Thursday, August 15, 2013

Today’s Video Cameras Can Do it All, But Do You Want Them To?

Why Optical Filters and Matte Boxes Still Matter in the Digital Age: Pt. 1


This week’s guest post comes from Tiffen's MPTV Filter Group Consultant Carey Duffy. Carey has long been a recognized expert in filtration and has done extensive work with the London Filter Company and Tiffen to spread 'filter awareness.' Carey shares his experience with us on why optical filters and matte boxes are still a vital piece of the cinematographer’s toolkit in today’s digital age.


For more on Carey and everything filters, follow him at @Carey_Duffy on Twitter. Visit www.tiffen.com for more information on the filters seen here in this post.

Filters and matte boxes are key for controlling light entering the lens. Pictured 
 top, scene shot with no filter. Pictured below, scene shot with SMOQUE filter. 


Modern digital cinema cameras and DSLRs are truly incredible advances: they house large digital sensors that deliver amazing latitude and enable you to peer into shadows and seemingly see in the dark. Some have lossless codecs and even shoot totally uncompressed. 8 bit, 10 bit, even 12 bit color resolution eliminates banding and keeps colors accurate. 

There are log formats for maximizing latitude, high ISO for even more light sensitivity-- the features and power are endless. The post-production coloring and editing tools seem to compensate for just about everything you couldn’t do in-camera. Even with this kind of technology arsenal, optical filters and matte boxes are still standard-issue equipment used by virtually all the top productions today. Why? Because camera filtration and matte boxes can achieve things not possible in post and are an essential tool in getting the best results possible.

Filters are one of the many tools a cinematographer employs to control the light entering the camera lens, either as a physical necessity, for aesthetic effect, or even to achieve both these ends. The question of how to control the intensity of light, especially for exterior shots, means that the use of filters as a primary tool becomes paramount. 

Although the transition from film to digital has made the cinematography process less complex with the introduction of built-in features such as ISO adjustment, filtration not only gives better results, but offers many things that just aren’t possible in post production or with fancy sensor electronics. Let’s take a look at how filters can give you the best results.


ND Filters

Many people ask me what is the main reason cinematographers, directors of photography (DoPs), or videographers use multiple filters. Often, the simple answer is for correcting the exposure while keeping shutter speed and aperture constant. Neutral Density (ND) filters cut down the amount of light without adjusting camera settings. This is vitally important if you are shooting with a lot of light and want to keep your aperture open for a nice shallow depth of field (such as keeping your interview subject in focus while throwing the background out of focus). Adjusting the camera’s shutter speed or lens aperture would result in a different look and feel. Built-in camera ND is often not exact enough to maintain your camera settings.


                                                         

Graduated and Attenuated ND Filters

A variant of ND filters are graduated and attenuated filters, which cut down the amount of light in a specific area. This is typically used to cut down the bright sky so the exposure is more even between the action on the ground and the overhead light. Graduated ND goes from some ND to clear, whereas attenuated filters have a graduated ND across the entire filter, never going to clear.  There are also choices for hard and soft edge graduated ND as your scene dictates (hard edges are often used with architecture to even out exposures or for dramatic effect). What’s most important about using these filters is the ability to rotate and slide to position the graduation so it matches the horizon. A matte box enables you to get that exact filter placement (more on matte boxes later).


Infrared (IR) Cut Filters and Hot Mirrors

The dramatically increased sensitivity of modern digital cinema cameras have come at the expense of increased infrared (IR) light pollution. This typically shows up as a red or purple cast in the blacks and shadows. When adding ND filtration, the IR pollution effect is often increased, leading to footage that is colored strangely, and difficult to correct in post. An IR Cut filter (as referred to as a “hot mirror” filter) helps eliminate the infrared light pollution. This can be an extra filter added into the filter stages, or ND filters can be purchased with the IR cut built in to them (since ND tends to make the IR pollution worse).


Top: no filter. Bottom: with IR filter. Note cleaner blacks
                                                       

Polarizers

Polarizers are one of the most frequently used camera filters in cinematography. This is because they are both technically fixing and aesthetically enhancing. From a technical improvement, polarizers are used to remove unwanted reflections, such as through glass, shooting over water, etc. 

Polarizers have to be rotated to find the angle at which they cut the glare or reflection, so they need to be set up and work at a specific place. This is why polarizers are recommended primarily for locked-off video shots rather than shots where the camera is actually moving, since moving the camera would also change the polarizer’s angle and spoil the effect. 

