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p.2 #2 · p.2 #2 · Has anyone replaced their drop-in filter on 400/600GM? | |
DavidBM wrote:
Not a good idea, I think. These long tele lenses are calculated with a flat element in the image path, whose refraction is taken into account. Removing it will give you induced field curvature and possibly a little outer field astigmatism. This is the reason there is a clear/UV filter there is the first place. They want you to be able to use a polariser or ND filter, but if they calculate the lens with a filter, then shipping it without one will be problematic. If they calculate the lens without a filter, then using a pol or ND will be problematic. So they calculate it with a filter, and put in a more or less clear one when it ships so performance is as expected (filters at the front of the lens have no or negligible effect, but at the rear you need to calculate for them)...Show more →
Yes, I am well aware of the theoretical downsides of removinng the flat piece of glass from an optical path designed to have it there. The point of focus shifts slightly so I believe the blue red and green light rays don'align as intended either anymore, leading to chrometic abberation.
I am not saying it is best to remove the filter from a theoretical point of view, but here are some alternative thoughts:
-flat pieces of glass are far more critical when it comes to light passing through them then the actual curved lens elements, which are far easier for light rays to pass through. An optical path without a flat piece of glass is more optimal.
-flat "neutral" filters are never truly neutral. I once decided to do a test with three filters, a Nikon filter, a sigma filter and a B&W filter. They all added their own particular color cast, the Sigma being positively warm, the B&W being slightly greyish, and the Nikon most neutral. The color cast changed the actual colors slightly as well.
-on low res FF cameras, for which the whole drop-in filter concept was designed, you see no difference in fine detail, but with a high resolution camera like the Canon 7DII, the improvement in fine detail by removing the stock filter was remarkable.
-super telephoto lenses use exotic glass elements, but the drop in filter is in all cases a generic consumer piece of glass from the standard filter range. The quality can vary substantially.
-the practical impact of leaving out the filter varies, and the question is whether this impact is worse than the inevitable impact of putting a flat piece of fairly ordinary multicoated glass in the optical path.
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