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Film Makers Internet Forum – Frequently Asked Questions
This is the third draft of the FAQ for the Film Makers Internet Forum. The two previous drafts were prepared for the original Filmmakers Mailing List. The last one was issued on July 21st, 1994 so it’s a bit old! Wikipedia is probably a better resource nowadays, but who knows – maybe this will help.
It is intended to answer the most commonly-asked questions, and to serve as a first-reference for forum members. Before asking a question in the forum, take a quick look here to see if it has already been answered.
The following people contributed material for this document:
- Henry Breitrose
- Scott Dorsey
- David A. Roth
- Liane Rozzell
- Bob Stewart
- Dermot Tynan
- Frank Wylie
What is the Film Makers Internet Forum?
The Film Maker’s Internet Forum is a 1990’s reincarnation of the original Film Makers Mailing List which lasted for about ten years before dying out.
The new forum is intended to take up where the mailing list left off. It is not intended for movie reviews, or the eternal discussion of Blade Runner, unless such discussion relates in some way to the actual filmmaking process.
What was the Film Makers Mailing List?
The original mailing list was created by Dermot Tynan in the late 1980’s as a way for filmmakers worldwide to exchange comments, notes, gripes, and technical reviews with each other. Questions and comments on film stocks, cameras, problems with equipment and projection were commonplace. The discussions ran the gamut of independent filmmaking, including such topics as scriptwriting (although this is better served through the screenwriters mailing list), preproduction, camera operation, lighting, sound recording, editing, negative cutting, printing, processing, film festivals and distribution.
Where else can I find resources on film and video available through Internet?
Look in the guide compiled by Lisa Wood and Kristen Garlock. It is available on the University of Michigan gopher. Here is the URL: gopher://una.hh.lib.umich.edu:70/00/inetdirsstacks/filmvideo:woodgarlock
Note: I haven’t checked this out in a while so I don’t know if it still works…
What does the term “Aspect Ratio” refer to?
Aspect ratio refers to the ratio of width to height within a frame and is expressed in a formula such as, 1.33:1 Academy. The first set of numbers “1.33” refers to the width, in units, in comparison to the height of the frame unit, which is always “1”. The colon separates the two units and should be expressed as “to”. So, the above reads as “one-point-three-three to one”. For every one unit high the image is, it correspondingly is 1.33 units wide. This size determines the mathematical dimensions of the rectangle that forms the frame, thus effecting the composition and staging of a scene. Most 16mm cameras, with the exception of a Super 16mm camera, have the 1.33:1 aspect ratio. The Super 16mm camera has an aspect ratio of 1:66:1, due to it’s enlarged image area.
Why should I know about aspect ratios?
If you have any intention of producing film-based subjects which will be distributed in any form or fashion, you should know about aspect ratios. Rarely are films exhibited in the old 1.33:1 aspect ratio, (unless we are talking about film festivals or television) so you had better know what aspect ratio you film will be converted to in order to be distributed. For a typical theatrical release, 16mm and Super 16mm films are blown-up to 35mm, with 1.66:1 being the norm for European release and 1.85:1 being the norm for US theatrical release (although, I think the 1.66:1 frame is being seen more frequently in the US). With Super 16mm, this is not a problem if you are going to 1.66:1, as that is its native aspect ratio and you don’t loose anything. However, on Standard 16mm, the physical limitations of trying to get a longer rectangle (1.66 or 1.85) out of a more “box-like” frame (1.33 is almost square), you must loose a portion of the upper and lower frame. Try it yourself. Draw a box that is 3 inches high and 4 inches wide (1.33:1). Now, starting in the upper, left-hand corner of the existing frame, draw a 1.66:1 box, and a 1.85:1 box over the top of the 1.33:1 box (simply add 1.32” and 2.08” extensions to the right-hand side of the 1.33:1 box). See the problem? Your original image must fit within the new aspect ratio and the only way to do this is to take a smaller portion of your original image. The size of the blow-up frame is reduced (while maintaining the proper aspect ratio) to fit inside the existing image. This is accomplished by cropping a portion off the top and bottom of the frame. Any information within this area will be lost. If you don’t compose the original scene with this image loss firmly in-mind, you will have real problems. Of course, since you are using a smaller portion of the original image- area, you will decrease your effective negative-area, increase grain and lower resolution, but them’s the breaks. Nonetheless, with current 16mm negative filmstocks, you can expect a very nice image when properly blown-up from standard or Super 16mm.
