Depth of field effects: The distance between foreground and background is called depth of field. The depth of field is the distance in front and behind the point of focus. Objects outside of the depth of field are out of focus. In most cases, the depth of field extends one-third of its distance in front of the focal point, and two thirds its distance behind the focal point.
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Suppose a lens is focused at area, and lens required sharpness at X distance, and the subject is approximately at X distance, now if we move subject sometime near and far away from the camera at same position, than a very minute changes happen to sharpness . So in anywhere not in camera but our eyes also have some zone through which subject is acceptable sharp and that zone itself is the distance between the lens and the subject and that is called depth of field. However, due to limitations in the human eye, subjects that are slightly nearer to the lens and also subjects slightly further from the lens are perceived to be in focus.
These below are the factors which affect more and less depth of field effects.
More Depth of field effects
Less depth of field effects
To achieve more depth of field we have to use shorter focal length of lens.
To achieve less depth ofd field effects we have to use longer focal length of lens.
To achieve more depth of field we have to use small aperture size.
To achieve less depth of field we have to use large aperture.
Larger acceptable center of confusion (COC).
Smaller acceptable center of confusion (COC).
Distant plane of critical focus.
Close plane of critical focus.
Smaller film format.
Larger film format.
Focus split: Selecting near and far distance which need to be held equally and acceptably in focus and setting the focus distance to suit.
Factors affecting Shallow depth of field effects in cameras
High contrast lenses have less depth of field effects than the low contrast lens. Consequently, a smaller diameter of center of confusion should be used in depth of field calculation.(eg- 1/2000 instead of 1/1000 )
Lighting which sharply focused details, brightly lighted objects will appear to have less depth of field effects.
The use of lens filter in between the lens and the subject, which will reduces maximum definition and make a greater degree of soft focus.
As usual but very common, depth of field effects situation arises with uses of wide angle lens at moderate focus distance.( for e.g. 3ft to 12 ft)
Object at infinity: This is occur especially when photographed using long focal length lens( more than 55 mm lenses) trend to appear tend to appear soft because of atmospheric elements(like moisture, smoke, dust, haze, smog). There is solution too for such situation that is only either use HAZE filter’s available in market or using light yellow filters to increase contrast and sharpness in film. This affects when you filming or photographing in any hilly area, mountain area, beaches, waterfall, or any natural area sometime forest too).
In case of shutter release Priority: A fast shutter speed used in photography in order to capture fast moving object( e.g. like if you want to take a shot of moving car), whereas a slow shutter speed used to create blur effects on fast moving object( for e.g. a fast moving car in night and you want to capture back light of car as beam using slow shutter speed) , so therefore a fast shutter speed requires a larger aperture size let’s say (f1/16or f/11 ) which allow small amount of light to enter the lens hence it extended depth of field effects and in case of slow shutter speed requires small aperture let’s say (f2.8, or f2) which allow more light to enter the camera and hence it produce shallow depth of field effects. When aperture is more open than it allows more light to enter the camera and fall on camera CCD, hence to control amount of light we need a more shutter speed to accumulate and to prevent the photograph from over exposed.
A telephoto lens gives you a narrow depth of field effects, by moving a lens as close as to the subject and use largest aperture size of camera lens, while to get the large depth of field use Wide angle lens and move your camera and lens as far as possible and use small aperture size.
The degree of magnification that the image may be subjected to when shown or projected close or near to the audience sheet to the screen. I mean to say that when Audience close to a cinema screen will be more aware of soft focus than those who are far away from the cinema screen.
Focus Bias: This can occur when one subject is in focus and rest is in out of focus like in case of macro photography with small aperture and long focal length for eg a 200 mm lens with aperture value of 2.8 or 2.0.
Effects of aperture and the depth of field:
The depth of field effects is being controlled by lens aperture value. If you increase the f-number than depth of field effects also increased but decrease the amount of light entering to the camera and when you decrease the f-number than depth of field effects also decreased (it gives shallow depth of field effect). In motion picture it’s very limited use of it. Generally cinematographer take a single aperture settings for a shot and for another shot they choose another setting as par locality and they adjust exposure settings with the help of light meter device which tells actuall amount of light presents in that area. Aperture settings and exposure settings are changed more frequently in still cameras than motion picture cameras. In photography and cinematography, variation in depth of field effects used to create various kinds of special effects.
Camera movement and the depth of field effects:
In film making, a person who looks and takes care of responsibility of lens changing, filter changing, point to focus, maintaining depth of field, setting focal length is known as focus-puller. Focus-puller is one of the assistant person in shooting crew members or camera man. A good focus-puller has very adequate and vast knowledge of cinematography. He has to focus and shift focus smoothly in such a manner that audience can’t find jerk on them. In film industry most hard job is of focus puller. He has to perform his duty very accurately because suppose if any role given to actor to act on that, and he done that, but while recording focus puller didn’t check focal length and filter than actor has to act again. Doing reshot is tough to achieve same expression what he achieved in previous shot. So this was example how much focus puller important to the cinematography. His work is more technical rather than creative. He has to keep in mind all the guidelines given by the director of photography during shooting a shot.
