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Archive 2011 · Diffusion cup color shift
  
 
Elan II
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p.1 #1 · Diffusion cup color shift


At long last, I own a color temperature meter. Something I always wanted.

I was taking some test readings of gels on my SB-24's when I noticed one was 300k warmer than the others. It was the only one with a diffusion cup on it, an ebay generic. Always assuming those are 100% neutral, my first thought was that this one flash had an aging bulb. I took the diffuser off anyway and the reading became the same as the other units. It was the diffuser that shifted the color by 300k.

Next, I tested my SB-600 with a Sto-Fen diffuser. The color shift with the diffuser was 550k! My SB-800 with the factory diffuser had the least amount of shift: 250k.

To complete the picture I should add that the SB-24 diffuser is one year old, the Sto-Fen is five years old and the SB-800 diffuser is three years old -- same as the flash.

I always knew modifiers like umbrellas and softboxes effected the color temperature, but never suspected these little hard plastic cups. With three brands tested, I think +300k is a good estimate, worse as they age.






Dec 20, 2011 at 02:45 AM
BrianO
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p.1 #2 · Diffusion cup color shift


Yep; I've observed the same phenomenon.

Custom white balance for JPEGs and post-capture CC for raw captures is the way to go. I always take a color reference of some sort with me wherever I go.



Dec 20, 2011 at 05:37 AM
cgardner
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p.1 #3 · Diffusion cup color shift


All modifiers shift WB to some degree. You can measure it with just the camera and gray card by using the K temp method for setting WB. Just shoot the gray card in a series of exposures bracketing the K temp setting then check the RGB values of the card in the resulting image. The the K temp of image with the equal RGB values will the temp of the source.

You can do it even quicker with ACR. First shoot the card setting Custom WB off the gray card with bare flash, then without changing the settings put the modifier on shoot the card again. In ACR if you click on the two card images with the eye dropper tool the Temperature setting will show you the K difference and the exposure slider ( and histogram) will show the exposure difference.

The fact that cameras can be adjusted for neutral color balance with a gray card before or after shooting makes a color meter unnecessary. Regardless of how you measure the color temp of the sources you want to avoid mixing different color temp modifiers unless you specifically want to see differences in the photo, such as warmer or cooler shadows (fill) or accents (hair light) vs the key light.




Dec 20, 2011 at 01:45 PM
Littlebike
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p.1 #4 · Diffusion cup color shift


I too have wanted a color temp meter but have not pulled the trigger, I am still using a GretagMacbeth card.

Here is a thought/question about your testing, could the color temp shift have been a result of reflected light? The job of a diffuser is to diffuse, as a result the diffused light is free to reflect off, and be colored, various materials in the room thus potentially warmer temps.

I would think the thing to do is take your flashes outdoors in the evening and perform the tests again.



Dec 20, 2011 at 02:17 PM
Elan II
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p.1 #5 · Diffusion cup color shift


BrianO wrote:
Yep; I've observed the same phenomenon.

Custom white balance for JPEGs and post-capture CC for raw captures is the way to go. I always take a color reference of some sort with me wherever I go.




I shoot architectural interiors using strictly Kelvin white balance. The mixture of light sources is ambient natural (window) light, ambient artificial light and my fill. The last two are controlled by me, but the natural light can shift color while I'm still on the same setting. It's going to really help me to have the ability to take frequent readings of the average color temperature and make adjustments as I go.






Edited on Dec 20, 2011 at 03:36 PM · View previous versions



Dec 20, 2011 at 03:25 PM
Elan II
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p.1 #6 · Diffusion cup color shift


cgardner wrote:
All modifiers shift WB to some degree. You can measure it with just the camera and gray card by using the K temp method for setting WB. Just shoot the gray card in a series of exposures bracketing the K temp setting then check the RGB values of the card in the resulting image. The the K temp of image with the equal RGB values will the temp of the source.

You can do it even quicker with ACR. First shoot the card setting Custom WB off the gray card with bare flash, then without changing the settings put the modifier
...Show more



Chuck, see my reply to Brian. I NEED a color temperature meter. It's going to be quite a time saver for me. Thanks for all the good work I see you do here btw.






Dec 20, 2011 at 03:29 PM
Elan II
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p.1 #7 · Diffusion cup color shift


Littlebike wrote:
I too have wanted a color temp meter but have not pulled the trigger, I am still using a GretagMacbeth card.

Here is a thought/question about your testing, could the color temp shift have been a result of reflected light? The job of a diffuser is to diffuse, as a result the diffused light is free to reflect off, and be colored, various materials in the room thus potentially warmer temps.

