Image Stacking Composites - Tests
This is the first full scale test. Using rise, fall and shift as before to capture the greatest area possible of the lenses image circle. Combined with racking the focus to generate enough images to describe the entire field of view as 'in focus'. This test image is riddled with errors. Most of these errors are surmountable but require an further increase in tolerances of the current imaging process. Currently the source images for each composite are captured via a Canon 350d attached to a 4x5 studio camera with large range movements. Because of the area of the 350d sensor is so small a great number of individual exposures are required.
The sensor is effectively in the back of a box, the shutter-box. Because of this not all of the image circle (approx 301mm at f22 from 210mm lens) can be captured. Also due to the angle of the mirror for the viewfinder only a very small area of the total image circle can be seen and focused. This never effected the early images, as i used an angle finder to amplify the viewfinder and acquire focus. This point of focus would then be same across the entire set of images made via the movements. By introducing the need to change the focus to acquire enough information to make a all in-focus composite, many of these early limitations become major problems. The issue with not being able to use the viewfinder outside of a small image area makes it incredibly hard to find all the required focus points for the image stacking. I have found a few ways to circumvent this but non of them is as satisfactory as being able to use the viewfinder. As the camera is mounted about 80mm from the 4x5 insert its not possible to use the ground glass focus screen. The next major hurdle is repeatability.
With each whole image being constructed from upwards of 300 source images the margin for error is high. Each image must overlap approximately 25%. This leaves room to wriggle. But as soon as focus stacking is introduced the number of images skyrockets to anything upward of 650. Additionally the overlap needs to be even greater to offset the change in focal length caused by shifting the focus. This focal shift manifests itself in the focus stacked images having a soft edge often reducing the overlap below 15% and intern increases alignment errors. All of these errors can be seen in the test.
A live-view camera would allow for more accurate on the fly focusing but the best solution would be to use a medium format digital back. These have an adaptor that would allow the use of a magnify-able ground glass screen and also benefit from a large image sensor that would reduce the amount of total shot required.
Upon reflection i have decieded the best way to improve the image and get it nearer to my vision will be the use of a scanning back. This will increase the effective sensor size to 100mm x 80mm approx. As a scanning back will require being tethered to a laptop, instant review of the images will remove most of the aforementioned errors. An with potentially less than 30 images required, high tolerance repeatability will be easy.
Of course all of these errors and ideas pertain to a single vision of the images i want to create. Their are many other routes and experiments to be explored. Im particularly keen to explore some form of automation of the process. It not inconceivable to develop a system that manipulates the camera movements via stepper motors. I also have yet to try using any of the movements that manipulate the perspective and plane of focus of a recorded image. But all in good time.
at 6:45 PM
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