A method for estimating a bandwidth limiting penalty of a chain of bandpass optical filters in an optical telecommunication system. The method is advantageous for systems carrying an optical signal at a bit rate of about 10 Gb/s, wherein average bandwidth of the filters in the chain is of the same order of magnitude as bandwidth of the optical signal. The method comprises selecting a worst case filter, checking the system by testing and/or simulating it as being provided with the selected worst case filter instead of the chain of filters, and then judging about the bandwidth limiting penalty of the chain of filters based on results of the checking.