Essential differences between movie lenses and camera lenses: stepless aperture, focus throw (270°~300°), parfocal and breathing effect

1. Optical light flux accuracy: F value (geometric aperture) vs T value (real transmitted light flux)

• Nominal F-number of photographic lenses (F-stop): It is the purely geometric optical ratio of the focal length of the lens divided by the effective entrance pupil diameter (F = rac{f}{D}). It does not take into account the absorption loss of the lens glass material and the multiple reflection light attenuation of each group of lens surfaces. A photographic lens with a nominal f/1.4 may have an actual light transmission rate of only 80% (equivalent to T1.6). The actual brightness of 50mm f/1.4 lenses from different brands varies greatly.
• Cinema lens nominal T value (T-stop / Transmission stop): It is the true transmitted light flux measured and calibrated by a precision photometer. At nominal T2.0 for all cine lenses, the photon flux received by the sensor is absolutely consistent. This ensures that when the crew shoots with multiple cameras at the same time or changes lenses with different focal lengths, there will be no jumps in the actual light and dark of the picture, and there is no need to level the exposure one by one in the later stage.

2. Mechanical focus stroke: stepless damping and 270°~300° ultra-long rotation angle

This is a mechanical miracle set up to cooperate with the focus puller (Focus Puller) to manually and accurately follow the focus:

• Still Photography Lens: In order to pursue the high-speed focusing of the autofocus motor, the internal focusing stroke is extremely short. The manual focus ring can be rotated 45°~90° to complete the entire distance from the closest focusing distance to infinity. When manually following the focus, if your finger moves slightly by 1 mm, the focus point will fly out by 2 meters, making it easy to lose focus in shallow depth of field. Moreover, most modern lenses are "electronic focus rings (Focus-by-wire)", which have different rotation speeds and different rotation angles, and cannot be marked with scales.
• Cine lens: Using fully mechanical rigid linkage, the focus stroke is as high as 270° to 300°. The focus ring is engraved with precise imperial and metric luminous scales. The ultra-long physical stroke allows the focus operator to subdivide the focus into dozens of micro-steps between 1.5 meters and 1.6 meters to achieve silky-smooth, slow and controllable focus transfer (Rack Focus).

3. Parfocal design: focus deadlock during push-pull zoom

In video storytelling, zoom in/out is a classic lens language:

• Zoom defocus of camera zoom lenses (Varifocal): Most camera zoom lenses (such as 24-70mm) are zoom non-parfocal designs. When you have a clear focus at the 70mm end and push the focal length to 24mm, the picture will be completely out of focus and blurry in an instant, and you must refocus.
• True cinema parfocal zoom lens (Parfocal): It has an extremely complex mechanical compensation lens group inside. After the photographer accurately focused on the actor's eyelashes at the telephoto end, allowing the focus to be pushed and pulled at will throughout the entire range, the actor's eyes always remained as absolutely sharp as needlepoints, without the need to recalibrate the focus.

4. Breathing effect (Focus Breathing) and standardized gear outer diameter

• The physical nature of the breathing effect: When a photographic lens rotates the focus ring, due to the internal optical element group moving back and forth, the field of view (Field of View) of the picture will be slightly zoomed, which looks like the lens is "breathing and expanding". When the focus of dialogue between two actors switches in a feature film, the screen zooming back and forth will seriously disrupt the immersion. Cinema lenses use complex optical compensation to compress the breathing effect to an absolute zero level that is imperceptible to the naked eye.
• Standardized Industrial Housing: Cinema lenses from the same series (e.g. 24mm, 35mm, 50mm, 85mm) usually have the exact same outer diameter (e.g. 95mm or 114mm) and exactly the same position of 0.8 modulus focus and aperture gear rings. When the crew changes lenses, the matte box and wireless follow focus motor do not require any positional readjustment and can be seamlessly hot-swapped and replaced within seconds.

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