Hi Zhe, I'm using a commercially calibrated camera (LMK mobile air) so my experience with calibration is limited. But I¹ll try to answer your questions. 1) I can average the results (1.84) as the correction factor, but is it correct? Some values have great difference (1.74 VS 1.93). I don't think you should use an average. My understanding is that you should always use the calibration factor for the main light source in your scene. The spectrum of the different light sources is different and that affects the measurements. 2) I notice that I use the same model as Coltide, however, our results are very different. Is this normal? (The production of each camera and lens cannot be exactly the same?) Apparently yes, they can be different. Not sure if it¹s the production that is different or it is simply the fact that mechanical systems do not always behave in the same way. Even your own system will not always produce the same result. That¹s why you should also test the reproducibility of your HDR luminance value. You test that by doing not one but several captures and using the average of those for calibration rather than the result of one single capture. In that way you would be accounting for the camera's mechanical uncertainties. It looks like there are also sources of error relating to the luminance meter measurement itself, as described in Inanici 2006, Evaluation of high dynamic range photography as a luminance data acquisition system. So that might add to the differences between yours and Clotilde¹s calibration. 3) Does lighting source influence the accuracy of the factor? Focusing on daylighting data collection, should I take images under consistent electric lighting conditions or daylighting conditions? (Daylighting conditions have dynamic changes, but electric situations offer lower results.) Different light sources will provide different calibration factors. I was told by the seller of my system that a measurement under white LED has errors of 5 to 12% in relation to the measurement done with a halogen-based calibrated camera. But LED has a lot of flicker, doesn¹t it? I¹m not sure how people using your calibration method deal with fluctuations of the lighting conditions during capture, in the case of daylight scenes from variability of the sky and in the case of the electrically lit scenes due to flicker. Your LDR shots are done over a period of time and light can change from one second to the next. I wonder when does your luminance spot measurement take place in relation to the LDR captures and if that might affect the result? Do you take illuminance measurements during luminance collection to see how light varies? In any case, I think you should always make you collection under very stable lighting conditions, whether daylight or electric light. One option is to calibrate for the most stable electric light source that you can find and then make the correction for the daylight spectrum independently, if you have access to a spectrometer. Hope it helps.
Best Regards Raquel Raquel Viula PhD candidate TU Delft | Faculty of Architecture and the Built Environment | Architectural Engineering and Technology Julianalaan 134, 2628 BL Delft, The Netherlands | P.O. Box 5043 2600 GA Delft M +31 (0)62 39 82942 | Email [email protected] From: Zhe Kong <[email protected]> Reply-To: High Dynamic Range Imaging <[email protected]> Date: Monday 27 February 2017 22:25 To: <[email protected]> Subject: [HDRI] Question about the overall correction factor for raw2hdr Dear all: Sorry to bring up this topic again, but I run into the same issue Coltide has (Coltide's original post is here: http://radiance-online.org/community/mailing-lists/archives/hdri). I also use Canon 5D 2 with SIMGA Fisheye lens 8mm f/3.5. I want to figure out the -C factor when using raw2hdr to combine LDR images. I took the images (f/11) under different circumstances: two HDR images were taken along a side window, two images were taken under electric lighting condition, and the last one were taken far away from a glazing. For each scene, 12 images were taken and assembled. The luminance values from the HDR images and the lighting meter are below (cd/m2): Side window: 123.0(HDR) 225.0(measurement) 1.83(-C factor) Side window: 101.0(HDR) 195.4(measurement) 1.93(-C factor) Electric light: 1.9(HDR) 3.3(measurement) 1.74(-C factor) Electric light: 1.4(HDR) 2.6(measurement) 1.79(-C factor) Away from glazing: 9.8(HDR) 18.5(measurement) 1.88(-C factor) Questions: 1) I can average the results (1.84) as the correction factor, but is it correct? Some values have great difference (1.74 VS 1.93). 2) I notice that I use the same model as Coltide, however, our results are very different. Is this normal? (The production of each camera and lens cannot be exactly the same?) 3) Does lighting source influence the accuracy of the factor? Focusing on daylighting data collection, should I take images under consistent electric lighting conditions or daylighting conditions? (Daylighting conditions have dynamic changes, but electric situations offer lower results.) Any suggestion or comment is appreciated. -- Zhe Kong PhD Student University of Wisconsin - Milwaukee School of Architecture and Urban Planning 2131 E. Hartford Ave, Milwaukee, WI 53211 Office 327 _______________________________________________ HDRI mailing list [email protected]http://www.radiance-online.org/mailman/listinfo/hdri
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