Why We Need Cinema Lenses

2012-01-25T23:29:00.000-08:00

Why We Need Cinema Lenses

By Ryan Patrick O’Hara

Lenses are not cinema lenses by simply being used on a cinema camera. Cinema lenses are optically designed for top performance and mechanically designed for the specific and demanding use on cinema production. Cinema lenses are designed for the rigorous and specific demands of a cinema film set. These lenses may not be appropriate within other realms of the industry.

Putting a geared ring around a photography lens focus barrel and de-clicking the aperture ring does not a cine-lens make.

As the low-cost/large-sensor movement marches forward leaving a lens-hungry camera-saturated landscape in its wake, a few lens manufacturers are stripping down and dressing up photography lenses and advertising them as cinema lenses. Once upon a time in Hollywood, the difference between the two was obvious. Now, it is not so clear and the distinction will continue to blur as manufacturers try to release lower cost cinema lenses, which will undoubtedly forsake important characteristics of cinema lenses to hit a price point. The future will see photography lenses being called cinema lenses and cinema lenses behaving more like photography lenses.

This article is written on behalf of cinematographers, camera assistants, and directors who, in the age of 'good enough', may enjoy a reminder of what makes a real cinema lens, why they exist and the benefits they provide. Slated for review will be the optical and mechanical design points of a cinema lens and why/how they differ from photography lenses. Perhaps then it shall become clear why these lenses typically carry such prestige and price.

Before continuing, it must be recognized that there exists amazing photography lenses and wonderful cinema lenses. They differ in design because of how they are used. The photographer captures a single frame, a frozen moment in time. His images stand on their own, telling a story with just one frame. The cinematographer works within the medium of time. His images are juxtaposed together by the art of editing. His images tell a story with multiple shots usually meant to create the illusion of continuous passage of time.

As obvious as this statement is, it fundamentally changes the needs of the man behind the camera.

Optical Requirements:

Sharpness / Contrast
It is not terribly difficult for any manufacturer to make a decently sharp lens in the 21^st century. Of course after a certain point the difficulty of designing and building a sharper lens does get exponentially more difficult and expensive. Lenses with many elements (such as zooms), very wide focal lengths, and fast aperture lenses still provide for challenging designs. The fact of the matter is this author can not remember the last time a cinema or quality photography lens made in the last 10 years was rejected for not being sharp enough for 1080 or even beyond. This matter is addressed because it often seems many consider sharpness as the definitive reason why cinema lenses are better. Thus, these individuals become convinced that photography lenses make perfectly good substitutes to cinema lenses when they discover a photography lens of close, equal or sometimes greater resolving power. Despite some cinema lenses being among the sharpest lenses in the world, there are plenty of wonderfully sharp photography lenses. There is much more to cinema lenses than sharpness. Sharpness is important but typically over-rated when evaluating a lens or lens set as a whole.

Exposure Vignetting
It is the goal of every lens manufacturer, cinema or photographic, to reduce the uneven field illumination or 'vignetting' of a lens as much as possible. However, all lenses vignette to some degree; the good ones simply remain undetectable to the eye. Cinema lenses demand higher tolerances for vignetting than their photographic counterparts. Why? Because vignetting in cinema lenses is often more noticeable in cinematography than in photography. In photography, slight vignetting is hard to notice upon a still image. It can be disguised within the image if subtle enough. On the contrary, when a lens vignette appears within a moving image, the vignette becomes considerably more noticeable as objects enter and leave the effected area of frame or when the camera pans, tilts or travels causing the vignette to move with it.

