Introduction
85 millimetres is a focal length that has become a cornerstone in the world of photography and cinematography. When measured from the optical centre of a lens to its principal plane, an 85 mm focal length is considered a short telephoto distance on a full‑frame sensor, providing a moderate field of view that is well suited for a wide range of photographic applications. Because of its versatile character, the 85 mm focal length is widely used by professional photographers, documentary crews, and hobbyists alike. This article provides a detailed overview of the optical principles, historical development, manufacturing processes, performance characteristics, and practical uses of the 85 mm focal length.
History and Development
Early Lenses
The concept of a 85 mm focal length can be traced back to the early days of optical design. In the nineteenth and early twentieth centuries, the development of photographic lenses was driven largely by the need for sharper image reproduction and reduced aberrations. Early lenses of this length were primarily used on large format cameras, where the longer focal distance allowed for a more manageable working distance between the subject and the photographer.
Standardization of 85 mm
With the introduction of the 35 mm film format in the 1940s and 1950s, the industry began to standardise lens focal lengths to match popular sensor sizes. The 85 mm focal length emerged as a popular choice for medium‑to‑long focal length prime lenses due to its ability to provide a flattering perspective for human portraits. By the 1970s, a variety of manufacturers had begun producing 85 mm lenses, many of which featured advanced optical designs to minimize distortion and aberration.
Optical Principles
Focal Length and Field of View
In optics, focal length determines the magnification and field of view of an image. On a full‑frame sensor, an 85 mm focal length offers a field of view equivalent to approximately 47 degrees diagonally. This intermediate perspective strikes a balance between a wide‑angle view and a narrow telephoto view, making it ideal for capturing subjects with a natural appearance.
Lens Design and Element Count
To achieve optimal image quality, modern 85 mm lenses typically incorporate a multi‑element design that may range from 10 to 16 individual optical elements. These elements are arranged in groups to correct various aberrations, including spherical, chromatic, and coma. The number of elements, as well as the configuration, is a key determinant of a lens's sharpness, distortion characteristics, and weight.
Depth of Field
Depth of field (DoF) describes the range of distances from the front to the back of a scene that appear acceptably sharp. An 85 mm focal length, when combined with a wide aperture (e.g., f/1.4), allows for a shallow depth of field, which is often used to isolate the subject from the background. The relationship between aperture, focal length, and subject distance is governed by the hyperfocal distance equation, enabling photographers to predict the extent of background blur.
Image Quality Factors
Image quality is influenced by several factors, including resolution, contrast, color fidelity, and the presence of aberrations. The optical design of an 85 mm lens directly impacts these variables. For instance, the use of aspherical elements can reduce spherical aberration and improve edge-to-edge sharpness. Additionally, lens coatings play a critical role in minimizing flare and ghosting, thereby preserving contrast and color accuracy.
Manufacturing and Materials
Glass Types
High‑quality lenses use specialised optical glass with specific refractive indices and dispersion properties. Low‑dispersion (LD) glass is often employed in 85 mm lenses to mitigate chromatic aberration. Some manufacturers also use extra‑low dispersion (ED) glass in premium models, which further enhances colour accuracy, especially under high‑intensity lighting conditions.
Coatings
Anti‑reflection (AR) coatings are applied to each lens element to reduce internal reflections that can cause flare and reduce contrast. Modern multi‑layer AR coatings may involve more than ten individual layers of dielectric materials, each precisely engineered for maximum transmission across the visible spectrum.
Construction Techniques
Precision machining and assembly are essential for maintaining the optical alignment of the elements. Lenses of this focal length are commonly constructed using a combination of screw‑mounted and dovetail‑mounted designs. This structure facilitates accurate focus ring movement and allows for the inclusion of image‑stabilisation mechanisms in some models.
Applications
Portrait Photography
One of the most prevalent uses of the 85 mm focal length is portrait photography. The perspective offered by an 85 mm lens compresses facial features subtly, producing a pleasing representation of the subject. The moderate magnification also allows photographers to maintain a comfortable distance from the subject, reducing the chance of motion blur in handheld situations.
Cinematography
In film production, an 85 mm lens is valued for its ability to capture actors with flattering proportions while providing a shallow depth of field that isolates them from the background. The lens's moderate perspective also lends itself to establishing shots that require a cinematic, natural look.
Sports and Wildlife
While not as long as lenses typically employed for high‑speed sports or distant wildlife, an 85 mm lens can still be valuable for capturing candid moments in sports environments. Its compactness and relatively lightweight make it easier to carry during extended shooting sessions.
Macro and Close‑up
With the addition of a teleconjugate extension tube or a close‑up filter, an 85 mm lens can function effectively for close‑up photography. The inherent focal length allows for a higher magnification ratio compared to standard 50 mm lenses, making it a useful tool for photographing small subjects such as insects or detailed product shots.
Industrial and Scientific
In industrial inspection and scientific research, 85 mm lenses are employed for their ability to provide a consistent field of view and magnification across a variety of imaging systems. Their optical performance characteristics - particularly low distortion - make them suitable for machine vision applications that require precise measurements.
Lens Variants and Models
Prime vs Zoom
Primarily, 85 mm lenses are manufactured as prime lenses, offering a single fixed focal length. Prime lenses generally provide superior optical performance due to simpler optical designs and larger apertures. In contrast, a few manufacturers have introduced zoom lenses that include an 85 mm focal length within a broader focal range, typically 70–200 mm. These zooms sacrifice some optical perfection for the convenience of variable focal length.
