DSC's October TechTip
for October 2009 comes from

Tony Salgado
Digital Imaging Technician

The Subject - "Establishing a repeatable baseline reference"

The introduction of digital cinema cameras such as the Sony F23 and F35 series cameras which allow recording in various gamma modes such as S-LOG, Hyper Gamma, and user gamma modes can lead to potential post production confusion when photographing a gray scale chart on how to interpret the exposure values represented on the individual chips.

A CamAlign chart shooting technique which minimizes such misunderstandings can be accomplished by indicating on the chart the precise IRE or millivolt values which the peak white, middle gray and black chips represent for the specific camera gamma mode chosen to photograph a scene. The CamAlign chart can be photographed during the camera prep or in advance of principal photography, as it will be used to establish a unity baseline reference to aid in determining the post production grading LUT.

For example in S-LOG mode (loosely referred to as flat pass viewing) on a DSC CamAlign ChromaDuMonde chart, the white chip on upper left and lower right represents 65 IRE (reference the Green channel), 100 IRE when viewing the Sony ITU 709 180% dynamic range LUT and 90 IRE when viewing the Sony 709 800% dynamic range LUT. Indicating the precise IRE or millivolt values on the chart will become even more critical with the ability to create custom user in camera viewing LUTS which can be recalled on set. If any of these viewing LUTS are chosen by the DP and/or DIT for the initial grading of dailies etc. it will be critical to establish a firm visual “roadmap” with a CamAlign chart to insure a unity baseline reference throughout the course of the entire production.

In addition, it is highly recommend to write the HDCAM SR playback frequency frame rate, sampling rate and camera gamma mode information on the test chart, slate and tape labels to aid in avoiding any confusion later during post.

Example: 1080 23.98 psf 4:4:4 SQ S-LOG, 1080 23.98 psf 4:4:4 HQ S-LOG, 1080 29.97 psf 4:2:2 Hyper Gamma 7

It is always in the best interest of the DIT to protect the DP’s original creative intent by shooting a DSC CamAlign chart so that post production will not have to guess or question what the DP had in mind in terms of exposure. In the world of production, many items can be left open to interpretation, however the visual intent of the DP should not be one of these.

We are grateful to Tony Salgado for this month’s Tech-Tip on Establishing a repeatable baseline reference.

DSC's TechTip
for August 2009 comes from

Art Adams
Director of Photography

The Subject - Matching Multiple Cameras

"Matching Multiple Cameras"

This question was recently posted on the Cinematography Mailing List:

 "I need to match two Sony EX3 cameras to an HVX-200. How do I do it?"

My suggestion was to shoot a perfectly lit and exposed DSC chart on all three cameras, using the same criteria for setting exposure (placing the white chip on the chart on the same luminance value for all three cameras) in order to facilitate post color matching. A colorist should be able to match the cameras by looking at the star pattern the chart creates on a vectorscope and bending the points of each camera's star to match. It's important to match the cameras' gamma and knee settings in advance, as a colorist may have a more difficult time matching those. The DSC chart's gray chips form an "X" pattern on a waveform monitor, and the goal is to create matching "X" patterns on all three cameras. Match the most adjustable camera (the EX3) to the least adjustable camera (the HVX-200) by manipulating the gamma and knee settings. The knee circuit will interfere with the top part of the "X" pattern so it's probably best to shut that off and adjust it separately after gamma matching is complete.

If you have some time to spend in prep it might be possible to match camera colorimetry in advance using the EX3 user matrix. Using the less adjustable HVX-200 as a reference, adjust the user matrix for each EX3 so that the star pattern, created by the DSC chart on the vectorscope, matches that of the HVX-200. Always use a single waveform/vectorscope and monitor when matching cameras. No two monitors are the same, so looking at all three cameras through a single monitor eliminates that variable. Route the HVX-200 into the "A" input on the waveform/vectorscope and one of the EX3's into the "B" input, and then toggle between them while making adjustments. Once the first EX3 is matched to the HVX-200, repeat the process with the second. The Leader 5750 waveform/vectorscope is a great tool for this as it is simple to capture a still frame from one camera's star or "X" pattern and overlay a live image from the other camera on top of it. Then it's just a matter of adjusting the second camera's pattern so that it overlays the first.