For more information, I suggest you locate a copy of, “Four Aspects of the Film”, by James L. Limbacher, Brussel and Brussel, New York, 1968. This book, which cries-out to be re-printed, covers in great depth the history of 3-D, color, sound and widescreen cinema systems. If you find a copy, buy it!
What is Cinemascope?
Is Super-8 dead and gone?
No. Definitely not. The film stocks are harder to find, as are processing facilities, but at least one feature film has been shot completely in Super-8. Films such as “Flatliners” used extensive S8 footage as well, albeit shot to look like the viewfinder of a camcorder.
What is Super 16mm?
Super 16mm was developed in the mid-1960s by Swedish cinematographer Rune Erickson as a means of shooting high-quality documentaries without the associated weight and bulk of 35mm cameras. Erickson’s solution was to extend the picture-area (on single-perf filmstock) into the area normally reserved for the soundtrack. This extension added 40% more image-area to the frame and changed the aspect ratio from the standard 1:1.33 academy ratio to 1:1.66. Now that the image was larger, and the optical center of the image shifted, the lens mount had to be re-centered to cover the entire negative area. Additionally, it was found that not all 16mm lenses would cover the full Super 16mm frame. Lenses that do cover the full frame of Super 16mm are, with few exceptions, new generation primes and a few zooms.
Super 16mm was initially invented by Erickson for direct blow-up to 35mm (via C.R.I.), but with the introduction of Super 16mm Rank Cintel flying-spot scanners (and the growing acceptance of letterboxed images), has enjoyed an upsurge of popularity among music video and made-for-video producers. Also, with the oncoming HDTV standard, television producers are rapidly switching-over to the Super 16mm format as its aspect ratio closely matches that of HDTV.
Popular misconceptions aside, shooting on Super 16mm does not automatically entail large savings in production costs for theatrical-release, feature productions. Often, production costs for an Independent feature in 35mm are equal to, or less than, a Super 16mm production of the same scope. The expense of a negative blow-up often negates any cost-savings incurred in Super 16mm production
For more information: “Why Shoot In Super 16?” by Rune Ericson, American Cinematographer, February, 1981.
“How Would A Feature Shot In Super 16 Compare With One Shot In 35mm On A Theater Screen?”, by Irwin Young, American Cinematographer, September, 1980.
“The Ballad of Gregorio Cortez”, by J. Greg Evans, American Cinematographer, October, 1983.
Is Super-16 alive and well?
Yes. While the format is certainly controversial, with people like Jon Jost claiming it is a waste of money to shoot in S16, a lot of European television production companies now shoot S16 rather than 16mm. The arguments against usually circle around the fact that the cost of a blow-up can be more than the added cost of shooting in 35mm to begin with. Also, greater care must be taken in the shooting and handling of the stock due to the increased image area where there are usually sprockets. On the other hand, TV production companies find it more convenient to shoot S16 in preparation for the ensuing HDTV standard. Places such as the BBC now have archives
Has anyone heard of a Russian film camera which is very cheap?
Yes. The 16mm Krasnogorsk-3 movie camera can be bought for from ICBM in New York. Here is an excerpt from the review written by Frank Wylie. For more complete information, look in the FTP archive.