There are different causes for vignetting, one of which being the law of reflection and refraction mostly impacting wide-angle lenses. When incoming light hits an uncoated lens from a zero-degree perpendicular angle of incidence, roughly 4% of the light will bounce off the reflective glass surface and 96% of the light will continue through the lens. In fact, William Stull, ASC, wrote an article in 1940 about coated and uncoated lenses. His figures were 5.22% reflection upon every air-to-glass surface in a lens. A typical prime lens consists of 9-14 elements. If a 14 element prime lens were to be uncoated, by the time the light reached the sensor only 56% of the light would remain. That is almost one stop of light loss. Of course modern lens coatings help reduce this number greatly, but remember that figure is simply the light entering perpendicular to the lens front. Wide-angle lenses are made to accept incoming light from a wide field of view. The light entering the lens from the peripheral of the field of view will be not be perpendicular to the lens, but rather be approaching from an obtuse angle of incidence/reflection. When light from these angles reaches the front element, more light will reflect off the front element and less light will find its way through. As the angle of incidence/reflection moves away from perpendicular, the amount of light which reflects off the glass surface increases. Thus, this is one of the reasons why wide angle lenses have vignetting.

Law of Reflection/Refraction

"Cooke lens coatings reduce reflections to .1%. A brew of silicon and

titanium, hardened with magnesium fluoride coats each element. It is
1 nanometer thick, which is 1 millionth of a millimeter. Your whiskers
will grow longer than that in 1 second after the swipe of your razor."
- Jon Fauer, Film and Digital Times "Cooke Book" pdf

To correct for this vignetting, a lens must be carefully designed and at times undergo shading to help counter the loss. It is this author's understanding that larger lens designs handle vignetting much better. Presumably this is another reason why photography lenses are plagued with uneven field illumination when wide open. Photography lenses need to be small and light. Making the lens larger means larger optical elements, which will raise the price tag quite fast. Thus this can be very difficult and expensive problem to solve. This is perhaps why many photography lens manufacturers allow a certain degree of vignetting in their lenses. Allowing a certain amount of vignetting can allow a lens to be made smaller and for less cost. Vignetting usually decreases as a lens is stopped down, so photography lens manufacturers usually allow for a degree of vignetting in their lenses wide open, and after being stopped down it all but disappears.

Regarding cinema lenses, consider the following:
Alfonso Parra, AEC, tested the Cooke Panchro/i’s, and found they lose, at most, 2.1% of the luminance on their corners at most. The lens with the least only lost 0.99%. This number is small enough to go unnoticed by the most trained eye. If the math is correct, 2% luminance loss would be roughly 1/25^th of a stop lost at the corners.

Alfonso Parra Testing Light Uniformity
(photo from Alfonso's Cinematographic Evaluation)

Now examine some popular photography lens choices. On an APS-C camera, which is very similar to S35mm size sensor, a Zeiss ZE 18mm f/3.5 Distagon photogrphy lens vignettes by as much as a stop and a half in the corners at f/3.5. The Zeiss ZE 21mm F/2.8 Distagon at f/2.8 also vignettes as much as a stop and a half in the very corners! The Zeiss ZE 28mm F/2 Distagon breaks the trend and pulls just a half stop loss in the extreme corners of a APS-C sensor when at a f/2.8.

Most Canon L-series zooms vignette about half a stop in the corners of APS-C sensor cameras when at a f/2.8. The Canon EF 20mm f/2.8 vignettes just under two stops wide open at a f/2.8. The 24mm F/1.4 L series Canon lens wide open at f/1.4 shows a stop and a half loss in the corners, and about 3/10ths of a stop loss when closed to a f/2.8. Many of the L-series primes do well with vignetting once you surpass the 35mm realm or close them down. However, they all exhibit exposure vignetting when wide open. All of them. Wide open, modern cinema lenses are built and designed to eliminate vignetting.

At wide open, even the Canon EF 35mm f/1.4 L-series prime has over a stop of vignetting loss in the corners, as does the 50 f/1.2 L-series. Of course, when stopped down to f/2.8, these lenses have a corner loss of around 3/10ths of a stop; an acceptable range.

The Tokina 11-16, which is a great lens for the money and Duclos makes a great PL’d version for cinema, vignettes a stop and a half at the extreme corners at 11mm and a stop of loss at 16mm.