One local rental house took this a step further and matched their EX3's colorimetry to a Varicam, as they considered the Varicam's colorimetry to be more pleasant.

We are grateful to Art Adams for this month’s Tech-Tip on Matching Multiple Cameras.

DSC's TechTip
for November 2008 comes from

Dan Mulligan, DP - UK

The Subject - Do You Waveform Monitor the Lutted Image or the S-Log?

"Do You Waveform Monitor the Lutted Image or the S-Log?"

I waveform the S-Log, the LUTs on set can wack out the waveform (imagine waveforming the Rec 709 output, ouch!) and they really are, for most reasonable purposes, for comfort viewing only. You can go very deep into LUTs and start some very sophisticated looks, using the waveform to really balance your RGB values, but that needs careful and thoughtful application and a good working relationship with your post team, but you really can achieve some lovely results.

The S-Log is my neg and that’s what I want to waveform and look for black level and clipping. Once exposed, then toggle to your desired LUT for that scene/shot.
 
For most log images on set, which are mostly partially colour temperature corrected in camera, then a simple bit of contrast to the blacks will tighten up the image enough for a pleasant on-set look and the waveform will confirm your stop value/black level loss with your desired LUT applied, but for me its the S-Log/Neg I'm interested in first - LUTs come next.

Mike Richardson
holding a ChromaDuMonde for Dan

You can always tell which professionals use their CDMs all the time . . .

. . . they're covered with gaffer tape

DSC's TechTip
for June 2008 comes from

George Walkley

The Subject - Matching the Sony EX-1 with more costly Broadcast Cameras

"Matching the Sony EX-1 with more costly Broadcast Camera's"

We have been creating custom profiles recently for our customers who have purchased XDCAM, HDCAM, and Sony’s new PMW-EX1 and other cameras.  We need to match them all for particular clients for particular shooting environments.   It is absolutely necessary when creating profiles to have the correct aides to guide you and to have the end user with you (important) so that he/she is comfortable with the final look you create.
 
All of the menus are just numbers that you adjust for the response you want.  Unless you have a calibrated monitor, calibrated chart that is correctly lit and so forth, you can tweak all you want and not come up with the result you would like.  As a tech I understand this, but rather than confirming that factory specs are giving a correct response, I dive into having fun creating picture profiles or scene profiles with the customer, while keeping away from undesirable responses. 
 
One thing that is a must with HD is avoiding washed out high-lights that get blown out as soon as the scene is saturated with light.  All the cameras listed above plus some others really need some help as they come out of the box.  DSC Labs’ charts help with developing chroma profiles, black stretch adjustments, flare adjustments, etc, with confidence.  Because the EX1 is loaded with things to tweak, including an individual color with phase and saturation ability, the ChromaDuMonde chart really helps me confirm that I’m getting the result I want without getting unsatisfactory results in other areas of the setup.
 
I have gotten tons of compliments from our clients that shoot major outdoor and other types of projects with a variety of cameras on the same shoot.  The cameras will all have the same profile characteristics - PMW-EX1, PDW-355’s, and HDW-730’s for example.  What’s amazing is the Sony EX1 and just how great it looks, especially for the money.  I’m not trying to sell an EX1 but I have to admit the blacks look awesome, the chroma is fabulous and the grey scale is superb.
 
These new cameras allow you to tweak your ‘---‘ off, but you need a guide to do it properly!  A good chart is a must.

DSC's TechTip
for April 2008 comes from

David Blackham

The Subject - Continuous Light Sources

"Continuous Light Sources"

There is a growing trend to use fluorescent, LED and discharge light sources for film and video production.  This is good news for the environment as these light sources use less power than conventional studio lights.  As the power consumption is less, so less heat is generated on set.   Environmental issues should not be ignored when considering technical and creative requirements for production - being environmentally responsible has practical benefits too!

One of the problems faced by these light sources is the lack of continuous spectrum activity, exemplified in fluorescents, LED and other discharge fixtures. It is impossible to add light that was not present in the first place but spikes can be removed with suitable gels (e.g. minus green for green spikes). Often, light sources originally designed for film and television are used with video – this is common on productions with lower budgets, and may be why there are so many brands of CFL bulbs.  If selected with care, many of the quality bulbs have a high CRI and are useful sources of light. Video and digital cameras have a greater tolerance to low CRI light sources and they can be easily balanced during image capture, but only when a neutral DSC test pattern is as a reference.  CRI figures are not a guarantee that the source has a continuous spectrum, but can be helpful when selecting potential light sources.  Much of this information can be found on the internet.