‘The camera’s design and features are rather strange and unusual, until you pick it up and study it. The Russians have blended modern and proven technology to produce a hybrid that should provide most of the features and accessories the beginning filmmaker would want or need for U.S. dollars. The following are some of the features; Spinning Mirror Reflex system. Spring-wound motor, variable-rate 8-48 fps. Single frame cable-release socket. Built-in, non-coupled TTL light-meter. “D”-type (?), screw-mount lens mount. 17-69mm f1.9 zoom lens included.’
Can I use a recorded piece of music in my film?
In short, without permission; no. However, a lot of people do. Particularly film students who just have to have that Elton John song playing in the cafe scene. No-one will arrest you for including a piece of commercial music in a film, but you’ll effectively have limited the distribution of the film. Most film festivals for some reason don’t mind if there is commercial music in a film, but no cinema will show the print, and no TV station will air it. You are basically infringing on a copyright. Do it at your peril.
What if I have someone perform an identical version of it?
Again, no. Not only is the recording itself covered by copyright, but the music as it was composed and arranged is also covered by a separate copyright. So, while the Beatles won’t sue you for your rendition of Let It Be, Michael Jackson probably will (he owns the actual music).
What about rights to classical music?
There is a lengthy document available on the Internet, concerning copyright. You should read that first, for a thorough understanding of the law. A copyright lasts a certain fixed number of years after the death of the owner. Most if not all classical music is now in the public domain. That does not mean, however, that you can just lift a section from the London Philharmonic Orchestra CD. Again, the actual performance is covered by copyright, and you cannot reproduce it without permission. Furthermore, you can’t get your favourite Philharmonic to play it either, as chances are, the particular rendition you heard is a specific arrangement by a specific conductor. You also need permission to use that arrangement.
What are “synchronization rights”?
Essentially this is permission granted by a copyright owner to a filmmaker to “synchronize” a copyrighted work (be it music or whatever) to a film. It means that the copyright owner still owns the recorded work, and you still own the film, but the synchronization of the two via an optical soundtrack, video tape or whatever is permitted. It does not entitle you to screen the eventual film. That is covered by separate ASCAP and BMI agreements with the cinema and/or TV station.
What are good books on filmmaking?
Like anything else, everyone has their own favourite filmmaking handbook. There is an extensive list of film books on various topics available for ftp. My favorite is the Pincus book.
How much should I pay for a 16mm camera?
How much ya got? 16mm cameras sell for as little as on up. I don’t know where the limit is, and I’d say Panavision and Arriflex will continue to do sterling research in finding out. A used sync-sound camera will cost in the region of 000 to 500. A used Bolex might sell for anything from to depending on the model, etc. Shop around. Get advice. Get Jessie Chambless’ catalog, and work backward from there.
What is the Chambless Rule?
Fair market value on Bolex gear is generally determined as being Jesse Chambless’ price, divided by three.
I need a sync camera for less than 1,000. What should I get?
For that price range, you should look at the following outfits:- Arriflex BL w/ 10-100 Zeiss
- CP 16A or CP 16R with 12-120 Angenieux
- Eclair NPR or ACL with 12-120 Angenieux
- CP GSMO w/12-120 Angenieux (some people HATE this camera, beware!)
- Bolex 16 PRO & 16 PRO-100 w/12-120 Angenieux or 10-100 Schnider-Variogon (a rare, and very WEIRD camera, but a good performer. Designed for News-film market, as was CP16A).
What do the terms “over-cranking” and “under-cranking” mean?
“Over-cranking” simply means to accelerate the frame-rate of the camera above the “normal” filming speed. Typically, this is any speed over the standard 24 fps sound-speed (25 fps in Europe). When the frame-rate is increased upon shooting, the images will appear to slow-down a corresponding amount upon play-back at normal speed. Images shot at 48 fps, projected at 24 fps, will cause an action to take twice as long to occur as in real-time. Images shot at 96 fps, and projected at 24 fps, will take four-times as long to occur as in real time, etc.
