The Filters We Need With Digital...

Started Oct 5, 2008 | Discussions thread
Iliah Borg Forum Pro • Posts: 24,133
Re: Now that is just weird...

Joseph S Wisniewski wrote:

Iliah Borg wrote:

I have just bought a
Tamron SP 300mm f2.8 LD IF and while searching for sites with more
info about it I came across a lens test with a remarkable
finding...As you probably know, Tamron used to offer an optional
112mm "normal" filter for the front of the lens. I dont know what a
"normal" filter is but apparently the tester has found his Tamron
300/2.8 to be considerably sharper with this filter on the lens than
without it!!!

I had a similar experience with Canon 1D MkII and long lenses. A UV
filter improved AF performance of the camera.

Iliah, that is just plain weird.

Can't agree more.

Not that I'm doubting you, I've seen
enough weird stuff in this world to know that "weird happens".

But I can't imagine how, unless the AF system was unusually sensitive
to UV, so that whatever was making it through the lens was enough to
drift the AF system off a bit (since the lens isn't corrected for
near UV, or even for violet). Even then, a long lens should block
pretty much as much UV as a filter...

Were you using a stronger UV filter, a UV(1) or something that cuts
off near (or even above) 400nm, like those 415nm UV filters I use for
blacklight sessions...

I use regular slim MRC Schneider 010 type. The experiment was quite simple: a resolution target lit by sun, 50 shots AF on with filter, 50 shots AF on without filter. Lens was focused to infinity manually between shots. Resolution was consistently higher on the shots with the filter. Shots without filter exhibited slightly erratic focus behaviour, with 20% of shots being 8% lower resolution and 40% being 4% less resolution then the shots with the filter on. Only 10% of shots without the filter were of the same resolution then the peak resolution obtained with the filter. 70% of shots with the filter achieved maximum resolution; 20% of shots with the filter were 4% less then maximum resolution, 10% of shots with the filter were 8% less them maximum resolution. On the contrary, 10% of the shots without the filter resolved 30% less then maximum.

I hesitate to attribute this to the effect of UV on the AF system. I started with a hot mirror because the reports were that AF misses were mostly under hot outdoor conditions. Getting better AF behaviour with hot mirror I decided to recheck with a simple UV blocking filter, and it turned out AF improvement with UV filter was nearly the same as with hot mirror filter. My colleague used a Tiffen clear glass filter on his Canon camera with the same AF improvement results.

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