Understanding Iris, F stops, Aperture, Gain, Shutter, Depth of Field & Lighting
Approximate 11 minute read
Even people that work with video cameras all the time are often confused with the function and the interdependent relationships iris, f-stops, aperture, depth of field, gain, shutter and lighting levels have on each other.
In this post I will attempt to unravel these terms and discuss how their functions are used – practically in a church multi-camera production environment.
On ENG style broadcast video lenses there are typically four rings (or barrels) the camera and/or video operator (shader) may manipulate. On other styles of lenses similar controls exist but may be located in different places or in a different order on the lens. On broadcast video lenses the barrel closest to the front of the lens allows the camera operator to adjust focus. The next ring down toward the middle of the lens adjusts the lens’ focal length. Adjusting the focal length smoothly while on air, usually by manipulating a zoom rocker control on the lens itself or on a rear lens control, allows the camera operator to “zoom in” or “zoom out.” The next ring down toward the camera is the iris ring. The iris ring in a multi-camera production environment is remotely controlled by a video operator (shader). The last ring controls back-focus, which adjusts the focal point of the back of the lens onto a camera’s imager. If the back-focus is improperly set, the lens will not maintain focus while zooming.
An analogy from nature would be your eye is the lens and your eye’s pupil is the iris controlling aperture. Your eyes’ aperture (via the pupil) changes size letting in more or less light automatically when in varying lighting conditions. This prevents your retinas from becoming over exposed and aids you in seeing when you are in a dark environment.
In a similar way, a video operator (video shader) can control a lens’ iris ring to allow more or less light to enter the camera as needed.
The increments an aperture is adjusted by are referred to as stops (or f-stops, f-numbers, or f-ratio). F-stop numbers are etched onto the lens’ iris ring. The following is a representation of various f-stop settings and the relative size of the aperture.
Each adjustment of the iris ring setting a higher f-stop number (for example going from f/2.8 to f/4) reduces the amount of light hitting the camera’s sensor by one-half. Conversely, each adjustment of the iris ring setting a lower f-stop number (for example going from f/11 to f/8) doubles the amount of light passing through the lens to the camera’s imager.
Some lenses also allow aperture increments in 1/4, 1/3, 1/2, 2/3 and 3/4 stops.
Since I am partially Italian, I like to think about technical things in terms of food! If we think about iris settings and therefore aperture f-stops in terms of pizza pie slices (no, not pan or Chicago style pizza; a round, nice, thin crust – NYC style slice … the way God intended), our slice becomes larger when we set the iris ring to a larger aperture (which is a smaller f-stop number). And our pizza slice becomes smaller when we set the iris ring to a smaller aperture (a larger f-stop number).
As said, the aperture is the variable opening at the rear of the lens which determines how much light travels through the lens and is focused onto the camera’s imager.
F-stop numbers are inversely proportional to the amount of light focused onto the camera’s imager. A small f-stop number (say for example f/2.8) corresponds to a large, wide aperture size – which then results in shallow depth of field. Conversely, a large f-stop number (for example f/16, or f/22) results in a smaller aperture hole and then therefore a deep depth of field.
As the quantity of light reaching the camera’s imager is controlled by the lens’ aperture, adjusting the lens’ f-stop is (one of a few) tools available for adjusting for the proper video exposure of a shot. Thus, when adjusting the aperture due to inadequate lighting or lighting inconsistencies, we are (possibly unintentionally) also changing the shot’s depth of field.
Depth of Field
Depth of Field (DoF) may be best described as a front-to-back zone or a distance visible within an image in which the content is razor sharp (when of course the front element of a lens is appropriately focused on a subject).
As soon as our subject (the person or thing we are shooting) moves closer or further away, increasing or decreasing the subjects distance to the camera, the subject will eventually move out of the optimal sharp focus DoF zone. The extent to which the subject becomes less in-sharp-focus is directly proportional to the distance the subject moves out of the zone. The more the subject moves out of the in-sharp-focus zone, the more out of focus the subject becomes.