When using new fluorescent tubes from any manufacturer, a common practice is to burn the tube in for at least 24 hours before use.  This allows the tube to stabilize and become consistent in its output.  Try and use light sources with a high CRI and suitable CCT when budgets allow. Occasionally, light sources that deviate from 3200k or 5600k are welcome on set, so long as the quality of light from the source remains constant in terms of CCT and spectrum. A meter is not always necessary to determine colour temperature as many Digital cameras have onboard indicators.

A guide to monitoring continuous spectrums of light is easy with DSC Laboratories test patterns. The front lit ChromaDuMonde 28R is a necessity to identify light sources that may not have a continuous spectrum. If the camera is calibrated with a continuous source (such as tungsten lamp) then a similar colour temperature fluorescent source can be compared on a vectorscope display. This may not be standard practice, but this technique is useful as it requires basic tools found on common sets.  Monitor calibration is of equal importance - especially when shooting on location.  I use a Spyder 3 monitor calibration device made by Datacolor. Location viewing environments may not be the perfect calibration spot, but the device is simple and, when paired with a DSC Pattern, easy to use when trying to achieve perfectly neutral imagery.

DSC's TechTip
for March 2008 comes from

D. Gregor Hagey CSC

The Subject - Red One and CMOS Static

Learn more about Gregor here

"Red One and CMOS Static"

Every time you change a lens on a digital camera such as the Red One, be sure to check the sensor for dust. The single CMOS sensor has a static charge that attracts dust (just like DSLRs). Use a loop to magnify the sensor before blowing gently with a blower bulb or compressed air.  If blowing won't remove the dust, then use a brush designed to clean DSLR sensors, but use it with great care. 

When dust is left on the sensor it appears as a soft grey blob in the image. This is not always visible on small displays and might not be noticed unless you see your work on a large display or projector.

DSC's TechTip
for December 2007 is from

Reid Robertson
Reid is a Digital Camcorder Specialist with Panasonic Canada.

The Subject - Back Focus

Always remember when performing a "flange-back" adjustment (as Back Focus is sometimes called) the Iris should be FULLY OPEN. When the Iris is wide open, the depth of field is minimized, giving you the optimum focus point during your adjustment.

When there is too much light, use the ND filters in the filter wheel to cut it down to a useable level. If there are no ND filters available, use the "Poor Man's ND": higher Shutter speeds (this trick comes from the CCTV world). I cheat and also use the Viewfinder Peaking set to maximum, to find the best focus during the adjustment.
Rebuttals on Back Focus:
Just received your tech tips #9. With all respect, the suggestion to use max vf peaking, is suspect, in the same sense that doing a back focus at f/16 would be wrong. The idea is to make it more difficult, and more critical, than less so. My recommendation is always minimum (or just the least bit) of viewfinder peaking. - A. L. New York
**********
Hello DSC, a comment from someone who used to work for a HD colour viewfinder manufacturer - on Sony 730/750/900 we used to find that setting the VF peaking at about +70 (It goes from -99 to +99, with 0 being the default) gives a much better image on which to judge focus. Note, setting this peaking does NOT affect the image recorded to tape, it purely lifts the transitional edges slightly in the VF so that they "Snap In" at the point of critical focus. - RB London UK
**********
I guess by "difficult" the commenter means "difficult to be satisfied with what you see" rather than "difficult to obtain the desired result"?