“Under-cranking”, conversely, is lowering the frame-rate of the camera below the “normal” filming speed. Typically, this is any speed over the standard 24 fps sound-speed (25 fps in Europe). When the frame-rate is decreased upon shooting, the images will appear to speed-up a corresponding amount upon play-back at normal speed. Images shot at 12 fps, projected at 24 fps, will cause an action to take half as long to occur as in real time. Images shot at 6 fps, and projected at 24 fps, will take a quarter of the time to occur as in real-time.
Where can I find a list of Film Festivals?
Claddagh Films maintains a fairly complete list of film festivals around the world. It can be found on their web site.
What is SMPTE?
SMPTE is the Society of Motion Picture and Television Engineers, which sets such standards.
What’s the difference between SMPTE Leader and Academy Leader?
What’s the difference between centre and edge sound recording?
The specifications for center and edge track 16mm magnetic recording were set by SMPTE, and you should be able to find them somewhere in a back issue of JSMPTE. Originally, 16mm magnetic recording was done on double-sprocketed fullcoat mag film, and the track was located in the center. My guess is that the advent of magnetic striping for single-perf reversal television news raw stock caused the track position to be shifted to edge, where it lives now (mostly) in the U.S. In Europe, center position is standard.
Has anyone heard of stereo 16mm mag?
Yes. Usually this means that the stereo signal is transferred to the centre track and the edge track. Make sure that the dubbing studio and telecine house can handle this form of stereo before you bother. Note that you cannot produce a stereo 16mm optical print.
What is Photokinestasis?
Photo=picture kine=motion stasis=still Photokinestasis is the process of re-photographing still photographs (usually on an animation stand) and imparting movement to the image via selective cropping and camera movement. Many “historical” documentaries depend heavily on this process to impart a sense of the dynamic movement to historical documentation (i.e., photos, posters, letters, etc.).
What is Pixelation?
Pixelation is an animation technique whereby inanimate objects appear to move of their own volition. This is accomplished by moving the object slightly between each single-frame exposure. Cinema pixelation predates video and should not be confused with any video term.
For scale models, what is the formula to find the proper frame-rate?
filming speed = square root of the scale of the model x normal fps .sp Example: You have a 1/48th scale model you want to appear to have the mass of a full scale model in motion. You find the square root of 48, which is 6.92. Multiply that times the normal frame rate, 24 fps, and you get 166 fps.
What is the tonal range of film and video?
Film can reproduce a tonal range of 130:1, while video can reproduce a tonal range of 40:1 (approx.).
How much latitude does a modern negative stock have?
According to Eastman Kodak ,up to 8 stops with their new EXR stocks.
What is “push” processing, or, “pushing the film in the lab”?
When a lab “pushes” a filmstock, it over-develops the film to compensate for underexposure. Simply put, it allows the film to remain in the developing solution longer than normal to allow the marginally exposed silver particles to develop more fully and produce a more dense image. The amount of “push” to be applied to the filmstock is expressed in f-stops or ISO rating. A one-stop push (exposing ISO 50 stock at ISO 100 and push-processing one-stop) would allow you to shoot in half as much light as your normally-rated filmstock would allow, but will increase the grain size, increase contrast and, for color, tend to desaturate the color of the image. Modern filmstocks can often be pushed comfortably one-stop, but you will notice loss of quality at, and above, a 2-stop push. You must run camera tests to see if the quality loss is objectionable! The above applies to negative stocks.
What is “pull” processing or, “pulling the film in the lab”?
As you might have guessed, it is exactly the opposite of “pushing” the filmstock. Filmstock is overexposed and then under-developed by the lab. “Pull” processed film often tends to exhibit tighter grain and more saturated colors.
My camera has worked fine in the past. Why should I run camera tests before each production?
Let’s put it this way; What’s a few dollars in ruined test footage compared to a ruined day (week, month) of shooting? It is simply astounding that anyone would spend the time and effort to write a script, gather a crew, buy filmstock and start shooting without even running a foot of film through the camera to find out if it is working properly! It happens every day, and a lot of people get burned through this omission. Remember last week when Uncle Charlie borrowed your XYZ Super Cam to shoot a few pickups for HIS feature? Well, he used it to beat a nail back into his shoe and now you’ve got a Zeiss Super Speed prime with a loose rear element, only… YOU DON’T KNOW IT! So when that UFO crashes into your set during a master shot, don’t go crying to Zeiss when it appears as other-worldly blobs on your rushes. You have been WARNED!
