Knocks and Shocks
Another thing which is bad for optics is mechanical shock caused by dropping or knocking. Often these accidents happen without knowledge of the owner when the instrument is being carried in its case. It is true that the higher quality equipment such as roof prism, rubber armoured binoculars (e.g: Swarovski) will stand considerable shock but all optical equipment should be treated with care as a matter of course.

Placing optical equipment unprotected into the boot of a car (where vibration is often severe and continuous) is not good practice unless it is placed in something like a blanket in a box.

The most obvious effect of a knock is double vision in a binocular and an experienced technician must take corrective action. It is better if the collimation (optical alignment) is a long way out rather than a little because then the binocular will not be used. If it is a little out of alignment, the owner may continue using the binocular because his eye muscles can compensate for the misalignment and this will lead to eyestrain and possibly a headache at the end of a day. As a rough check for misalignment when in the field, get someone else to look through your binoculars as their eye muscles will not be trained to compensate for any fault in your binocular. Another rough way to check that your binoculars are in collimation is to look through them quickly and see if they "pull" your eyes when viewing something geometric such as an aerial in the middle distance. You will briefly see two aerials before, within a couple of seconds, your eyes compensate for the double vision.

Of course, many accidents can be avoided if the binocular lanyard is always worn around the neck. Most telescopes accidents happen when the tripod, with telescope, is left unattended and the ensemble is knocked or blown over Measures which will prevent or reduce damage to the telescope are the use of a tailored, well-padded stay-on case and the extension of the lens hood (which acts as a shock reducing "crumple zone").

Apart from mechanical shock, thermal shock can also damage your optics. If the instrument is quickly taken from one temperature extreme to another, thermal shock may fracture a lens or cause cement to break down.

Evidence of thermal (or mechanical) shock can be a rainbow pattern in a lens or prism unit and the image will become unacceptable. Avoiding thermal shock is one reason for taking your optical equipment with you in your hand luggage instead or putting it in an aircraft hold. A breakdown in optical cement can often be corrected but fractured or chipped optics need replacing.

Servicing
The question is often asked, "how often should my binoculars (or telescope) be serviced" but a definite answer cannot be given because servicing intervals depend on the type of instrument, how much it is used and the conditions that it is used under.

Ingress of sand and grit must be dealt with straight away. A build-up of dust and dirt gradually reduces light transmission and so does oxidation of the optics. The reduction in performance happens almost too gradually to notice until the owner compares the instrument with a new one. On the other hand, fogging, caused by the ingress of moisture into the instrument, is immediately obvious and this, like a complete service and clean, can only be carried out by a good technician who will completely strip the instrument down and, first, hot-air dry all components. The more expensive equipment will be much less susceptible to these problems and will need servicing less often.

One should be careful to entrust one's binoculars and telescopes only to a competent servicing technician of good repute. Examples of poor workmanship are legion. There are often ways of externally adjusting an instrument when, more properly it should be dismantled, the internal fault corrected and the final adjustments then made. Poor workmanship will also involve damaging screws, misfitting seals, failure to relubricate and so on. Avoid such mistreatment by seeking out the right person for the job.

Use with Spectacles
When buying your optical equipment, a good dealer will have taken care, in the case of spectacle wearers, that the equipment is suitable and that the method of operation is understood. Retractable eyecups and good eye relief (where the instrument focuses an acceptable distance from its eye lens) are normal requirements. However, even with the best equipment, some problems associated with reflections can arise under certain conditions. It should be realised that we are trying to form the best optical system comprising the instrument, the spectacle lenses and the user's eyes. The reflections, which sometimes cause problems such as ghost images, blurring and reduced contrast, chiefly comprise the following. Reflected light into the eye from the back of the spectacle lens or the instrument eye lens, internal reflections between the front and back surfaces of the spectacle lenses and, finally, reflected light from the cornea of the eye interacting with the spectacle lenses and eye lenses. Good spectacle lens coatings can reduce these problems as can the use of flat-fronted spectacles and retractable eyecups. Naturally, one does not wear spectacles unless necessary (e.g. because of astigmatism) but they can become more problematical as the binocular or telescope power increases. A good answer may be to use contact lenses.