Unleash Unlimited Depth Of Field

This Article Features Photo Zoom

Finished Image. Craig Blacklock defied the limits of depth of field and diffraction to create this image. He created the series of exposures shown here and on the next page, shooting each one for maximum sharpness in a limited area and then composited the final image together.

For the first time since the invention of the medium, photographers are now able to create images that rival our visual memory. For years, we’ve been using high dynamic range (HDR) techniques, combining bracketed captures to extend the range of exposure. Using the stacked focus technique, we can extend depth of field, as well.

When we view a composition, our eyes scan the scene, refocusing on every detail, then our brain composites everything we’ve seen into a visual memory with the entire scene sharp. Painters have depicted the world this way for years, but prior to the advent of stacked focus, photographers were limited to what a lens could resolve in a single capture.

The stacked focus technique allows us to overcome this limitation by making multiple captures, each at a slightly different focus distance, then use software that selects the sharpest pixels from this stack of captures to extend the depth of field through the entire photograph or a specific range within the photograph. In addition to extending the depth of field, the sharpness is enhanced, because each capture creates another focus plane that is in true focus—not just acceptably small circles of confusion, which appear sharp in small prints, but may look mushy in larger ones.

This technique is most commonly applied to close-up work. It’s also useful in landscape photography, however, when we want the depth of field to begin closer than that achieved when the lens is focused at the hyperfocal distance for the smallest usable aperture.

Which ƒ-stop Is Best For Creating Stacked Focus Images?
Depth of field is inversely proportional to the square of the magnification. This is why we have such a hard time getting much in focus as we move in closer to a subject or use longer lenses. Of course, we can regain some, or all, of that lost depth of field by closing down the aperture. Depth of field doubles every two stops we close down the aperture. But eventually we run into the fuzzy brick wall of diffraction. As light passes by the aperture blades, it’s diffracted. This scattering of the light causes an overall degradation of sharpness. The smaller the aperture, the more pronounced the effect. Images made at ƒ/22 will be slightly softer at the plane of focus than those made at ƒ/16. Those made at ƒ/32 will be dramatically softer than those made at ƒ/22.

So, we must consider both the circle of confusion and the effects of diffraction when selecting which ƒ-stop to use in stacked focus images. Since we’re trying to extend depth of field, we’re looking for that sweet spot where we get the most depth of field from each capture, without significant loss of sharpness due to diffraction.

For full-frame-sensor cameras, this is an effective aperture of ƒ/22, and for APS-sensor cameras, it’s ƒ/16. Keep in mind, the ƒ-stops marked on the lens are based on the lens focused at infinity. As we focus closer, the marked aperture effectively becomes smaller (example: marked aperture ƒ/11 is effectively ƒ/22 when the lens is focused at a 1:1 reproduction ratio).

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This Article Features Photo Zoom

How To Use The Stacked Focus Technique
Like many things we now do in photography, there’s a capture portion and then a processing portion done with specialized software. I’m presenting four examples and how to apply the technique to each.

Overall Extended Depth Of Field In Close-Ups. This is the most common use for the stacked focus technique. We use it for compositions that are treated like miniature landscapes with sharpness throughout the composition. It’s not appropriate for subjects that are isolated against distant backgrounds. (For those compositions, see the Extended/Selective Depth Of Field example.)

Because stacked focus gives us unlimited depth of field (starting with the near limit a lens can focus at), we no longer have to take depth of field into consideration when composing. This gives us creative freedom to work from any angle.

Here’s the step by step for capturing the images:

1 Securely mount the camera on a sturdy tripod. Lock it down well.
2 Set the camera to manual focus, aperture priority, mirror locked up, and use a two-second self-timer or remote shutter release. (Vibrations are magnified in close-ups; if either the camera or subject moves during the capture process, start over.)
3 Compose your shot with the focus at the closest part of your composition.
4 Make your first capture.
5 Carefully shift the focus back one depth of field zone and make your next capture. I simply estimate the depth of field I’m getting by zooming in on the LCD image from a test capture. The more images you make, with smaller increments, the sharper your results will be. Don’t be frugal here; the last thing you want is to process the image and discover you skipped over zones, resulting in bands in and out of focus. The higher your magnification, the more images you’ll need to make. Something the size of a coffee cup may only need three or four captures, while something the size of a postage stamp may require 30.
6 Continue to shift the focus and make captures until you’ve worked your way to the back of the composition. It’s not necessary that they be done in even increments. If there are very important details you want exceptionally sharp, you may want to do extra captures precisely focused on those.

