WARNING - This is a long post. Don't read it if you rnot interested in the finer points of this problem. The solutions are at the end if you are in a hurry.
Never use Auto white balance. This can change between frames and cause exposure problems.
With non DSLR cameras avoid upping the ISO at all costs. This will introduce noticeable noise. ISO is a setting to make the CCD more sensitve to light. It is used only in low light or very fast shutter speeds in combonation with small apertures to capture stop motion in bright light or manage depth of field in motion shots. In low light Use 50 ISO and choose a larger aperture and slower shutter speed.
Always take the largest megapixel image you can.The more data the better your final image will be. If you want clear, crisp images you need to start with the most amount of data possible. When sizing down or resampling you will be loosing data so starting with more data in the beggining means more data at the end. Actually it means more "quality" data not necessarily raw megapixels.
AE-lock will work as many use this feature, however you really should learn how to use the camera in full manual mode and choose your aperture and shutter speed according to the situation. Meter if you like first to see what the camera "thinks" is the proper exposure and then go to manual and set the aperture and shutter to that reading. Then start shooting. This will cause the camera to use the setting you picked on every shot until you change them. AE-lock simply locks the aperture and shutter speed to what the camera picked on the first shot so that the other shots are taken with the same settings.
Do not use auto focus. Set focus to infinity. If you use auto focus and you get closer than say 2 or 3 feet to an object like the corner of a wall that shot will be out of focus at distance. This refers to the "hyperfocal distance" of the lens. Look that term up for a better explanation.
Note: Sun iris focusing onto the CCD is really a problem for NON slr cameras because they do not have a shutter mirror but this can still occur in DSLR cameras if shooting repeatedly in direct sun.
This is a classic example of "light refraction" within the lens and an overworked ccd sensor.
Wide angle lenses especially one at 8mm gather the light from a 180 circle. Next comes the "resolving" power of the lens. This takes the light and compresses it into an area that is rendered onto the CCD. The more resolving power the crisper the image will be. All of this light is being compressed and when it is invariably refraction occures. Most of the time we hear of refraction as related to aperture performance. For example small apertures (large f numbers) on wide angel lenses generate refraction at the sharp edges of the aperture blades. This is due to the oblique angle at which light is bent through the lens. Some of it cant quite make it through the aperture hole and strikes the edges instead of passing directly through the aperture hole. The light is striking these edges and bouncing around in the lens. This can cause a "soft image" in ALL situations but it is the direct sun at issue here because of the AMOUNT of light not necessarily the aperture size.
In this case the light or rather the brightness and more importantly "amount" of light is the one of the culpits. The sheer volume of light is huge. Think of it like playing with a magnifying glass. On a bright sunny day you can focus the light through the glass to the point at which it will burn a hole in a leaf or fry an ant (for you "Sid's out there- lol). On a cloudy day there is not enough light to generate enough heat to burn. The dot is the same size but there simply is not "enough" of it.
When you push this much light through a lens invariably this brings out the inherint faults of the lens itself. The other end of the scale is low light and again faults in the lens will rear their ugly head.
Lens makers attempt to coat the elements within the lens with non reflective materials in an attempt to allow as much light as possible to pass through the elements without actually reflecting off the elements. Their is a point at which some light will still be reflected and this is related to the amount and brightness of the light. When this reflection occurs you get "refraction" within the lens. The refracted light is bounced around in the lens at can again be sent back through the lens. If this happens you will sort of over expose the image somewhat. With non wide angle lenses something say around 35mm or higher this is not usually a problem because you would rarely if ever point the lens directly at the sun. And if you did you would use some kind of light restricting filter anyway.
But on a wide angle lens this is not something you can control because the lens is "wide angle" and will therefor most likly get the sun in the image. At least as far as panoramic images are concerned.
Note for Sigma users: The sigma 8mm actually generates a full circular image but on 1.5x sensors this circle is cropped creating what we call a "drum image". When the lens is pointed or alighned vertically with the sun this cropping also contributes to the exposure problem. If you looked at two circular images from this lens one being taken directly at the sun and the other not at the sun both images would appear normal with normal exposure according to your choosen settings. Even if both images were taken with a corresponding aperture/shutter speed comb ( meaning an equal exposre for both) the sun image would still be over exposed compared to the second image.......culprit is again light refraction. When you crop a circular image you are taking out some of the light distrubution in the image circle. The image was meant to make a circular image not a drum image. This is a contributing factor but not the real reason for the problem.
The REAL culprit is the CCD in this case. The CCD heats up when it is working. Heat is the enemy of the CCD. The CCD begins to heat up as it is used and ambient heat from the camera body will further intensify this problem. You have about 5 minutes in direct sunlight before you camera will start to suffer from heat stress. It will even cause the camera to just suddenly shut off if you let it get too hot. Use a white towel to shield the camera until you are ready to shoot and when done immediatly cover it up or you are asking for trouble.
When the lens is aligned vertically with the sun and slightly off center as well the direct sun will be focused onto the ccd. The spot on the CCD will "immediatly" and I mean immedialty begin to heat up tremendously. The CCD becomes the ant and the lens becomes the magnifying glass. When this occures that spot transfers heat to the rest of the CCD. As the CCD gets hot it causes the electron collection diodes to basically stop working and when this happens they can not properly record color data. You loose data just like you do if you over expose an image. You can avoid this somehwhat by choosing a really fast shutter speed of 1/3000 or higher but not entirely because you still have to shoot 4 or more frames to complete the shot. Another option is to use a gel filter (for sigma 8mm and nikon 10.5) on the back of the lens to control the amount of light in combonation with a fast shutter speed. It is worth noting that this prolem can occur in doors as well and does when using long shutter speeds in a dark room with poor lighting. The more you over expose the scene the worse this problem will be. It is best to go for a slightly under exposed image and color correct this later than to shoot what should be a"correct" exposure.
So my friends the problem stems from four things.
1. light refration
2. Amount of light being resolved
3. CCD temperture
and 4 (which I havent mentioned yet). - The angle or obliqueness at which the primary light source (ie sun) is being collected. When the sun is placed at the edge of the lens circle refration is increased. Light will bounce of the lens elements more easily at a sharper angle than strait on at the center of the lens.
1. As Dave states above set up the shot so that two of the images are evenly spaced as related to the sun. If this was a two shot situation you would "split" the sun by putting it directly at the edge of the frame so that the primary light source is evenly split between two frames. In three shot this is not possible. So you must split it between three frames. To achieve this one must make a judgemnt about how the light in the scene is being reflected from the environment. Examples are scenes with snow or white sand vs non reflective surfaces such as pavement and bricks or a white wall of a building that you happen to be next to. In four shot you have an easyier job because you have more overlap in frames and blending of the the images is easyier. In fact you are in luck if there is a bright reflective surface in the scene because this will help in reducing differences in light refracton between frames. But solution 4 below is by far the best.
2. Use a gel filter in combonation with fast shutter speed.
3. Use only a fast shutter speed and be quick about shooting the scene. Cover the camera before shooting and immediatly after the scene is shot.
4. Find something to obsure the suns iris such as a tree trunk, light pole, tree limb, tree leaves, building overhang or some other natural obstruction.
By the way. Direct sun focused through your lens can in fact damage your CCD if allowed to be there to long. I never leave my lens uncapped outside while moving to different locations. You can get distracted by an inquisitve person or get caught up in composing you scene and all this time the sun is beaming onto your CCD.....bad...bad.
I hope you enjoyed this little short story. or Novel if smooth is reading this LOL