Talking about Depth of Focus in Photography

Depth of focus compared to depth of field

Depth of focus is the distance the film plane can be moved while the image points remain acceptably sharp without refocusing the lens. 

While the term depth of focus traditionally was used, and (sometimes still is), to mean field depth, in today's world it is most often set aside for image-side depth. Field depth is a calculation of depth of adequate sharpness within the subject or object space space.

Depth of focus, conversely, is a calculation of the distance amount existing behind the lens and still allow the sensor or film plane to remain in sharp focus. It can be thought of as the opposite of field depth, taking place on the exact opposite lens side.

Where field depth can often be calculated in macroscopic units like meters and feet, focus depth is most often measured in microscopic entities like fractions of a millimeter or in thousandths of an inch.

The identical aspects that establish field depth also establish focus depth, although these aspects can have different results than they do in field depth. Both field depth and focus depth increase when using smaller apertures. For more far away subjects (beyond macro distance), focus depth is relatively uncaring about subject distance and focal length, for a particular f-number. In the macro area, focus depth expands with longer focal lengths or nearer subject distance, while field depth decreases.

Determining factors

In small-format cameras, the smaller circle of confusion limit yields a proportionately smaller depth of focus. In motion picture cameras, different lens mount and camera gate combinations have exact flange focal depth measurements to which lenses are calibrated.

The choice to place gels or other filters behind the lens becomes a much more critical decision when dealing with smaller formats. Placement of items behind the lens will alter the optics pathway, shifting the focal plane. Therefore, often this insertion must be done in concert with stopping down the lens in order to compensate enough to make any shift negligible given a greater depth of focus. It is often advised in 35 mm motion picture filming not to use filters behind the lens if the lens is wider than 25 mm.


A rough formula often used to quickly calculate depth of focus is the product of the focal length times the f-number divided by 1000 (with result in same units as focal length); the formula makes most sense in the case of normal lens (as opposed to wide-angle or telephoto), where the focal length is a representation of the format size. The precise formula for depth of focus is two times the f-number times the circle of confusion times the quantity of one plus the magnification factor. However, the magnification factor depends on the focal length and format size and exact focus the lens is set to, which can be difficult to calculate. Therefore, the first formula is often used as a guideline, as it is much easier to calculate. It relies on the historical convention of circle of confusion limit being equal to focal length divided by 1000, which is deprecated in modern photographic teachings, in favor of format size (for example, along the diagonal) divided by 1000 or 1500.

Internal focusing

An internal focus lens (sometimes known as IF) is a photographic lens design in which focus is shifted by moving the inner lens group or groups only, without any rotation or shifting of the front lens element. This makes it easy to use, for example, a screwed-in polarizing filter or a petal shaped lens hood. During macro photography, using an internal focus lens reduces the risk of the front of the lens accidentally hitting the subject during focusing as the front element does not move.
    IF (internal/inner focusing
    One of the middle lens groups in front of the diaphragm moves during focusing resulting in fast AF and light manual focusing. No change in physical length of the lens body which also remains quite small. Shorter minimum focus distances but often some decrease in focal length towards closer focus distances. Non rotating front element.

    RF (rear focusing
    Only one or more rear lens groups behind the diaphragm move during focusing. Due to the small size and weight of the group it enables faster AF. There may be a change in physical length when zooming. A lens with a constant length and RF is in fact called an IF lens. Non-rotating front element..

    Extension system
    All groups or the front group(s) are shifted for focusing. The physical length changes usually during focusing and zooming often combined with a rotating front element. Relatively slow AF with long and heavy lenses. Relatively small max. magnifications (larger min. focusing distance). In the case of an all-group shifting mechanism there's only a relatively small increase of aberrations (=minor decrease of image quality).

Manual focus

A manual focus camera is one in which the operator has to adjust the focus of the lens by hand. Before the advent of autofocus, all cameras had manually adjusted focusing; As a result, the term is a retronym.

The focus itself may be adjusted in a variety of ways. Larger view cameras and the like slide the lens closer or further from the film plane on rails; on smaller cameras, a focus ring on the lens is often rotated to move the lens elements by means of a helical screw. Other systems include levers on the lens or on the camera body.

There are a number of ways in which focus may be determined. Simplest is simply using a distance scale and measuring or estimating distance to the subject. Other methods include the rangefinder, which uses triangulation to determine the distance. On other cameras, the photographer examines the focus directly by means of a focusing screen. On the view camera, this ground glass is placed where the film will ultimately go, and is replaced by a sheet of film once focus is correct. Twin lens reflex cameras use two lenses that are mechanically linked, one for focusing and the other to take the photograph. Single lens reflex cameras, meanwhile, use the same objective lens for both purposes, with a mirror to direct the light to either the focusing screen or the film.

Focusing screens, in their simplest form, consist of a matte glass or plastic surface on which the image can be focused. Other devices, such as split-image prisms or microprisms, can help determine focus.

Soft focus

Soft focus is a lens flaw, in which the lens forms images that are blurred due to spherical aberration. A soft focus lens deliberately introduces spherical aberration in order to give the appearance of blurring the image while retaining sharp edges; it is not the same as an out-of-focus image, and the effect cannot be achieved simply by defocusing a sharp lens. Soft focus is also the name of the style of photograph produced by such a lens.

Because soft focus is a technical flaw, many older lenses had soft focus built in as a side effect of their construction. Newer lenses are optimized to minimize optical aberrations, but there are lenses such as the Canon EF 135mm lens f/2.8 with Softfocus and Pentax SMC 28mm f2.8 FA Soft Lens, to name but two, which have adjustable levels of spherical aberration at wide apertures. The effect can be disabled entirely as well, in which case the lens is sharp.

Nikon produces a series of DC ("Defocus Control") lenses which are sometimes confused with the soft focus effect, but these are not soft focus lenses, as they do not introduce spherical aberration over the whole field.

The soft focus effect is used as an effect for glamour photography, because the effect eliminates blemishes, and in general produces a dream-like image.

The effect of a soft focus lens is sometimes approximated by the use of diffusion filter or other method, such as stretching a nylon stocking over the front of the lens, or smearing petroleum jelly on a clear filter or on the front element of the lens itself.

It can also be approximated with post-processing procedures. Specifically, highlights in an image are blurred.

See also

  • Cardinal point (optics)
  • Defocus aberration
  • Depth of field
  • Far point
  • Focus (geometry)
  • Fixed focus
  • Bokeh

Facebook Comments