What can I do with the footage, other than just shoot glamour shots of my dog Sparky?
Well, Sparky could give you vital clues as to how your filmstock will react under a variety of conditions. First, determine if you will have any unusual lighting situations in your upcoming production (who doesn’t?). Second, try to re-create a similar situation to film your tests under. (Don’t get lazy here, it IS worth the effort!) Third, include a color chip-chart and gray-scale in the shot. These are valuable indicators of how the filmstock will react to the prevailing conditions and exposure settings. Skin tones are not enough, and Sparky is not a good substitute anyway. Fourth, bracket your exposures. Don’t just shoot one shot of each angle at the “proper” exposure, shoot seven or eight shots and bracket from three stops under to four stops over the “proper” exposure. These shots don’t have to be of a long duration, just enough to examine during projection. Now, and this is the IMPORTANT PART, have the lab process this stock normally and have a TIMED workprint struck for examination. A one-lite workprint will simply confirm, “Yup, it’s under and over exposed!”, while a timed workprint will show you what you can get away with. By the way, NEVER evaluate your footage in any type of viewer. You must project it to get a proper sense of what it will look like on the screen. You will find that some interesting and very beautiful effects can be had by over/under exposing filmstock and having the timer compensate during the printing.
So many people fail to understand the symbiotic relationship between the laboratory and the D.P., that it often remains THE secret weapon of “THE REALLY GOOD D.P.”. A good lab can save, or enhance, your production in so many ways it not even funny, but you have to learn how to talk to the lab personnel before you can take advantage of their expertise. Get to know a timer at your lab. Ask stupid questions. Let them help you. That’s what they get paid for. You should find that most lab people are very helpful.
Colour Temperature
“Strictly, color temperature describes the actual temperature of a black body and, thereby, completely defines the spectral energy distribution of the object.”, American Cinematographer Manual, 1980, p. 309
I believe this means that a light with the color temperature of 3200 degrees Kelvin is distributing the spectral energy, ie. light, equivalent to that of a black body heated to that temperature.
The higher the color temperature the shorter the wavelength. Hence, light of higher color temperatures is “bluer”, that of lower temperatures “redder”. Most incandescent sources are in the 3000-3600 range. Sunlight varies greatly, direct sunlight at noon 5400, shade (or skylight) can be much greater. Sunlight on an overcast day is actually of a higher color temp. then direct sunlight.
Our brain tends to create that most light of a moderately wide spectrum appears as white light to us. Film does not have this ability to compensate. Film balanced for tungsten, ie. incandescent light, will make sunlit scenes appear very blue. Film balanced for daylight gives an orange cast to incandescent light. Flourescent light can be anywhere between 3000 and 6000, with some particularly ugly greens in between.
In general you need to use lighting that is all of a similar color temperature to avoid these effects, or filter the individual sources and/or the camera.
How do I shoot Sunset for Sunrise?
Spot meter the atmosphere just adjacent to the Sun and close-down 2 stops. Film in reverse.
How do I shoot Sunrise for Sunrise?
Spot meter horizon just above horizon where Sun is rising (use compass and solar-rise tables to pin-point location and time). Close-down 3 stops and film normal.
How do I shoot B&W Day-for-Night?
Shoot in bright, harsh light (morning or late evening best) and use a combination of a #23A and #36 OR #25 and #36 Kodak Wratten filters. Under-expose 1 1/2 to 2 stops (after compensating for filter factors) and try to shoot toward blue sky. This does not work very well on overcast days.
How do I shoot Color Day-for-Night?
