UV Lens Test: What should be included?

[nfoto]

Theoretically the focus shift should be approx. 1/3 of the filter thickness.

[Alex H]

But that would also depend on the refractive index of the glass used. Of course, with thin filters this focus shift is mostly unnoticeable under most shooting circumstances. However, it can show up when doing testing as Enrico described.

[nfoto]

That's why I used the term 'approximate'.

 

A few test shots with different filters should be performed to clarify the magnitude of the shift. This can potentially be an issue when you do visual focusing and rely on the result being the same in UV. For normal shooting around 1:5 to 1:10 or smaller I think the potential error is inside the variation caused by subject movement and obtained depth of field by stopping the lens down. For critical macro work, the filter effect can no longer be ignored.

 

Again, using Liveview for focusing will accommodate these filter-based shifts if you check focus each time a filter is swapped.

[enricosavazzi]

[...]

A few test shots with different filters should be performed to clarify the magnitude of the shift. This can potentially be an issue when you do visual focusing and rely on the result being the same in UV. For normal shooting around 1:5 to 1:10 or smaller I think the potential error is inside the variation caused by subject movement and obtained depth of field by stopping the lens down. For critical macro work, the filter effect can no longer be ignored.

[...]

Quite true, I have had this problem already in close-up photography rather far from the macro range (0.2-0.3x). This is especially likely to become noticeable with sets of filters of substantially different thickness. Astronomy filters are usually available in matched sets with the same optical thickness, but with our UV-pass filters the thickness is extremely variable. It can be equalized by sandwiching with fused silica flats of suitable thickness, but this is not problem-free either, and even after solving the reflection problems even a flat glass plate introduces chromatic and spheric aberrations that may become noticeable with fast lenses and/or at high magnification.