Hubbry Logo
Infinity focusInfinity focusMain
Open search
Infinity focus
Community hub
Infinity focus
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Infinity focus
Infinity focus
Infinity focus
View on Wikipedia
from Wikipedia
The infinity focus marked as ∞ on a Minolta lens

In optics and photography, infinity focus is the state where a lens or other optical system forms an image of an object an infinite distance away. This corresponds to the point of focus for parallel rays. The image is formed at the focal point of the lens.

In simple two lens systems such as a refractor telescope, the object at infinity forms an image at the focal point of the objective lens, which is subsequently magnified by the eyepiece. The magnification is equal to the focal length of the objective lens divided by the focal length of the eyepiece.[1]

In practice, not all photographic lenses are capable of achieving infinity focus by design. A lens used with an adapter for close-up focusing, for example, may not be able to focus to infinity. Failure of the human eye to achieve infinity focus is diagnosed as myopia.

All optics are subject to manufacturing tolerances; even with perfect manufacture, optical trains experience thermal expansion. Focus mechanisms must accommodate part variations; even custom-built systems may have some means of adjustment. For example, telescopes such as the Mars Orbiter Camera, which are nominally set to infinity, have thermal controls. Deviations from its operating temperature are actively compensated to prevent shifts of focus.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Infinity focus

View on Grokipedia
from Grokipedia
Infinity focus is a fundamental concept in and referring to the lens setting where parallel light rays from objects at an infinite distance converge precisely on the or plane, rendering distant subjects sharply in focus. This configuration is denoted by the (∞) on most camera lenses and is essential for capturing scenes where the primary subjects, such as horizons, mountains, or celestial bodies, are extremely far away. In practical terms, "infinity" begins at distances where light rays entering the lens are effectively parallel, typically starting at several hundred feet or more depending on the lens and sensor format, though true optical infinity implies no . The mechanism of infinity focus relies on the lens's and the , where for an object distance approaching , the image distance equals the focal length (1/f = 1/u + 1/v, with u → ∞, so v = f). When the lens is adjusted to this position, the extends from the —a calculated point determined by focal length, aperture, and —to , ensuring sharpness across vast distances without needing to refocus. However, factors like changes, lens design tolerances, and attachments (e.g., filters or adapters) can slightly shift the point, sometimes requiring manual fine-tuning via live view magnification or tools like a for precision in . Infinity focus is particularly valuable in to capture expansive scenes with everything from midground to horizon in clarity, in for sharp stars and galaxies, and in wildlife or architectural shots involving distant elements. It contrasts with focusing by prioritizing broad over selective sharpness, often paired with smaller apertures (e.g., f/8 or higher) to enhance overall image quality. While modern systems approximate infinity focus reliably, manual lenses and older designs provide direct control, making it a timeless technique for photographers seeking maximum environmental detail.

Definition and Principles

Optical Definition

Infinity focus in refers to the configuration of a lens where objects located at an effectively infinite produce a sharp on the focal plane, such as a or . In this setting, the lens is positioned such that incoming rays from these distant objects, which arrive as parallel bundles due to their vast separation, are refracted to converge precisely at the . This adjustment ensures optimal sharpness for subjects beyond a certain , typically considered infinite when the rays are collimated rather than diverging. The principle relies on the behavior of light rays from remote sources: as an object's approaches , the rays emanating from any point on it become parallel to the , forming a that the lens focuses at its rear focal point. A converging lens bends these parallel rays inward to meet at this focal point, creating a point image without the need for further accommodation. This contrasts with finite-distance objects, where rays diverge from the object point, necessitating lens movement or adjustment to shift the convergence location along the for sharpness. At focus, the lens is typically set to its farthest extension or a calibrated position, maximizing the between the lens and to match the for parallel input. While the term and specific adjustments for developed in the 19th century, the principle has been fundamental to optical instruments like telescopes since the early 17th century. The application of infinity focus in emerged in the during foundational developments, exemplified by the work of mathematician Joseph Petzval, whose 1840 portrait lens advanced the correction of aberrations, enabling improved sharpness for portrait and . When a lens is set to infinity focus, the extends from the to infinity, providing sharpness for all objects beyond that near limit.