Camera’s depth of field limits
The sharpness of depth of field can be increased by decreasing by Center of confusion, but it’s not as easy to decrease because it also has some disadvantage that is motion blur and diffraction. As we know center of confusion has inversely relation with f-number like a smaller center of confusion need a greater f number, but need a long exposure time. if we increase f-number than diffraction also increases which will soften or soft all the part of image, thus image will no longer be in sharp.
The f-number which will overcome from center of confusion is the minimum acceptable sharpness value. If we increase f- number than the sharpness at limited depth of field improved. As we know there is also maximum f-numbers through which sharpness of depth of field in image gets over. The best f-number should be chosen in between of minimum and maximum f-numbers.
Depth of field and Macro or close-up:
One of the main difficulties with close-up photography is that keeping the image enough in focus. The closer the lens focuses, the less depth of field you have. When using a macro lens at its maximum magnification, depth of field may only extend a fraction of an inch. With floral close-ups, a small aperture is almost always needed, but even so, it is important that the lens focuses on a key focal point in the composition, since not in the entire frame.
Depth of field and shooting on bright hot afternoon:
While shooting at outdoor especially in day light, the director of photography may wish to reduce amount of light and the depth of field for a close-up shot. For reducing light, a very good filter available called neutral density filter commonly known as ND filter. Suppose the light meter indicating F/22 but we have till f/16, than ND filter play a very important role. This will reduce the amount of light entering to the camera and enable you to shoot on bright sunny day. On other case, if the director of photography wants a close-up shot, and you have to reduce your depth of field, because by reducing depth of field, it will out of focused the background and puts more importance to the subject or highlight the subject.
Circle of confusion
While shooting the film, the focus puller must be know that which part of scene should be sharp, so therefore we need something to measure the sharpness of image in relative to film limits of the eye’s resolution at the time of viewing or at time of projection on screen.
For example: if a image is captured through 35 mm film projected at theater, audience must be at distance of 1/3 back from screen. Now we know that eye resolving power is 1 minute of arc. From audience I mean 1/3 back from screen, the smallest size of a dot appearing on a screen which is in perfectly focused or sharp. Now we can easily measures the distance between audience and the screen through any inch tape, from screen the previous sharpen dot which appeared smallest at back now that will be largest dot from the screen, the arc of 1 minute comes up with physical dimension on the screen for the largest dot that the audience previously seen as a smaller sharp point.
Figure 1 (need to put a diagram over here)
Now the diameter of dot on screen is not actual size of dot, to find actual size of dot we have to calculate diameter of big dot appearing on screen, than we have to divide with enlargement factor needs to bring the size of our 33 mm film up to size of our cinema screen or projected area. This will bring the size of maximum or big dot.
When lens is focused at infinity:
If any point is focused from the infinity, the distance between plane of focus and the lens become the focal length of the lens.
How image size varies at different points behind the camera lens:
Figure 2 (need to put a diagram over here)
You can see the image is not at extract plane of focus. The image was there somewhere after plane of focus. So in this case size the dot size forming big. Now imagine if the audience sees this dot size sharpen, clearly in focus, than we can easily calculate the amount of sharpness in which part of overall image looks sharp.
Near and far points of focus:
Figure 3 (need to put a diagram over here)
In this case point B creating a sharp image on image plane or plane of focus, while point A creating image after crossing image plane or plane of focus, which means out of focus or blur or less sharpen image.
In film plane, both near object and far away object (Let’s take Point A and Point B) expanded and become confused to larger dot than the original dot. Hence this is the circle of confusion.
Figure 4 (need to put a diagram over here)
Thus, a center of confusion (COC) is the diameter of largest dot which is still considers being sharp, focused, and also seen by the audience in Film Theater when it projected on screen.
Focus split is selecting the near and far distance which need to be held equally and acceptably in-focus and setting the focus distance to suit.
Let’s take x as the distance of near object from the camera lens
Let’s take y as the distance of far away object from the camera lens
Than what could be the focus area?
Hyper focal Distance:
Hyper focal distance is maximum depth of field that starts from infinity to the camera. If a lens is set to hyper focal distance, than the depth of field effects is half of hyper focal distance from infinity. Hyper focal distance is the distance in which lens is focused at infinity, so that the object in background and foreground both will be at sharper focused.
Hyper focal distance depends on many factors, some of like
Focal length of the camera lens
The aperture f- value
Diameter of circle of confusion
Hyper Focal distance = (FL2)
f X COC
Where FL means focal length, f-number means aperture size and COC means circle of confusion
After knowing hyper focal distance, it’s easy to calculate near and far point
Near point depth of field = H X S
Far point depth of field = H X S
Where H = Hyper focal distance value
S = Distance from camera to the subject
F = Focal length of camera lens
COC = Circle of confusion
Focus Bias: Focus bias occurs when one object is in focus and rest is in out of focus (for e.g like in case of macro photography, with larger aperture and longh focal length, i.e f 2.8 with 200 mm focal length)
In cinematography, focus plane yet depth of field effects is calculated using the entrance puple position of
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