4 Small Structures and Macrophotography
This chapter discusses the special photographic techniques that are required for depiction of subjects smaller than 1 inch in diameter. Special macrophotography lenses and their characteristics are described, as is the use of a ring flash for appropriate illumination. Manual and hybrid focusing techniques are detailed to address the depth of field limitations inherent to the situation. Image composition for medical macrophotography is described.
composition, depth of field, focus stacking, focusing, hybrid focus, manual focus, macrophotography, ring flash
The head and neck are rife with small, important anatomic structures that can be difficult to capture photographically using conventional techniques. For subjects smaller than 2 inch in diameter, the use of macrophotography techniques is required. This requires specialized lenses and flashes, as well as different focusing and composing techniques (Fig. 4.1).
Lenses for Macrophotography
For medical purposes, the best macro lenses have a focal length of 80 to 200 mm. Shorter focal lengths produce an incredibly short working distance that is impractical for medical uses. Macro lens capabilities are also expressed using a maximum reproduction ratio, that is, the ratio between the actual size of the subject and the size of the image on the physical sensor. A 1:1 ratio means that a 35-mm object will occupy an entire full-frame sensor and thus will be magnified approximately 10× life size if printed at 8 × 10 in. It is important to realize that the actual reproduction ratio changes as the lens is focused. At the minimum focus distance, the object is magnified more, but at larger distances it is less magnified, or not magnified at all. Additionally, close focusing distances result in dramatic diminution of depth of field, even if an incredibly small aperture is used (Fig. 4.2). As an example, the depth of field with a 105-mm lens at 2 m is 6 cm at f/2.8, and 73 cm at f/32, but at the minimum focusing distance (30 cm) and f/2.8 depth of field it is 0.1 cm, and at f/32 only increases to 1.1 cm. For this reason, small apertures are always used for medical macrophotographs, which necessitates use of a large amount of supplemental light.