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I photograph fish, and soon I want to buy an external flash unit. But I'm not sure if I should but one that works off-camera or as a TTL flash.
I've used table lamps so far, but the light seems to go through the fish rather than bounce off it and show its colors.
However with pop-up and no lamp, I can actually get a better representation of the colors. I assume the table lamp washes out their colors in general.
Please advise on how I should light my subjects.
But I'm not sure if I should but one that works off-camera or as a TTL flash.
Any flash can be used off camera with the right equipment. For taking photos of fish in an aquarium (or even fish on ice at your local fishmonger), a simple TTL cable is all you really need in addition to the flash itself.
However with pop-up and no lamp, I can actually get a better representation of the colors. I assume the table lamp washes out their colors in general.
At least some of the color in fish, and all the iridescence, is due to structure rather than pigment. If you look at an individual fish scale in certain light, it just looks clear, but under the right light at the right angle, you get colors.
I think the reason the table lamp gives you poor results is that the light is too soft -- it's probably filtered through a lampshade, and it also sends light in all directions and gets reflected back by other surfaces in the room. Instead, you want very directional light, also known as hard light. When all the light comes from one direction, it all interacts with the fish in the same way, creating a lot of the same color. When the light comes from many different directions, any colors that are produced will mix with other colors produced from light at other angles.
The pop-up flash worked better because it's a much harder, more directional light. Using an off-camera flash will give you the same kind of directionality, but it will let you try lots of different angles to get the one that works best for you. It also gets the flash away from the lens, letting you create some shadows that will add definition and depth to your subject.
If you're shooting fish in a tank with lighting, you may run into cases where the light reflects off the front of the tank and causes glare. There are a few options to mitigate that:
lens hood: If your hood is flat across the front (i.e. it's not a tulip-style hood) you can put the front of the hood right up against the glass, and that should eliminate glare from any lights.
snoot: Like a hood that goes on your flash, a snoot limits the angle at which light can exit the flash, preventing reflections that are visible to the camera.
cardboard: Just a simple piece of cardboard can be used to block reflections from the camera. Get a helper to hold the edge of the cardboard vertically against the front of the tank between the flash and the camera. Any light reaching the camera from the flash will then have to bounce off objects in the tank.
In order to get all of the colors, make sure you illuminate using a white light source, which actually contains all of the colors. I say this because many table lamps tend to have more warm light, so that if the color of your fish is blue, purple or even green, they will not be illuminated properly which will result in a very color-distorted image.
Also, make sure that you don't use a strong and point source of illumination, try to create a more even and soft illumination that will not flatten your image.
Some lamps have color rendering index in their description. You want one with this index as high as possible.
Others can show a curve. How much of which color they are emitting. That's even better. You want it to be as flat as possible. For fish, especially in blue half of the chart.
Both of the above can be used to evaluate both light bulbs and camera flash.
Now, for direction, it gets tricky. One lamp can't emulate natural light. At the lake you have at least three "sources": sun, sun's reflection in the water, and light diffused by atmosphere and land. That's why you need three lamps to be prefect. Strong from above, weaker from below, and diffused "background".
For your result with pop up flash vs desk lamp, there are two factors. First, built in flash lamps are actually pretty decent, if weak. Marketing move to make you want buy bigger stuff. But beneficial for those who won't. Second, if your fish is kept flat, parallel to your lens surface and sensor, light that comes from camera's general direction is reflected back in camera's direction, and significant part of it gets into lens. Desk lamp at your left means most of the light is reflected to your right. And that's a problem because most interesting colors come from reflections on semi translucent layers in scales.
By the way, have you ever seen "whoa!" good photo of fish lying flat on a desk? Letting it flex, move, in a good natural background might help a lot (also, lets you free your subject if you choose not to eat it).