The point of optimum sharp focus is typically (very close to) the middle of the DoF focus zone. When focusing on subjects relatively close to the camera (a short distance) the optimal sharp focus point tends to start at about 50% percent into the focus zone’s front-to-back distance. When we stop-down the aperture using a higher f-stop number, the depth of field zone increases in size and the optimal sharp focus point will shift forward a bit, moving slightly closer to the front of the zone (closer to the camera), with then a larger in-focus distance behind the optimal focus point.
Depth of field is typically described subjectively in one of two ways, “shallow DoF” and “deep DoF.”
A shallow DoF is when the front-to-back focus zone is very narrow in distance. Many times shallow DoF is a mere few inches whereas deep DoF will have a total image where all (or nearly all) of the content front-to-back is in very sharp focus.
While some people (usually film school students without a lot of real world experience or a deep understanding of movie production history) associate shallow depth of field as “a cinematic look.” And I would certainly concur shallow depth of field can be a valuable tool useful to direct the audience’s attention in a scene, by no means is deep depth of field less cinematic. Most famously, cinematographer Greg Toland in Orson Welles’ Citizen Kane shot nearly entirely in deep depth of field (between f/16 – f/8 and with wide field of views) allowing the audience to explore a scene no different than people see in real life. Nearly all Alfred Hitchcock movies were shot deep depth of field. Brian De Palma (Director of Carrie) often used split-diopters to insure that objects both very close and very far from the lens remained in-focus. So, the next time Cecil B. Dabbler sermonizes about shallow depth of field, bring up Greg Toland. OK, I am done with my rant …
Whether or not the DoF is shallow or deep, DoF is impacted by three interrelated factors: (i) the camera’s imager size, (ii) the lens’ aperture size – the f-stop – determined by the iris ring’s position, and (iii) the field of view chosen.
Larger imager cameras (for example Micro 4/3 or Super35mm) lend themselves to shallow depth of field esthetics contemporary often used in feature film, narrative, commercial and music video projects where the content is scripted and where talent and camera positions are planned and blocked in rehearsal.
Field of view also impacts the depth of field achieved. For example, a 2/3″ imager camera placed at 30′ distance to the subject with a 97mm focal length lens, or the same camera at 40′ distance with a 129mm lens, or the same at a 50′ distance with a 162mm lens will in each case achieve a MCU (a medium close up) shot field of view. In each example cited there will be a 2′ 10.9″ depth of field at f/5.6 aperture even though the lens focal length and camera position were changing. Because the sensor size, aperture and field of view in each case was consistent, so was the DoF.
Conversely, moving a camera closer while keeping the lens focal length the same (thereby achieving a tighter shot field of view) will cause the depth of field to become shallower. Or doing the opposite, moving a camera further away from the subject while keeping the lens focal length the same (thereby achieving a wider field of view) will cause the depth of field to deepen.
Depth of Field Examples
Here we see a 2/3″ imager camera at 50 feet camera to subject distance shooting a bust (MCU) shot. To achieve this field of view a 2/3″ camera will require a 162mm focal length lens. If we shoot with the aperture wide open, at a f-stop of f/2.8 the depth of field will be a shallow 17.4 inches! That means if we are centering the lens’ optimal focus on the subject’s eyes, only 8.70 inches beyond the eyes will be in sharp focus! And, 8.70 inches of the subject’s nose (or the distance closer to the camera) will be in sharp focus. Unless that is the subject moves closer or further away! But then preachers/teachers/pastors never ever move around, right?
Now if we do nothing but stop-down the iris by one stop – going from f/2.8 to f/4 – the DoF (our sharp focus zone) increases by 6.66 inches (or 38%). But in doing so we’ve also cut the amount of light hitting the imager by half.
