Focal Length

Lead Summary

Focal length — measured in millimetres from the optical centre of a lens to the point where parallel light rays converge — is the single number most consequential to how a photograph looks. It determines the angle of view captured on the film, the apparent compression or expansion of depth, and, critically, how much of the scene falls within acceptable focus. Yet focal length is not an absolute: a 50mm lens is a "normal" lens on a 35mm camera, a wide-angle on a 6×7 medium-format body, and an ultra-narrow coverage lens on a 4×5 large-format camera. Understanding focal length means understanding it as a ratio — relative to the format it is paired with.

This article covers the optical and practical mechanics of focal length in film photography: how it interacts with format size, field of view, depth of field, perspective, and the physical constraints of lens-mount adaptation.

Core Concepts

Field of View and Format Size

The same focal length produces different fields of view on different film formats because field of view depends on both the focal length of the lens and the dimensions of the recording medium. A 50mm lens on 35mm film provides what photographers call a "normal" field of view — close to human perception. The same 50mm lens on a 6×7 medium-format body acts as a wide-angle lens with a broader field of view. On a 4×5 large-format camera, 50mm becomes an extreme ultra-wide. Conversely, a 150mm lens on 4×5 provides a normal field of view, while 150mm on 35mm is a moderately telephoto focal length (Focal Length Equivalents — Aram's Wiki).

The practical implication: larger formats require longer focal lengths to achieve the same field of view.

The Normal Lens

A "normal" lens is defined as one with a focal length approximately equal to the diagonal measurement of the film frame. This standard applies consistently across all film formats:

• 35mm film (43mm diagonal): 50mm
• 6×6 medium format (79mm diagonal): 80mm
• 6×7 medium format (~87mm diagonal): ~100mm
• 4×5 large format (153mm diagonal): 150mm

Perspective

Perspective in a photograph is determined by actual focal length, not by field of view or format size. Two lenses producing the same angle of view on different formats still create different perspectives: the longer focal length produces more compression, flattening the apparent distance between foreground and background. A 150mm medium-format lens and a 90mm 35mm lens can frame the same subject from the same camera-to-subject distance while rendering the depth of the scene differently (Format strengths — Ming Thein).

The 50mm focal length is particularly noted for producing rendering close to human eye perception with minimal distortion, making it well-suited for portraiture, documentary, and product photography where representational accuracy matters.

Depth of Field

Depth of field depends on actual focal length and f-stop, not on field of view. When photographers match field of view across formats by using proportionally longer focal lengths on larger formats, the depth of field at equivalent f-stops scales with format size. The equivalents are:

Format	Equivalent f-stop (to 35mm f/11)
35mm	f/11
6×7	f/22
4×5	f/45
8×10	f/90

If a photographer shoots the same focal length at the same f-stop on different formats, the larger format produces shallower depth of field in the final print, because more magnification is needed to reach an equivalent viewing size (DPReview TV Mythbusting — Medium Format DOF). This is why medium format cameras using 120 film produce shallower depth of field compared to 35mm cameras even at narrower apertures — the larger negative size enables greater subject isolation (The Advantages of Medium Format Over 35mm — PetaPixel).

Mechanism & Process

Crop Factor and 35mm Equivalence

To compare focal lengths across formats, photographers use the crop factor — the ratio of the 35mm frame diagonal (43.3mm) to the format's diagonal measurement. The 35mm equivalent focal length is calculated by multiplying the actual focal length by the crop factor:

35mm equivalent = actual focal length × crop factor

Since medium and large formats are larger than 35mm, their crop factors are less than 1 — meaning a given focal length on those formats covers more scene than it would on 35mm. The key crop factors are:

Format	Frame Diagonal	Crop Factor (vs 35mm)	Normal Lens
35mm	43.3mm	1.0	50mm
645	69.7mm	~0.62	80mm
6×6	79.2mm	~0.55	80mm
6×7	87.3mm	~0.50	100mm
4×5	153mm	~0.33	150mm

On a 645 camera, an 80mm lens produces approximately the same field of view as a 50mm lens on 35mm, making 80mm the practical normal lens for that format. On 6×6, the calculation yields 44mm (80mm × 0.55 ≈ 44mm), though 80mm is still conventionally treated as the normal lens because it approximates the format diagonal.

Wide-Angle and Telephoto as Relative Categories

The categories of wide-angle and telephoto are relative to format, not absolute. A 35mm lens is wide-angle on 35mm film, normal on 6×7, and telephoto on 4×5. A 105mm lens is standard on 645, wide-angle on 6×7, and shorter than normal on 4×5. Understanding how a given focal length will behave requires knowing both the lens specification and the format it will be used on.

Wide and telephoto are not properties of lenses — they are properties of lens-and-format combinations.

