mercoledì 10 ottobre 2012
How to make HDR panoramas.
Vedretta degli Sfulmini (TN) Italy - August 18, 2012... HDR panorama.
Somebody asked me how this is done...
HDR PANORAMA MADE WITH SETS OF BRACKETED SHOTS
I guess we all know that normal panoramas require taking overlapping sequences of pictures of the scene to be stitched together with some panorama software afterwards. HDR panoramas are just a bit trickier since you have to shoot a set of bracketed photos on each position instead of taking single pictures. Here is how it's done (or how I do it, anyway).
1) If possible, set the camera on RAW. Sensors have a 12/14 bits depth, so you want to make sure you don't lose 4/6 bits of data right away by using 8 bit JPGs!
2) Set your camera on manual exposure and choose an exposure setting good for the average luminosity of the scene. In case of a well lit scene at around midday, you can use the Sunny F16 Rule. If the scene includes the sun early or late in the day, I usually expose beside the sun, so that it's just outside the measuring area of the camera. Needless to say BE VERY VERY CAREFUL WHEN YOU LOOK TOWARDS THE SUN THROUGH A LENS!!!!
3) Set bracketing on. How many shots per set and how many F-Stops bracketing is needed will depend on the contrast of the scene. If in doubt, set the maximum amounts available. Most of the time you will end up using just part of the bracketed pictures anyway, for example, only the 2 darker ones of each set or whatever, depending on histograms. Actually, when there is not too much contrast, HDRs from single pictures are (sometime) the best choice.
4) Since you'll have to take sets of 3, maybe 5, shots, if possible, set the shooting mode to hi-speed, continuos. I know it would be better to use a tripod but, if clouds and weather are changing rapidly, then you must shoot quickly or clouds and luminosities will not match between pictures when you build the panorama. It will just take some practice to press and release the shutter at the right times. Well, actually pressing is easy. Releasing after the 3rd shot (if you set the bracketing to 3) is (just a bit) trickier. If you shoot 4 pictures instead of 3 by mistake, reset bracketing and start over. HDR software will correct camera shifts introduced by handheld shooting, but not 100%.
5) If using a tripod, remember to turn off the vibration reduction of the camera/lens or the pictures will not match when creating the HDRs! I went crazy over this until a good soul on a forum told me to try and remove VR!!! Actually, if the shutter speed allows it, turn VR off anyway but remember to turn it back on again when finished! With a tripod, a remote control to trigger the shutter would be a great, cheap, accessory, if supported by your camera. Otherwise use the self timer to make sure the camera has stopped oscillating when the shutter opens. My camera can be programmed so that when the self timer goes off, 3 shots are taken in sequence. If bracketing is on, the 3 shots will be bracketed as well. Check if you have this feature! Finally, with SLR cameras, check if you can have the mirror go up sometime before the shutter opens. This will minimize micro shaking.
6) I always forget to do it, but it would be a good thing to set the focus on manual so there will be no focus differences along the panorama. Nothing worse than having parts of the panorama focused at different depths. Just auto focus where you think it's right, then turn auto-focus off without touching the lens.
7) Now you are all set: shoot like you would a normal panorama with the difference that you will take more than one bracketed shot per position. One thing you may notice while you shoot is that sometimes you press the shutter and nothing happens: I believe it's because the camera is busy sending all these pictures to memory. I grew accustumed to wait a couple of seconds between each set of shots. How much time you must wait depends on your camera and memory.
8) Go home, download the RAWs to your PC and convert them to 16 bit TIFs with your favourite conversion software, making sure that you use the same settings throughout. To be sure, I don't apply any settings and convert them straight. The color space of the TIFs depends on your way of working. I usually convert them to sRGB, but I guess AdobeRGB would be OK, if your workflow needs it.
9) Look at the results with a photo retouching program: the important thing to look at are the histograms. This part is very subjective: let's say you took 3 bracketed pictures per set. If all the overexposed pictures are way overexposed and the correctly exposed pictures already have enough details in the light areas, then you could probably throw away the overexposed shots and create your HDRs using just the other two. There are times when all the pictures in one set fit the histogram and therefore it's better to make HDRs from these single shots (see below). Some times it's better to keep the darker shots anyway to give more depth to the clouds... it takes time, experience, trial and error...
10) Once you have chosen what to keep, put all the TIFs in an empty folder. This will be useful later for batch processing. I used to create HDRs directly from RAWs but it looks like the RAW to TIF converters bundled with cameras are better (less noisy, anyway) than what HDR programs can do. But then, again, some people say that converting to TIFF will lose information... I wish somebody who knows what they are talking about made this clear once and forerever!
11) Set the folder containing the TIFs to show the preview icons then choose the set of bracketed pictures that best represents the whole panorama (or where you want the best results anyway). Open your HDR software and create an HDR from this set... to your liking. Keep in mind that other parts of the panorama may be lighter or darker, so keep this sample HDR, respectively, a bit darker or ligheter too. If the software allows batch processing, close the HDR without saving it and batch apply these settings to all the sets. If it doesn't, save the HDR and, one by one, create all other HDRs with the same identical settings. Again, save them as 16 bit TIFs.
12) One big problem with HDRs is that they stress vignetting, digital noise and chromatic aberration (sounds awful, doesn't it?). Noise and aberration can somewhat be taken care of in the final panorama, but vignetting is a big pain in the neck... actually I should say "in the sky"! Vignetting on a single picture can be OK and most retouching software can take care of it but, if each picture in a panorama is vignetted, the final panorama will have darker vertical bands in the sky, impossible to remove. Even the picture above has this problem, even though it blends a bit with the clouds. This is something that just occurred to me yesterday: before creating the panorama we should apply the same amount of de-vignetting to all the HDRs. I don't think there is a batch mode for this, so it must be done by hand, one by one :-( That's probably why I never tried it, removing the idea even before it occurred!
UPDATE 1. If you convert the RAWs to TIFs with Photoshop ACR, open all the files at once and then apply the same amount of de-vignetting to all of them. Actually, if ACR has the settings for your lens, you can do this automatically: it works very vell for my lenses! If you remove vignetting correctly when converting the RAWs to TIFs, there will be no vignetting when you put them together with Photomatix (or whatever you use). While you are at it, remove Chromatic Aberration and Distorsion as well, again, based on the lens, if available.
UPDATE 2. After shooting hundreds of HDR panoramas, I noticed an aspect that never came to mind before: WHITE BALANCING. Unless you set white balancing to something fixed at shooting time, you must remember to set the white balancing to the same value for all the shots when you convert them from RAW to TIFF. I guess that this is not a big problem, since I never noticed it in the final panoramas, but it's there and I must say that my fist HDR panorama where I took care of this aspect seems smoother than usual... maybe it's just an impression... I'm not sure... we'll see how it goes with the next ones.
13) Using the HDR TIFs created above, stitch the panorama with your favourite panorama software and then pass it on to your favourite photo retoucher since, for sure, tonal values and noise will be way off! Not to mention fixing all the spots where the panorama software didn't quite match one picture to the next one, especially when things like electricity cables are present. If you do all this in 16 bits, it's even better!
You are done! With some practice, it won't take so long as it may seem by reading this!
One final note: camera shutters are usually guaranteed to take about 100,000 shots before giving up! With HDR panoramas you will reach that time much, much more quickly!!!
And don't forget to calibrate your monitors!
Comments and suggestions are welcome, bye, Giorgio.