If the scene and video image was properly exposed at f/2.8 changing the aperture to f/4 will require adding (doubling the) light on the scene or increasing the camera’s gain (thereby adding noise).
If a neutral density filter was being used at f/2.8, reducing the density of the filter or eliminating the ND filter altogether at f/4 may make up for the stop of light loss. Or, if the camera’s shutter was being used at f/2.8, slowing the shutter, or turning off the shutter may also make up the one stop of light loss.
If we go from f/4 to f/5.6 the DoF increases another 10.84 inches (a 45% increase).
If we go from f/5.6 to f/8 the DoF increases another 14.5 inches (a 42% increase). The total DoF in-focus zone is now 49.4 inches, beginning at about 48’0″ from the camera (toward the subject) and ending at 52’2″ from the camera (just behind the subject).
If we go from f/8 to f/11 the DoF increases another 20.5 inches (a 41% increase).
And then from f/11 to f/16 the DoF increases another 29.3 inches (a 42% increase).
Lastly f/16 to f/22 the DoF increases another 42.1 inches (a 42% increase), to now be 11.77 feet.
In general, when working with 2/3″ imager cameras in a multi-camera church application, many find the sweet-spot to be an aperture of f/4 to f/8 (f/4, or f/5.6, or f/8), with no shutter, no added gain, with a stage illumination of 40 – 80 foot candles depending on how light sensitive your camera is.
OK, so how does this compare to a Super35mm imager camera?
We see here that at f/2.8 with a MCU field of view the Super35mm sensor camera has a miniscule DoF of only 5.9 inches! That means less than 3″ before the optimal focal point, and less than 3″ after the optimal focal point will be in sharp focus! Time to hire a 1st Assistant Cameraman to ride focus! Even at the smallest aperture (the deepest depth of field possible with a MCU field of view) the DoF will only be 47.2 inches.
What about a Micro4/3rd imager camera?
What about a 1/2″ imager camera?
And a 1/3″ imager camera?
These examples illustrate why for multi-camera live production purposes most churches who seek professional advice will wind up purchasing a 2/3″, 1/2″ or 1/3″ camera. If the camera will be relatively close to the subject (therefore a very telephoto lens may not be needed) and there is no planned requirement for a CCU or 3rd party fiber transport (CCU) capability or gen-lock; and when low-price is the highest overriding priority, a 1/3″ imager camera may prove to be a great value. However lack of gen-lock, CCU or paintbox capability, or not having the option of interchangeable lenses (especially very telephoto lenses) can be a serious liability in live multi-camera production, so therefore a 2/3″ or 1/2″ imager camera may strike a better balance for churches when the budget allows.
ISO is a term typically used on digital cinematography and hybrid/still photo cameras. Unlike aperture which refers to to the size of the lens opening allowing more or less light to hit the camera’s imager, ISO refers to how light sensitive the actual imager is. Typical ISO settings are 100, 200, 400, 800, 1600 and 3200; the higher the number the more light sensitive the camera is. There is a downside to setting high ISO numbers however, and that’s noise in the image.
Adjusting the camera’s ISO from 400 to 200 will make the camera one-half as light sensitive therefore requiring twice as much light. Whereas adjusting the camera’s ISO from 400 to 800 will make the camera twice as light sensitive therefore requiring half as much light.
Broadcast multi-camera production video cameras however generally do not have an ISO setting but rather offer a GAIN setting. For those audio people out there, think of camera gain as a microphone pre-amp. It will amplify the audio but it will also amplify any background ambient sounds and any noise the microphone circuitry and signal path is picking up or generating. In this sense camera gain acts similar to ISO – the higher the number, the noisier the image. Gain settings are expressed however in decibels. 3dB of gain is equivalent to 1/2 stop of exposure. 6dB equals one stop. 9dB is 1-1/2 stops. 12dB is two stops. 18dB is three stops.
