A filter is designed to block out unwanted or irrelevant stuff so that what you get through the filter is purified. (or alternatively what is trapped in the filter may be the bit you want with the impurities washed away after passing through)

A prism breaks stuff up into its components so that you can analyse it.

Of course you can use a series of filters to achieve a similar end - although filters tend to be discrete, whereas a prism can expand a continuum like the colours in a rainbow.

Look through a red filter and you will only see the red things, a blue filter will only show you the blue and the rest will appear dark. Use a prism and all the colours will be present but separated out on a scale so you can, for example see how much of each wavelength is present.

It seems to me that when Marx first used dialectics to analyse the means of production and consumption he was using a prism view to separate out economic relations of his time into different types for analysis.

Nowadays, all to often, followers of Marx use his ideas as a filter through which to view the world today.

Sometimes this is useful, clearly issues around the ownership of the means of production and patterns of consumption, distribution of goods (and services) within an economy are of interest if you want to understand and influence today's society just as Marx did in the 19th Century.

On the other hand when the ideas are used as a filter to force a particular 19th century analysis of mid-period capitalism onto the more complex relations existing in today's late capitalist society then much valuable information is discarded.

Just to take one example, the notion of distinct classes in society might have been more or less true a hundred years ago, but in today's world we all tend to occupy multiple class positions at different times and places. Sometimes, in some relationships, I might self-identify as a rentier owning some piece of capital and exploiting other by extracting rent value from it, in other relationships as a labourer selling my labour, or member of the bourgoisie, or as a peasant or serf. Classes have become pluraised across vast swathes of society.

It may be the case that we have a predominant class position which we occupy most of the time, but using Marxism as a filter to see only that relation does not help understanding of why the revolution has not come, and will not happen in the way Marx predicted for mid-period capitalism.

I suspect that genuine Marxists who have understood the texts (I haven't) recognise this and do correctly use the dialect method as an analysis tool, a prism through which to see the structure of the world. It is the ideologues within the Marxist camp who tend to misuse Marxist ideas as a filter through which to force an analysis that applied to the mid 19th century onto today. This leads them to shrily insist that black is red - which the rest of us intuitively see it is not and so we dismiss them as irrelevant.

In the process there is a possibility that we are throwing out something useful with their dirty bathwater.