it can also be due to prism imperfection.
i observed it in the old days in my film DLR. even seen some dirty marks through the viewfinder but no matter how i clean it, under the prism thro the lens mount or directly cleaning the viewfinder area, it doesn't go away.
then again it doesn't do anything to the image either. so i leave it as that.
a camera technician said it's due to some mould or else some fault lines in the prism and it's quite typical of any camera of any brands, many of which are getting the prism from the same manufacturer.
as we know it's made of glass and it's very hard to get rid of micro 'air bubbles' or micro air pockets or micro 'fault lines' in glass, even at the highest standards of glass making.
these faults get magnified in the line of sight when we view it through a viewfinder.
the other thing is the focus screen. it is highly likely that over time mould or even cracks developed in the focus screen. it is actually easy to resolve by simply replacing the focus screen.
i observed it in the old days in my film DLR. even seen some dirty marks through the viewfinder but no matter how i clean it, under the prism thro the lens mount or directly cleaning the viewfinder area, it doesn't go away.
then again it doesn't do anything to the image either. so i leave it as that.
a camera technician said it's due to some mould or else some fault lines in the prism and it's quite typical of any camera of any brands, many of which are getting the prism from the same manufacturer.
as we know it's made of glass and it's very hard to get rid of micro 'air bubbles' or micro air pockets or micro 'fault lines' in glass, even at the highest standards of glass making.
these faults get magnified in the line of sight when we view it through a viewfinder.
the other thing is the focus screen. it is highly likely that over time mould or even cracks developed in the focus screen. it is actually easy to resolve by simply replacing the focus screen.
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