Here is maybe the most delicate part which is done way more easily in digital rather than in film-based photography, in three steps. First of all, you need to measure the average quantity of light shining on your subject. Then, you need to measure the difference of contrast between the different important parts of the panorama whether you'll have to make light exposure corrections between two consecutive photos, or even make double-exposures to correct a window for instance. Then, you'll have to set the white balance - essentially if you're working in JPEG - because the harmonization of a color panorama is always delicate in this file format, destructive.
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General measurement of the light
Firstly, you need to know what general exposure time will be better, globally, for the whole panorama. Depending on the contrast hence the light shining on the panorama, only one exposure time could work but in most cases, you'll have to make small exposure corrections in order to reduce the differences of local luminosities that are too important. To realize it, you may use the camera's built-in photo cells, in certain modes called matricial measurement, ESP, multizones, etc. working quite well in this case.
In a first time you need to evaluate an average value in the whole field of view that will give you a reference exposure time / diaphragm opening pair. To do that, I always take one measurement and one photo to analyze the histogram in the average zone of the scene shot - between the darker and the lighter part. If I notice a big difference between the left and the right for instance (one part in the sun and one in the shadow), I take a measurement in those two parts. This will tell me what correction I should prepare to reduce such differences.
Important! between two consecutive pictures, you don't want to correct more than half a diaphragm (1/2 diaph.). I like to make corrections of a third of diaphragm (1/3).
If you're shooting in JPEG, the choice of the final exposure will be made according to high lights that must above all not be "burned". Indeed, when high lights have been overexposed, they can't be corrected afterwards. In RAW format, you'll be able, depending on your camera, to overexpose up to 1,5 diaphragms, while keeping raw material in the high lights. This will mainly depend on the quality of your camera's sensor.
At this point it is important to avoid measuring the zones that are obviously more or less lit than the whole of the scene because this could cheat the photocell. You need to avoid, for example, to put a very luminous source in the field of the cell, as a street lamp or the sun.
Once you have your reference time, it's important, if you want to realize a nice exposure and if you're not in a hurry - well, you're the one setting the priorities - to realize a more accurate measurement to take local contrast differences into account.
Local measurement - contrast differences
Once taken the general measurement of the light on the whole panorama, you need to check local differences of contrast. These can indeed be important, in any case superior to the dynamic of the sensor and it would then be wise all or part of those differences.
Except in cloudy or foggy weather, as soon as you're shooting a very wide angle as is the case in panoramic photography, you notice very often that there are important differences of contrast between the right and the left part of the image. To schematize, one will be in the sun and the other in the shadow.
In panoramic photography by stitching, an old proverb - yes, old already!!! - tells us to expose all the photos with "almost" the same exposure time in order not to have luminosity differences in the sky visible between two images, for instance, so that this exposure difference can't be seen in the stitching zone. To control this exposure, you need to work in MANUAL MODE. And some software are now able to harmonize small differences of luminosity and even less important differences, like with Autopano Giga. It will then be possible, in that case, to make corrections of 1/3 diaphragm on certain shots. The easiest case is thus the one with the right or left part of the photo being in the sun and the reverse in the shadow.
On ten photos and by making a correction of 1/3 diaph. between each shot, you can make a global correction of close to two diaphragms. In full sun it will be entirely enough in most cases. It won't even always be necessary to make so important corrections.
By making exposure corrections of 1/3 diaphragm between two shots, you can get a full diaphragm back in three pictures.
In another case, also usual since I'm referring myself to interior photography, there'll be big differences between the room and the openings - windows, doors, etc. -.
The process will be quite different. It will be about taking a first serial of photos for the inside of the room and another one for the openings - spotmeter measurement - hence with big differences sometimes. The technique used will then be quite similar to HDR photo. In a first time, you can stitch together the photos at various exposures, either with Photoshop, either with a software like Photomatix and in a second time stitch this photo corrected for high lights with the rest of the serial. This second method is described in details in the Cahiers du Designer n° 17 published by Eyrolles " Photos panoramiques par assemblage " and in the page dedicated to editing before stitching of the virtual tours part .
Color temperature and white balance
Always an issue in film-based photography, the management of the color temperature of the photos is eased dramatically by the white balance of digital cameras.
In film-based photography, the photographer had to choose between two wide ranges of color films: the daylight films and the artificial light ones. Depending on whether he was working inside or outside, he opted for one or the other. But even if he picked a film for artificial light, the variety of inside lights often compelled him to use a thermocolorimeter to measure the exact color temperature lightning its subject. Thanks to an appropriate range of gelatine filters, he could correct all or part of the defects of a particular lightning.
This problem is reduced to a simple white balance with a digital camera, whether be it measured or chosen among predefined settings (sun, shadow, cloudy sky, tungsten, halogen etc. over here).
I strongly advise you to choose it from the menu over there or to measure it inside thus to deactivate the automatic white balance. Indeed, when you're turning around your subject, the color proportions of the subject change and can cheat the photocell - even more if you're shooting with a Canon rather than a Nikon according to my experience -. So, as for measurement of the light, in colorimetry it's better to work in manual mode or predefined mode for the white balance to remain identical in all pictures to be stitched. Caution!
in JPEG, white balance corrections are very destructive and thus limited.
In RAW format
You can stay in automatic white balance without a problem. Indeed, when you reveal all the pictures of a same panorama, you'll only have to apply a one and only white balance to all the pictures and this in a click of the mouse with most demosaicing software. Very powerful and efficient, plus this operation is non destructive in RAW. A huge advantage of the RAW format in panoramic photography!
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