In any ideal optical system, light, after bouncing off an object and passing through the system converges at a single point on the image plane (the location of the film or sensor). However, this is not always the case in any normal optical system.
As a general rule, the light rays that pass through the center of the optical system are more in focus than light rays that pass through the outer parts of the optical system. The more curvature in a part of the lens, the more the wavelengths of light split and hit the image plane at different locations. This is called chromatic aberration.
Another kind of aberration is monochromatic, which results without dispersion. The different types include Tilt, Piston, Defocus, Spherical, Coma, Astigmatism, Curvature of field, and Image distortion. You can read more about them here.
Bearing in mind the effect of curved glass on light and the resulting aberrations, this is why aperture has so much to do with how much of your image is in focus. The larger the aperture (the smaller the f/number), the more of the glass is exposed and the more the aberration takes place and lessens your DOF.
This can be either an advantage, or a disadvantage, depending on what you are trying to capture in your photograph. Usually with macro photography it is hard to get around having a shallow depth of field, which is why using the Helicon Focus technique can be so useful.
Description:
Helicon Focus is a powerful program designed to compile a series of images featuring a different area of focus in each image of a shallow depth of field (DOF) and to create a new image with an increased DOF.
This lesson will teach how to effectively photograph a series with different focus points and compile them with Helicon Focus.
Field Study:
What you need: tripod, remote release recommended
Find something with a large DOF – it could be an object, a flower, or architecture.
Set up your camera on your tripod and set it to manual mode.
Open your aperture to the largest it can get by setting it to the lowest f/number. Mine is f/3.5, but it depends on the lens you are using. Use the light meter on your camera to set the appropriate shutter speed for a good exposure for whatever point is about in the mid-range in the field.
Turn off auto focus and switch to manual focus.
If you have a remote release device, set it up now.
Set up your shot: start with focusing manually on the nearest point to the camera to determine your starting point for your series of images.
While trying to maintain camera stability, take a picture and alter your focus slightly so that a piece of the area in view is focused just slightly beyond what was in focus for the previous exposure. It is best when the sharp areas overlap. Make sure the differences in the focus areas are very small. You ideally want to end up with about 8 shots in the series.
I have found that on the low range, 3 or 4 images are acceptable for things with harder edges and objects of a more geometric design, like the piano image below. For objects with more detail such as flowers, you really want to have more for the program to work successfully.
Digital Darkroom:
What you need: Helicon Focus software. Click here to download a 30-day free trial or purchase the software.
First, process your images as you normally would for lighting adjustments, but keep them high resolution. I convert mine to JPEG because the files are smaller, but Helicon Focus can read RAW files if you choose to use those instead.
Next, I like to sort my images by putting a separate series in a unique folder. This makes the file selection process easier inside the program.
Open Helicon Focus. In the “Files” tab, select the “browse” option on the far right above the file list. You should be able to locate the folder where your photos are stored.
You will see a frame come up on the right showing each of the photos check marked. Verify that each one of these photos is a part of the same series.
When this is done, click “Run” above the list of series photos on the right. See below:
The program will then go through the series of photos and combine them. The progress is shown on the bottom right of the window with the red progress bar. You can run it several more times making adjustments of the Radius and Smoothing parameters.
The Help menu within the program recommends that if your image contains course lines, such as in my piano picture, that you should use a higher Radius, and that if your image contains fine details to use a lower radius. The Radius determines the pixel distance around each point that is used to determine if that point is in focus. I have also found that the Radius you use also depends on how many photos you have in your series. If you have less than the recommended amount, say a flower picture of only 4 stacked images, that it is also good to use a higher radius. Though my first
recommendation would be to go and shoot the series again, this time making sure you have at least 6.
The Smoothing parameter determines how the images are combined, the lower values being for a sharper image and the higher values for less noticeable transition areas. Again, it is usually better to use a higher value only when you don’t have enough images. As a rule of thumb, think more images, the sharper the better. Then you can use the lower tolerances and still have it come out okay.
Here is what the result of the piano sequence looks like. I had 6 images in the series, used a Radius value of 8 and a Smoothing value of 4.
In the case of images with more detail, such as flowers, you might experience a sort of “halo effect” around some parts of your image like the example below. This can be fixed manually through using the tools on the “Retouching” tab.
The "Copy Source" brush allows you to select an area of one of the original photos from the series and copy it to the equivalent location on the processed image. This can be used to eliminate noise by adding a version of the background to an image that was blurred out in one of the original images. Keep in mind you can change the size, hardness and sensitivity of your brush.
Another tool is the Clone brush, which is very similar to the clone stamp tool in Photoshop. You click the area of the processed image you would like to use as the “source” and then copy it to a “target” area. To select a new source area, hold down Ctrl and Left click the mouse. This tool is useful for creating a sharp background where a halo has appeared around part of your in focus image.
One tool that is really cool to use is the “Edge Sensitivity” function of the brush. When you push the level up, it is able to detect edges in your image and not copy or clone over it as long as it stays between the inner and outer circles. I highly recommend taking advantage of this tool.
At any time the “Erase changes” brush can be used to undo a change by holding down the mouse button and dragging it over an area that was previously modified.
