Managing the challenge of narrow depth of field in highly reflective surface environment
By Avi Roth
In the field of commercial photography, be it in motion or static, every category presents a challenge of its own. With jewelry photography, an object of highly reflective surface (HRS), the challenge is amplified due to its relatively small size captured mostly in close-up environment with shallow depth of field, surface complexity (specular), and limited ways of lighting due to space restriction between the lens and the object. Jewelry photography demands knowledge and experience to overcome the compounded difficulty of controlling reflections and limited depth-of-field (perceived sharpness) in close-up jewelry photography.
In addition to the problems mentioned above, the success of reproducing and obtaining a high quality digital image of jewelry will depend on the equipment used in the reproduction process. There are basically three types of camera systems one can employ:
a. Digital SLRs like Canon, Nikon, Olympus, Sigma with good sensor resolution (6-14 MP)
b. Medium format cameras like Hassleblad, Rollei or Mamiya with a larger sensor digital back (11-39 MP)
c. Large format view cameras like Sinar or Toyo with live video studio solution (11-39 MP)
The purpose of this article is to discuss some of the principal challenges posed by the complexities of reflective surface reproduction, particularly in digital close-up photography. Below I will present some solution to these challenges from the lessons learned and experienced in my daily practice and from my own research as to what is the best, fastest, and simplest solution to an otherwise complex assignment of jewelry photography.
Jewelry surface (HRS)
The most difficult task of all HRS digital captures is jewelry. In essence all jewelry objects are built with reflective qualities of various degrees and can be compared to mirrors. Mirrors are objects with good specular or diffused reflection; that is, image forming qualities. Imagine pointing your lens towards a mirror at a close distance. What will you see? Exactly, yourself holding a camera and your surrounding area. Jewelry objects in essence embody three kinds of mirrors: flat, convex and concave determined at random by the jewelry’s very shape. Curved mirrors magnify or shrink images therefore distort the reflected image. Most jewelry objects typify spherical mirrors structured in an interesting relationship of concave and convex surfaces.
Solutions:
Understanding the basic reflective properties of mirrors can be helpful in solving some basic problems related to jewelry photography because of their common reflective properties.
a. Jewelry reflection has no physical existence. Conceal the camera or choose a shooting angle for the jewelry to reflect a controlled surface such as a white board or soft box.
b. The location of the camera/lens matters in terms of how and what will the jewelry reflect. Use longer lenses to extend the working space between object and lens.
c. When light strikes the jewelry surface the angle of reflection equals the angle of in incidence. Place your light source indirectly to the angle of reflectance.
d. The reflection on the jewelry is half the size of the reflected image – the jewelry is always halfway between the reflected image and the reflection. Creating and placing the jewelry inside a large soft tent enables the application of various light sources to minimize and control unwanted reflections.
e. If the jewelry surface is flat and perfectly shiny, specular reflection will result. If the surface is rough, diffused or distorted reflection will result (bending) Place the jewelry in a light zone of several loosely arranged large white panels, and by modifying their position relevant to the object will enable the control of light source there by controlling unwanted reflection.
Overall Sharpness:
A narrow depth-of-field (DOF) can create spectacular effects when photographing insects, flowers, etc., but in jewelry photography if the entire image does not appear in full focus, the image becomes an unusable one. Unusable in the sense that some parts of a ring or bracelet needed to provide useful data for the purpose of presentation will be unacceptably less sharp than other parts. For example, the front part of a bracelet (focal plane of the lens) is in sharp focus; but as we move towards the back region the loss of sharpness becomes unacceptable (blurred). DOF and image sharpness changes with sensor size. Smaller digital sensors obtain maximum sharpness at wider apertures (f:8-f:11) while larger ones at (f:11-f:22). Capturing small objects like jewelry via close-up photography presents a problem since only a very narrow portion of the object relative to the focal plane will be acceptably in focus. One way to extend depth of field would be to take images using a narrow lens aperture. This solution has a serious disadvantage. Small apertures bring more light diffraction, degrading the image resolution. This is one reason why many consumer digital cameras (with very small sensors) won’t let you stop down past f:8, while studio solution sensors are often stopped down to f:32 and beyond.
Solution:
The application of double exposure solution or photo stitching in jewelry photography partially solves the limitations of shallow depth-of-field. The technique known as “focus stacking” is another. Using double exposure when limited depth-of-field is available is a powerful technique to help photograph and present jewelry.
Depth-of-field determines what part of the jewelry has the sharpest focus and its amount. Unfortunately, at larger apertures depth of field limits attention to a particular focal points i.e. the top of a ring, while minimizing details at the shank. A greater depth-of-field would presents all the elements of a ring sharp at once, relatively equally. Depth-of-field is determined by the following: lens aperture (ƒ-stop), lens focal length and your distance from the focal point. In jewelry photography, of these three the aperture has the most influence and the least modifiable. Therefore to increase depth-of-field and achieve total sharpness over the entire image, move farther away from the jewelry by using a longer lens.
1. Capture two images, first focus slightly below the ring top, then a second focused lower at the center.
2. Bring both images into two separate layers in Photoshop to combine the two into one sharp image.
There are of course other solutions such as the Helicon Focus. This program is designed to manage the shallow depth-of-field problem in close-up imaging. It also aligns the images that change their size and position from shot to shot. This function is especially important with close–up jewelry photography.
However, the most pragmatic answer to the depth-of-field challenge is the large format studio camera. These cameras such as Arca Swiss, Linhof, Sinar, Fuji, Calumet or Toyo have expandable bellows, tilt and swing capabilities, and are allowing the capture of high-quality images with extended depth-of-field beyond the optical limitations of the lens.