How can I test a camera or lens at infinity focus in a small room?

This sort of testing is normally performed with an infinite conjugate optical system. One example of such is a/an (auto)collimator.

Collimators and AutoCollimators can be used interchangably for the task of testing a lens at infinity. The testing procedure is simple.

1. Obtain an (auto)collimator which is focused to infinity and contains a resolving power target
2. Focus the camera on the "infinite" distance image from the collimator
3. Read the resolving power indicated by the target

This system has the advantage of not only measuring the resolving power at infinity but also allowing you to focus the camera with the aid of the collimator. To use the collimator to assist focus, simply perform a binary or progressive search while focusing. The peak resolution represents infinite focus.

Autocollimators may also offer the ability to focus at distant, far conjugate distances. For example, with a 300mm lens the hyperfocal distance could be thousands of feet. A properly configured autocollimator could be focused to, a specific distance to allow you to focus.

Collimators can be found on ebay for a couple hundred and, if not damaged, tend to last forever. Edmund Optics and Thor Labs carry collimators in a $1k range. Newport is a mid-range supplier. The Cadillac of production collimators is the Moller Wedel but expect to pay for that quality.

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A point source system projects a collimated beam of light into your camera. By measuring the diffraction via a PSF measurement of the image produced by the camera, an MTF value can be generated. Systems which use point sources generally include a goniometer for off-axis testing but that is not strictly necessary.

It is possible to construct your own Point Source using a normal bulb (LED preferred) and the collimating eyepiece of a telescope. The precision of your device will be a product of your facilities for alignment and calibration of the focus rig. PSF/MTF measurement is fairly easy to implement if you are computer savvy and have access to the raw images. OpenCV and various Python distros include libraries to perform such a task.

Cost for such a system could be as little as $50 (the cost of the eyepiece) assuming you've already got a suitable hardware rig and computer. On the other hand, point source focus testing is generally considered the most precise form of optical system testing (most metrology utilizes Point sources) so there are a variety of commercial systems. The cheapest is probably about $5k and prices range up to $1million including a truly impressive systems from Optikos and Trioptics.

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Optical Mirrors are the simplest solution. If the size of your cleanroom represents a large fraction (say, 1/2 or 1/4) of your hyperfocal distance, consider using mirrors. 1 mirror will allow you to double the distance to your target and you can keep adding mirrors. Use high quality mirrors such as first surface optical mirrors. I recommend enhanced aluminum for visible applications due to its superior performance in the blue region. For many folds (greater than 3) consider upping the quality of the mirrors to 7/lambda or 10/lambda.

The longer the focal length of the lens, the smaller the mirror needed but the higher the quality needs to be in order to match the increased angular resolving power. To determine the size of optical mirror needed, image your target through a handheld or wall mirror. Mark the edges of your resolving power target with tape using a live feed from the camera to judge the limits of cropping. Then you can measure the taped off area.

Optical mirrors can be had for a couple of dollars but I suggest going for a reasonable level of quality in anything more than 1 fold. As such, expect to pay $30-$90 per mirror. Edmund Optics or Thor labs are good sources for new mirrors. Avoid purchasing new mirrors from ebay or amazon where counterfeits are common. Used scientific mirrors, however, tend to be of high quality and it's easy to see if something is wrong with the mirror in an image of it.