Also just to clear up a common misconception: a “circular” polarizer does not refer to the filter being round. There are plenty of square or rectangular circular polarizers. Circular polarizers are almost exclusively used nowadays instead of linear polarizers. There is a lot of complicated stuff behind this, but essentially circular polarizers work correctly with today’s modern autofocus and metering systems.


                                                    Left: no filter. Right: with polarising filter


Contrast Control Filters

All of the filters mentioned so far generally deal with controlling light at constant camera settings, or to help control exposure. Contrast control filters have a very important part to play in the ability to control uneven exposure “looks” within a frame.

                                                     Left: no filter. Right: with polarising filter 

For example, a stronger density is generally used on a long focal length establishing shot, since the light hitting the image sensor is only passing through a small part of the filter’s area. Conversely, a lighter density of filter is used for a wider close-up shot as more of the filter’s area is being used. For this reason contrast control filters come in a number of different varieties and densities. 

They are not designed by stop densities but by increment, to take into account the different focal lengths of lenses used and to create continuity across a scene. Switching between densities depends on focal length and the image in frame. It’s just one part of the effort a cinematographer needs to put in to ensure the continuity of the look across all of the film that is being produced for a particular project.

Low contrast filters create a small amount of ‘localized’ flare near highlight areas within the image, and this reduces contrast by lightening nearby areas of shadow, leaving highlights almost unchanged. Soft contrast filters include a light-absorbing element that, without exposure compensation, will reduce contrast by also darkening highlights, and cinematographers use these when lighter shadows are not desired. In both cases, the mild flare produced from bright highlights is sometimes used as a lighting effect.






Color Conversion Filters

With electronically controlled white balance, color conversion filters are less used nowadays, but there are great choices for converting daylight to tungsten, or vice versa, without any adjustments in post-production. Sometimes color conversion filters can be used to create a special effect look.



Top: no filter. Bottom: with 85b CC filter

Matte Box: the Essential Accessory for Filters


microMatteBox
On professional sets and productions, you almost always see a matte box used in conjunction with filters. Like filters, a matte box performs some vital functions that simply can’t be replicated in post, and help you get the best images possible. 

In addition to holding filters, a matte box protects the lens and the filters from unwanted light sources that create flares and lens reflections that can ruin a shot. When you put filters in front of your lens, you increase the number of optical surfaces where light can bounce around and create havoc. 

The matte box provides both a shade (the ‘deep box’ in front of the filter) and flags/side wings to extend that shade so light is blocked from directly hitting the filters and lens.

For motion pictures the standard size of filters is 4x5.65”, also known as Panavision size.
Filter Trays
This wide size is ideal for the wider HD aspect ratio, especially when using wide angle lenses.

Matte boxes are much more versatile than screw-in filters because they enable you to:
  1. Make fast lens changes without having to remove/re-attach filters.
  2. Reduce the amount of filter handling. Less of a chance of dropping or scratching.
  3. Invest in one set of filters, regardless of size or style (still versus cinema).
  4. Protect your filters and lenses from unwanted dust, dings/knocks, and falls.
Filter Tray



There’s quite a number of matte boxes available today in varying price points. The best matte boxes are those that have the following features:
  1. Support the full 4x5.65” standard filter size for motion picture production
  2. Have a swing-away design for fast lens changes
  3. Solid construction including metal fasteners, knobs, and swing-away construction
  4. Multiple 360 degree stages
  5. Add/remove stages to tailor the size and weight of the matte box to your needs
  6. Filter stage locks, so filters don’t accidentally fall out.



Having a tool designed to offer a wide range of flexibility in sculpting how individual glass filtration effects can be applied consequently becomes of primary importance. A matte box will enable a filmmaker or cinematographer to build up an extremely personal and useful inventory of filters that they will constantly use and add to, allowing the filmmaker or cinematographer a playground of in-camera creativity at the heart of image capture.

Putting It All Together


This feature is merely intended to be an introduction to the world of filters, so you can consider what filter tools you should be employing to burn-in your look on your moving images. Each production, scene and location requires individual consideration if you are going to achieve your desired look.

Knowing what tools will enable you to achieve the look you are after is part of the job of being a cinematographer. If you have filters and a matte box readily on-hand they can also provide a crucial last-minute fix to unexpected problems that digital post-production software can’t touch.



Check out the Redrock microMatteBox after the jump. 


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What is your favorite filter or special effect filter you use? How important is it for you to "get it right" in the camera?

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