Tuesday, November 5, 2013

Exposure Blending Tutorial: Why Auto-bracket Your Images?

Part One: Why Autobracket Your Images?

To download an expanded version of this guide as a free eBook, click here.

This is a three part series on the somewhat complicated, but incredibly beautiful art of exposure blending your images. This article here helps to explain the theory behind it all, and my next two articles will give you step-by-step instructions on how to:

1. capture images for exposure blending properly, and

2. how to blend the bracketed images manually in Adobe® Photoshop CS6.

At the end of this tutorial, you'll see how to take an evaluative image (left), and blend multiple exposures together for a complete tonal range (right):

This free eBook will detail how to go from the before to the after in a step-by-step instruction.

What is Exposure Blending?

Exposure blending is one of the most powerful ways to create a stunning photograph. As you've probably noticed, it can be difficult to capture an entire landscape accurately (especially those with a sky) because your camera sensor is limited to one aperture and one shutter speed at a time – it's not possible to have two different settings within the same frame. You'll often note that your photos rarely live up to what you actually saw – exposure blending is one way to overcome the limitations of your camera and photograph a landscape with the full tonal range that you see in the field.

Simply put, in order to have a well-exposed landscape you'll need one image exposed for the brightest tones in your photo (typically the sky) and another for the darkest tones (the ground, usually) when the lighting isn't suitable to capture it all in one exposure – think of sunsets and how differently the natural light level is when you compare the sky to the ground. Once you have your two extremes, you'll blend them together to make one perfectly exposed landscape.

I love to use visuals so let's look at my image below. As you can see, the setting sun is beautiful but caused some problems in exposure…the image where the foreground is exposed well (slower shutter speed) has an extremely overexposed sky, while the photo exposed for the sky (faster shutter speed) has the foreground in complete shadow. To get a proper exposure, I would need to combine these two images to get my result shown – and that, my friends, is exposure blending.

In addition to these two photos on the extreme ends, a middle exposure (one that's been light metered for the entire frame using evaluative metering, which finds a middle ground between your fastest and longest exposure) is useful to have as well for any middle ground elements you need to blend in. At times, the gap between your fastest and slowest exposure may be so great that you end up skipping the optimal exposure for those elements that fall in between the two. An example of this would be the water in the above scene. Notice how the image exposed for the ground (left) has the water slightly overexposed, and the image metered for the sky (middle) has some noticeable shadows. A middle exposure was needed to balance this out, and is what you see in the final blended scene (right). For intricate images with vast tonal ranges, it's not uncommon to have 5 or more photos of the same scene at your disposal in order to avoid any lost data.

The range of your exposures will depend entirely on how bright your highlights are and how dark the shadows appear – in other words, how vast your tonal range is. For example, a cloudy day will have very little difference in values, but a sunset image may have 5 or more stops in your complete tonal range.

How do I combine different exposures? It's actually quite simple once you learn the technique – and DSLRs have made it a lot easier with auto-bracketing.

How Auto-Bracketing Works

To get three different exposures of the same scene, you don't have to calculate your settings and manually input a different shutter speed. Auto-bracketing is a common feature with digital SLRs, which allows you to capture one photo exposed normally (your base image), and then will automatically expose two more – one directly after the other – of a slower and faster shutter speed than your base. The difference in exposure will depend on your camera and chosen settings, but usually 2 full stops is the maximum increase/decrease from your base photo that you can use auto-bracketing for.

This is where the term "auto-bracketing" comes from since you're bracketing your images off automatically by capturing the same scene at different exposures within "X" amount of stops.

This allows you to capture three sequential images of different exposures without having to adjust your settings manually – it's very handy, and helps to streamline your workflow when you're presented with a wide tonal range that one exposure can not contain.

For more intense differences in lighting – as you'll often find for sunrises and sunsets – you'll still have to adjust your settings manually to capture more of a range in exposure than two stops (or whatever your camera's auto-bracketing limit is). I'll explain how to do this in the next article, but first I want to show you why you should auto-bracket instead of adjusting just one RAW file.

For brevity, a RAW file is the digital equivalent of a film negative before it is printed onto paper – the "raw" state of your image that

you can't really use as a photo, but all the information you need to create a photo is there. A RAW file is just that – you can't upload a RAW file to your website or email it to others without special viewing software, but you can take that RAW file and turn it into a universal image.

RAW is preferable to JPG in many ways, most notably because of image quality. When you take a photo in JPG format, it is instantly compressed – and this compression will take detail away from your photo, thus limiting your processing capabilities.

With a film negative, you can adjust the exposure by exposing the negative onto your paper for longer or shorter amounts of time. RAW allows you to do the same – at least, for a limited amount of stops. Since you're working with the unprocessed digital "negative", you can recover some blown highlights or blocked shadows by adjusting the exposure of your RAW file before converting it into a more universal file format for display or print, such as JPG or TIFF.

