Nikon long lenses - drop-in plain glass filter, why?

[gramps]

As with all Nikon drop-in filters, the supplied plain filter is part of the optical formula; using a filter holder without glass is a slight no-no.

(Thom Hogan)

When discussing fitting UV type protection filters to the front of a lens many photographers warn of a negative impact on image quality, so what is it about the 'optical formula' that makes it necessary to have the plain glass filter in place at all times in these lenses, anyone know?

 

[holty]

if your pictures are anything to go by whatever you use keep on using it as your pics are brilliant
if you use a clear or a uv on the front of the lens and it does not affect the end result then thats the best formula
just as the best camera is the one you have with you when an opportunity arises
i don't use any on my 3 holy trinity lenses as i always have the lens hood on the front

 

[chuckles]

Glass in the optical path will always have an effect, even if very, very slight. If you then need to use a 'proper' filter ie red, UV or yellow then allowances must be made for that glass. So, on the strength of that logic, you can't have no filter. It will throw the formula the formula out for the rest of the optical path.

[edited to add]
The above is certainly the case for internally fitted filters - can't see that it should be any different for externally fitted ones

 

[gramps]

Thanks guys, it's only the drop-in filter that I am asking about, I don't use others on my 500 f4 ... my mind can't get around why it wouldn't be better not to have that extra bit of glass in the way.

 

[Terrywoodenpic]

Thanks guys, it's only the drop-in filter that I am asking about, I don't use others on my 500 f4 ... my mind can't get around why it wouldn't be better not to have that extra bit of glass in the way.

Light is bent by refraction at every glass air surface.
In front of the lens the light is not yet focused so though it is bent it has no effect on the optical focus.
When a filter is placed between the elements ,this bending is used in calculating the optical properties of the lens.
If you do not replace a filter with an equivalent plain glass you upset the optical path sufficiently to be noiced as a drop in quality.

 

[gramps]

Light is bent by refraction at every glass air surface.
In front of the lens the light is not yet focused so though it is bent it has no effect on the optical focus.
When a filter is placed between the elements ,this bending is used in calculating the optical properties of the lens.
If you do not replace a filter with an equivalent plain glass you upset the optical path sufficiently to be noiced as a drop in quality.

Helpful, many thanks.