Brands and Models
Major lens manufacturers have released a diverse range of 85 mm lenses over the past decades. High‑end models often feature a maximum aperture of f/1.2 or f/1.4 and incorporate advanced optical elements such as aspherical and ED glass. Mid‑tier options typically range from f/1.8 to f/2.8, balancing cost and performance. Budget models may offer apertures of f/2.8 or f/3.5 but generally contain fewer optical elements and less sophisticated coatings.
Digital vs Film
With the shift from film to digital sensors, 85 mm lenses have been adapted to accommodate various sensor sizes, including full‑frame, APS‑C, and APS‑S. Some lenses include an "FTZ" (full‑size to zoom) coating to reduce flare on smaller sensors. Additionally, modern autofocus systems have become integral to many 85 mm lenses, providing faster and more accurate focusing compared to older manual focus designs.
Performance Evaluation
Sharpness
Sharpness is measured by the ability of a lens to resolve fine detail across the frame. Modern 85 mm lenses are capable of delivering sharp images at wide apertures when used on a full‑frame sensor. Edge-to-edge sharpness is often assessed through test charts and is an important metric for photographers seeking the highest image fidelity.
Bokeh
Bokeh refers to the aesthetic quality of the out‑of‑focus areas of an image. The 85 mm focal length allows for smooth, creamy bokeh when coupled with a wide aperture. Lens designers achieve this effect through the curvature of the lens elements and the shape of the aperture blades, which influence the contour of the out‑of‑focus highlights.
Distortion
Barrel and pincushion distortion are common optical aberrations that can cause straight lines to appear curved. A well‑designed 85 mm lens typically exhibits minimal distortion, particularly in the centre of the frame. Minor distortion can be corrected in post‑processing using lens profile data.
Vignetting
Vignetting manifests as a gradual darkening of the image corners relative to the centre. Lenses of this focal length often exhibit mild vignetting when used at wide apertures, which can be mitigated by stopping down the lens or using image‑stabilisation. Advanced lens coatings also reduce the severity of vignetting by enhancing light transmission to the periphery.
Aberrations
Optical aberrations - including spherical, chromatic, and coma - can degrade image quality. High‑end 85 mm lenses mitigate these effects through the use of aspherical elements and ED glass. Lower‑cost models may present more noticeable aberrations, particularly at wide apertures, but still deliver acceptable performance for general use.
Technical Specifications
Aperture
Maximum aperture values for 85 mm lenses range from f/1.2 to f/3.5. The larger the aperture (smaller f‑number), the more light the lens can transmit, allowing for faster shutter speeds in low‑light conditions and the ability to achieve a shallower depth of field.
Maximum Magnification
At close focusing distances, an 85 mm prime lens can achieve a maximum magnification ratio of approximately 0.3x. This ratio indicates the size of the subject on the sensor relative to its actual size and is a critical metric for close‑up photography.
Weight and Size
Depending on construction, an 85 mm lens may weigh between 250 g and 1,200 g. Compact, lightweight models are commonly used in handheld photography, whereas heavier lenses with larger apertures provide superior optical performance but may require a tripod or stabilization system for optimal use.
Maintenance and Care
Cleaning
Proper cleaning of an 85 mm lens begins with removing dust using a blower or a soft brush. Lens cleaning solutions are then applied to a microfiber cloth to gently remove smudges and fingerprints. The cleaning procedure should be performed with the lens caps in place to avoid accidental damage to the elements.
Storage
During periods of inactivity, the lens should be stored in a dry, dust‑free environment. Using a lens cap or a lens hood while the lens is stored helps prevent scratches and dust accumulation on the front element. A lens bag or a padded case also protects the lens from impacts.
Repairs
Any lens requiring professional repair should be sent to a certified service centre. Typical repairs may involve resetting the focus ring, replacing damaged lens elements, or realigning the optical assembly. Replacing a damaged element typically involves a complete reassembly to ensure the optical performance remains consistent.
Common Issues and Troubleshooting
Focus Problems
Focus issues such as hunting or focus lag can result from a worn focus ring or a malfunctioning autofocus motor. If the issue persists, a professional inspection of the internal focus mechanism is recommended. Adjustments or replacements of the focus motor may be required.
Lens Shading
Lens shading can be caused by dust on the sensor or the lens, or by reflective surfaces in the environment. If the shading is consistent across the frame, it is likely a sensor issue. However, inconsistent shading may indicate a problem with the lens’s internal coatings or element alignment.
Barrel Distortion
Barrel distortion that deviates significantly from the lens’s specifications could indicate an optical misalignment or a damaged element. Calibration through lens profile adjustment or a return to factory settings may correct the distortion, but in severe cases a professional repair may be required.
Future Developments
Image Stabilisation
Several 85 mm lenses now include in‑lens image stabilisation, which counteracts camera shake and allows for slower shutter speeds in handheld shooting. Stabilisation mechanisms involve moving internal lens elements or an electronic stabilization system that compensates for motion.
Digital Integration
Increasingly, lens manufacturers are integrating digital processing elements into the lens body. These include micro‑processors that manage autofocus performance, lens data logging, and in‑camera corrections for vignetting and distortion. This integration improves the overall shooting experience and simplifies camera‑lens compatibility across multiple camera brands.
Conclusion
Over the span of 50 years, the 85 mm focal length has cemented itself as a versatile, high‑performance lens, particularly well‑suited for portrait work and cinematic imaging. Its moderate perspective, expansive aperture options, and reliable performance characteristics make it a staple in the arsenals of both amateur and professional photographers alike. Advances in glass technology, multi‑layer AR coatings, and autofocus systems continue to refine the optical quality of 85 mm lenses, ensuring they remain relevant in an era dominated by high‑resolution digital imaging and advanced shooting applications.
No comments yet. Be the first to comment!