Let us define "difficult" in the most common connotation of "difficult to do." Then, less sensitive = MORE difficult to see and do. More difficult = LESS sensitive! You want it more sensitive, yes! But less difficult to see the right adjustment. That's why you don't use f/16, which makes it LESS sensitive and MORE difficult. Max peaking, on the other hand, lets you see a slight decrease in sharpness as you move away from the optimum point. MORE sensitive and LESS difficult. - WB Illinois
**********
Here's the correct way to do it, real easy.
Open up the iris completely, and get your camera to deal with the light. Use any means available such as shutter and filter wheel to get the camera to deal with the lens wide open. Next zoom in completely and focus on your subject. Then zoom out completely and adjust back focus. Repeat a couple of times since these adjustments interact slightly.
This is the best way to do it, because this provides you with an individual back focus setting for your specific shot. Not a problem normally except if you have a really good lens coupled with a very good camera. Normally, you should just do this for a shot at infinity, and leave the lens set up this way for the next person using it.
Viewfinder peaking helps find the "sweet spot", always maximize it if you need to. Main problem with Back Focus is that people don't get that the Iris has to be WIDE open doing this.
Reason is simple. Before lenses we had pinhole cameras. They were really cool. A box with a pinhole in it and a piece of film on the other side. No problem with any focusing of the lens whatsoever, and always a great shot. Might take half an hour to get a good shot without anyone moving their face. Probably why old photos show grumpy looking people. - JK CA
**********
As far as I can tell, the cameramen I work with, who've also been doing this for 25+ years, do just the opposite of what this fellow above suggests.

They crank the living daylights out of their viewfinder peaking. Their objective is to make the image jump off the eyepiece and into their eyeballs when they've hit critical focus.

I do the same thing when I'm adjusting back focus, or doing other focus critical alignments.
LC - NY
**********
Setting VF peaking to max for back focus alignment, was also very suspect to me. Depending on the condition of VF and camera, it's someVerdanaecessary to use a certain amount of peaking in order to get the best result, but only in the very wide position of the zoom.
That's how I do it sometimes, but as soon I zoom to the tele position, to adjust the focus, I turn down the peaking. That's more work to do (especially when repeating the procedure many times...), but it pays.
I think, the max VF peaking might only be suitable for an "emergency setting" in the field, when no Siemens star is available and whatever is in front of the lens has to work as a test chart. Best regards from Switzerland - MR
**********
Gee, if we are going to turn the viewfinder peaking down, I guess we should go back inside the camera and turn off the enhancement contours like my engineer friends did back in the Norelco PC-70 days! That would help even more to make it nearly impossible to get meaningful results under field conditions.

Viewfinder peaking is on a display, therefore it enhances what you CAN see. If you would look at a waveform monitor you would see the same thing that peaking shows on the viewfinder - a very stark "snap" in and out of focus. Peaking (and the waveform monitor) is also helpful in cases where you are too far from the chart and it is too hard to see detail at the wide angle setting. How far should we be from that chart, anyhow? And repeat the process at least twice. DG Iowa

DSC's TechTip
for September 2007 comes from

Greg Foad

The Subject - Are Six Colors Enough

Learn more about Greg here

Image A

Image B

"Are Six Colors Enough?"

Believe it or not, the camera that shot this 28 color (Image A) ChromaDuMonde was accurately aligned to a six color (Image B) CamAlign chart (note - the six primary colors are still in their boxes).  With the advanced handles on many HD cameras, matrix alignment can be misleading.  

The testing lab at DSC reminded me of a crash test dummy plant.  Various test charts and wacky designs lay everywhere - some had been yellowed from radiation exposure, others bent in testing shipping packaging, and others were just designs that never made it past the testing stage.  I was there to help DSC capture image scenes onto a WFM7100 USB buffer for an application note they were writing for Tektronix.  We used my Sony F900 to test color reproduction using different test patterns.

What we came across was quite astounding.  We first used a standard CamAlign six color pattern with crossed grayscales.  With only six colors to work with, the multi matrix controls on the F900 allowed me to put each color in its box very quickly - camera alignment took no more than 10 minutes… or so we thought.  The green and cyan dots seemed to have an oblong shape, so I was skeptical that we had aligned them correctly.  Sure enough after putting up a 28 color ChromaDuMonde, we saw the vastly misshapen hexagon.  One goal of camera alignment is to adjust its taking characteristics to accurately reproduce the widest possible range of colors – not so in the first image.  Clearly, we would be better off using one of the factory presets then using this skewed matrix.