I would think the thing to do is take your flashes outdoors in the evening and perform the tests again.




The meter I have (Minolta Color Meter IIIF) has a setting to measure reflected (flash) light only, so the ambient light was not counted. I took the readings roughly 18" directly in front of the flash heads, so probably no bounced contamination. Good points though. I will be doing more testing. I've already tested one softbox and was surprised to see only a +200k shift. But it is brand new, so testing an old one is next.






Dec 20, 2011 at 03:36 PM
 

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RustyBug
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p.1 #8 · Diffusion cup color shift


+1 @ bounce contamination ... maybe you can mask the diffuser(s) (for testing) so that you are only receiving direct light (or test in a neutral setting). Realizing that only a "portion" of the flash power is being emitted directly, the impact of indirect can be surprising. The "flash" only reading will still capture the flash (i.e. not ambient) that is reflected off nearby surfaces.

For me, when the EV value of direct sunlight is being reduced as the angle changes, the amount of influence the indirect overhead sky imposes becomes much more significant as the relationship between direct & indirect changes (same source @ sun) ... i.e. the direct is no long "overpowering" the indirect as dominantly. As such, I have to be "on guard" for the amount of blue that is mixed in with the "golden hour", particularly when there is a clear blue sky involved.

Littlebike's point that you are diffusing your flash to change the light direction ... also changes the relationship of direct vs. indirect influence. While most recongnize this regarding illumination levels/ratio/contrast ... for your color critical work, his point may be more viable than an initial credence might otherwise seem to be. It may not be the amount of color shift from the diffuser itself ... but the change in relationship between direct & indirect influences.

Conversely, you could flag/mask/block the direct light coming from your diffuser for comparison testing of the reflected flash influence from it's surrounding surfaces.



Dec 20, 2011 at 04:03 PM
Elan II
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p.1 #9 · Diffusion cup color shift


RustyBug wrote:
+1 @ bounce contamination ... maybe you can mask the diffuser(s) (for testing) so that you are only receiving direct light (or test in a neutral setting). Realizing that only a "portion" of the flash power is being emitted directly, the impact of indirect can be surprising. The "flash" only reading will still capture the flash (i.e. not ambient) that is reflected off nearby surfaces.

For me, when the EV value of direct sunlight is being reduced as the angle changes, the amount of influence the indirect overhead sky imposes becomes much more significant as the relationship between direct & indirect
...Show more



I made a quick black snoot and took readings 6" in front of it at three different power levels. The difference actually grew a bit. Btw, I bought these diffusers for all the flashes I have, but I've yet to use one off camera. They work nicely when the flash is mounted on the camera, but on a stand I prefer a true diffuser or a bounce.






Dec 20, 2011 at 05:39 PM
Elan II
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p.1 #10 · Diffusion cup color shift


I should add that a cloud passing over the sun can change the color temperature by +1500k in less than a minute. I used to wait those out, but with a Kelvin meter I'll be able to make a quick, accurate adjustment and keep shooting.






Dec 20, 2011 at 05:46 PM
RustyBug
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p.1 #11 · Diffusion cup color shift


+1 @ dynamic ambient

Here's an illustration of the influence of indirect light where direct light "is" vs. "isn't" overpowering it.












Dec 20, 2011 at 06:05 PM
rico
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p.1 #12 · Diffusion cup color shift


Having become aware of impure color sources, I can't see anything else. Kind of ruined my enjoyment of outdoor photography unless there's heavy overcast, and owning a color meter just feeds the obsession. Note the green cast on the underside of that golf ball. Now, please excuse me, I must return to my light tent!


Dec 21, 2011 at 05:34 AM
BrianO
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p.1 #13 · Diffusion cup color shift


rico wrote:
Having become aware of impure color sources, I can't see anything else. ...owning a color meter just feeds the obsession.




I want a Sekonic color meter, but I spent all my meter money on a flash meter a couple of years ago.



Dec 21, 2011 at 06:53 AM
RustyBug
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p.1 #14 · Diffusion cup color shift


You forgot to mention the red on the upper left or where colors intersect/mix/blend.

Even without a color meter ... once you've learned to recognize them, "ignorance is bliss" kinda comes to mind as a thing of the past. But now ... I understand better why a GOLD reflector can be the tool of choice instead of a white or silver (neutral) one, i.e. reflecting indirect overhead sky (blue) light.



Dec 21, 2011 at 06:04 PM





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