the default colors of Black and White, use the color block reset button…color-block-greyNow with the mask selected, click on the Blend/Gradient Tool gradient-tool and set the FG to Black, BG to White, Blend Mode to Multiply(on the tool NOT the layer), and the Shape to Radialgradient-options-radial-mult

Now click on the center of your subject and pull up, then work your way around the image to reveal more of your subject…gradient-in-use*** Make sure the tool’s blend mode is set to Multiply, otherwise each click will only reset the gradient rather than add to it. ***

If you were to turn off the visibility to your base layer, your blur layer would look something like this…blur-transparent-maskMy two images look like this after the mask and gradient…

150223_NAT_008-gimp-bg-blur60-mask-tut

Image Size: 5202×3465 px … Blur Radius: 60 px … Mask and Gradient Applied

150223_NAT_008-gimp-bg-blur20LR-mask-tut

Image Size: 500×333 px … Blur Radius: 20 px … Mask and Gradient Applied

And that’s it!  You now have a Shallow Depth of Field type of effect to your image.  Easy Peasy.

I wouldn’t do this to every image, and I’ll admit that there are some images that this just won’t work on.  But for the rest of them, it’s a nice effect.

I hope you learned something, and will try it out.  Let me know if you do, and how it went! 😀

Until next time…
~nic


* GIMP is Open Source software that is available for all platforms (Linux, Mac, and Windows), it is a photo editor that does many (if not all) of the things you can do in Photoshop. You can download it for free here. The GIMP online manual can be found here.

wilber

Like Wilber? You can get him here.


3 thoughts on “Building Blocks – Gaussian Blur to Create Shallow Depth of Field (A GIMP Tutorial)…

  1. Wow, Nic! I had no idea this could be done in this way. With my rudimentary skills, I would have tried some convoluted way of using the selection brush to grab that flower head. Actually, I would have tried and given up, because it would have looked fake. This is so cool! I admit I will have to sit down with your explanation on my iPad screen while I try to play with a photo on my laptop screen, but your instructions are very clear, so it shouldn’t be too difficult for me! Lovely effect, well done!

    • Thanks, Stacy! It was one of those things that happens… you know a light bulb just went off one day. I too used to try to paint out what I didn’t want blurry and it always looked bad and I usually tossed it out. I’ve been playing around with the different shapes for the gradient (Blend Tool) and it works really well as a vignette…. then light bulb… what if I had the blur fade away gradually just like it does in the camera. Voila! An idea is born. I tried it out and it worked really well. I hope you try it out too! Let me know how it goes. 😀

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