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Control Depth of Field in Macro Photography
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Control Depth of Field in Macro Photography

Multiple shots plus Photoshop gives you focus flexibility

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I love to shoot macro, especially of flowers.  As I focus on a macro subject I am always enchanted by the soft background due to the very shallow depth of field I see through the viewfinder at macro magnifications.


 

 

When you look through the viewfinder, you are seeing the image with the aperture wide open, giving the minimum depth of field.  This makes the image brighter and makes AF faster; it isn’t until a fraction of a second before the shutter fires that the lens is stopped down to the aperture you have set.  In this article, which I've prepared exclusively for the Adorama Learning Center, we're going to see how we can also control depth of field using Adobe Photoshop and Lightroom, both of which are available through Adorama.

In order to see what the image will look like with that aperture, you can press the depth of field preview button on the camera body, which will cause the aperture to be stopped down to the set value.  The image will be darker (because smaller apertures admit less light) and for a very small aperture it may be too dark to see much.  In this case, switch to Live View (if your camera body supports it) and the image will be displayed with normal brightness (unless you are in Manual mode or have an exposure offset, where it will be displayed indicating the set exposure).

But the limited depth of field that is so beautiful for the background may not work well for the main subject, which is often frustratingly three-dimensional.  That was the case with this small flower, which was only half an inch across but about a quarter inch in depth.  If I stopped down to get more depth of field on the main subject, I would loose the soft beauty of the background elements.  Here is the scene at f/13.


Of course, if the background elements are much more distant than the subject’s distance from the camera, they will be soft anyway. But when they are close, as in this case, I can shoot at a wider aperture and call Adobe Photoshop and Lightroom to the rescue. 

I can use the Auto Blend Layers command to combine several wide-aperture shots focused at different distances and the sharpest parts of each will be combined. This is a wonderful technique to mimic a tilt-shift lens, which lets you place the plane of focus at an angle to the sensor.  It is very successful with a planar subject such as a dew-covered spider web that is angled away from the sensor plane.  (And of course you can use the same technique to composite focus with larger objects such as a still life or a landscape.)

I decided to try the technique with this flower.  I will use it to show you the technique, and also to show you a pitfall.

In the camera

You need to place your camera on a tripod for this technique, which is something you should do anyway for macro work. Even with an image-stabilized lens, the more magnification you have in a shot (the more “macro” it is) the less depth of field you have for any given aperture, and the more critical it is that the distance between the subject and the lens be very steady. Even the slightest back-to-front movement from hand holding between the time of focus and the time of shooting is going to throw the focus off.

After you have a steady platform you simply need to make a series of shots at different focus points. Leave everything the same except the focus. Go to manual focus (which you should also be using for macro work, so you can control where the focus is targeted) and just advance the focus minutely for each shot in a series, or use a macro focusing rail to move the camera slightly forward or back. How many shots you need may take some experimenting for each subject, depending on the depth of field and how 3-D the subject is, but it is usually between 10 and 20. Too many shots is better then too few.

You can see focus more accurately using Live View, if your camera has it, but you risk slight camera movement pushing buttons. With a solid tripod you should be OK, though. You can add steadiness to a tripod by being sure the legs are spread fully (pull each one out individually to be sure it is fully extended and “dug in” with some tension on it). You can also hang your camera bag from the hook on the center column. Be sure everything is tightened, including the rotation collar if the lens has one, and the center column tightening adjustment. Leave the center column in its lowest position.
 
Go to Live View and use the joystick to move the center rectangle to the critical focus area. Zoom in, focus and shoot. Try to keep a high shutter speed or use a remote release with mirror lockup or Live View. If the shutter speed is in the range of roughly 1/30 to 1/2 sec, vibration from mirror slap can be an issue.  Using flash as the main light source, overpowering ambient light, will give an effective very fast shutter speed.

Then, tweak the focus to the next distance, which may entail moving the Live View center point. I want to minimize camera movement between shots so I usually just do my best to eyeball focus through the viewfinder as I turn the focus ring just enough to see a change. 

With the technique of multiple shots, subject movement is much more of a problem than with a single shot because now you have the added issue of movement between exposures as well as during an exposure. You want to minimize that as much as you want to minimize camera movement. If you are dealing with a small flower on a long stem you will probably need to move it indoors.

Pointing a camera at something always seems to trigger a slight breeze when there was none before. When you are in Live View and zoomed in you will see movement that is hardly noticeable through the viewfinder. But if you can see it there, it will be significant unless you have a very fast shutter speed.

In the digital darkroom

The series of shots can be assembled as layers in Photoshop and the sharpest portion of each one selected, automatically.

I’m using Lightroom here, which I love. I select the series of shots in the filmstrip and make any desired raw adjustments to the “most selected” one, the one on the left in the figure below. 

 

Then I click Sync (circled in the figure above) and the same adjustment is applied to all the other selects. 

Then I go to Photo > Edit In > Open as Layers in Photoshop.

 

 

If you are using Bridge, select the desired files and click File > Open in Camera Raw. Make the desired adjustments to the first image and click Select All then click Synchronize, as shown in the figure below. Choose the desired settings to sync (in this case, all). But don’t click Open Images; instead choose Done and your raw settings will be saved for each file.

 


Then back in Bridge, with the same images still selected, click Tools > Photoshop > Load Files into Photoshop Layers.

From either the Lightroom or the Bridge workflow you will now be in Photoshop.  Look at the Layers palette / panel and select all the layers by holding Ctrl (PC) or Cmd (Mac) and clicking each one. All will then be highlighted, as shown in the figure below. 

 

 

Then click Edit > Auto Align Layers and choose the Auto default.  When it is done turn off the visibility of each layer using its eyeball icon (circled in the figure above) to check the alignment.  (I always find it’s perfect, but it’s important enough that I check it anyway.)

Then go to Edit > Auto Blend Layers and check Stack Images. (Make sure all layers are still selected.)  The sharpest portions of each layer will magically be masked to give a seamless composite. You can see the masks on each layer in the figure below. 

 

 

Scroll around the image at 100 percent to see if you missed any focus areas.  In this case, I found softness around the white center structure.  This is the pitfall I referred to above.


The softness was because the white area was very blurry in the shots in which focus was on the petals behind it. That blur caused the white area to “expand” and covered up the petal behind it. This is not surprising because the distance from the white structure to the petals behind it is relatively large.

For many images some repair work could be done by cloning, but in this case it would be difficult to work around the detailed white edges. The best fix for this will be to re-shoot the focus sequence at a higher f/stop, then layer on top a shot at a wide aperture with the desired background softness and mask this layer to reveal the composite focus flower. Be aware that at some point around f/16 to f/22 you will begin to see some softening of fine details due to diffraction of light around the very small aperture opening.

I always preserve my layers as much as is practical but in this case you might as well flatten the focus stack layers at this point. You won’t be likely to improve on the masking. Here’s my final result, after I added a few adjustment layers to enliven the colors.

 

 

There are other programs that combine focus points, such as Helicon Focus, but I find with recent versions of Photoshop (CS4, and even more so with CS5) that I am happy with its results. In this case Helicon Focus did no better than Photoshop in dealing with the problem area around the white structure. It has more features than are available with focus stacks in Photoshop but I don’t have enough experience with it to give you more information.

 

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