Why are filters only added to the front of the lens and not the back, too (or even the camera)?

Question

I see how some filters have to be attached to the front of the lens, because they require some amount of interaction, like a graduated neutral density or circular polariser filter.

But what about the other static¹ filters? Wouldn't it be possible to have a filter thread at the back of the lens?

The disadvantage would be that one had to remove the lens in order to change the filter. The advantages would be to have smaller filter sizes or possibly only one filter size for all lenses.

Camera manufacturers solved the problem for telephoto lenses with drop-in filters instead of attaching the filter to the rear end. This makes me wonder if there's a technical reason that filters are not attached to the back end of a lens?

Is there not enough room for a filter? Is it too much development for too little gain feature wise?

The number of filters that could be added to the back is certainly limited, but so is the number that can be added to the front without getting issues like vignetting.

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¹ for lack of better word: those filters that don't require any interaction

Answer 1

I can think of a few reasons.

If you want to mechanically put it "inside" you lens/camera and still attach your lens to your camera (lens attached to camera, as usual):

• You still have to find room for electric connection between your camera and your lens. As you can see on most lenses, there is not much surface available.
• You need space to screw you lens with your finger. You will probably need a tool to screw it inside your lens/body without adding fingerprint on the filter.
• You will have one more chance of adding small pieces of metal/plastic in your body/lens as you screw/unscrew filter on the thread.

As mentioned by Itai, you might make it easier by using a slot/tray in which insert you filter but:

• The body of your lens will have to be reinforced to compensate the structural "hole" => heavier lens
• It makes your lens bulkier, you have to add a new mechanism to lock/unlock the filter in position
• Making it weather-sealed might be troublesome
• It has to be "cleanable" to remove dust

If you want to mechanically add a filter between you lens and you camera (lens attached to filter attached to camera):

• It will change you lens basic characteristics as it adds distance: adding a filter between you lens and your camera will have the effects of an extender (focal length and aperture will change for example).
• You will add one more source of movement between lens and camera if your can tighten the whole 3 elements correctly.
• The filter will have to be resistant enough to maintain the lens on the body. So you will probably end up with a huge filter with the same mount as a lens.
• Your filter will have a hard time being compatible with all lens/body because they all have different electrical connections.
• Sealing might be a problem for weather resistant body/camera combo (the effort exercised on the filter will be greater).

For both options, you probably won't be able to stack filters.

Of course adding a filter inside the lens or the body will have the advantage of having a unique diameter of filter for all lenses, and (example from Itai) it may be the only solution to fisheye lenses or lenses with a huge front element.

Answer 2

Your assumption is incorrect. Filters are sometimes placed at the back of the lens. Many lenses only support front filters, many only back ones and some even support both. Manufacturers prefer to use filters which go in the front because they are more convenient:

• Easy to add and remove.
• You can interact with the filter simply.
• You can stack multiple filters since there is room, up to a point where it causes vignetting or degradation.
• Minimize risk of dust entering where it would be difficult to clean.

Sometimes they give up because the design of the lens does not easily allow filters in the front:

• Extremely wide and fisheye lenses often cannot accommodate a filter without vignetting.
• A bulbous element may stick out from the lens barrel, making front filters needing a collar.
• A wide front elements makes a suitable enormous filter very large and expensive.

In such cases, manufacturers often have a slot to slip in a thin gel filter. This occurs in bright telephoto lenses such as the Sigma 500mm F/4.5 EX DG which takes a 46mm drop-in filter at the back. There is no standard size here either, although conceivably, it may be possible based on the size of the lens mount. Still, there are no adapters between rear filter sizes, so you need different sizes for now.

You can have both, even though it is unlikely to find a suitably large filter. The Pentax DA 560mm F/5.6 AW supports 112mm filters in the front and 40.5mm in the barrel.

There is even a way to interact with specially designed internal filters. The Pentax 645 DA 25mm F/4 comes with a polarizer which can be inserted in the middle and there is a control-dial to rotate it on the holder. You can see it clearly here.

Answer 3

Regarding the technical reasons to opt for front filters vs. a slot/rear filter, I think I'm right in saying:
A translucent light modifier placed in front of the front element typically is a safer bet because:

1. It is completely out of focus and any scratches/dirt will therefore be much less likely to show up in the photograph.
2. It has less of an impact on the optical formula of the lens; as opposed to a filter placed behind the rear element, which interrupts the perfectly focused image as it exits the lens. Also for this reason the precise composition of the rear filter is important, as variations in thickness, coatings or refractive index will have a marked impact on lens performance. Note that the few lenses that have rear filters usually require a marque brand clear filter to be in place whenever not using coloured filters, else the lens performance is allegedly not optimal.
3. Many lenses have recessed rear elements, making it hard to insert a filter close to the rear element. The closer to the film plane the filter is, the more likely it will contribute to unwanted internal reflections.
4. Lastly, you'd need to remove the lens to change filters and nobody wants to do that!

Answer 4

Sometimes filters are used on the back of a lens.

The Canon EF 17-40mm f/4 includes both a gel filter holder on the back of the lens and filter threads on the front of the lens.

Answer 5

If you place a window of glass in front of a lens focused to infinity, there is absolutely no effect on imaging aside from some stray light considerations. On the other hand, if you put a window of glass in the path of converging light, you will get aberrations. See this post on the lensrentals blog if you would like some examples.

The negative affects of a filter in the converging beam are present at any focusing distance. It is true that there are mild repercussions at nearer focusing distances for the in-front-of-lens case, but to reiterate, they are mild.