Cat Islander wrote:
As I look at updating my Nikkor glass I am running into some of the lenses take slip in filters. I have always used a screw in uv or polarization filter. Please opine
Cat Islander
Take only pictures
Leave only footprints
I believe you might be referring to
"drop-in" filters.
Lenses that use drop-in filters usually have exceptionally large front element or other reasons that a standard screw-in filter cannot be used.
Many lenses that use a drop-in include the filter in their optical formula, so some sort of filter must be installed all the time. (Of course, the filter holder serves to keep stray light from entering the slot in the side of the lens, too.) The manufacture usually provides a "plain" drop-in filter with the lens.
Drop-in circular polarizers and, less common, variable ND filters have some means of externally rotating and controlling the filter. This makes them a dedicated unit (non-interchangeable filter) and rather pricey, but necessary if you want to use this type of filter with that particular lens.
Some examples I personally use include Canon 300mm f/2.8 and 500mm f/4 lenses, which have 120mm diameter and 130mm diameter front elements, respectively. No one makes filters that large and even if they did, the cost would be pretty steep and the filters would be extremely fragile and difficult to use! Both those lenses use drop-in filters. I have C-Pol and UV/Sky filters for them. There's also choice of using standard 52mm screw-in filters in one type of filter holder, or less expensive gel filters in another type of holder. The lenses came with a gel sky filter installed in a gel-type filter holder. I purchased screw-in type holders to use with them, which allow me to use standard glass filters (easier to clean and more durable than gel filters).
More recently Canon has offered some adapters that allow using a DSLR (EF or EF-S) lens on the R-series mirrorless cameras (RF mount)... which cleverly include a provision for a drop-in filter. Currently versions with a C-Pol or a Variable ND are offered. I think this was a brilliant idea, additional use of the adapter that makes it possible to easily use filters with lenses such as the Canon EF 11-24mm, 8-15mm Fisheye, and 17mm Tilt-Shift.... all of which have protruding convex front lens elements that preclude using standard screw-in filters. Now those lenses, when adapted for use on R-series cameras, can be used with drop-in filters too!
"Slip-in" filter holders are provided on the rear of some lenses that have similarly protruding front elements (regardless the type of camera they're used upon). These are a couple slots on either side of the lens' rear element can only be used with thin gel filters, which you will need to cut to size to fit. There is a wide variety of types of gel filters.
"Slide-in" filters... as referred in some previous responses.... are yet another thing. I can't imagine Nikon specifying using this type of filter though. They are exclusively third party manufactured. Cokin, Lee, Tiffen, Singh-Ray and others make slide-in filters and the accessories needed to use them. This involves a filter holder into which one or more oversize, square or rectangular filters can be fitted, along with an adapter of some sort that allows the filter holder to be mounted onto the lens. These filters come in various standard sizes... 84mm, 100mm, 125mm and more... as well as in optical plastic or glass. Many are uncoated, but some have multi-coatings similar to glass screw-in filters. This type of filter is difficult to shade effectively and they're rather bulky. Good quality, multi-coated glass filters of this type also can be quite expensive. There are more affordable, but more easily damaged and likely uncoated, optical plastic from several manufacturers.
Among the most popular slide-in filters for use with film were graduated neutral density. These were often used in one-stop, two-stop or three-stop strengths for scenic shots, to "hold back" a too-bright sky, balancing the exposure with the rest of the scene. These are largely unnecessary now with digital. It's still possible to use them, but it's also possible to produce the same effect with multiple exposures or even multi-processing a single image, then combining the two (or more) images in post-processing. Doing this digitally is actually a lot more controllable and much more precise than was ever possible with film and Grad ND filters. The same post-processing technique can be used for images where no filter would be possible. (I have a set of Grad ND, holders and adapters from way back... But haven't used them in years. I can do a much better job more easily with post-processing. I only keep the filters in case I want to shoot some film... but haven't done so in quite a while!)
On another subject...
There's virtually no need for a UV filter on a digital camera. People use them as "protection" on their lenses. Personally I think that's pretty silly, to think a thin piece of glass is going to offer much in the way of protection. Lens hoods and lens caps do a much better job... and are even more necessary if using a filter, to "protect" the protection!
Back in the days of film we used UV filters a lot because most film was overly sensitive to the UV spectrum and that caused a bluish color cast to images. Sometimes we'd use even stronger 81A, 81B and other warming filters. We weren't using the filters "for protection". We were using them to color correct our images. But I think people interpreted it that the filters were being used for protection and that's how the whole thing of using a UV filter got started.
A circular polarizer, on the other hand, is one of the most useful filters for digital photography. It can provide effects in images that are difficult or impossible to do in post-processing software. (Many other types of filters are easily emulated in post.)
However, as useful as it is, a C-pol also can be problematic for some shots and should never be left on a lens full time (I've seen some of the lighter tint C-pol being advertised as dual purpose polarizer and protection filters). Polarization improves some images... But it also can ruin other types of shots.
All my filters (except for the plain drop-ins) are stored in my camera bag separate from my lenses. I only install them when they actually serve a purpose. (Sometimes that purpose is "protection", such as when shooting at the sea shore, where salt spray is nasty stuff to clean off lenses!)