Variants & Subtypes

Portrait Focal Lengths

For 35mm photography, the choice between 85mm and 105mm portrait lenses is primarily one of working distance and perspective rather than optical quality. The 85mm enables closer engagement with subjects, making it ideal for head-and-shoulders and half-length portraits with flattering perspective and tight depth of field. The 105mm requires more distance but provides longer working distance for portrait sessions. Optical quality differences between comparable 85mm and 105mm designs from Zeiss, Nikon, and Canon are minimal; working distance needs typically determine the choice.

In medium format, the equivalent portrait range shifts accordingly. On a Bronica ETR 645 system, the 150mm f/3.5 Zenzanon provides a 35mm equivalent of approximately 93mm — a flattering telephoto perspective for headshots and fashion photography, representing the practical sweet spot between subject isolation and working distance on the 645 format.

On rangefinder cameras, the choice between 35mm and 50mm focal lengths is driven by genre: 50mm suits portraiture due to less distortion and flatter perspective on faces, while 35mm's wider field of view is advantageous for landscape and architecture. For street photography, 50mm's greater working distance is preferred by photographers uncomfortable with close proximity.

Ultra-Wide Focal Lengths

Ultra-wide focal lengths require careful handling. The Hasselblad SWC/M's 38mm Biogon lens provides approximately 90 degrees of diagonal angle of view on 6×6, equivalent to about 21mm on 35mm — substantially wider than the standard 80mm. Despite the extreme angle, the Biogon's retrofocal design allows it to focus as close as 30cm, and at f/22, depth of field extends from 1 metre to infinity, making precise focus less critical.

The Ricoh GR21, released in 2001, was the first compact camera to feature a 21mm ultra-wide lens. The combination of a 21mm f/3.5 lens in a compact body enabled close-focus street photography with extensive depth of field — photographers could work within arm's length of subjects.

Notable Examples

Macro and Close-Focus

Macro photography exposes the relationship between focal length, bellows extension, and working distance. For film scanning with a macro lens, optimal focal lengths vary by sensor type: 70–105mm for full-frame cameras, 50–60mm for APS-C sensors.

On the Hasselblad V-system, extension tubes are light-tight cylinders without optical elements that increase magnification by moving the lens further from the focal plane. The 80mm lens is considered the practical sweet spot for macro work because it retains useful working distance at higher magnifications while dramatically reducing depth of field. The Hasselblad Automatic Bellows, providing 63.5mm to 202mm of extension, achieve 1:1 reproduction scale when paired with the Carl Zeiss CF 135mm f/5.6 Makro-Planar lens.

The Mamiya C series TLR system achieves 1:1 magnification with its rack-and-pinion bellows mechanism using standard lenses without supplementary optics, focusing as close as 30cm.

Large Format: Bellows Extension and Focal Length Range

In 4×5 large format field cameras, bellows extension determines usable focal length range. Standard lenses require bellows extension equal to the focal length for infinity focus — a 120mm lens needs 120mm of draw. Telephoto designs allow shorter draw than their stated focal length (a 300mm telephoto may need only 200mm extension), while wide-angle lenses of 75mm and wider require approximately 200mm maximum draw. For subjects at normal distances, photographers need approximately 1.2× the focal length of their longest lens; for close-up work, approximately 2×.

On monorail cameras, interchangeable bellows systems allow selecting between standard and longer bellows — the Cambo SC-2, for example, provides a maximum draw of 18.9 inches, enabling both long telephoto work and extreme close focus.

Image Circle and Camera Movements

Large format lenses must project an image circle larger than the film diagonal. For 4×5 (153mm diagonal), a lens must cover at least 153mm, but 210mm or more is practically recommended to allow useful tilts and shifts. When movements are applied, an insufficient image circle produces vignetting at the corners. A single focal length — say 150mm — can be designed with a tight 230mm image circle for minimal movements, or a generous 386mm circle for maximum flexibility.

Mechanism & Process: Focusing and Focal Length

Rangefinder Accuracy Scales with Focal Length

Rangefinder focusing accuracy is directly affected by focal length choices. The Leica M3's longer effective baselength provides approximately 25% more accurate focusing than later M bodies (M4, M6), making it particularly suitable for precise work with longer lenses. The M3's 0.91x viewfinder magnification makes rangefinder alignment marks larger and easier to align when using 90mm and 135mm lenses.

However, the Leica CL and Minolta CLE have significantly shorter rangefinder bases than standard M bodies — the CL's effective base is only 18.9mm. This limitation makes it difficult to focus accurately with fast telephoto lenses at wide apertures: the 90mm f/2.8 Tele-Elmarit-M cannot be focused accurately wider than f/4. Practical workarounds include focusing at infinity and stopping down — smaller apertures increase depth of field enough to compensate for minor focus inaccuracies.

