With the release of the first lens in the Irix Cine line on the horizon, many of our followers may be wondering about the difference between a still camera lens and one designed for cinematography. There are significant differences and I'll do my best to explain them in this article.
Let's start with the differences you can see, shall we?
What's with the Gears?
One of the first things still photographers will see as unusual in the photo of the new Irix 150mm Cine Dragonfly are the gear teeth on the lens housing. It looks as if you might be able to connect something to this lens, right? It should come as no surprise that those teeth are for exactly that purpose.
Focus Pulling
You've probably heard the terms "focus pulling", "focus pushing" and "follow focus" but might not know exactly what they meant. Here's a quick explanation:
In video recording you're often going to be working with subjects that move closer or further away from you, as well as dealing with movement of the camera for effect. In order to keep a subject in focus, you'll need controlled, constant movement of the focus ring on the camera. "Pushing" or "pulling" focus is the act of turning the focus in one direction or the other to maintain focus as those movements occur.Keeping the focus action smooth is critical during these adjustments. The reason you don't notice subjects going in and out of focus in movies is because this is handled smoothly. (In some cases cases, you might actually notice the focus shift from one subject to another, in dramatic fashion.)
In either of those cases, focus pulling and pushing is much easier to manage with a video setup that includes a follow focus attachment that allows adjustment of the focus ring with a side-mounted mechanism, as shown below:
In fact, during complex camera movements, it's typical for a "focus puller" to be assigned to the camera crew who uses an extended follow focus mechanism to maintain the focus, allowing the videographer, or "cameraman" if you prefer, to concentrate on the panning or tracking movements he's responsible for.
As you can see in the photo above, the follow focus mechanism needs to be attached directly to the lens, which can be easily facilitated on many rigs with an industry-standard foot mount like the one you can see in the first reference image. In other cases, the foot can be removed or placed on top of the lens to adapt to other setups.
Aperture Differences
The other gear ring you'll see on the Irix 150mm macro Cine Dragonfly lens is on the aperture ring and it's there because of another important difference in cinematic lenses.
De-clicked, manual aperture control
While this isn't actually visible, it's related to the gear ring, so we'll discuss it here. The aperture mechanism on a cinema lens has two specific features that most modern DSLR lenses don't have.
First, the aperture diaphragms in most DSLR lenses are controlled by the camera's electronics. This is a convenient feature for still shooters and most DSLR lenses don't have an aperture ring at all. In the cinematography arena, however, it's necessary to adjust the aperture while the camera is actively recording . To allow for this, cinema lenses have an aperture ring that allows diaphragm adjustment at the lens.
Older camera lenses had aperture rings with click-stops at measured intervals to facilitate accurate exposure control. Those stops were the basis of the "f-stop" aperture measurements we still use for exposure calculations in still photography.
Unfortunately, if an aperture diaphragm "clicks" from one setting to another while a camera is recording video, the sudden difference in light reaching the sensor will cause flickering or unpleasant, harsh lighting changes. To compensate for this, a cinema lens uses a "de-clicked" aperture mechanism that allows constant, smooth motion over the entire range of aperture size. By adding the gear to the aperture ring, it can be operated smoothly as the lighting of a scene changes, and may also be operated by a focus puller.
T-stops vs f-stops
One more visible difference lies in the T number you'll find on cinema lenses. You'll see it plainly marked on the barrel of the Irix 150mm Cine lens as T 3.0 in the image above.
If you know your exposure basics for still photography, you know that the f-numbers on a still camera lens represent a fraction of the lens focal length and that's how the aperture size is determined when you use them. When you're dealing with continuous recording, however, this system isn't very practical.
Cinema lenses use an aperture scale that's based on actual light transmission to the sensor. T-stops are the measurement of this transmission (hence the "T") and are more accurate for manually calculating exposure. Lens elements block light transmission , so each cinema lens design will transmit light differently.
Fortunately, most other characteristics of the aperture scales are the same: Higher T-numbers represent more light reduction, so the higher the number, the smaller the aperture. Put simply, T-stops can be thought of as f-stops with the light loss within the lens factored in, so T-numbers will always be higher than the corresponding f-numbers for a given lens. For example, the Irix 150mm still camera lens has an f-number of 2.8 for the maximum aperture, while the cinema version has a T-number of 3.0.
Unseen Differences
You can see from what's already been discussed that there are considerable differences in the two lens types. I haven't, however, covered those differences you can't see! I'll try to explain them quickly, since this post is already fairly long.
Internal focusing: While you can see this feature, it isn't obvious until you notice that the lens doesn't extend and retract while you focus. This keeps the lens at a constant length and distance from the subject. The result is less worry about encroaching on a close subject. It's also easier to use with rigs that include mat boxes or similar devices.
Extremely precise focusing: Yes, this has already been mentioned, but there are a couple of other factors in this we haven't yet mentioned. Most cinema lenses are equipped with a focus ring that has a very long throw. Focus rings are also clearly marked, to allow precise, repeatable settings from test to take and between takes. The Irix 150mm T/3 Dragonfly lens has both of these features.
Rugged build: Cinematography lenses are often at the center of bustling, hectic environments and harsh conditions. The Irix Dragonfly build delivers a robust, easy to grip housing with weather sealing.
Extreme optical quality: The video and cinema industry requires a lens with exceptional sharpness and clarity with minimal aberrations. The first lens in the Irix Cine line is based on the proven optics of the 150mm macro Dragonfly, and is very much up to the task.
Built to industry standards: To facilitate the myriad attachments and extensions that can be required on cinema rigs, manufacturers need to consider more in their cinematic lenses to ensure compatibility. Gear pitches, front filter sizes and objective end diameter are only some of the standards that are carefully met in the new Irix Cine lens line.
That's a wrap!
I hope the information above helps answer our readers' questions about cinema lenses and how they compare to still camera lenses. The differences aren't hard to understand, but they are important in an industry that places unique demands on the gear and its operators.The Irix Cline line will be engineered with all of these differences in mind, along with the high standard of quality built into all of their products. We look forward to announcing a date and price soon!