While both user and multi matrix settings appear in the paint menu of the camera, they have very different functions.  The user matrix was originally intended to help warm or cool an image, depending on the shot.  The algorithms behind this circuitry grab clusters of colors and pull them fore and aft, i.e. R-B, B-Y, etc.  The multi matrix controls uniquely allow selective color enhancement and hue adjustments up to 20 degrees.  These controls are powerful, as they only select one color independent of any intermediaries.  So, the correct alignment sequence is to adjust the user matrix first, and then use the multi matrix controls for fine tuning.

So, this tech tip begs the question – are six colors enough?

DSC's TechTip
for June 2007 comes from

Bruce Alan Greene
Originally appearing on the CML site it is reprinted with kind permission from Bruce and the CML

The Subject - Varicam Detail

Learn more about Bruce here

"Varicam Detail"

On my last major Varicam project I conducted a film out test and included a variety of detail settings in the test.  After viewing the film print on the big screen I selected a level of detail that I believed added enough "punch" to the image without any noticeable detail artifacts visible in the print. In the test I was surprised by how much added detail the film print would tolerate, though to be fair, I've developed a subtle eye for "Sharpening" through years of Photoshop work.

For the Varicam, I decided that "Master Detail" could be set as high as "0" before I didn't like the look. To be on the safe side, I shot with master detail set to minus 2. At the factory defaults for all the detail parameters on the Varicam, master detail = -7 is the same as "detail off". Master detail set to "0" is a significant amount of detail on the Varicam.  For the most part I ended up happy with my detail setting until...I had a shot of a woman against a bright blue sky. On my 17in LCD monitor, I could just start to see an outline where the woman met the sky. I then dialed down my setting a couple notches. 

My advice for anyone brave enough to turn on the detail is to view your test image and test charts on a large LCD monitor that has a true pixel to pixel representation of the image. I think if one judges by CRT then one is apt to add too much.  Also, it is very important to use a waveform monitor when adjusting the detail. You will see changes in the detail settings on the waveform that you will miss by looking only at a monitor. 

I hope this is of some help to those interested in this discussion.

Bruce Alan Greene
Los Angeles

DSC's TechTip
for April 2007 comes from our

User Database Reports
This is a compilation of similar ideas from a number of different users.

The Subject - Noise, Green, Cyan, and Saturation

"Noise, Green, Cyan, and Saturation"

When data acquisition for scientific applications is the main purpose of image capture, precise camera setup is of even greater importance.  For that reason, we are used to hearing very specific guidelines from clients that the norm does not always follow – for instance, some of our clients capture only in a progressive field mode, as testing has shown interlaced to be less reliable (an arguable, but tested opinion!).  Another pertinent example includes lining up a DSC Labs ChromaDuMonde with 80% Green and Cyan saturation, as opposed to lining them up directly in their vectorscope boxes.  Some clients align the phase of the green and cyan chips to be in plane with their appropriate vector boxes; however when boosting the saturation values, they do not max that setting out on the camera just to get the greens and cyan's square within their boxes.  As many cameras come from the factory with greatly warmed G’s and Cy’s, clients have found themselves maxing out our saturation values in order to get the colors deadset in their boxes and this can create a fair amount of noise in the darker areas of the image - this will vary from camera to camera.  Operating with a buffer is a good engineering tip. 

One client recently described the process as follows - 'For simplicity's sake, let’s use a Varicam to quantify this process.  Take a DSC Labs ChromaDuMonde 28 color pattern, and using the Color Matrix and Color Correction settings, align Y, R, Mg and B up, whilst aiming the G and Cy chips at their targets.  Then, we increase the saturation of these chips within the color corrector until we are at about 20% of where they should be.  As the Y, R, Mg and B chips are already near their boxes, setting these colors is relatively easy.  The G and Cy chips however, are much farther from their targets, and therefore require a significant boost in gain, even though we do all of our set ups with a zero gain setting. When all of the colors are lined up, the end result is a bit noisy, as if we were lining up with an increase in gain.  The same increase in dynamic range as used in the ‘gain’ setting must be applied to the saturation settings within the camera’s color correction controls.  This process may be considered subjective, and we are not preaching it as gospel, however it produces a satisfactory colorspace with a limited amount of noise, which works perfectly for us.'

DSC's TechTip
for February 2007 is from

David Mullen, ASC
Originally appearing on the CML site it is reprinted with kind permission from David and the CML.