Mathematical Basis

The mathematical foundation of infinity focus in lens optics relies on the thin lens equation, which relates object distance uu, image distance vv, and focal length ff: 1f=1u+1v.\frac{1}{f} = \frac{1}{u} + \frac{1}{v}. For an object at infinity, u→∞u \to \infty, so 1u≈0\frac{1}{u} \approx 0, simplifying to v=fv = f, meaning the image forms at the focal plane of the lens. Ray tracing illustrates this convergence under the paraxial approximation, where rays are assumed to make small angles with the optical axis. A principal ray parallel to the optical axis (representing light from a distant point source) passes through the lens and bends to intersect the focal point on the opposite side, at a distance ff from the lens. Other parallel rays in the bundle, such as one passing through the lens center (undeviated), also converge to this point, demonstrating how the lens curvature refracts collimated incoming rays to a single focus. In paraxial ray tracing, the change in ray angle θ′\theta' after the thin lens is given by θ′=θ−yf\theta' = \theta - \frac{y}{f}, where θ\theta is the incoming angle and yy is the ray height at the lens; the ray height itself remains unchanged across the . For rays from infinity, θ=0\theta = 0 (parallel bundle), so θ′=−yf\theta' = -\frac{y}{f}, and the rays propagate to the focal plane where the height h′h' relates to the field angle by h′≈−fθh' \approx -f \theta (in the small-angle limit). This approximation assumes rays near the axis (∣θ∣≪1|\theta| \ll 1 ) and neglects higher-order terms, enabling linear calculations. Real lenses deviate from this ideal due to aberrations. arises from wavelength-dependent , causing parallel rays of different colors to focus at slightly different points along the axis, with f(λ)f(\lambda) varying across the . occurs because marginal rays (at larger heights yy) experience stronger than paraxial rays, shifting the effective focus for off-axis bundles; the focal shift is approximately Δf≈12Ky2\Delta f \approx \frac{1}{2} K y^2, where KK is the aberration coefficient. In the ideal model, however, perfect convergence at v=fv = f is assumed without these effects.

Applications in Imaging

Photography

Infinity focus plays a pivotal role in , where it ensures sharpness across distant horizons, mountains, and expansive vistas without requiring precise manual ranging of subjects. In , it captures clear details of far-off buildings and structures, maintaining edge-to-edge sharpness in wide scenes. relies heavily on this technique to render stars, the , and other celestial objects as pinpoint sharp elements, avoiding blur from slight focus errors during long exposures. Achieving infinity focus differs between manual and autofocus lenses. With manual lenses, photographers simply rotate the focus ring to the infinity (∞) symbol, which serves as a reliable hard stop for distant focusing. Autofocus lenses, however, may overshoot this mark due to manufacturing tolerances or environmental factors, often requiring —such as adjusting the focus ring while monitoring a distant target—to ensure precision, especially in challenging low-light scenarios. When focused at , the zone of sharpness extends from the to all objects beyond, creating extensive that is particularly effective at small apertures like f/8 to f/16. These apertures reduce the , allowing foreground elements closer to the camera to fall within the sharp zone while preserving distant clarity, though excessive stopping down beyond f/16 can introduce softening. Practical examples highlight its application: in night sky photography, precise infinity focus ensures sharp, pinpoint stars without blur or bloating, yielding crisp points of light against , as seen in wide-angle shots of the . For street photography, wide-angle lenses are frequently preset to near-infinity focus at apertures around f/8, ensuring spontaneous urban scenes remain sharp from midground pedestrians to distant architecture. To verify infinity focus accurately, use live view mode to magnify the image and adjust the focus ring until a distant star or high-contrast point appears as the smallest, sharpest pinpoint, checking for neutral color fringing. Distant reference points, such as buildings or mountain peaks, serve as effective aids for confirmation, particularly when shooting in daylight or twilight before transitioning to darker conditions.

