Stereo Imaging

Michael Perryman

Viewing suggestions

The images are prepared for 'cross-eyed' viewing. If you are not
familiar with the technique, view in a darkened room at the maximum size
possible, and from a distance of around 70-80 cm. You are aiming to make
your eyes converge, so looking just beyond your nose, but while actually
focusing on the screen. The technique is non-intuitive because in normal
vision the eyes are focusing and converging at the same distance.
The aim is to get the two images to merge into one, and it may help if
you first concentrate on making any text in the image fuse into one
single image. The effect may not come immediately, but persistence
usually pays off. Once you can 'lock on' to the stereo image, you can
look around the scene, and enjoy the stereoscopic depth. And once you
know how to trick your eyes to get the required effect, you can quickly
go from image to image

Fairy Cave Quarry, Mendip (2020)

Fairy Cave Quarry is a disused limestone quarry between Stoke St Michael
and Oakhill in the Mendip Hills of Somerset. Quarrying was first started
on the site in the early 1920s. In 1963 the quarry was acquired by Hobbs
(Quarries) Ltd., and production began on a much larger scale.
Excavations cut back into the hillside above St Dunstan's Well Rising,
and various caves were intercepted in the process.
All the caves in the quarry are unusually well 'decorated' with
speleothems. Withyhill Cave is one of the most beautiful in the country,
and contains a profusion of stalactites, stalagmites, helictites and
flowstone features. Thanks to Dave King (SMCC), our guardian, who also
let me use his camera to take these specific shots.

Technical

Most of these stereo images have been taken with a pair of identical
Olympus E-PL1 digital cameras (from 2010), mounted on a tripod with a
lateral separation of 6 cm.  I use two LEDs rated at 5000 ANSI lumens
for illumination. During a typical 10 second exposure, I 'paint' the
features that I want to pick out, skipping quickly over nearby or high
reflectivity surfaces. One trial exposure is usually enough to adjust
the painted areas in a follow-up exposure. I capture the images in raw
format, then adjust the colour temperature and saturation using Affinity
Photo. The image pairs are then aligned using a least-squares adjustment
of selected features, which accounts for the principal optical
aberrations including rotation and translation, but ignores the lateral
residuals due to the parallax terms. I carry the cameras in a waterproof
Pelican case, and my unsuspecting assistant carries the sturdy Benbo
tripod, which provides great stability in difficult positions.