Monday, 5 August 2013

Focus Stacking an Introduction

Focus Stacked from Three Exposures
Focus Stacked from Three Exposures

Focus stacking is a technique used in photography to extend the depth of field in a photograph beyond what the lens’s highest f/stop is capable of rendering.

Normally to increase the depth of field photographers stop down the lens (use narrower apertures) but beyond a certain limit narrowing down the aperture will cause diffraction which significantly degrades the quality of the photograph. Also choosing higher f stops increases the exposure time.

Application of Focus stacking

Focus stacking is not suitable for every occasion that requires more depth of field. Typically focus stacking is commonly used in the following genres of photography.

Macro photography:

Lack of depth of field is the most severe issue faced by macro photographers. The greater the magnification the shallower the depth of field; often times the lens’s minimum aperture (f/22, f/32 etc depending upon the lens) fail to achieve sufficient depth and stacking multiple images remains the only option.

Landscape photography: 

Landscape photographers often desire everything in their frame (from a few inches in front of the camera to infinity) to be in sharp focus, combining two of more photos to obtain the required depth of field helps avoid softness at the focal plane caused by diffraction.

Low-light photography: 

Stacking multiple photographs help photographers avoid using prohibitively long exposure times, to freeze motion in parts of the frame while capturing expansive depth of field in others without using flash etc.

Limitations of Focus Stacking

Even though focus stacking is a very powerful technique and at times is the only way to achieve the required depth of field; it definitely has certain disadvantages:

It requires elaborate set up, depending upon the subject being photographed, and the intended final use of the picture, photographers need to use accessories like, sturdy tripods, focusing rails, remote shutter release etc to precisely capture the many shots required to stack. Setting up all this requires a lot of time and effort.

To capture the various exposures necessary for the final stack the subject need to be motionless.

Focus stacking, especially those shots that require a large number of photographs need specialist equipment like focusing rails, it also requires specialized image editing software like Adobe Photoshop or Combine ZM to combine all the photographs into one.

Capturing, storing and processing many photographs instead of just one with sufficient depth of field require more storage space and computing power.

Alternatives to Focus Stacking

There is another technique called Depth of Field Stacking in which different images are taken at varying F stops and later combined to extend the available depth of field. Although this technique is easier than focus stacking, it cannot increase the depth of field as much as focus stacking can.

Alternatively the tilt movements of a Tilt/Shift lens could be used to capture various images by re-positioning the depth of field to different areas.

Focus Stacking Technique

Focus stacking requires several photographs taken at different focusing distances to be stacked into a single photograph which has the combined depth of field of all the individual shots. Focus stacking is a three stage process; the first is capturing the individual photos required for the stack. Second stage involves aligning all the individual photographs properly and the third stage is the actual merging and blending. We will look into each of these stages in detail.

1. Capturing The Photos

The most important decision a photographer need to take is the number of photos required for a particular subject. The key is to ensure that the depth of field of each subsequent photo overlaps with the depth of field from the previous photo. This will ensure that there is no softness in the final composite. More closely spaced focusing distances often produce more consistent and natural looking sharpness, but this can take a lot longer to capture and requires more storage space. On the other hand if the photographs captured have too widely spaced depth of field it will result in a wave like depth of field effect which could ruin the final composite.

Optimal F-Stop

Using higher f stops significantly reduces the number of photos required for the stack, but this can also decrease the sharpness of the final image. The trick is to find out the narrowest aperture that could be used (in the lens that you intend to use ) without causing diffraction. Generally for most DSLR lenses it will be somewhere between f/8-f/16.

Shooting Technique – Step By Step

  1. Set up your camera on a stable tripod.
  2. Connect remote shutter release if you have one.
  3. Compose your frame.
  4. Shift your lens to manual focus.
  5. It is recommended to shoot in fully manual mode and use RAW as your file format.
  6. Set your lens to the optimal f stop.
  7. You could either start from the portion of the subject which is closest to you or from the part of the subject that is farthest from you.
  8. In macro photography you have the option of either using the focusing ring to change focus or physically moving the camera back and forth. Both methods have advantages and disadvantages. Experiment and find the one which better suits your style.
  9. Live view mode in your camera is a great tool which helps focus stacking much easier. It is even better if you could shoot tethered to a laptop as the bigger screen allows you to precisely align the focus.
  10. Take as many photographs as required, make sure you overlap each frames depth of field by at least 25%. This is easier than you might think especially when shooting in live view or tethered.
  11. Get through your series as fast as possible as any change in subject position / lighting conditions could create many complications while post processing.

focus stacking
Photo captured by focusing on the farthest part of the subject

focus stacking
Photo captured by focusing on the middle part of the subject

focus stacking
Photo captured by focusing on the nearest part of the subject

2. Aligning the Photos

Effective focal length of a lens changes with focusing distance and as a result angle of view also changes, this makes the individual photos we took to be slightly misaligned. The effect is more like zooming in or out, when you focus nearer it appears as if you have zoomed in and when you focus farther it appears you have zoomed out. 

In Photoshop, click File > Automate > Photomerge.

Select the images you wish to stack together, make sure Blend images Together box is clicked and you could combine aligning and merging together.

Alternatively you can

Paste all your photographs as layers on a single document.

Click  Edit > Auto-Align Layers

3. Merging and Blending the Photos

Focus Stacking in Adobe Photoshop
Focus Stacking in Adobe Photoshop

With simple subjects which have well defined layers merging and lending is a pretty straight forward process just the same as aligning layers.

In photoshop click Edit > Auto-Blend-Layers

Select blend method as stacked images, make sure create seamless tones and colors are checked.

But in situations that have irregular layers you will not get satisfactory results from the auto blend method and will have to manually edit which photo contributes where by using the layer masks.

Zoom in and inspect your final result at high resolution for imperfections and make corrections using the layer masks if required. Once you are satisfied with the results 

Click Layer > Flatten Image and you are done.