Opto is another great compressor, which is constantly used in recording studios. In this case we have the optical compressors which use a photocell system for compression. This type of compression is slower than other models with a sometimes irregular release, which compresses very naturally.
These types of compressors have a relatively slow attack time and a medium release, with this confinement we can find different models such as the Teletronix 2A or the CL1B from Tube Tech.
The usual uses of this type of compressor is for vocals or bass, since they achieve a very natural dynamic control. On the one hand, you have greater control over the RMS of the tracks, for this reason this compressor is usually used in a chain with other compressors (it is not always necessary, depending on the result you are looking for). On the other hand, they are not usually transparent compressors, so the different colors can be used as colors (harmonic distortion) when making a production.
Some engineers use a chain for voice recording in which they use a FET compressor such as an 1176 to get a little control and some aggressiveness, then go through an optical compressor to get more dynamic control. It is important to note that optical compressors are usually quite warm, so depending on how aggressive the pre-compression is, we can soften the voice with the use of this type of compressor in the chain.
If this is your first time using this type of compressor, look for compressions between 1dB and 5dB to start hearing the differences and see how the compressor reacts. For vocals, when I use it, I usually find the sweet spot in relatively aggressive compressions (LA 2A from Universal Audio) around 5dBs.