@newsense52 wrote:
It looks like apperture value is adjusted that is fitted to the situation to create a best result? Would you suggest Av mode?
What I am concern is the color when flash is used - it magnified the true color of the object focused like yellow becomes white and yellowish becomes orange...etc. ( under flourescent light ). Outdoor has no problem of color under the natural light when using auto mode.
About clarity or sharpness of object focused ( exposure ) is depending on quality of camera sensor or resolution?
This is a "white balance" problem -- not an exposure problem. More specifically, you're having an issue with "mixed" lighting (by "mixed" I mean the light sources are not creating identical color temperature lights).
Light can cast a color tint on your subjects (as you've discovered) but it's actually possible to correct for this either in camera or using software on the computer.
The easiest tool to do this accurately is called a "gray card" and it is as the name implies... a card... and it's gray. But it's not just any gray, it's a perfectly "neutral" gray -- meaning this shade of gray doesn't favor any color. If you took a photo of this card in perfectly neutral "white" light, then inspect the RGB color values of the pixels, those pixels should report an equal level of "red", "green", and "blue" for each pixel. If one value is a higher than the others (suppose the "green" value is higher then the red or blue for your pixels) then it means your light source is pumping out more "green" and less "red" or "blue".
For any typical subject, you might not know precisely what color it should be. But if you have a gray card, you DO know precisely what color it should be (and so does the camera). So it analyzes the pixels, determines how much color cast your light is creating, and then the camera corrects for that lighting (and you can do this on the comptuer too... it doesn't have to be done in-camera.)
But where it really gets tricky is when you have different lights and they provide different color cast. If there's just one light and it's too green, then the computer can dial back the "green" on every pixels.
What if you have one light and it's pumping out a slight "orange" color cast... and another light is pumping out a slight "blue" color cast. When you try to correct for the blue cast, the orange cast gets EVEN MORE orange (fixing one makes the other worse.)
This is basically what happens when you use "flash" (daylight color balance) in a room with florescent lights (which might be have a slight green color cast or a slight pink color cast - depending on the specific bulbs).
But you can put a colored "gel" on the front of the flash to tint the light so that it matches the color of the florescent light. You might think this is making things worse because you used to have one accurate light and one inaccurate light and now you have TWO inaccurate lights... but it turns out that once you make the lights CONSISTENT (the color temperatures agree) you can now use that gray card to fix the overall white balance.
So... if your florescent light is a pale green color cast and you wrap a pale-green gel on the flash, the whole image looks pale green. But now you take a photo of the "gray" card and the computer realizes that every pixel is a bit too green. So it subtracts a bit of green (in the correct amount based on what it measured using the gray card) and ... like magic the entire image now shows accurate color as if it were shot in "white" light.
The two major vendors of these gels are Lee Filters and Rosco filters.
They come in sheets of various sizes (or even rolls). It's an acetate-like material but it's been color-tinted. You cut a piece and wrap it on the front of your flash head (imagine taking kitchen food plastic wrap and wraping it on your flash - except this is more like an acetate and it doesn't "cling") and you use a rubber-band to secure it.
While many of their gels are for "creative" lighting, they also make filters designed for color correction of various types of light.
It can be a little confusing because there are two ways to solve the problem... one way is to gel the lights that have the wrong color temperature. But if your at some venue and the whole room is full of florescent lighting, you can easily run around and gel the whole room. That type of correction is intended for studio situations where you can control the lighting on the set. The other option is to just gel your flash to match the room lighting. It isn't important that the room lighting is "wrong" -- it's more important that the flash MATCH the ambient light and now the gray card can be used to correct the entire scene because every pixel needs the same amount of color correction.
Tim Campbell
5D III, 5D IV, 60Da