The Minolta CLE's viewfinder addresses this with framelines for 28mm, 40mm, and 90mm — the 28mm frameline making it more practical for wider-angle photography than the CL (which only covers 40mm and 90mm). The M4 introduced a native 135mm frameline, completing a practical working range of 35mm, 50mm, 90mm, and 135mm without auxiliary viewfinders.

SLR vs Rangefinder: What You Can Frame

SLR cameras allow use of the complete spectrum of focal lengths, from ultra-wide to super-telephoto, because the photographer views directly through the taking lens via the reflex mirror. Rangefinder cameras have physically separated viewing and taking lenses, creating parallax problems with very wide or very long focal lengths — effectively limiting their practical lens range.

SLRs also directly display depth of field in the viewfinder at any aperture setting, and provide focusing aids like split-image circles. This is not possible on rangefinders.

Zone Focus: Depth of Field as a Technique

Zone focusing exploits the relationship between focal length, aperture, and depth of field as a deliberate tool. On the Ricoh GR1/GR1s 28mm lens at f/8 with a 1.5m preset focus distance, the depth-of-field zone extends from approximately 88cm to 5.14m — a practical range for close-distance street photography without continuous autofocus.

The Hasselblad SWC achieves similar results on medium format: at f/22, depth of field extends from 1 metre to infinity. Photographers dial in a focus distance by eye without a rangefinder, relying on the 38mm Biogon's extreme depth at small apertures.

Mechanism & Process: Infrared Focus Shift

Infrared film photography introduces a direct effect of wavelength on focus plane position. Infrared light focuses differently than visible light due to its longer wavelength and differential glass refraction. The lens must be focused slightly closer than the visible-light focus point — approximately 1/400th of the focal length (focal length × 0.0025 mm). Many manual-focus SLR lenses include a red infrared focus mark to facilitate this adjustment, though most modern lenses lack this feature. A complementary strategy is stopping down the aperture, which increases depth of field enough to compensate for the inherent uncertainty in infrared focus shift.

Mechanism & Process: Flange Focal Distance and Adaptation

Every lens mount system is built around a flange focal distance (FFD) — the distance from the lens mount registration surface to the film plane, which must be maintained to hundredths of a millimetre for accurate focus across all focal lengths. FFD is the foundational constraint governing all lens mount adaptation.

The adaptation direction matters: when adapting from a longer FFD to a shorter one, a simple mechanical spacer is sufficient — the lens simply sits further from the film plane than its native design. But when adapting from a shorter FFD to a longer one, the lens cannot be positioned far enough back, requiring an optical correction element (glass) to maintain infinity focus. This correction element introduces additional glass and typically reduces image quality.

A canonical example: Canon FD lenses have a 42mm flange distance. Nikon F-mount bodies have a 46.5mm FFD. Because FD cannot be positioned 4.5mm closer to the film plane than the Nikon standard allows, a 1.4× correction lens is required — degrading sharpness and increasing effective focal length. The same asymmetry applies elsewhere:

• Pentax K-mount (45.46mm) and M42 screw mount (45.5mm) are nearly identical, making M42 lenses fully adaptable with a simple mechanical ring without optical correction.
• Olympus OM (46mm) and Pentax K (45.46mm) differ by only 0.54mm — too narrow for a mechanical adapter, requiring optical correction that makes the combination impractical.

Canon's transition from FD to EF mount reduced the flange distance from 42mm to 44mm (shorter), which means FD lenses cannot be adapted to EF bodies without optical correction elements — an intentional design incompatibility that prevents FD-to-EF adaptation without quality loss.

Controversies & Debates

Does Longer Focal Length "Compress" Perspective?

The popular claim that telephoto lenses "compress" perspective is partially misleading. Perspective depends solely on actual focal length and camera-to-subject distance — not on format or field of view. When a photographer uses a longer focal length from the same position, background elements appear larger relative to the foreground subject because the angle of view is narrower. However, if the photographer moves farther back to restore the same framing (matching field of view), the perspective does compress because the camera-to-subject distance has increased. The focal length itself is not the cause: physical distance is. The compression effect is real, but it is a consequence of where you stand, not which focal length you choose.

Medium Format Depth of Field: Is It Really Different?

Medium format cameras are widely described as producing "shallower" depth of field. The nuanced reality: depth of field depends on actual focal length and f-stop, not on format size directly. A medium-format camera shooting the same absolute focal length and f-stop as a 35mm camera from the same position produces shallower depth of field in the final print, because the larger negative requires less magnification — but equivalent angles of view require longer lenses and proportionally larger apertures to yield equivalent depth of field. "Medium format has shallower DOF" is true in practice because achieving equivalent framing requires longer focal lengths, and those longer lenses at the same f-stop number produce shallower DOF.