We present two ferrofluids in which high variations of optical transmission can be induced by application of moderate magnetic fields (obviously a desirable feature of functional substances aimed at photonic applications). The samples tested are representative of two different behaviors observed in ferrofluids. Exposed to a magnetic field, the size of the aggregates are much smaller in sample 1 than in sample 2, and once the magnetic field is turned off, aggregates disappear in sample 1 while they stay in sample 2. The transmission of samples 2 mm thick, as a function of magnetic field (up to 72 G), are compared. For ferrofluid 1, its transmission increases up to 40% or decreases up to 75% by applying a magnetic field perpendicular or parallel to the incident light respectively. For ferrofluid 2, the presence of a magnetic field parallel to the incident light can multiply its transmission by a factor up to 160. In both samples, the transmission dependence on the applied magnetic field shows an acceptable linearity, which is a very interesting feature face to eventual sensing applications. Besides, it is shown how aggregates favor a drastic temporal response after DC magnetic field switch on and off.