| Joined: | Sun Apr 1st, 2012 |
| Location: | South Lakeland, UK |
| Posts: | 4066 |
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hmmm roberts statement re: digital corrected lenses gives me food for thought ,i wonder if it makes a difference to performance of older lenses ,i.e a 35mm film slr never had a filter in front of the film did it ,the shutter opened and the transmitted light fell on the film ,job done .
this is throwing so many variables into the equations ,
The effect is very easy to demonstrate, If you wind the window of your car down a bit, and put a pencil or a matchstick, anything slim, on the other side of the glass and visible above the glass, view it at 90º to the glass then move your head gradually sideways, the object will appear to split, the light goes through the glass at 90º no matter what angle you view it. The thicker the glass the more pronounced the effect. This is how/why lenses work.
The LPF is very thin but we are talking very fine optics here, even the 0.6 mm LPF has an effect which is also more pronounced with lenses which have a deep intrusion into the mirror chamber. Some lenses particularly the longer lenses even have recessed rear elements which effect less distortion. It's nowhere near as simplistic as I describe but it does have an effect and it also affects the edge distortion and edge drop off of light, giving a vignetting effect because of the stretching of the light waves at the edge of the frame.
This is also the reason why lenses with a built in filter holder must always have a filter fitted, even if it's clear glass because it's part of the optics of the lens.
The old 35mm film lenses will still work and produce great images but for the most part without correction they suffer from slight distortion and apparent vignetting to a greater extent than their modern digitally corrected brethren.
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Robert.