Zeiss ZE 21mm f/2.8 Distagon at f/2.8
(center box = APS-C frame)

Canon EF 14mm f/2.8 L-Series at f/2.8
(center box = APS-C frame)

Canon EF 20mm f/2.8 at f/2.8
(center box = APS-C frame)

Tokina 11-16mm f/2.8 at f/2.8

Distortion
Similar to vignetting, cinema lenses must have more strict tolerances for distortion. Distortion of a lens, within a photograph, is often less noticeable than when presented in motion. Pin cushion or barrel distortions go from subtle whispers to bloody screams the moment movement is introduced. Cinema lenses cannot get away with the amount of distortion a photography lens can, especially when going to the big screen. Very high amounts of distortion moving about encompassing a viewers' field of view can cause motion sickness in a theater. Thus, cinema lenses are made to be as rectilinear as possible, often times demanding more complicated, challenging and expensive lens designs. "Fisheye" is a funny term. Meant to sound like a deliberate design point, the term is basically one to describe a very wide lens that has absolute horrendous distortion. In general photography lenses and cinema lenses handle distortion well, however, the cinema world is more demanding and they separate from the pack, especially on the super wides.

Pincushion and Barrel Distortion

Zeiss Ultra 8Rmm vs Nikon 8mm
Price tag difference around $22,000

Color Matching
Many variables exist that effect the way a lens reproduces color. These variations are usually slight in nature, but important all the same. It is a well known fact that photography lenses are not designed to be color matched. Why would they? Color matching is another perfect example of why cinema and photography lens designs differ based upon how they are used. A photographer works with single independent images. A photograph is taken with one lens, it does not matter if other lenses in the photographers kit have unique characteristics. However, the cinematographer works with multiple shots woven and juxtaposed together to create an illusion of continuous time. When cutting between a medium-close up and an extreme close up, one can’t have the medium shot look neutral-cool and the close-up suddenly have a warm/pink tone. The inconsistency will either consciously or subconsciously weaken the illusion and possibly awake the viewer to the fact they are watching a contrived work of fiction. Cohesion of the image is incredibly important and the illusion must live on. Of course a modern day digital intermediate color-correction session can fix just about any lens color rendering inconsistency. Unfortunately such a process takes time and money. If one were to shoot an entire feature with mismatched lenses, there would be two choices: either release the film with bad color timing, or spend the money and effort to fix it in post. Color timing sessions can be well over hundreds per hour. There is enough work to be done in post. Matching lenses in post color correction when they could have simply been matched on set, is the last thing a production needs to spend money on.

Color Matching Test from Alfonso Parra's Cooke Panchro/i Evaluation
(photo from Alfonso's Cinematographic Evaluation)

Chromatic Aberration
Chromatic Aberration, also called CA, is the optical occurrence when a lens fails to bend all wavelengths if light equally. Light is made of many different wavelength frequencies that create the colors we see. A lens must capture incoming light then bend and straighten it to fall upon the film/digital sensor plane as straight as possible. When the optical elements within a lens bends the light, they can consequently act as a very mild prism and separate some wavelengths from others. These offset wavelengths will fall just slightly off from their counterparts resulting in a color fringing in the image. This is why aberration is a thin line of color. The color can change depending on which frequency the elements offset. As discussed above in the vignetting section, wide angle lenses have very extreme field of views. These lenses must take incoming light from very radical angles of incidence (thus more radical angles of refraction), and bend them toward the film plane without allowing any wavelengths to be slightly offset creating CA. Thus, chromatic aberrations are often found on the edges of wide angle lens frames.

Lens test showing multiple types of chromatic aberration color and strength.
(mild orange left, mild purple middle, and strong blue right)

Mechanical Requirements:

Cinema lenses provide productions with the best lens mechanically. Better mechanics means the lenses are accurate, durable, reliable, and quick. Cinema lenses need little work around and are streamlined to save time, reduce the possibility of error, and deliver a image.

Size Consistency

Length
Cinema lenses are designed and manufactured to be either very similar or the exact same in length. When using a mattebox mounted to support rods, having to readjust the mattebox and bellows placement upon every lens change takes additional time. This time can add up to minutes over the course of a day. If on steadicam, having the lens change require a mattebox adjustment can require a greater re-balance of the rig. This adds to the time the lens change requires.

Every Zeiss Master Prime pictured is 8" long.