Cinematography

In cinematography, infinity focus is frequently utilized in establishing shots featuring skylines, horizons, or expansive landscapes, where the lens is set to maintain sharpness on distant elements throughout camera movements such as pans, ensuring visual continuity without the need for constant refocusing. This technique is particularly valuable in dynamic sequences, allowing cinematographers to capture broad environmental context while prioritizing background clarity. Cine lenses are engineered with hard infinity stops to provide reliable and repeatable focus at infinite distances, a feature essential for precise control during and compatibility with follow-focus systems. In contrast, zoom lenses often exhibit shifting points across focal lengths, necessitating the use of focus scaling charts or calibration to accurately achieve without overshooting or undershooting. Challenges in maintaining infinity focus arise from environmental factors like temperature fluctuations, which cause lens elements to expand or contract at different rates, potentially shifting the focus plane during extended takes and requiring real-time adjustments via follow-focus systems. Modern cameras mitigate this through focus peaking aids, which highlight high-contrast edges in the to confirm alignment even in low-light or hazy conditions. This approach often integrates infinity focus with shallow to separate foreground subjects from backgrounds, ensuring crisp rendering of remote elements like mountains while blurring nearer planes for emphasis.

Astronomy and Telescopes

In astronomy, telescopes function as afocal optical systems, specifically designed to image celestial objects at effectively infinite distances, such as stars, by processing parallel incoming light rays. The objective lens or mirror collects these rays and converges them to form a real intermediate image precisely at its focal plane, where an or camera can be attached for or . This configuration ensures that the system outputs parallel rays, maintaining the afocal property without introducing a net . For visual observation, the is positioned such that the intermediate at the objective's focal plane coincides with the 's front focal point, resulting in parallel output rays that allow the observer's relaxed eye—focused at —to view a magnified comfortably without accommodation strain. This infinity-corrected arrangement, analogous to designs in advanced , relies on parallel rays emerging from the objective to enable projection of the final to , optimizing long-duration astronomical viewing of deep-sky objects like galaxies and nebulae. In , adapting a DSLR camera to a via the afocal method requires setting the camera's lens to focus to capture the parallel rays exiting the 's , thereby aligning the camera's focal plane with the 's output without inducing additional ray convergence or distortion. Proper collimation of the 's is essential in this setup, as it aligns all components to ensure incoming parallel rays from converge accurately at the focal plane; any misalignment shifts this convergence, producing blurred star images with non-concentric patterns when defocused. Refractor telescopes, such as the NexStar 102SLT, exemplify infinity focus in practice, employing an objective lens to gather parallel light from distant sources and adjust focus via a moving mechanism to place the image at the focal plane for insertion, enabling sharp views of deep-sky objects. The telescope's directly influences the resulting , with longer lengths providing narrower but more magnified perspectives of celestial targets like star clusters.

Hyperfocal Distance

The is defined as the closest distance from the lens at which a subject can be acceptably sharp when the lens is focused at , with the extending from this point to . This concept is particularly relevant in the context of infinity focus, where the far end of the sharpness zone reaches distant objects, but the near limit is determined by the hyperfocal point to maximize overall scene sharpness. The HH is calculated using the formula H=f2N⋅c+f,H = \frac{f^2}{N \cdot c} + f, where ff is the of the lens in millimeters, NN is the (), and cc is the circle of confusion diameter in millimeters, typically 0.03 mm for full-frame sensors. The addition of ff accounts for the lens's in the distance measurement, though it is often negligible for longer focal lengths. For example, with a 50 mm lens at f/8 on a full-frame sensor (c=0.03c = 0.03 mm), H≈10.5H \approx 10.5 m, meaning everything from approximately 10.5 m to will be acceptably sharp when focused at . In practice, focusing at or beyond the maximizes the for scenes with extensive subject distances, ensuring sharpness across a broad range without needing precise adjustments for distant elements. size influences the circle of confusion value: smaller sensors like (with c≈0.02c \approx 0.02 mm) yield a shorter when using an equivalent for the same (e.g., 35 mm on versus 50 mm on full-frame), allowing closer foreground elements to remain in focus. In , photographers often set focus at the rather than pure to include sharper midground details, such as rocks or , while maintaining distant horizons in acceptable focus.

