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Lenses 101

disassembled camera lens

Just a heads up: This is a long blog post. It’s from a course we’re developing for aspiring cinematographers. We believe it may be helpful to share some of the things we think about when we prepare to film your videos. That being said, put on your school goggles (right? “school goggles”) and dive in.

A lens is a device that modifies or changes the direction of the movement of light. A camera lens has a number of smaller lenses housed in groupings that arrange the movement of light so that an image can be focused onto the sensor in a camera. These smaller, solo lenses are called elements. Typically lenses will have a few standard reference points which are: Focal length, field of view, T stop, and minimum object distance, a couple more features of lenses are aperture, flare, zoom and fixed focal length (aka, prime lenses).

Let’s start with focal length! The focal length of a lens is basically a measurement of magnification expressed in millimeters – the higher the number, the greater the magnification. Likewise the opposite is true: the smaller the number, the less magnification. In order to describe the physical attributes of focal length more specifically, we need to address the no parallax point (NPP).

We’re not switching gears here. Don’t worry, the No Parallax Point is just the optical center of a lens. It is the point at which light rays taken in and focused by the front elements of a lens converge. Once the light rays converge, the back elements of the lens transmit and focus the light rays onto the camera’s sensor. And! Just like the lenses in our eyes, the image hits the sensor upside down and the camera flips it right side up. So how does the NPP affect the focal length? The focal length is measured from the NPP to the sensor. The NPP of a 50mm lens is 50mm from your camera’s sensor.

Focal length is the measurement that determines the field of view (FOV). It’s easiest to think about Field of View as a cone. That’s right, the geometric shape – the field of view is expressed in degrees, and those degrees describe the angle of the cone’s vertex which can be found, you guessed it, at the NPP. The angle at the NPP creates a conical boundary for light rays traveling through the lens. Any light rays outside of this boundary are not collected by the lens. The greater the angle, the shorter the focal length, and the smaller the angle the longer the focal length. That’s why Field of view is also referred to as angle of view. Oh yeah and did you know your lenses capture a circular image? It’s the sensor that creates those right angles.

Another factor that determines the FOV is Sensor Size. As the sensor size decreases, the angle of view of your image will change because the sensor is not able to collect as much of the transmitted image.

Lenses are usually categorized into 3 different groups: Wide, Normal and Telephoto. Wide Lenses typically have focal lengths of about 4mm – 35mm. Depending on the sensor format a wide lens may be considered a Fisheye lens. Fisheye lenses can achieve a very large field of view with something called barrel distortion which renders objects with straight features, like buildings, as curved. Still some wide angle lenses may have barrel distortion if they’re not rectilinear lenses. Rectilinear lenses produce straight lines for objects with straight features. Normal Lenses range from about 45mm – 50mm. Telephoto Lenses have focal lengths above 50mm.

Straight lines on the building appear to bend at the edges of the frame. Steven Soderberg chose lenses that were made in the 1950’s and 60’s, the same era in which the film No Sudden Move takes place.

Moving around the lens here: we have arrived at the aperture. The word aperture means opening, and it’s easy to confuse this with f-stop and entrance pupil, so it’s easier to think of the aperture as referring to the iris mechanism. The Iris is made up of a number of opaque blades or leaves that contract [disallow] and expand [allow] the transmission of light. The number and shape of the iris blades determines the overall shape of out-of-focus objects in a composition. The out-of-focus objects in a composition are called the bokeh. So in other words the bokeh will resemble the shape of the iris at a given F Stop/T Stop. Some lenses have irises with a triangle shape opening and others with circles or hexagons depending on the number of blades.

iris mechanism

The primary function of the iris is to control exposure. On the lens, the quantification of exposure is standardized and expressed in two different ways: F Stop and T Stop. Photography lenses are typically rated with F Stops which represent an equation that produces a certain, theoretical transmission value. But it is expensive to T-stop-test photography lenses and the exposure values between lenses can be easily adjusted with electronic parameters in-camera.

Cinema lenses need a little more precision as everything captured is continuous; constant light sources illuminate perpetually moving objects that reflect light rays focused by a lens which projects an image onto photosites in a sensor where they are digitally written into an ever-flowing video stream. Whew!