Unlike still photography operating techniques where shutter is primarily used to adjust for exposure (while maintaining a desired DoF with aperture), when using broadcast video cameras shutter is (typically) used to achieve a given desired motion esthetic.
Very slow shutter speeds (or turning the shutter off) may give the impression of motion being blurred. Very high shutter speeds lend themselves to a motion esthetic which is temporally fast-paced, in some cases to the point of appearing visually staccato. Lower shutter speeds (or turning the shutter off) allow more light to reach the camera’s imager. Higher shutter speeds reduce the amount of light and therefore affects exposure.
On digital cinematography cameras shutter speeds are often expressed as occurring in degrees. For example, a 180 degree shutter rotates twice per video frame (visually creating the appearance of doubling the camera’s frame rate).
Video cameras tend to express shutter not in degrees but in fractions of a second, subject to the camera’s selected frame rate . If the video camera is set to shoot at a frame rate of 59.94i, the shutter menu choices may appear as 1/100, 1/125, 1/250, 1/500, 1/1000, 1/2000.
If the video camera is set to shoot in a frame rate of 29.97p, the shutter menu choices may appear as 1/40, 1/60, 1/120, 1/125, 1/250, 1/500, 1/1000, 1/2000.
And if the video camera is set to shoot in 23.98p, the shutter menu choices may appear as 1/32, 1/48, 1/50, 1/60, 1/96, 1/125, 1/250, 1/500, 1/1000 and 1/2000.
If for example one is shooting at 23.98p, a 1/48 shutter (on a video camera) is equivalent to a 180 degree shutter (on a digital cinematography camera).
In the first example below we see one second in time with twenty-four frames. Each camera frame is exposed to light 1/24th of a second (41.66666666667 milliseconds).
If we dial in a 1/48th (aka 180 degree) shutter, we now have a ratio of two shuttered images (each 1/48th of a second, or 20.8333333333333 millisecond) per actual video frame, giving the audience an effect similar to shooting at 48 frames per second.
If in the top example (no shutter) we are shooting a young girl jumping rope, during each 1/24th of a second (41.66666666667 milliseconds) video frame the child will have moved. This motion will appear blurred as her motion from point A to point B occurred while the frame was still being captured and processed. By engaging (or increasing) shutter (or increasing the camera’s frame rate), we are reducing the duration of time for each frame, therefore the child will have moved a shorter distance (per frame or shuttered duration) thus the image appears sharper. The trade off however from using shutter, or increasing frame rate is in light, and therefore in exposure.
In most multi-camera broadcast video camera applications, shutter is left OFF.
One large pie please …
When we put all the slices together into a pie; light, aperture, shutter and gain; it’s easy to see that if we want to achieve a shallower depth of field we will need to shoot at a wider aperture (a low f-stop number). Increasing our aperture “pie slice” will then require that either the shutter speed is increased, or lighting levels be reduced (or ND added), or camera gain being reduced, or some combination of all three to maintain a proper exposure. Of course, if we make the depth of field too narrow the average camera operator (church volunteer or professional) will not be able to track focus.
Conversely, to achieve a deeper depth of field we will need to shoot at a smaller aperture (a higher f-stop number). Decreasing the size of the aperture “pie slice” will require that either the shutter speed is decreased (or turned off), or the lighting levels increased (or ND reduced or eliminated), or gain increased (or some combination of two or all three ) again to maintain a proper exposure.
By balancing production decisions regarding a camera’s imager size, aperture, shutter, gain and lighting (or ND filtration) levels; you can achieve the esthetic look you are seeking, whether that is a particular motion rendition, depth of field, cleaness/graininess of the image, etc.
Tom D’Angelo has worked in television production and AVL corporate theater for nearly four decades. He is Emmy Award nominated (Best Director category, Mid-Atlantic) and has been part of various teams nominated or winning national Emmys. As the Media Director at a megachurch in the 1980’s he developed a love for the Church and church performing and technical artists.