The Subject - Tips on HD

Learn more about David here

"Tips on HD"

When shooting digitally and recording to a fairly compressed format like HDCAM / DVCPRO-HD, I sort of subscribe to the "fifty-fifty" thinking (although the true breakdown may lean more towards in-camera than post) by which I mean "get close to the look you want in terms of the original photography / recording and finish the look in post." You don't have to get 100% of the look in-camera if it is more efficient, especially time-wise on the set, to complete the look in post. On the other hand, the compression of HDCAM, for example, limits how far you can push an image in post before you start to pick-up artifacts, and some types of image manipulation are more artifacty than others.

For example, adding more contrast, which may involve crushing the shadows and/or clipping the highlights, is probably best saved for post, whereas lowering contrast, increasing shadow or highlight detail, should be done in-camera with proper lighting / exposure.

If you are unclear as to how to manipulate the Color Matrix, then yes, I would get a DIT involved. As far as doing a silver retention process to the final prints, there will be some desaturation and increase in contrast in the shadows, with deeper blacks, so keep that in mind when shooting and when doing the final color-correcting. I recommend doing a test before you finalize any color-correction decisions. Luckily if you choose Technicolor's ENR or Deluxe's ACE print process, you have a lot of flexibility in controlling the degree of silver retained, although on a reel-by-reel basis, not shot-by-shot.

DSC's TechTip
for June 2006 is from

Robert Goodman
Emmy-nominated director and  the author of the "Goodman's Guide" series of field guides for SD and HD cameras

The Subject - Creating a Look

Learn more about Robert here

"Creating a Look"

Creating special looks for any camera starts with a baseline. I establish that baseline using a ChromaDuMonde chart.  Once you set the camera up to what is essentially normalized technical accurate reproduction, you can begin to deviate from that norm.

Manufacturers generally set up the camera at the factory to reproduce pleasing skin tones by altering the color gamut of the camera. This is no different than Eastman Kodak color balancing Kodachrome so it reproduced bluer skies and reds more brilliantly than were often present in the actual scene.  Fujichrome made greens look more lush and skin tones creamier than reality.  And you can do the same with most of today's digital cameras.

Under DAYLIGHT BALANCED lighting, shoot a ChromaDuMonde chart and use the camera's controls to reproduce it on a Vectorscope so all the main targets are in their boxes.  You'll need to increase the gain to approximately 1.875 (for charts after May 2007 use X2 vectorscope gain) instead of the CAL (calibrated) setting because of saturation.  Make sure you have a evaluation grade monitor preferably a CRT or one of the carefully calibrated LCD monitors, which are costly though less costly than the evaluation grade HD CRTs.

With the proper tools, you can begin to create your own looks.  Want a warmer, more pleasing look than the "technically accurate" look you just created?  Adjust the camera's controls to skew the color reproduction of the blues and cyans towards the reds and yellows.  Want a cooler look? Skew the color reproduction of magentas, reds, and yellows towards blue and cyan.  It will take a lot of trial and error to get what you want.

Pay careful attention to the impact of changes on the intermediate colors.  Try to keep the spacing between intermediate colors on the vectorscope uniform.  Remember that when you skew the color reproduction, you are eliminating colors so don't go too far overboard if you want to reproduce a nearly full color gamut.

Have fun and let yourself go overboard.  In making adjustments you'll discover the limits of the camera and perhaps a stunning look.  Every shot doesn't need to reproduce color accurately.  A cinematographer's job is to depict emotional truths.   Accuracy is for scientists.

A few final words: Style is over-emphasized; if the audience is watching what the cinematographer is doing they aren't involved in the story.  Subtle color changes resonate and alter the mood of audiences in ways that heavily filtered images never can.

Robert Goodman has conducted workshops on digital production and post for AIVF, Guild of Professional Photographers, IFP, ITVA, SMPTE, Sony Corporation, Women in Film, and at film festivals in Atlanta, Austin, Los Angeles, Minneapolis, New York, Philadelphia, Portland, San Diego, and San Francisco.

Mike Brennan
Mike is a leading DP in London, England and editor of High Definition Magazine.