While it’s possible to process the stacks with other software, including Photoshop, Helicon Focus produces the best results I’ve seen, with the most control. It’s also extremely easy to use. I would recommend the Pro version. At $200, it’s one of the greatest bargains in photography. As I ask my students, what would you be willing to pay for a lens that gave you infinite depth of field at any focal length?

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This Article Features Photo Zoom

Here’s the step by step for processing the images:

1 Process the entire stack of RAW images with your normal editing software, making sure they’re all processed identically. With current and older versions, it was necessary to save RAW images as TIFFs, PSDs or JPEGs, as the program didn’t see the edit changes to RAW files. I’m currently testing a beta version of Helicon 6, and it appears it will be able to read many types of RAW files, eliminating the need to save out a separate set of images.
2 Open the Helicon Focus software and load in your stack of images either by clicking on Add Images and navigating to the folder that contains them, or simply drag and drop them into the Source Images area of the program.
3 Select the Render Method. I always use Method B.
4 Select the Radius and Smoothing. For detailed images of nature, I personally start with a Radius of 3 and Smoothing of 2. If that results in halos around images, or significant focus skipping in large, smooth areas, I increase those numbers. The default is a Radius of 8 and Smoothing of 4. The lower you can keep the numbers, the sharper your results will be.
5 Click on Render. You’ll be treated to a show of the stack being aligned and analyzed by the program. It then will produce an output image made up of the sharpest pixels from the stack.
6 Zoom in on the image to see if any areas were skipped or if double images were created by the subject having moved between exposures. If there are, you can click on the Retouching tab at the top of the display area. This gives you two screens. One shows the source image highlighted in the Source Images, and the other shows the output image. Adjust the brush as needed and clone from the source image you’ve selected into the output image.
7 Click on the Saving tab and save in your desired format to the desired location.

Extended Depth Of Field In Landscapes. Here’s the step by step for capturing the images:

Steps 1 through 3 are the same as with close-ups, making sure to compose while focused on the closest element in your composition.
4 Focus at the hyperfocal distance for the lens and ƒ-stop you’re using and make your first capture (the composition will have changed some, due to the change in magnification when you changed focus, but don’t adjust it!).
5 Carefully pull the focus forward one depth of field zone and make your next capture. Continue to shift the focus and make captures until you’ve worked your way all the way to the front of the composition.

The steps for processing the images are the same as for close-ups.

Extended/Selective Depth Of Field.
This is used when you have a subject with some depth to it that you wish to have entirely sharp, isolated against a soft background. A common subject for this would be a tall flower stalk. The problem we traditionally encounter when using selective depth of field (selective focus) is trying to find an ƒ-stop that provides ample depth of field on the subject, while rendering the background sufficiently soft. We solve this by first using the stacked focus technique to create the subject component of the image, then making an additional image for the background at whatever ƒ-stop provides the desired amount of blur. This technique also can be used where the subject blends into the background and we want a very short transition zone from very sharp to very soft.

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Capturing the images:

The capture process for the subject is identical to that of a close-up in which we want overall depth of field. However, we stop making captures once we reach the back of the subject (not the back of the composition). Often, this only takes two or three captures, depending on the magnification and depth of the subject. In my example, I made two captures, one focused on each of the flowers.

Whenever possible, I remove the subject from the composition prior to making my capture of the background. The reason I do this is that the blur of the subject, if left in the frame, would extend outside the border of the sharply rendered subject and eventually need to be cloned out.

Here’s the step by step for processing the images:

1 Process the stack of subject images in Helicon Focus, then open the saved output image in Photoshop.
2 Open the background image in Photoshop and drop the subject image onto it.
3 Add a layer mask to the subject layer and make a selection of everything but the subject.
4 Fill the selected area of the mask with black to reveal the soft background image around your subject.
5 Adjust the mask edge as needed in the masks panel.

Processing Stacked Images Using Adobe Photoshop Photoshop is more limited than Helicon Focus for working with stacked images, but it allows you to experiment with the technique, and in some cases, Photoshop does a fine job. If you want to try Photoshop, here are the steps: 1 Process the entire stack of RAW images with your normal editing software, making sure they’re all processed identically. 2 Select images in Bridge, then go to Tools > Load Files into Photoshop Layers. 3 Highlight all of the layers in the Layers palette. 4 Go to Edit > Auto-Align Layers. 5 Go to Edit > Auto-Blend Layers. 6 Check Stacked Images and Seamless Tones and Colors in the pop-up menu. When output, all images in the Layers palette will have a mask that’s partly black and partly white, controlling which area of each image is used. If needed, you can retouch masks to correct for areas of skipped focus.

See more of Craig Blacklock‘s photography and find out about his books and workshops by visiting his website at www.blacklockgallery.com.