Well this paragraph in a technical lens review makes me wonder

[dabateman]

I just read the Imaging Resource technical review of the new Olympus 150-400mm f4.5 lens. But what grabbed me was this paragraph:

"Many people don't realize that all the glass in a lens element that's between its two surfaces doesn't actually serve any purpose, other than to keep those surfaces in their places. The only part of the lens element that does any work in actually bending the light is a microscopically-thin layer right at the surface; everything else is just along for the ride. (This is the basis of Fresnel lenses, first used for crafting massive lighthouse optics.)"

 

This would not only apply to the glass elements inside a lens, as Olympus has thinned many to speed up autofocus as less mass to move, but to our filters.

If we place an ever so slightly off axis polished surface filter, it will change the angle and effective focal length of our lenses.

I have been assembling fused silica lenses lately and a 500mm and 1000mm focal length element looks nearly flat. But will add up with the other elements to change the focal length and if off axis, change the aberrations.

With so few elements in our UV lenses this will have an effect. How I made a 35mm lens is with a 125mm, 100mm and 88mm elements. My 110mm lens is 500mm, 300mm and 200mm elements.

 

Basically the focal length seems to result from the focal length of an element, divided by the total number of elements, summed and divided by the total number of elements. This I can do in my head. As a 62.5mm is (1/2(100mm) + 1/2(150mm))/2. There is most likely a more complex formula, but this seems to always work.

 

But the impact is the filter will effect your sharpness, aberrations and resolution tests that people have started.

 

If you think the lens is soft in a corner, rotate that UV filter stack and see if it changes.

 

This maybe why my ball lens and EDIX lens that I bash around so much gave me mixed sharpness a couple nights ago without filters. They looked different with filters. As in I may have corrected a filter issue in making the lens that I don't have the liberty to do with off the self lens.

 

You can make a super cheap UVB dreamy fast lens with mirrorless cameras by buying a 40mm BK7 (Chinese K9 cystal glass ball) place it in a slim M42 camera adapter and stack a 5mm M42 ring and M42 to 52mm step up ring to hold it. This is very soft depending on the clarity of the crystal ball, but really fast f stop wise and k9=bk7 can see above 300nm.

[bvf]

Well, I've probably not understood what you're saying, but what the hell, here goes ...

 

I don't think that having a filter at a slight angle to the lens axis (if that's what you mean) will have any effect other than potentially shifting the image very slightly sideways. As the surfaces of the filter are parallel to each other they will not have any optical effect. To do what I think you're saying you'd need a filter (or just plain glass) with surfaces that were not parallel - a prism, in fact.

 

Go on - tell me I've completely missed what you're getting at ...

[Andy Perrin]

Bernard is right. If you have a flat filter with both sides of the filter parallel, refraction only moves the image parallel to itself. It's the third example here

 

I think it's a different story behind the lens, because then the rays are far from parallel.

[dabateman]

Well I typically don't sleep, so not surprised if I muddle up my words or thoughts.

What I was getting at was that since only the top microscopic lens surface determines the focal length, than our filters rated as seconds (Omega) and discount (from Midoptic). May not be perfectly flat and might have tilts, aberrations and add focal length (say 2000 or so mm).

Thus if your testing your lens and see it looks soft with a Baader but sharp with a 345bp. Or if one corner is better than an other, than try rotating the filter 90 degrees to see if there is a difference.

 

My Edix lens also started to look better. Maybe I am too used to the softness of my own lenses now.

[dabateman]

Bernard is right. If you have a flat filter with both sides of the filter parallel, refraction only moves the image parallel to itself. It's the third example here

 

I think it's a different story behind the lens, because then the rays are far from parallel.

 

Yes exactly. I am saying our filters may not be perfectly flat and may add issues to lens tests in UV using these filters.

One of the Omega seconds filters I received was so badly glued that it produced a double image. I returned it. But now I wonder what other issues there might be.

[bvf]

Thus if your testing your lens and see it looks soft with a Baader but sharp with a 345bp.

 

 

OK, get what you're saying. But I would have thought that the filter seconds are being discounted for flaws in the coating rather than optical flaws(although you haveat least one example to the contrary).

 

If your lens gives a soft result with a broadband filter like a Baader but is sharp with a narrow band filter like a 345BP25 that's more likely to be a sign of chromatic aberration. That's why in my lens tests I tested resolution through both broadband and narrowband filters.