Theoretically which is better for image quality Drop in Filters or Front Filters

[Brian Hirschfeld]

I'm just thinking about it in theoretical terms, assume that there is a lens which has the ability to have drop in filters (decidedly in the rear of the lens like any super-telephoto) but also has a normal filter size on the front. Theoretically speaking which would be better for image quality.

Would a drop in filter system / filter system like on the Leica R 19mm / Mamiya 37mm f/4.5 yield better results because the filter is filtering focused light? or is there some reason why it is better for the filter to come when the light is initially entering through a regular front threaded filter system?

I'm thinking about this as a point of discussion for my upcoming article when comparing the Mamiya 500mm f/4.5 APO and the older 500mm f/5.6 lens as well as when revisiting the Mamiya 300mm f/2.8 APO and Mamiya 300mm f/4.5 APO lenses the 500mm review. Thanks for the help,

[theguywitha645d]

Drop in filters are usual used because the front element is too larger where it would be too expensive or impossible to attach a filter. Because the filter is in the lens, they become an optical component and so the lens is always supplied with an optical blank--a piece of glass needs to be in that position.

Neither type of filter should be in a position that has "focused" light. You do not want defects or dirt impacting the image. An internal filter is near the aperture for that reason and a front filter is within the focal length.

The internal filter can have an advantage of being in the space the aperture occupies as the light is in a sense collimated. A front filter on a wide will have a longer optical path length for off-axis light.

But I don't think the filter position is that important as far as quality--I doubt you would ever be able to see the difference. However, the quality of the filter is important.

http://www.lenstip.com/113.1-article-UV_filters_test.html

[Jim Kasson]

Brian,

I don't know if this is the usual situation, but the few lenses I've owned that supported drop-in filters incorporated the filter into the lens design. There was no option for "no filter at all". If you didn't want some particular filter effect, there was a clear (to my eyes, at least) piece of glass that you dropped in. The instructions warned that using the lens with no filter would produce fuzzy images.

In such a lens, I don't think you'd want to add a front filter if you had a drop-in that could produce the effect you're looking for, since you'll have two additional air-glass interfaces and increased possibilities for flare, plus whatever optical losses (losses over what; that's a little tricky since the filter is part of the lens design) you've already incurred with the drop-in.

Jim

[theguywitha645d]

The reason for the blank internal filter is optical path length--take it out and the focal plane shifts. The filter in front of a lens shifts the object plane. A shift in the object plane of a millimeter or less has no impact on your images. Placing a filter behind the lens will shift the focal plane, but if you have a camera design that can compensate, one with a ground glass or AF system after the lens, then there is no problem. A rangefinder would not be a good design for sticking a filter behind the lens.

[Jim Kasson]

...if you have a camera design that can compensate, one with a ground glass or AF system after the lens, then there is no problem.

Makes sense. Thanks. So you're better off leaving the "blank" filter out, assuming the camera can focus to infinity without it in place? One less set of air-glass interfaces, the possibility that the "blank" filter might not be truly flat, or might not be aligned exactly perpendicularly to the optical axis? Or are these effects too small to matter?

Jim

[theguywitha645d]

No, you want to keep the blank in the lens. I was refer to putting a filter behind a lens, rather than in it, as a comparison to a filter in front.

[Jim Kasson]

No, you want to keep the blank in the lens. I was refer to putting a filter behind a lens, rather than in it, as a comparison to a filter in front.

Gotcha. It turns out that I do have a lens that has its filters at the very back: an old and not-very-sharp 500mm f/5 Nikkor. Upon inspection, it looks like taking the blank out will allow dust to get into the lens. I think I'll leave it in.

Jim