The Subject - Aberration Issues

"Aberration"

For a few years I've assumed that it could be worth spending many dollars on high quality HD zooms to improve the performance of wide shots that all seem a little soft on HD.  In the past few years I'd noticed lateral chromatic aberration on standard quality HD zoom lenses (colour fringes either side of verticals), mostly on wide shots and assumed a high quality $70k HD lens would reduce the colour fringes.

What has been annoying is that all the frame grabs I had ever seen of my DSC charts in my studio look pretty good, as do everyone else's frame grabs, but location wide shot frame grabs still suffer the lateral aberration.

First, lens manufactures have a hard time focusing Red Green and Blue light to an equal distance through the optical block. In fact they get a helping hand from the camera manufacturers who created a standard where the red green and blue CCDs are not set at equal distances on the optical block; this has enabled TV lenses to be designed with 40x or 100x zoom ratios and film lenses (where all wave lengths have to be focused on the same plane), become virtually impossible to make past 25x.

However, it is still a devilish task to create a zoom that is perfect at all focal lengths, impossible in fact, but each zoom lens does have what DPs call a sweet spot.  This is a focal length where RGB rays are as good as they can get.  There is a residual lateral aberration in the form of an inherent problem in the prism - (well, camera manufacturers pass the responsibility to the lens manufacturers and lens manufacturers blame the prism itself!)

The problem becomes increasingly evident below 12mm and is really quite a problem at 5mm or 6mm.  Having tested many lenses on many cameras, it is evident on all lenses and cameras. 

This sweet spot varies with each lens.  A sweet spot on a Canon 40x lens, for instance, is 100mm whereas a 21x may be 14mm.  Both focal lengths aren't too long or too wide so that a test chart can't be shot in a lab without knocking down a wall.

So most of the charts I shoot, say testing gamma curves, are usually well above the focal length where the aberration is a problem and also are in the sweet spot of the lens!  So I have a brilliant collection of frame grabs with the DSC chart neatly filling the frame where there is near zero aberration, and a collection of wide shot frame grabs with problems.

So why hasn't this aberration been noticed on our test charts?  It is only recently we have pixel perfect registration in LCD monitors and, combined with 1920x1080 10 bit recording rather than 1440x1080 pixel 8 bit recordings, we are seeing more fine detail.

The main reason is that we rarely shoot a test chart on a wide lens where it fills the frame.

Pretty basic!

Since there are no 10 foot wide charts my tech tip for testing the wide end of zoom lenses is to shoot the DSC chart on the left and right edge of frame at 4 meters.

If you have a technical suggestion to share - please send it to "TechTips" <dsc@dsclabs.com> for consideration in a future "DSC TechTips".

Charlie Goldman
Charlie Goldman is a leading video engineer on popular shows such as Canadian Idol, Cirque du Soleil, etc.

 The Subject - Color Bar Symmetry

"Color Bar Symmetry"

Producing consistent accurate color reproduction can be a challenge, particularly under different lighting conditions.  Whether I am shooting the Olympics, Rock concerts, or Ice hockey I use the same basic technique.

It is helpful to understand the engineering behind DSC colorbars.  All DSC color chips are mid saturation and represent real life colors.  Each color is designed to produce levels of 80 and 40 IRE analog or 560 and 280 mV digital.  An increase in vectorscope gain of 1.875 will put all the signals in their boxes when a camera is aligned for accurate reproduction.

In analog systems, this simply means increasing vectorscope gain to position the burst opposite the outer corner of the yellow box, then adjust the matrix to set CamAlign's RGB and CMY color chips in their boxes.  A number of scope makers now incorporate the 1.875 factor for DSC charts in their new digital scopes or provide a calibrated readout of gain level.

Older digital scopes do not have a calibrated gain function, so in these situations I adjust the matrix to produce Red chip waveform levels of Red 560mV,  Green 280mV and Blue 280mV.  I then match the other colors to the same saturation on their appropriate vectors.  Be more concerned with the lower values than the higher, because errors at the 280mV levels will have a much larger visual effect than variations in the 560 mV level.  Dave Corley gave me this tip years ago, told to him by a Sony engineer in Chicago.

Charlie Goldman will be featured in an upcoming "Spotlight" at http://www.dsclabs.com/

- If you have a technical suggestion to share - send it to "TechTips" <dsc@dsclabs.com> for consideration in a future "DSC TechTips"