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Sat May 14, 2011 10:29 pm

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http://www.flickr.com/groups/strobist/d ... 679167124/

This thread is the epitome of misinformation, ignorance, confusion, lies and lack of industry knowledge regarding color balance and other factors, particularly wrong and libelous misrepresentations regarding AlienBees and certain European brands. So I thought I would take some time to explain it in simplified general terms photographers can understand.

Color distribution: Light is measured along two axis . . . the Yellow/Blue axis (usually express as Color Temperature) and the Red/Green axis (often referred to as Tint).

A light source that has a Color Rendition Index (CRI) of 100 (such as an ordinary tungsten lamp or pure direct sunlight) produces a smooth distribution curve containing all color wavelengths. As color temperature is raised, the shorter wavelengths (blueish) are accentuated in relation to the longer wavelengths( Reddish), but the distribution curve remains smooth, thus the light is free of tints, color spikes and other anomalies that would cause tint errors.

Xenon Flash Tubes produce a characteristic distribution of color spikes that are distributed quite evenly and thus produce a very high, but not perfect, CRI.

While a Xenon flash tube can be optimized to produce a zero R/G tint at a specific operating point, changing the power level by any means will invariably result in changes to the color temperature, tint and flash duration. This is a physically reality and cannot be avoided, regardless of the cost or design.

Methods of power control in Pack and Head systems:
When flash power is adjusted by switching flash capacitors in or out of the circuit, both color temperature and tint remain relatively constant, while flash duration becomes shorter at low power (few capacitors) and longer at high power (more capacitors). This typifies the performance of basic pack and head systems.

The limitations of such systems are that power can typically only be accomplished in 1 f-stop increments, and the sharing of a capacitor bank by multiple heads results in uncontrollable changes in flash durations, ratios of power delivered to the various heads, etc. Another factor is that mixing of different head types (IE standard heads, ringflash head, short duration heads, etc, results in a completely unpredictable relationship of all parameters.

Manufacturers of pack and head systems have gone to great lengths to minimize these factors, at the cost of operating complexity and price.

Power Control in MonoFlash systems:
The vast majority of monoflash units. regardless of cost or country of origin, control flash power by varying the voltage applied to a fixed bank of flash capacitors. All such systems, including AlienBees, Profoto, Elinchrom, WL Ultra and most other brands, follow the same characteristics:

Color Temperature drops at 75° to 90°K per f-stop of power reduction (typically 350 - 400° variation from full to 1/32 power).

R/G axis tints typically shift by 7 to 10 units over the same power adjustment range.

Flash duration typically lengthens by about double over the same power adjustment range.

Exceptions/modifications to these rule are found in our WLX1600/3200 models, which combine voltage variation with capacitor switching to achieve.

Profoto D1 misleading marketing and specifications:
It is often stated on forums by AB detractors that "Profoto D1 maintains 5600°K +/-50° color constancy over the entire power control range." The source of this misleading information lies in Profoto's specifications of "Color Accuracy = +/-50°K" and Color temperature = 5600°K" Uninformed readers assume this to imply the DI maintains 5600°˚ +/-50°K over the entire power control range. Nothing could be farther from the truth.

In reality, the color accuracy spec pertains to the shot to shot color repeatability at a specific power setting, and the 5600° spec also applies only to one specific power setting.

Actual testing in our labs of the D1 revealed a D1 full power color temperature of 6100°K and a minimum power (1/64 power) color temperature of 5400°K . . . a 600° actual color shift, and slightly over twice the flash duration relative to full power. Over this same range, the R/G tint shifts by about 10CC units.

By contrast, in parallel tests. AlienBees color shifted 375°K from Full to 1/32 power while the RG tint shifted by 7CC units. The flash duration doubled from full to 1/32 power.

Another note is that Profoto publishes only misleading t.5 flash duration times . . . merely specifying it as "flash duration" and omits the more meaning full t.1 duration.

AB1600 VS Elinchrom RX600:
AB detractors often make the statement that Elinchrom lights produce essentially no color shift and faster flash durations than AB1600.

In the same series of tests of D1 above, AB1600 performance over the Full to 1/32 power range was compared to RX600 over the same range. The result showed the two units were virtually identical, yielding 350 - 375°K color shift, exactly the same R/G tint values with both shifting 7CC units, the same t.5 and t.1 flash durations, and identical recycle times and identical performance when operated from our Vagabond Mini Lithium battery pack. They are virtual performance clones in these parameters, with the RX600 offering only somewhat better power setting repeatability at the low end of the power range as a result of the digital VS analog power setting method.

IGBT Controlled Flash units:
At this writing and to the best of my knowledge, there are only three IGBT controlled pro-level studio flash systems on the market (though there are rumors of a fourth special purpose version coming from Hensel.

The systems are the Paul C. Buff Einstein 640 monoflash, Broncolor Scoro and Grafit pack and head systems, and somewhat obscure Photogenic Solaire monoflash.

