A visa is a document allowing the crossing of a border or stay in a foreign country that must be issued to foreigners by consulates of a given country, apart from cases of so-called visa-free travel. Depending on the country that issues the visa, a photo is needed. The photo confirms that the visa holder has the right to stay in the country. In addition to the visa, therefore, you often need to show a passport or another document that officially confirms your identity.
Current requirements for a visa photo are determined by the regulations of individual issuing states and are customarily published on the embassy and consulate pages. One may need a photo for an American, Russian or Canadian visa. The requirements for photos for a Chinese or Vietnamese visa are also increasingly sought after. This is mainly due to the increase in the popularity of exotic travel. It is worth noting, however, that the photo format and its other features often differ, e.g. the photo we use for an American visa will not be useful to us when we apply for a document that will allow us to travel to Vietnam.
Basically, all visa photos are so-called biometric photos, i.e. those that allow us to capture our individual features that allow for quick and reliable identification. Therefore, usually in photography we should be positioned frontally and look straight ahead. The background requirements vary, but in most cases it should be uniform and one-colour (usually light, but may also be that e.g. the background must be blue or dark blue). Rules regarding glasses and headgear are also usually the same, i.e. you should photograph yourself without a hat and without glasses (although they are technically allowed, you ought to take off your glasses even if your sight is impaired - this minimises the chance of the composite photo being rejected, as there is a risk that glare will be visible on the glasses or the glasses frame will cover your pupils).
Photos for visas differ primarily in size. Some countries allow only a specific photo format, others offer several possibilities. An example of a strictly defined size of the photo is a US visa, which must always have the dimensions of 50 x 50 mm. Vietnam, on the other hand, expects a 4 x 6 cm photo, but also allows the possibility to attach a photo with the dimensions such as are required for the passport in the country of origin of the tourist.