The College of Optometrists

how is volk indirect ophthalmoscopy not affected by lens or media opacties?

it is affected - its just that the opacities are well out of focus, 'cos the depth of focus of a 78ds lens is only tiny anyway. Also, the direct has a broad beam of light which glares the observer being reflected of the media changes. So it gives a better result than direct in cases of media changes.

thanks palfi

:-)

Doesn't it also have something to do with the difference between the real image of the indrect and the vertual image of the BIO?

It might have to do with the inverted image of the volk as well. If the point of crossing (almost like a point source) was within the lens the image would be less susceptible to light scatter from the lens opacities. I think the light rays are inverted at the nodal point of the lens which helps as well but not too sure.

My physics ain't that good so can anybody else explain why indirect better than direct with opacities?

Fundus biomicroscopy (‘Volk lens’

assessment) gives a better view of the

posterior pole through cataract than

direct ophthalmoscopy for two reasons.

Firstly, the illuminating rays entering the eye are focused while passing through

the lens thus minimising backscatter if a

clear optical path can be found, and

secondly the optimised optics of the

condensing lens maximise image quality.

saw a px at hospital and cataract was so dense that the ophthalmologist couldnt get view of fundus with volk

obviously volk is not unaffected by lens opacities but the degradation of image is less in early cataract than with direct. I find that a disc and macular view is often avialable even in quite dense cataracts that cannnot be viewed with direct. even if you then dilate, you still can't get the same visualisation with direct as with volk. so your hosp px must have had one very brown tint!