The following will help you optimize your scene's lighting effects, as well as troubleshoot common problems. Many of the tips and optimizing methods described in Troubleshooting Lights can be applied to lights transmitting photons.
The first render — The first time you render photons, the render takes a little longer than usual. This is because the Photon Map is being calculated for the first time.
Number of receivers — The fewer objects you have defined as photon receivers, the faster the photon render.
If not necessary, avoid defining every object in a scene as a receiver. For example, if a room's ceiling is not in the camera's frame, do not define it as a receiver.
Start small — When tuning a global illumination effect, start with small values and work your way up. Try using few Emitted Photons so they are clearly visible.
A value of around 1000 should be good to get you started. Notice how the effect smoothens as you add more and more photons.
Use a high intensity — When tuning a photon effect, start with a high Intensity value (in the light's property editor). A value of approximately 40 000 makes your photons immediately visible.
When to use final gathering — Try not to use final gathering in scenes that have complex volumic effects or many speculars. Final gathering works best in scenes with diffused illumination.
Using Global Illumination (Speed Optimization)
Global illumination simulation can be very time consuming to render. Unless you require effects like color bleeding from diffuse interreflection and/or caustic effects, you should consider using ambient light or final gathering to imitate a global illumination effect.
Troubleshooting Photon Effects
Here are some common problems and their solutions.
My photon rendering won't launch
Make sure that you have defined a caster, receiver, and photon light source. An error message appears at render time if you have forgotten to define one of these elements.
Why is my photon rendering taking forever?
If the render seems to be taking much too long, you may have specified too many Emitted Photons.
Try lowering the value until you see an effect, for example, down to 1000.
If you are using global illumination, caustics, and final gathering all at the same time, your computer may be taxed beyond its limits.
Try deactivating two of the effects, adjusting the settings of the active effect, and then adding the others. You may want to try adding each effect in a different render pass so your system doesn't crawl to a stop.
My render seems to abort just before or after the first frame
This problem is most likely due to an invalid path specified for the Photon Map.
Open the mental ray Render Options property editor and click the GI and Caustics tab. At the bottom of the tab is a text box with the path to the Photon Map file. You can select a new path by clicking the (...) Browse button.
My global illumination/caustic effect is too soft
Locate which light is creating the soft effect and try to increase the Number of Photons from the light shader property editor
(choose Modify 
Shader).
My global illumination/caustic effect looks splattered
This may occur if too few photons are hitting an object. Usually the photons are hitting another object first and then the effect becomes uneven as some of the photons bounce to the desired location, but others don't.
You can fix this problem by either increasing the Accuracy parameter or the Number of Emitter Photons parameter.
My caustics don't have the sharpness I was expecting
Make sure that the caustic caster has the right shape to create sharp caustic effects. Usually, you'll want to use a curved object or an object with varying thickness.
It is also recommended that the object has an index of refraction of at least 1.33 or 1.5. The caustic receiver should also be at an appropriate distance (too far or too close results in no effect or a blasted one, respectively) and the light itself should be bright enough to create the effect.