Unlike voltage controlled or capacitor switching systems, IGBT systems utilize Insulated Gate Bipolar Transistors to interrupt the current flow through the flashtube, much like is done on a smaller scale in most speedlights. By using a sophisticated microprocessor, the applied capacitor voltage and the timing of the flash tube shutoff are established for each 1/10f power setting to allow +/-50°K color balance shift over the entire power range, an extremely wide, accurate and repeatable power control range, and exceedingly fast t.1 flash durations that shorten dramatically as the power is decreased.

Einstein has two operating modes: Constant Color and Action Mode. In Action Mode, color temperature is allowed to rise as power is reduced in order to achieve exceedingly fast t.1 time as short as 1/13,500 second, while in color mode, minimum t.1 flash duration is 1/8000 and color temperature is held to +/-50°K

Scoro and Grafit systems achieve minimum t.1 flash durations of 1/6000 to 1/7500 second and Solaire achieves 1/3500 second. See the below link for further Einstein specs.
http://www.paulcbuff.com/pcb2009/e640detail.html

Further explanations can be found at http://blog.bronimaging.com/2010/01/bro ... trol-ectc/

It should be noted that absolute values of color temperature, from my experience, cannot be obtained from commercial color meters, and must be derived from actual shooting in RAW and determined by the use of an accurate, calibrated grey card target and appropriate software. Absolute color temperature readings are influenced by the room environment, RAW software used and other factors. I use WhiBal card and Adobe Bridge, in a neutral grey room

I strongly urge qualified forum members to comment on this content, and to conduct their own tests and post there findings or disagreements or further observations in this thread. Less qualified members are invited to request clarification of aspects that may be foreign or difficult to absorb.




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Mon May 16, 2011 2:01 am

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Joined: Tue Dec 08, 2009 11:49 am
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Interesting support info here from TK

Paul,
I was perusing FM at home one night, and decided to take a look at the ad link on the forum. This was an ad for some guy talking about how great his Profoto Acute B2s was. How much power control, color consistency, portable and all the other buzzwords. So I started digging into more info to compare E640/VML vs. the AcuteB 2s, (and a B/X1600/VML would not be a bad player in this either). While I do not have a formal stat. comparison made up yet (though I can, if you wish), I did find some interesting info in the Profoto manual. http://www.profoto.com/sites/default/fi ... _EN_LR.pdf on pg 13, it states what you already know, and others refuse to believe; there is at least a 300K color shift from one power setting to another. Also, that is -300K at -4f, it does not mention what happens at -7f. Shot to shot is +/-50K, which is all they state in newer manuals. This is from Profoto’s own publication. The fact they now omit this, rather than pointing out any improvements leaves me skeptical of any actual improvement.

Just thought you would find this interesting.
David




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Mon May 16, 2011 2:08 am

Joined: Fri Apr 29, 2011 2:34 am
Posts: 9

great write up Paul!

I've learned so much from reading your posts--flash duration, color consistency, igbt, capcitor switchting, etc

I'm curious what your thought is on mixing light sources?
I've seen pros use softboxes as main lights with ring flashes as fill
-my understanding is that softboxes warm the color temperature and the silver reflectors on bare ring flashes cool the color temperature
-in addition, since the ring flashes are bare, their power is usually pretty low meaning. Lower powers mean cooler color temperature.

Question 1 is, since the softbox light will be relatively warm and the ring flash light will be relatively cold, won't there be a big difference in the color temperature of the two lights? The diffused value will be warm light and the shadow/fill areas will be cool light. If we white balance for the main light, won't the shadow/fill areas be far cooler in color temperature leading to inconsistent/different skin tones?

I understand that this might be abit over kill of thinking, but from my understanding, there could be up to a 1K difference in color temperature and I would think that is noticeable.

Question 2 is, with that in mind, are we best off using the moon unit? I'm thinking that with the moon unit, the power of the strobe has to be turned up meaning the color will be closer to 5600K as well as the moon unit will warm up the color temperature. In doing so, the color temperature will be much closer matched to my main light which would be a softbox.




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Mon May 16, 2011 8:55 am

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Joined: Tue Dec 08, 2009 10:43 am
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First, cooler temperatures=warmer tones, and vice versa. So a low power setting on your ring flash fill example will warm the tones. If your ringflash is still relatively cool toned, you can further warm it by using our ringflash gels. You can increase teh warmth in 1/8 CTO increments.

Second, your light source is only part of the equation when it comes to color cast on your subject. Your perfect 5600K light source will take on color casts when it bounces off a red wall, blue carpet, green shirt, etc.

Third, yes, using a Moon Unit will both increase the necessary power setting for the fill (putting it closer to the main), as well as being filtered through similar fabric.




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Mon May 16, 2011 11:43 am

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Joined: Tue Dec 08, 2009 11:49 am
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derekchinese

Your points are valid, and apply to any system from he cheapest Chinese lights to Profoto and Broncolor.