Light and Color Stream

The “T” in T Stop stands for “transmittance”. It quantifies the actual amount of light reaching the lens after full transmission to the sensor. The distinction between a T stop and an F stop can be described such as this: F stop or the F number of a lens is a mathematical equation in which F stands for “function” – it explains the ratio between the diameter of the entrance pupil (which is found at the No Parallax Point) and the focal length of a lens. The ratio represents a value of light transmission expressed in units called F stops. You can sometimes see the maximum F stop of a photography lens expressed as a ratio and printed on the barrel or front of the lens: 1:1.8. The maximum F stop of a lens is the diameter of the entrance pupil divided by the focal length. The entrance pupil is determined by multiplying the Focal length by the maximum F stop which in this case is 25 x 1.8. this gives us an entrance pupil of 45mm, or to circle back: 45/25 = f/1.8 as the maximum F stop. Ok, those were a bunch of numbers that are kind of silly because although they help us quantify the measurement of light, that’s about all they do. So how much light is actually hitting the sensor?

A T stop measures light that reaches the sensor after it has been transmitted through the entire lens. It adds time to the equation. The quantification is more accurate which counts when cycling through multiple lenses on a production.

(From Left to Right) The ligature describes the Minimum Optimal Distance (0.8ft), the diameter of the bayonet mount for filters at 49mm, the maximum f/stop of the still photography lens at 1.8, and finally the focal length of the lens at 25mm.

MOD or the minimum object distance: A lens grouping moves away from the image sensor to focus on objects that are close to the lens. The lens grouping can only move so far before it cannot focus the image of the object(s) onto the sensor. Makro lenses are built to move the focusing lens grouping much further away from the sensor in order to achieve extremely close focus.

And Now: FLARE! Lenses are complex; there are usually multiple elements of glass in a lens, and sometimes rays of light enter the lens at an angle from which the elements scatter rather than focus them. On its way through the lens, this scattered light is reflected by one or more of the elements which produces points of light or a haze that have nothing to do with the image-forming light rays that have been properly focused on the sensor. Flares have a few different looks, one in the form of a haze, which is usually produced by a light that is just beyond the field of view. The haze washes out the image, compromising the saturation and contrast. The other look is characterized by one or many points of light that resemble the shape of the aperture (if an aperture has 9 blades the flare may look like a nonagon). Zoom lenses, rather than Prime lenses, may be more susceptible to flaring at wider focal lengths because they usually contain more lens elements. Flares aren’t necessarily a bad thing. They can add a lot of style if one uses them deliberately. You may also prefer the way one lens flares to another. You can get some really pretty flares out of different lenses.

lens flare

Fixed vs. Prime and Zoom Lenses. A fixed lens simply means the camera does not support interchangeability – you cannot change lenses because it’s built into the camera body. Camcorders have a fixed, zoom lens so that the operator can easily change the focal length via a zoom toggle lever. I would use these on shoots where speed is paramount. The camera can power up and the lens controls are built into the camera’s computer so iris and focal length adjustments can be made with a push of a button. The camera is much more portable and in some cases lightweight. And it’s just simple, sometimes limiting your options can inspire creative problem solving when it comes to interpreting your story. You can save money by not purchasing a bunch of interchangeable lenses that tend to be pricey.

camcorder
A Camcorder with a fixed, zoom lens.

With an interchangeable lens system, you’re not limited to one range of focal lengths that have been predetermined by the manufacturer. Most prime lenses (one focal length) are faster (they can admit more light), and are sharper because there are fewer lens elements. There’s just a bigger variety of characteristics you can assign to your image because every lens manufacturer is a little different. Some lenses may flare differently, their bokeh may vary because of the iris, and some lens coatings may yield a softer feel, while others a crisper look.

Zoom Lenses are full of glass. You can change focal length via a barrel on the lens or a toggle lever on a camcorder. As the operator zooms in and out, groupings of elements are physically moving through the lens and changing the NPP distance to the sensor. If Your lens has a varied maximum T stop, the amount of light transmitted through the lens will decrease as the operator zooms to a higher focal length. The T stop number will reflect this decrease in light transmission by automatically increasing. It’s definitely better to use zoom lenses with a fixed maximum T stop. That way you can maintain most of your exposure settings when zooming in and out.

video lenses

There is so much more! – the history of lens making, mounts, coatings, spherical lenses, anamorphic lenses, and a handful operation characteristics that affect the formation of the final image. Our small glimpse into the world of lenses should boot up the imagination and help identify some of the possibilities the right lens can offer your story.