Front Diameter
Cinema lenses are often used in tandem with a mattebox system. Unlike photography lenses, which use screw on filters and built in lens hoods, cinema lenses use a mattebox to keep extraneous light from flaring the lens and to hold filters in front of the lens. By using a mattebox, a lens can be changed much faster without having to remove and reattach filters. However, when using a mattebox, it is extremely important to keep light from entering the mattebox from behind, thus either a bellows ring, doughnut ring, or clip-on back must fit perfectly around the lens front. By having a lens set with matched front diameters, the previously listed devices need not be switched out for different sizes, thus the lens change needs no additional actions. This saves time and reduces the amount of support gear needed.

Weight
Although rare and extremely difficult, some select cinema lens sets have many focal lengths with similar or the same weight. Usually these are lenses in the typical range of lengths, as very wide or telephoto lenses have lens designs which often make them heavier in nature. Having similarly weighted lenses can help when the camera is on steadi-cam, a remote servo-head, in handheld mode, or any other delicate mounting operation when balancing the camera is very important.

Focus/Iris Ring Placement and Gears
Cinema lenses are designed to have geared focus and iris rings placed at the same point on the barrel for all focal lengths. Doing so saves the camera assistant time when changing a lens, as the follow focus module nor any FIZ motors will require being adjusted after every lens change, thus further saving time on every lens change. Additionally, photography lenses typically do not have geared focus or iris rings. They are textured as to provide a nice grip for the photographers hand, but are not geared as cinema lenses are. Geared focus and iris rings are a must if to be used with a professional follow focus or remote follow focus system.

Zeiss Ultra Speeds is a great example of what is likely one of the most consistent lens sets in regards to physical build. Every lens between 16mm and 100mm is exactly 143mm in length, have a 93mm front diameter, and matching geared focus and iris ring positions. All lenses are consistent T-stop of T/1.9, and six focal lengths within the 24-85mm range have the exact same weight of 2.2 lbs.

Zeiss ZE Canon lenses, a pretty nice photography lens set, is an example of how photography lenses are built for different types of use. Each lens is streamlined to be as small and light as possible, paying little attention to set uniformity. This particular lens set has different sized front diameters, different lengths, and dissimilar ungeared focus/iris ring placement.

When a lens is changed, the 1st AC will likely have to adjust the mattebox placement on the rails, exchange the bellows ring/back plate/doughnut, slightly adjust the follow focus, and require a greater re-balance on steadicam. This is not a big deal, as it's simply more work for the AC, but it can cost time over the course of a day. If lens changes are often, this can add up quite quickly, especially in on bigger films.

If this wasn't enough, many wonderful photography lenses no longer have an aperture ring! Manufacturers have moved the aperture ring from a physical and tactical ring on the lens to an electronic and internal function. For many lenses, the photographer must now use the camera to communicate with the lens and control the aperture electronically. On a cinema camera this immediately disqualifies the lens for use on most digital cinema cameras, as many do not have the means to communicate with a lens electronically. There are systems such as the Birger mount, which address some of these issues and does so quite well. If using internal aperture photography lenses on a cinema camera, this adapter seems to be the only sane option.

Where is the aperture ring?!

Focus Barrel Rotation & Distance Witness Markings
A photographer does not need to worry about pulling focus smoothly or tracking a subject accurately at all times. A photographer must quickly find his subject and snap the photo. He can freely focus in front and behind the subject, narrowing in his focus. His hand is on the barrel and his eye through the lens. He cannot see the lens markings on the barrel, nor does he need to. He finds focus by eye and releases the shutter on an intuitive moment. The cinematographer cannot do this. Maybe if shooting docu-style, this can be a semi-acceptable method of focusing, but for all general purposes, one does not want to call attention to the camera, and focus hunting during a shot is an effective way of doing so. The cinematographer must accurately and discretely use focus to manipulate and direct the viewers eye. The camera assistant must follow the performances of the actor and/or the movement of the camera to keep the subject in focus. He cannot hunt for focus during a shot, and thus needs assistance from the lens in order to help him accurately track his target. This assistance comes in the form of many accurate witness markings on the barrel.