Our silver fabrics are very close to neutral while white fabrics from almost anyone typically reduce color temperature by about 200°. Some (including the Wescott PLM ripoff, have UV brighteners that fluoresce and can raise color temperature by 1000° and turn everything blue.

1000° variations in a studio setup are most unlikely . . . 300° is more typical. For most work this is pretty insignificant, but if you are doing white or neutral grey seamless backgrounds you just have to be conscious of what can cause color shifts and learn to use the right light settings and accessories . . . with any system.

Many shooters underestimate the effect of the shooting environment, which typically causes more color shift than the lights and accessories. For example, what one thinks of as a white wall or ceiling is usually "warm white" with shades of red or green, and light bouncing around the room (even with grids, etc) can easily throw colors off by 300-400°.

From my experience, if you are concerned about achieving color consistency, you must start with a neutral grey room . . . floor, walls and ceilings . . . the darker the better, and always shoot in RAW with a white balance card (WhiBal is the best I have found).

When you go outdoors, the presence of foliage, water and sky conditions can easily shift colors by 2000°.

Most of the time, shooters get too hung up about color variations . . . they occur in nature and often add as much interest to shots as they cause. The exception to this is in product and catalog photography where image to image consistency can become a prime concern.




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Mon May 16, 2011 3:42 pm

Joined: Fri Apr 29, 2011 2:34 am
Posts: 9

thanks for the very clear explanations technical support and paul!

greatly appreciated




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Tue May 17, 2011 12:47 am

Joined: Thu Apr 15, 2010 6:22 pm
Posts: 178
Location: Aiken, SC

Further to what Luap said about color variations, I once saw a cool picture of a bride standing on a staircase. Her front was illuminated with a flashlight, and everything behind her was illuminated by sunlight spilling through a window. Mixed light sources can be quite interesting.




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Sat Jun 04, 2011 10:41 am

Joined: Mon Feb 21, 2011 8:25 am
Posts: 11

Thank you for taking the time to explain this Paul. It will certainly help me improve my lighting.

Luap wrote:
http://www.flickr.com/groups/strobist/discuss/72157626679167124/

This thread is the epitome of misinformation, ignorance, confusion, lies and lack of industry knowledge regarding color balance and other factors, particularly wrong and libelous misrepresentations regarding AlienBees and certain European brands. So I thought I would take some time to explain it in simplified general terms photographers can understand.

In addition to this, a student of photography will become very confused when wading through this sea of false data. It sure left my head spinning. Had I not learned a long time ago that the best source of information is the source of that information, I surely would have made some bad purchasing decisions. It's very amusing to read statements like "Elinchrom lights do not exhibit color temperature shifts". Do the laws of physics not apply to these flash units?

Luap wrote:
Color distribution: Light is measured along two axis . . . the Yellow/Blue axis (usually express as Color Temperature) and the Red/Green axis (often referred to as Tint).

I do not yet understand color distribution fully. How does color temperature and tint relate to the electromagnetic radiation spectrum that is light? Does this have something to do with Bayer array sensors?

Luap wrote:
It should be noted that absolute values of color temperature, from my experience, cannot be obtained from commercial color meters, and must be derived from actual shooting in RAW and determined by the use of an accurate, calibrated grey card target and appropriate software. Absolute color temperature readings are influenced by the room environment, RAW software used and other factors. I use WhiBal card and Adobe Bridge, in a neutral grey room

I purchased a WhiBal card. At $30, I think the return on investment will be through the roof. Now if I'm shooting at 1/200s, ISO100 and f/8+ in a dimly lit room where ambient light is not a factor, and I am controlling any spill, I don't think that neutral grey rooms are necessary. Would you agree?

Luap wrote:
I strongly urge qualified forum members to comment on this content, and to conduct their own tests and post there findings or disagreements or further observations in this thread. Less qualified members are invited to request clarification of aspects that may be foreign or difficult to absorb.

I have a new set of lighting equipment and plan to run some tests in a week. The only modifiers I have are reflectors and PLMs, but I hope to do a full suite of tests involving all my new equipment.




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Sat Dec 24, 2011 2:16 pm

Joined: Sat Dec 24, 2011 1:28 pm
Posts: 7

When I called the paul c. buff customer service number, the service rep. clearly told me that if I was concerned about color temp. shifts to buy the einsteins over the alien bees. She said that with multi light setup I could run into problems with different color temps. Based on what I have read, it seems like this can happen to any light, when the lights are set at different power levels (full power and half power, etc.) I am not very technical so I need this simplified for me... I basically need to know if its worth investing the extra money in the einsteins based solely on their color temp consistency when using multiple lights at different power output settings. Thank you




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Wed Dec 28, 2011 12:43 pm

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Joined: Tue Dec 08, 2009 10:43 am
Posts: 5266

This question was answered in the identical post here: viewtopic.php?f=8&t=3205




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