Calibrating diffusers

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gardenersassistant Veteran Member • Posts: 6,929
Calibrating diffusers

If you don't like long-winded posts, please stop here. (FWIW there are some related images of some insects here.)

A recent discussion led me to think more carefully about my diffusers, and specifically about whether they are producing a problematic colour cast. It turns out that they are, and it varies a lot between diffuser setups. It also turns out to be fairly straightforward to deal with, with (for my images at least, and to my eye) clear benefits in terms of image quality.

There are several factors that could change the colour of light falling on the scene from a flash setup, including:

  • The colour of the light the flash heads are emitting, and this may vary between instances of a particular model
  • The materials the diffusers are made from
  • Reflections of the flash light from surrounding surfaces
  • Reflections of natural light if the flash light is not dominating the illumination.

The eventual colours seen in an image can also be impacted by:

  • The close-up lens(es), if used
  • The camera lens
  • The camera sensor
  • Any in-camera conversion/processing including any in-camera auto-setting or manual setting of white balance
  • Any out-of-camera conversion/processing

I started by testing eight diffuser arrangements, keeping as many of these factors constant as I could, by using:

  • The same camera and close-up lens
  • The same flash unit
  • Dominant flash illumination
  • The camera and subject in the same position
  • The same subject (the large 18% grey area of a ColorChecker Passport)
  • Shooting raw
  • Using the same conversion/post processing (Lightroom, with default settings).

I first tested the 8 setups shown below. All used a Panasonic FZ330 bridge camera and a Raynox 150 close-up lens. From 1.1 to 1.8 the setups were as follows (All diffusion layers are separated with air gaps, of varying depth):

  1. Metz 58 AF-2 with 4 layers of expanded polystyrene, two pairs of two different sorts of expanded polystyrene
  2. Yongnuo YN24EX with 2 layers of expanded polystyrene
  3. KX800 with two layers of plastic paper on each flash head and a wide layer of plastic paper in front
  4. Second KX800 with no diffusion
  5. Second KX800 with three layers of plastic paper on each flash head
  6. Second KX800 with one layer of expanded polystyrene and two layers of plastic paper on each flash head
  7. Third KX800 with one layer of expanded polystyrene and two layers of plastic paper on each flash head, and a wide layer of plastic paper in front (This is my currently used setup, with the diffusion layers that have been in place for quite a long time and possibly discoloured with age)
  8. As previous, but with all three plastic paper layers renewed with fresh material that has been stored, boxed, out of the light.

The next eight images in the above illustration, 2.01 to 2.08, show the colour produced by each setup. These show huge variations in colour produced by the different setups.

The final two colours, 2.09 and 2.10 use the eighth setup (my currently used setup with mainly renewed diffusion material) on different cameras, an FZ200 bridge camera and a Panasonic G9 micro four thirds camera. Compared to the large differences between the different flash/diffuser setups, the differences between the three cameras using the same setup appear quite mild.

I next loaded the raw files in turn into PhotoLab, Silkypix and Lightroom and used their white balance pickers to measure the white balance setting needed to turn the colour to neutral grey. For each of the three applications the numbers varied somewhat from spot to spot. The table below shows typical values.

The table shows that for any particular setup the measurements given by the three applications varied. Looking through the setups the differences between the three applications' assessments do not vary in any consistent way that I can detect. These variations suggested to me that the best way to handle this would be to use white balance values which are specific to the camera, lens, flash unit, diffusion setup and application used to convert the raw file. I imagine the most reliable approach would be to take a grey card capture during a session and use that to set the white balance, using the white balance picker in the application used for the raw conversion.

Not only do the three applications differ as to the white balance numbers they give for a particular image, but as illustrated below they also use different numbers for their flash preset, and in fact Silkypix offers three different flash presets.

That was all interesting in demonstrating that diffusers could have a big impact on colours, with different diffusers have different effects, and different applications using different flash settings, but the thing that proved really useful from the practical point of view was to set the camera white balance so that it produced neutral grey from the 18% grey card. I did this for an FZ200 with the flash unit and diffuser setup that I have been using with the Raynox 150 and 250, setting it using the diffusers with the renewed plastic paper layers. I started with the white balance numbers for that combination of kit taken from the measurement table above, probably (I don't remember) the PhotoLab numbers. I then captured a test image of the 18% grey card, used the colour picker in Lightroom to get the Temp and Tint numbers (this was a mistake, I should have done this in PhotoLab as I would be using that for raw conversion). I then compared those numbers to the ones I got when I applied the white balance picker. After two or three iterations I got the "out of the camera" Temp and Tint as close as I could to the white balance picker values. (It was the wrong white balance picker, but as it turned out this was not a serious mistake, and I will correct it for future use.)

To test whether this change would make any difference I incorporated the new camera Temp and Tint values into my flash preset on the FZ200 and went into the garden to try and find some subjects. Fortunately I did find a handful, enough for the test to be meaningful, especially since they were all on two choisya bushes that have long given me colour problems. They have leaves which vary greatly in hue, saturation and lightness, from quite bright yellows through to quite strong greens, ranging from highly saturated to highly unsaturated and also varying in lightness. This is not just variation as the season progresses. It is variation from leaf to leaf, and even across a single leaf. I have had a lot of trouble getting the yellows to what seemed a credible state, this being especially difficult to judge because of the large natural colour variations in the foliage.

Aside: If shooting raw, which I do, why does it matter what white balance I set on the camera? It is easy enough to alter during post processing. In my experience it is certainly easy to change the white balance from raw, but I find it can be, and with these particular bushes it too often has been, difficult to get to colours that I am fully comfortable with; and occasionally more than difficult - there have been occasions when I have given up and left the yellows in a state I didn't at all like. An approach that gives me credible colours "out of the camera" (as far as there is such a thing with raw) without me having to struggle with it would be very welcome.

I used my (currently) normal workflow:

  • Raw into PhotoLab and use a preset specific to the camera type and ISO, this time using "As shot" for the white balance. Output DNG.
  • DNG into Silkypix and use a common (all cameras, ISOs etc) preset, with "As shot" white balance and output TIFF.
  • TIFF into Lightroom, with "As shot" white balance, and apply image-specific adjustments. This did not include any white balance adjustments. Round trip from Lightroom to DeNoise AI using "Auto" settings and back to Lightroom. Output to 1300 pixel high JPEGs using Lightroom output sharpening at the "Low" setting.

The images from this test session are in this album at Flickr and the invertebrate images from #1 to #17 and #27 to #31 used the flash setup. It seemed to me that the new arrangement worked well. Simply using the "As shot" white balance produced colours that looked credible to me (even though I was using not quite the right numbers). This is an approach that I will be using in future.

For those who don't like to click away from the site, I have posted eight of the test images in a post titled Some calibrated-diffuser insect shots.

Panasonic Lumix DC-G9 Panasonic Lumix DMC-FZ200
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