Shoot tungsten balanced raw stock without color balancing #85 filter. Use #81F Kodak Wratten filter to reduce bluish cast. Under-expose by 1 1/2 to 2 stops. Print lighter in lab. You can also shoot with #85 at 1 1/2 to 2 stops under, print lighter in the lab and have the timer add blue to the scene.
How do I shoot Night for Night?
Set up lights to reinforce basic lighting (i.e., street lights, moon, house lights, etc). Key lights should come from side or back and from low angle to throw long shadows. Use hard light sources for hard shadows (remove fresnel lenses from lighting units if possible). Gel lights with pale blue gels for color work. Gain depth by lighting in pools. Break-up large areas of light with flags or cookies. Wet-down buildings and streets for high-contrast textures that reflect light. Actors should wear light colored clothing to separate them from the background.
How do I shoot an overcast, boring sky?
Use graduated density filter to break-up the uniformity of the sky and tone it down. Reduces contrast as well.
How do I shoot in dull, flat exterior lighting?
Place a dark object in foreground for depth. Photograph terrain in morning or late afternoon to get shadows and depth.
What are filter factors, and how do I use them?
All filters absorb a certain amount of light. They are calibrated in terms of a filter factor, which can be converted into an F-stop using the following tables;
How can you tell that something was shot on film rather than video?
Perceptually, film-originated images can be differentiated from video, even after it has been transferred to video, due to a number of factors. At the most basic level, film is a chemical-based process (more organic), whereas video is an electronic process (less organic). Film images are (typically) viewed/transmitted to the eye as whole, individual images via reflected light at 24 fps (however, 35mm projectors interrupt the projection beam on each frame TWICE and give you a PERCEIVED frame rate of 48 fps. A neat little psi effect!), whereas video images are passed-on the eye directly from emissions of phosphors in a cathode ray tube (CRT) in half- frame portions (fields) at 29.97 frames per second (original NTSC mono video WAS 30 fps, but with the introduction of color and the limitations of existing bandwidth, NTSC decided to slow the frame-rate down to 29.97 to accommodate the color-burst signal rather than increase (and obsolete existing systems) the needed bandwidth). For simplicity, we will say video runs at 30 fps. Now, each frame of video is comprised of two fields of odd and even-line information (525 lines/field 1 lines 1..3..5/field 2 lines 2..4..6). The playback-rate of video is really 60 fields per second, alternating between odd and even field transmission. You could say, a fundamental difference between film and video is, to use a modern parlance, the “sample-rate” and “type”(organic-electronic). Film, like a non-interlaced monitor, tends to exhibit pronounced “flicker” at low frame-rates (early film=”the flickers”). Standard sound-film has a “sample rate” of 24 full-frames per second and Video uses an interlaced transmission of odd and even fields that comprise and effective “sample-rate” of 60 partial-frames per second. In film, frame 1 is frame 1 and will contain all the time/space information for that sampled moment. In video, each FIELD will carry the difference, in both time and movement, of the interval between the first field scan and the second field scan WITHIN THE TRANSMITTED FRAME. This, in my opinion, produces a heightened sense of fluid movement in video playback (hence its more “live” feeling). You can easily see this object-displacement effect when a freeze frame of video of a rapidly moving object is captured in full-frame mode on a time-base corrector/frame synchronizer. The object, a car for example, will appear to vibrate rapidly as the tbc/fs plays-back both fields. This is why most tbc/fs units have the option of capturing either a full- field freeze frame, or double-sample a single field. Due to the increased “sample-rate” of video, the APPARENT amount of information transmitted relating to motion is higher than film, even though it is far lower (film 1000+ lines or so).
- Film grain-structure (preserved on video transfer) vs random electronic noise (film grain is chaotic but consistent) in dark areas.
- Film can handle contrast ratios better than video due to the natural “compression” abilities of a chemical-based process, although the higher-end video cameras are rapidly being refined in this respect with electronic “knee-compression” circuits.
