Exposure Blending Part I

May 19th, 2011

In this post, I will write about what I feel is one of the main strengths of digital photography: Exposure blending. As we know, the digital sensor/photographic film can capture both shadow and highlight detail as long as the range in exposure values falls within its dynamic range. In cases such as sunrises or sunsets, with the sun included, this is almost never possible. In those cases, if exposing for the sun, shadows will be blacked out and detail lost, while exposing for the shadows, the sun will be blown out.

Although a significant number of digital and, by implication, film camera users, swear on Neutral density gradient filters (NDGs), I prefer exposure blending in most cases, since I feel it gives the photographer a much richer tool set for technical perfection and creative outlet.

I prefer exposure blending for two reasons:

1) Mind the line: No transition lines/obviously darkened areas of the image. Although NDGs keep exposure in check, in tricky scenes, which arise surprisingly often, the NDG line (the border between coated and uncoated regions of the filter) may be obvious in the final image. This is especially so for hard stop NDGs. These lines can be very hard and time consuming, if not impossible to remove in post processing. Alternatively you would have to be very skilled, since such lines could be avoided with 2 or 3 carefully arranged NDG’s, a time consuming effort. With exposure blending one can carefully combine exposures that were taken only needs to use the proper masks (which will be a topic of future posts). I manually blend my images and avoid the HDR approach, which in my opinion is seriously lacking, especially for landscapes.

2) Chase the light: With transition lines no longer a worry, you can now follow rapidly changing light conditions from different vantage points and with varying compositions, and still end up with technically excellent images. You won’t loose out on precious seconds of beautiful light as you fidget around with filters. You only need to ensure that the highlights and shadows are captured in correctly bracketed exposures, which can easily be checked with the exposure histograms on your camera LCD. The uninitiated will be pleasantly surprised by how rapid and simple this procedure is.

The two images below illustrate particularly tricky scenes which I was able to capture within a matter of seconds allowing me to move on to the next subject/composition, and still come away with technically sound images. This would have been very difficult and lengthy, if not virtually impossible to achieve with NDGs, unless I was happy with some fat transition line through my work.

The image above was taken on the frozen shores of Lake Ontario, Canada. This scene had a rather high dynamic range, from shadows within the “ice cave” to strong highlights associated with the rising sun and the bright snow. Had I used a NDG to keep the sky and shadows in check, a transition line likely would have passed through the top of the cave and/or part of the frozen lake surface. Instead I took three exposures to capture the dynamic range and then was able to blend these images with suitable masks, in a way which I felt best reflected the scene as I remembered it.

The second image, above, is an image of High Falls on the East Side of Algonquin Provincial Park in Ontario, Canada. This scene exhibited an extreme dynamic range, since it was taken in the late afternoon when the sun was still strong, and a number of areas in the image were in deep afternoon shadows. In this case the a strong NDG filter would have put parts of the trees on the top of the waterfall in deep shadow, and artificially darkened the sky, just to keep the sun in check. The transition from the lighter bottom part of the trees to the darker top part would have been particularly obvious. In this image I blended only two exposures, but had to double process one exposure and tripple process the other. Although it was a lot of work, it cetainly was worth it!

There are however cases where blending exposures may not be the best approach. When bracketing for exposure, images are taken at different points in time. This is problematic if there are areas of the image which incorporate moving objects and which need to be blended. Typical examples would be moving clouds or water; even a lake’s shimmering surface would be a case in point. For this a NDG comes out tops, since the exposure is kept in check in one single shot.

A strength of the NDG that is often overlooked, is the fact that it reduces flare in the camera. By flare I do not mean the obvious streaks or lines that are introduced, but the more subtle aspect of light bouncing around in the lens, thereby reducing contrast and sometimes seemingly changing the color temperature of the scene – light will appear a lot warmer than what it actually was. This can happen even if the sun is not included in the scene. The photographer will place the darker areas of the NDG over the brighter region of the sky and thereby will reduce the total amount of light hitting the lens, reducing flare. With exposure bracketing one needs to be particularly careful, since when one exposes for the shadows a large amount of light from the brighter part of the scene will enter the lens and thereby introduce flare, even in the shadows! One should shade the lens from the brighter part of the sky with one’s hand (flare caps as supplied by lens manufacturers are in my opinion quite useless), and then blend accordingly. If there is flare, the effect of blocking out the light can easily be seen in the view finder. The image below is an example where I used my hand to block out the sun when exposing for the shadows.

The image above is a view, taken in later afternoon, of islands surrounding the Greek Island Amorgos, which forms part of the Cyclades in the Mediterranean. This also is a blended exposure. However, when exposing for the foreground, a significant amount of light from the setting sun entered the lens and introduced flare throughout the image. This killed the contrast necessary to convey the beautiful golden glow on the foreground shrubs. By shielding the lens from the sun (this meant my hand was blocking the top part of the image) I eliminated flare, and by blending with an exposure for the sky and sun I was able to realize this image In future posts I will describe in detail how this image came about.

That’s all for this post. In the next series of posts I will give some examples of manual blending, mask generation and will introduce the reader to an extremely powerful tool, luminosity masks.

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