Photography lenses typically have short focus throws. One can go from close to infinity focus in a simple twist of the wrist. This is helpful when needing to focus quickly on a moments notice, as many field photographers do. However, short focus throws make it difficult to gently track a subject and increases the possibility of overshooting a target. The focus distance markings on a photography lens are often few in number, only generally accurate, and are without actual witness mark lines.

Modern cine lenses typically have a 300*+ barrel rotation. Cinema lenses are usually larger in size (for optical reasons) thus tend to have a long 300*+ rotation (300* rotation on a tiny lens can be less travel than a smaller rotation on a bigger girth lens). On high quality cinema lenses, each lens is custom engraved to ensure focus witness marks are as accurate as possible. Modern cinema lenses also have two focus scales, one for each side, so the camera assistant does not have to flip the lens in order to pull from the other side of the camera.

Build Materials
It can be argued whether photography or cinematography conditions are the hardest on equipment, (it's cinematography btw) but there is little argument that cinema lenses are typically better built. Cinema lenses are built for the most rigorous of production demands. They are made from machined metal and are designed to operate from sub freezing temperatures to dangerously hot climates. They can be easily serviced, repaired, and modified. The most typical of cinema lens mounts, the PL and PV mount, are amongst the most strong, sturdy and temperature resistant designs.

Cooke S4 lenses are made from machined anodized aluminum built to operate from -13° to 131° fahrenheit. They are not threaded lens barrels, but instead use a cam system, which eliminates the need for lubrication, such as grease.

Linear Iris in T-stops
Cinema lenses do not have 'clicked' iris rings like many photography lenses, thus one can set the aperture to land at any value between stops. Most modern PL lenses have linear iris rings, with every third of a stop marked. Because cinema lenses are in T-stops, achieving precise and matched exposures with different lenses is as easy as setting the iris ring. Photography lenses, often have clicked iris rings, meaning they must settle on one stop or another. Trying to split a stop will result in the lens likely trying to settle one way or another. If the photography lens does not have a clicked aperture, it will likely be rated in f/stops and without sub-stop markings.

T-stops Vs F-stops
Cinema lenses are rated in t-stops, 't' for 'transmission', instead of the familiar f-stops found on photography lenses. T-stops are values which represent the true amount of light passing through the lens. Each lens is tested and marked for their T-stop values. An F-stop is simply a formula. It calculates the amount of light that should pass through a lens based on the focal length divided by the entrance pupil. Thus it’s decently accurate except for one thing… it does not take into account the light lost from passing through the glass elements inside the lens! Thus there is always a varying degree of light loss from one lens to another. With F-stop lenses you are always playing within a margin of error.

Matching Maximum Aperture & Iris Assembly
Cinema lens sets are designed and built to have matching maximum apertures. This feature isn’t as important as it is helpful, but if a lens set does not have matching maximum apertures, it is the responsibility of the cinematographer to work within the least common denominator among his lens set, or he will find himself switching to a lens that cannot support the working exposure he has already set with his lights. However, the iris assembly is different and arguably more important to the image. When shooting semi-stopped down, on a long lens, and with shallow depth of field, the shape of the iris aperture can be defined in the out of focus elements commonly referred to as lens bokeh. Lenses with matching iris assemblies will provide matching out of focus bokeh shapes. Cooke S4's and Cooke Panchro/i's use the same iris assembly design, thus if you were to use them together, they would not only be color matched, but would produce the exact same bokeh renderings at matched apertures. Having a lens set where one lens has a triangular iris assembly, another hexagonal, another octagonal, and another with 12 blades, will result in very different bokeh shapes. If consistency is the goal, this could prove problematic.

Cooke S4/i & Panchro/i 8-leaf iris assembly
picture by arturo jacoby

Zeiss Standard Speed T/2.1 9-leaf iris assembly
picture by arturo jacoby

Lens Breathing
When one changes the focus of a lens, the optical elements inside shift in concert to bend the incoming light from the corresponding distance to a focal point upon the sensor. When the optical elements inside the lens reposition themselves during the focus rack, they can slightly alter the field of view of a lens, which will appear similar to a very slow and mild zoom. This is called lens breathing. In photography, breathing is not important what-so-ever. Besides changing the composition by arguably negligible amounts, breathing is not seen in the image. To eliminate breathing, the lens design must be changed to account for the optical effect, thus eliminating breathing is not a priority of photography lens manufacturers.

