Choosing the right optics is very important. Optics are similar to high end reflectors for T5s. Sure you can put a T5 bulb in your canopy and it will shine light, but if you put a quality reflector behind it, it will make a world of difference.
When choosing your optics there are a couple things to remember.
1.Most of the light is in the first 50% of the degrees. For instance, a 60degree optic has most of its light in the first 30 degrees. It is not linear.
2.Choosing the optic is determined by the width and height of your tank including the height your fixture will be off the top of the tank.
Here are some examples 180g 72x24x24in aquarium - 1 foot off the top and centered 1 foot from the end:
As you can see no optics on these CREEs is not ideal. At first glance, the 40 degree looks like the way to go but lets look at another angle.
Taking the first point mentioned above about where most of the light falls, you can tell that the 60 degree optics is the way to go for this setup. Even though it spills out of the tank a bit, the light is concentrated on your corals. You can also do this demonstration at home with a protractor. Draw a scaled down box of your tank. Then use a protractor to decide what degree optic is right for you at the height you need.
From what we have mentioned above, you should realize that the different optics will put out different intensity at the same distance from the optic. For instance, the intensity at 12in is going to be drastically different in a 60degree optic over a 40degree optic. A good way to visualize is to fill the cones up with a 100 marbles. The 60degree cone is going to have more space between the marbles than the 40degree cone. More marbles in a tighter area means that there is more intensity.
You should now realize that it is important to keep the same optics throughout your fixture otherwise you will get hotspots where you aren't expecting. Your first gut instinct is to put tighter optics on the outside of the fixture and wider optics in the middle. What happens is when the cones cross, there is more intense light at the outsides than there is in the middle of the tank. For instance, 12in from the optics at the edge of the tank is going to be drastically more intense than 12in from the optic in the center of the tank.
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