- Motion clues (how each process records and represents various forms of movement) are a big give-away. Film tends to look slightly more “strobe-like” when intercut or compared to video.
- Transfer artifacts (film 24 fps to NTSC 29.97 fps) tend to give-away a film segment. To convert the frame-rate from 24 to 29.97 fps, the transfer device must use a 3:2 pull-down sequence (each third frame is duplicated) and this is noticeable on a subconscious level. I can SEE this in many feature films transferred to video as a sudden “freeze” during fast pans.
There’s more, but the above are the basics.
What is pre-flashing?
To lower contrast, gain more shadow detail and heighten the sensitivity of a filmstock, you might consider employing a technique called “pre-flashing”. Pre-flashing is the application of a controlled amount of light to the raw stock before normal exposure to raise the “base-fog” level of the filmstock. This light can be neutral (white) or colored. On color stocks, neutral light lowers the contrast and desaturates the normally exposed image without imparting a color “cast”. However, flashing a color stock with colored light imparts a distinct color tone to the shadows and darker image areas without overpowering the highlights (depending upon the amount of flash imparted to the unexposed raw stock). Black and White stocks are usually only flashed with white light, but some interesting results could come from flashing with primary colors. Pre-flashed b&w stocks exhibit lower contrast and greater shadow detail, depending upon the strength of the flash.
How do I pre-flash a stock?
Do-it-yourself pre-flashing is not an exact science and tests must be performed to judge the relative usefulness of this process.
Locate a very smooth (no texture), white surface to shoot (try foamcore board). Be sure that there is no dirt or marks on the surface or these will show up as stationary defects in the frame. These defects will manifest themselves as areas of harder contrast within the lower contrast of the entire frame and look somewhat like shadows overlaying the frame. Make sure the surface is evenly illuminated. No hot-spots or light fall-off should be allowed. Point your camera at the surface, making sure the frame is entirely filled and de-focus the lens by placing it at infinity. NOTE: As with any test-dependent process, it is very important that you take detailed notes and note the EXACT location of your lights and camera. If you plan to use this process in any meaningful manner, you should prepare an area to flash filmstock and leave it set up for the duration of the project. A copy stand with attached lights (or equivalent) should do the trick and not tie-up valuable production equipment. Be sure to use the same lens each time.
With a light meter, properly set for your filmstock’s ISO, take an incident light reading and increase (open-up) this reading by one f-stop. This gives you the 100% flash exposure for your filmstock and allows you to establish a baseline for measuring percentages of flash you may give your filmstock. For example: If you have an ISO 100 color neg and get an f/4 reading from your white surface, you have established a f2.8, 100% baseline for that filmstock. If you were to expose this film at f2.8, you would give the filmstock a 100% flash and produce a white, or clear, image upon projection of the workprint. An exposure of f/4 would give you a 50% flash; an f/5.6 would give you a 25% flash, and so on. I suggest you shoot an entire 100 foot roll of film, starting at your baseline, and bracketing to the limit of your lens in 1/2 stop increments.
If you are careful, and set-up the baseline at maximum aperture of the lens being used (move the lights in and out), you can use the entire f-stop range to experiment with. Be sure to cap your lens between each “flash” and run off a few feet to distinctly separate the tests. After shooting this roll, carefully rewind the film to the head (in-camera if possible and with lens and viewfinder capped to prevent further exposure) and shoot the film “as normal” on a single, high-contrast scene. It is important to shoot the entire roll on the same scene (and same position, lens), so that you may judge the effect various percentages of flash impose on the contrast ranges within the shot. The sections of film where you capped the lens will give you a good reference between normally exposed film and the percentage of flash you have imposed on the filmstock. It is important that you have this film processed normally and have a TIMED workprint struck from the negative for evaluation purposes (for reversal, just process normal). It also would not hurt to have a color scale, or chip-chart, in the shot to observe the effect flashing has on various colors.
Flashing is not a process for the weak-of-heart, but it can produce some unusual and beautiful effects under high contrast situations.