Depth of Field Considerations

When a lens is focused at infinity, the depth of field (DOF) extends from the hyperfocal distance to infinity, ensuring that distant objects remain acceptably sharp while the near limit of sharpness is primarily governed by the aperture setting. Wider apertures, such as f/2.8, result in a shallower DOF even at infinity focus, limiting sharpness to only very distant subjects and creating a narrower zone of acceptable focus. The boundaries of this DOF are mathematically defined using the circle of confusion (CoC), which represents the maximum acceptable blur diameter on the before sharpness degrades noticeably to the . For 35mm format, a standard CoC value is 0.03 mm, derived from assumptions about print size, viewing distance, and , allowing consistent DOF calculations across lenses. Aperture plays a critical role in modulating this DOF at : stopping down to smaller like f/11 increases the near limit of sharpness, extending the focused zone closer to the camera without altering the far limit at . This effect arises because smaller reduce the of rays from off-focus points, minimizing blur within the CoC tolerance. Sensor or size influences DOF at infinity focus through its impact on the effective CoC and required for equivalent framing. Larger formats, such as , exhibit shallower DOF compared to crop sensors like , necessitating wider apertures to achieve subject isolation against distant backgrounds, as the longer focal lengths needed for the same amplify blur gradients. However, extreme stopping down beyond f/22 at infinity focus can introduce softening due to the diffraction limit, where wave optics causes light to spread beyond the Airy disk, reducing resolution in distant details regardless of the expanded DOF. This trade-off highlights the need to balance for optimal sharpness, as becomes the dominant factor at high f-numbers.

Lens Design and Calibration

Achieving Infinity Focus

Achieving infinity focus involves aligning the lens elements so that parallel light rays from distant objects converge sharply on the camera's sensor or film plane. This is typically accomplished through manual adjustment by rotating the focus ring on the lens until it reaches the infinity symbol (∞), which marks the position where the lens is optimized for parallel ray convergence from objects at optical infinity. For lenses equipped with autofocus (AF), photographers can override the system after the AF has locked onto a distant subject, switching to manual focus mode to fine-tune the setting. Some professional lenses include an infinity lock feature, which secures the focus ring at the ∞ position to prevent accidental shifts during shooting. To verify infinity focus, select a faraway landmark—ideally at a distance of several hundred , such as 150 for a 50 mm lens—and examine the image sharpness through the or on the LCD by magnifying the view. This technique ensures the focus is accurately set without relying solely on the infinity mark. On zoom lenses, the infinity mark may shift slightly across different focal lengths due to internal optical adjustments, requiring users to consult the lens's focus scale or employ test charts for precise alignment at the desired zoom level. Modern mirrorless cameras assist with digital tools like focus peaking, which highlights in-focus edges in real-time.

Common Calibration Issues

Temperature variations can significantly impact infinity focus calibration due to the differential expansion and contraction of lens materials, such as elements and metal mounts, which shift the focal plane. To mitigate this, many modern lens designs, including those from Canon, incorporate an infinity stop positioned slightly beyond the true optical infinity mark, allowing for adjustments as temperatures drop and components contract, ensuring sharpness at distant subjects without recalibration. When adapting vintage lenses to digital mirrorless bodies, such as mounting M42 screw lenses on cameras, flange focal distance mismatches often prevent achieving true infinity focus, requiring the addition of shims to the adapter. Shimming involves inserting thin materials like folded aluminum foil between the adapter's mount and body to precisely increase the effective distance, tested by focusing on distant objects like stars at wide apertures until sharpness is restored across the frame. Manufacturing tolerances in lower-cost lenses can result in infinity focus being achieved too early or too late relative to the marked scale, leading to soft images at distant subjects due to imprecise assembly or element alignment. Users can verify and address this by photographing high-contrast distant targets, such as the or , and adjusting the focus ring stop if accessible, though professional servicing is recommended for sealed designs to avoid damage. In zoom lenses, infinity focus may misalign across the range because the optical groups shift during zooming, potentially throwing off at wide or telephoto ends if the design is not parfocal. Recalibration typically involves DIY adjustments for enthusiast models, starting at the longest for primary alignment then verifying at the wide end, often by loosening internal screws to reposition the rear barrel while testing sharpness on a tripod-mounted setup aimed at remote landmarks. In contrast, modern lenses often recalibrate infinity via updates or in-camera micro-adjustments, enabling electronic compensation without physical intervention.