In cinema, tracking focus within a shot, or racking focus from one subject to another is a very common practice, thus cinema lenses take great strides to eliminate breathing. Not long ago, to eliminate breathing all together, Zeiss created a Dual-Floating Element design for their Master Primes. This design will be recognized at the 2012 Oscars with an Academy Award for Scientific and Technical Achievement.

Barrel Extension
As explained with lens breathing, when a lens changes focus or zooms, the optical elements inside adjust and shift. When designing lenses, it is often easier to allow the lens barrel to extend forward, in order to accommodate the shifting elements. Many photography lenses, when focused or zoomed, extend their barrel forward as the optical elements shift. Because cinema lenses have connected follow focus gears and a mattebox, telescoping lens barrels are not ideal, thus cinema lens designs provide for internal realignment. All shifting and repositioning of optical elements happen silently and unnoticeable inside of the lens housing. Everything remains as is.

Barrel telescoping can be from zooming or focusing. Typically barrel telescoping is worse from zooming, however poorly designed prime lenses can exhibit troublesome barrel telescoping when focusing a great distance across the barrel. Typically the issues arise when the lens pushes against the mattebox or the geared focus ring falls off the follow focus.

Despite lens hoods being added, one can see the telescoping nature of some photography zooms.
(images from www.the-digital-picture.com)

Consistent focus and exposure throughout zoom range
Cinema zooms almost always carry exposure from one end of the zoom to the other. As an example, take the legendary Angenieux Optimo 12x zoom is a perfect T/2.8 from 24mm to 290mm. Coupled with the other impressive optical and mechanical features of this lens, it's no surprise the thing is the size of a military shell.

Angenieux 24-290mm T/2.8 Cine Zoom

There are many photography zooms which hold maximum exposure throughout the zoom range, but there are photography zooms which forsake this feature in order to accommodate lens design within a small/light housing and low price. Yuck.

Conclusion

At the end of the day, a wonderful film can be made on photography lenses or cinema lenses. However, because these two mediums are very different in nature and thus the needs of photographers and cinematographers are very different, using photography lenses for cinema purposes is simply adding possible issues and concerns to an already full plate.

The same goes for using cinema lenses to take photographs. Using a Master Prime to take a photograph would be equally ridiculous. First of all, the lens is 8" long and weighs about 5lbs. Additionally, handheld photography is not the same as handheld cinematography. One has the luxury of taking the weight on the shoulder... the other is all taken to the wrist. Now imagine having to carry several around for a photo-journalism assignment. Focusing quickly would require multiple twists of the lens barrel, and likely lost time trying to reel in the focus, perhaps missing the spontaneous moment of the photo.

Thus, just as photography, there are types of videography that may also not benefit from cinema lenses. If shooting a documentary, wedding, or event videography that involves long hours of handheld shooting in spontaneous/unpredictable environments, perhaps a very lightweight photography zoom might be a more appropriate tool despite some shortcomings.

The design points described in this writing are the ideal design points of a modern day cinema lens set. However, not all cinema lens sets contain all of these attributes. Vintage cinema lenses and new lower cost cinema lens sets do not attain all of the above. Just as that is true, the same goes for photography lenses. There are photography lens exceptions to where some lenses exhibit attributes of cinema lenses. For instance, Zeiss ZM's have f/stop markings for 1/3 stops on the barrel.

Zeiss 15mm f/2.8 ZM (Leica Mount) with 1/3 stop mapped f/stop scale

Shoot a film with the best optics available to the production. Learn the strengths and weaknesses of that lens set and go about doing what is necessary to utilize those strengths and minimize the weaknesses. Cinema lenses simply allow for less weaknesses and more strengths, leaving the burdened mind of the cinematographer to other things. It's a luxury well worth having.

Ryan Patrick O'Hara: Written Articles

Why We Need Cinema Lenses