How To Create Seamless Long Exposure Effect In The Sky

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long exposure effect london canary wharf

Canary Wharf in London viewed from across the river

Long exposure effect is visually appealing and a demonstration of your skills as a photographer. Viewers are often impressed and assume it to be the work of an experienced photographer as you don’t see it often amongst beginners.

This means mastering the art of long exposure effect not only adds the wow factor to your image, it also boosts your creativity and confidence in creating more outstanding images.

Traditionally, an ND (neutral density) filter is needed to soften the clouds. This is no longer a prerequisite as digital photography has changed what we can achieve in post-processing.

If you didn’t use an ND filter during shooting and want the sky in your image to have that silky smooth effect, it’s possible to emulate that in post-processing. There are several techniques to create that but the main challenge is actually how to get that effect into every corner where the sky is visible without affecting other parts of the image.

The image above is an example of what I mean by “getting into every corner” (I'll explain more below).

In this post, I’m going to share a technique I’ve used recently to ensure the long exposure effect created in post-processing applies to all parts of the sky in a complex image.

Shooting Long Exposure During Blue Hour Is Challenging

Some of you might wonder why would you not use an ND filter when you want to soften the clouds.

Using ND filter is what I always prefer. But there are times when it's challenging to capture the moment with a filter on - one of these times is during the blue hour.

Blue hour refers to the short period of time before the sun rises and after the sun sets. In the morning, it’s the time between civil dawn and sunrise. In the evening, it’s the time between sunset and civil dusk.

Even though the sun is below the horizon during blue hour, there is still enough light to illuminate all things on earth. It’s called blue hour because the sky turns bright blue and dims to dark from there. It’s a truly magical moment for photography but you have to act fast.

One of the challenges I face during blue hour is shooting with an ND filter to create motion blur in the sky. The light intensity changes so rapidly that I find it difficult to achieve a fine balance between having a good exposure and getting the long exposure effect when the clouds are moving too slow.

canary wharf london unedited

Straight out of camera, unedited

I really wanted the long exposure effect when I was shooting Canary Wharf in London from across the Thames river. The weather was calm and my ND10 filter didn’t do the trick. I thought about using ND16 filter but the light was fading away (would have needed several minutes of exposure). Instead of messing around with filters, I quickly bracketed six exposures before the moment is over.

Why did I bracket exposure?

It has nothing to do with the long exposure effect. It's an editing technique I use to recover highlight details when I shoot night photography.

Blurring The Sky To Create Long Exposure Effect

Essentially, you want to render any details in the sky into soft, hazy pattern as if it’s painted by a paintbrush.

There is more than one way to create this effect in Photoshop or in the software of your choice. In fact, this is the easy part of the process.

The challenge is how are you going to apply this effect to the entire sky without affecting other parts of the image like the buildings or the foreground?

Let me elaborate that further.

When you apply a blur filter to the entire image intended for the sky only, blurred objects near the edges will bleed in to cause color contamination (image below).

motion blur for long exposure effect

The edges between the sky and the foreground are mixed together

Basic instinct tells us to apply a layer mask and paint it in or mask it with the Brush tool. That was exactly what I did at first but it was time consuming and it didn’t solve the issue with color contamination.

How To Create Selection and Mask Out Edges For Better Result

The only thing I could think of is to isolate the sky so that when I apply blur filter, nothing else would bleed into the field.

edges of the sky

Red line highlighting edges of the sky and the buildings

In this image, the edges where the sky meets the buildings are mostly horizontal or vertical straight lines. This makes creating a selection for the sky relatively easy.

As with other tutorials, I use Photoshop in my workflow but you can apply the same principle in any image editing software.

Step 1 - Create An Alpha Mask

Make a selection of the sky with any selection tool you're comfortable with. I used the Magic Wand tool and set the Tolerance to 20. To refine the selection along the edges, I used the Marquee tool to add and subtract smaller areas.

alpha channel

Click the icon to create an alpha mask for the selection

With the selection still active (marching ants on), go to the Channels Panel and click Save Selection As Channel. This generates a mask which Photoshop labels it automatically as Alpha 1.

alpha mask

Alpha 1 mask

Step 2 - Isolate The Sky As A New Layer

Go back to Layers Panel and make the selection a new layer. You can do this by placing your cursor within the selection, right click on your mouse and select Layer via Copy or use keyboard shortcuts to copy and paste (cmd+c then cmd+v).

You should now see a new layer with only the selection of the sky. The sky is now isolated from the entire image but you won’t notice any changes yet.

sky selection

Sky is isolated for blurring

Step 3 - Apply Blur Filter To The Selection

Before you apply any filter, make sure the new layer with only the sky is selected. Go to the Filter menu and apply the filter(s) of your choice to create motion blur that emulate the long exposure effect. You can refer to Adobe's support page for a list of filter that you can choose from.

Because the selection doesn’t have other parts of the image in it, you won’t get color contamination in the blurred sky.

If you 

blur filter for long exposure effect

Blur filters applied to selection

Step 4 - Mask The Layer With Alpha Mask

Go back to Channels panel to select the Alpha 1 mask you created earlier. Hold down cmd (Mac) or ctrl (PC) and click on the Alpha 1. You should see marching ants around which means the mask is now selected.

Return to Layers Panel, make sure the layer with only the sky is selected. Now click on the Layer Mask icon at the bottom to load the Alpha 1 onto the layer as layer mask.

What this does is to create well defined edges so the blur filter effect is confined to the sky. The image below shows why masking the layer with Alpha 1 mask is necessary.

Step 5 (Final) - Filling In The Gaps

Every image is different. Depending on the nature of the edges and the type of blur filter you apply, some areas may not be covered by the effect.

Zoom in to inspect your image by turning the layer on and off with the eye icon on the left of the layer.

If there are areas without the effect, use the Clone Stamp tool to fix it. These areas are usually small and you may not even notice it. But if you’re going for perfection, it’s a quick and easy job!

Is Neutral Density Filter Still The King?

I’ve always read the long exposure effect created using ND filter cannot be reproduced by software in post-processing. I still do have strong opinion on that but this experience made me think if this is entire true.

Even if the visual appearance of long exposure effect created in software is in par with that of an ND filter, I would still prefer the latter because it just saves so much time messing around in post-production.

No doubt ND filter remains the gold standard for this, but I feel we (as photographer) shouldn't dismiss other methods that can produce similar effects. It's never a bad thing to learn a new skill to add to your arsenal!

Although I show you how to apply this effect in a cityscape image, I do believe you can use it in any other types of images.

Hardware Vs Software

Software are getting more advanced and intelligent with every new version. I have no doubt that one day we’ll be able to do a lot more digitally than we are now.

Are you a follower of the old school where everything should be done right in camera? Or are you embracing the technology with an open mind?

For more tutorials on image editing technique, please check out the editing technique resource page!

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