References

  1. https://www.masterclass.com/articles/guide-to-infinity-focus
  2. https://photographylife.com/infinity-focus-photography
  3. https://www.cs.mtu.edu/~shene/DigiCam/User-Guide/4500/EXPOSURE/EV-depth-of-field.html
  4. https://wp.optics.arizona.edu/jgreivenkamp/wp-content/uploads/sites/11/2018/12/201-202-17-Camera-Systems.pdf
  5. https://www.highpointscientific.com/astronomy-hub/post/astro-photography-guides/achieving-perfect-focus-with-a-bahtinov-mask
  6. https://wp.optics.arizona.edu/jgreivenkamp/wp-content/uploads/sites/11/2019/08/502-03-Imaging-with-a-Thin-Lens.pdf
  7. https://visionbook.mit.edu/lenses.html
  8. https://pages.cs.wisc.edu/~dyer/ah336/papers/02_lenses.pdf
  9. https://www.britannica.com/science/optical-telescope
  10. https://www.getty.edu/publications/resources/virtuallibrary/0892365366.pdf
  11. https://pressbooks.online.ucf.edu/osuniversityphysics3/chapter/thin-lenses/
  12. https://www.cs.rochester.edu/u/brown/160_10_27_11/readings/optics_reading.html
  13. https://msbahae.unm.edu/Courses/Optics%20Lab/Aberrations.pdf
  14. https://wp.optics.arizona.edu/jgreivenkamp/wp-content/uploads/sites/11/2015/01/ch08.pdf
  15. https://www.lonelyspeck.com/focusing-at-night/
  16. https://www.planetary.org/articles/night-sky-photography-for-beginners
  17. https://richardhaw.com/2017/10/21/repair-infinity-focus-calibration-12/
  18. https://www.bhphotovideo.com/explora/photography/tips-and-solutions/calculating-hyperfocal-distance-in-photography
  19. https://amateurphotographer.com/technique/camera_skills/mastering-hyperfocal-distance/
  20. https://blog.mingthein.com/2012/04/24/zone-focusing-and-shooting-hyperfocal/
  21. https://www.wevideo.com/blog/what-is-landscape-videography
  22. https://www.studiobinder.com/camera-shots/framing/establishing-shot/
  23. https://timeinpixels.com/2015/11/difference-still-cine-lenses/
  24. https://photo.stackexchange.com/questions/40995/why-do-some-lenses-focus-past-infinity
  25. https://photo.stackexchange.com/questions/45799/how-much-does-a-drop-in-temperature-affect-focus
  26. https://www.bhphotovideo.com/explora/photography/features/who-killed-infinity-focus
  27. https://www.studiobinder.com/blog/depth-of-field/
  28. https://www.rp-photonics.com/afocal_optical_systems.html
  29. https://wp.optics.arizona.edu/mnofziger/wp-content/uploads/sites/31/2016/05/OPTI202L-Lab-4-Telescopes-SP11.pdf
  30. http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles2.html
  31. https://www.saao.ac.za/~wpk/exposure.html
  32. https://www.photographingspace.com/check-fix-telescope-collimation/
  33. https://www.celestron.com/blogs/knowledgebase/im-having-trouble-focusing-my-nexstar-se-telescope
  34. https://graphics.stanford.edu/courses/cs178-13/lectures/optics1-09apr13.pdf
  35. https://groups.csail.mit.edu/graphics/classes/CompPhoto06/html/lecturenotes/22_DepthDefocus_6.pdf
  36. https://sites.math.northwestern.edu/~len/photos/pages/dof_essay.pdf
  37. https://www.edmundoptics.com/knowledge-center/application-notes/imaging/depth-of-field-and-depth-of-focus/
  38. https://www.photopills.com/articles/depth-of-field-guide
  39. https://www.normankoren.com/Tutorials/MTF6.html
  40. https://lenspire.zeiss.com/photo/app/uploads/2022/02/technical-article-depth-of-field-and-bokeh.pdf
  41. https://cookeoptics.com/wp-content/uploads/2021/11/Depth-of-Field-Explained-FDTimes-June-2006.pdf
  42. https://phillipreeve.net/blog/tuning-adapters-infinity-focus-reflections/
  43. https://www.digitalcameraworld.com/news/how-i-fixed-my-lenss-focus-with-kitchen-foil-and-a-pair-of-scissors
  44. https://photographylife.com/how-to-calibrate-lenses
Add your contribution
Related Hubs
User Avatar
No comments yet.