You can artificially adjust the exposure of a JPG image in post process, but this is not a true exposure adjustment and will not recover any data for you. In the example images, we can see the true benefit of RAW when trying to recover these blown highlights. In Figure 1, you can see that while the foreground is exposed accurately, the sky is noticeably overexposed – a common occurrence.

RAW vs JPG comparison of adjusting exposure in Photoshop.

Figure 2 shows how much detail is recovered by reducing the exposure by two full stops in RAW editing. In contrast, Figure 3 displays the same reduction in exposure (two stops) for the JPG version. Since the JPG file holds no additional data, the exposure is artificially reduced simply by adding a layer of black tones – there is no recovery of data, and the photo looks noticeably worse than the RAW adjustment.

RAW vs JPG comparison of adjusting exposure in Photoshop.

RAW vs JPG comparison of adjusting exposure in Photoshop.

RAW does have its limitations however – it does not give you the power to shoot blindly with no regard to exposure, and it is always best to achieve a proper image in-camera. A RAW file will typically give you two stops of recovery capability – if you adjust the exposure more than two stops beyond the original exposure, you most likely will not recover any additional data and will start to notice the quality to deteriorate.

With RAW being such a versatile format for editing, why is there a need to auto-bracket when you can simply adjust the exposure in RAW and import all those exposures into one file? It would save you the time of setting up a tripod, taking three (or more) images, as well as saving on disk space – I thought RAW was supposed to be a magical exposure recovery tool?

It's always best to capture the optimal exposure for a scene in-camera simply because you'll have more data in an actual photo than an image adjusted artificially – even if it's in RAW. The more data that is retained, the more detailed your image will be. Auto-bracketing is superior to RAW for the following reasons:

1. You may need more range in your exposure than what RAW can offer you. Typically, you can only adjust your exposure +/- two full stops in RAW before you start to see damage – depending on what you're photographing, you may need 3, 5 or even 10 stops difference between your brightest and darkest photos.[/box]

2. While RAW is a great, low-damage, accurate way to adjust your exposure, it's still better to capture the image with your camera to make sure you have the highest quality photo.[/box]

I'm going to compare a scene where I underexposed my photo by two full stops using auto-bracketing with my camera vs. dropping the exposure in RAW.

Taking the overexposed version that was adjusted in RAW from above and zooming in on the recovered sky (Figure 1), you can see that there is still some overexposed areas. When compared to Figure 2 – the photo that I took two stops lower with my auto-bracketing – you can see that more detail was retained in the sky that RAW could not recover.

RAW vs. exposure blending comparison showing the difference between adjusting exposure in RAW and using a bracket of the same adjustment from the field.

RAW vs. exposure blending comparison showing the difference between adjusting exposure in RAW and using a bracket of the same adjustment from the field.

So while RAW provides a great recovery tool, it's more of a "last effort" way to adjust your exposure when you have no other alternative, or if you only need a very minor exposure adjustment. Adjusting your exposure in-camera will always give you a superior quality when compared to RAW adjustments, and is worth the extra effort of auto-bracketing.

Versatility of Bracketing

Although RAW can not replace auto-bracketing, it's still a file format that has many benefits over JPG and should be used when you bracket. Not only is there more data in RAW format, but you'll greatly increase your tonal range for editing. If you auto-bracket in JPG format, you'll typically be taking three images – your base image, one taken two stops faster, and another two stops slower – for a total range of four stops. However, if you shoot in RAW format, you can increase or decrease your exposures by two stops in post process. That safety net will add another two full stops to your range on both ends of your bracket, bringing your total tonal range up to eight stops (assuming you shoot three auto-bracket images in two-stop increments).

Blending exposures using one RAW file is best for those candid shots where you didn't have enough time to set up a tripod but need to correct some blown highlights, or simply want to brighten/ darken parts of your image – like in wedding or action photography. It's a fantastic way to save an otherwise unusable photo, but if you have the time and ability to auto-bracket your landscape for exposure blending, this is the method which will give you the highest quality output with the largest tonal range.

Exposure blending and bracketing is not just for combining bright skies with dark grounds – you can use it in many different ways. You can recover blown highlights in water reflections, use it as a no-damage way to dodge and burn your landscapes, or to blend a brighter foreground with a darker sky (like under stormy, overcast skies).

Another popular use of this technique is to blend different long exposures together – for example, blending an extended exposure that captures moving clouds with an image that shows stationary grass that would otherwise be windswept if taken at the same shutter speed as the sky: the possibilities are endless.

But first, you need to know how to get those exposures to begin with. In Part 2, I explain step-by-step how to capture different exposures the right way using a tripod.

To download an expanded version of this guide as a free eBook, click here.

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