El ocular cambia el campo de visión y el aumento.

Because ocular is a masculine noun here, so it takes el.

A useful thing to notice is that ocular can be:

• an adjective: un examen ocular = an eye exam / ocular exam
• a noun in optics: el ocular = the eyepiece

In this sentence, it is a noun meaning eyepiece.

Yes. In Spanish, ocular can absolutely be used as a noun in technical contexts, especially in optics.

So:

• el ocular = the eyepiece
• la lente ocular = the eyepiece lens / ocular lens

Spanish often allows words that feel adjective-like in English to function as nouns in specialized contexts.

Because the subject is el ocular, which is singular.

• el ocular cambia = the eyepiece changes
• los oculares cambian = the eyepieces change

The phrase el campo de visión y el aumento is not the subject. It is what gets changed: the field of view and the magnification.

Because cambiar is normally a direct transitive verb here. It takes a direct object without a preposition.

So:

• cambiar algo = to change something

Examples:

• Cambia el enfoque. = It changes the focus.
• Cambia el tamaño. = It changes the size.

You would not usually say cambia a el campo de visión in this context.

Spanish often uses noun + de + noun where English might use a compound noun or adjective.

So:

• campo de visión = field of vision / field of view

This structure is very common:

• gafas de sol = sunglasses
• centro de salud = health center
• campo de fútbol = football pitch / soccer field

You may also see campo visual, but campo de visión is a very natural and common way to express the idea.

It has an accent because of Spanish stress rules.

Visión is stressed on the last syllable: vi-SIÓN.
Words ending in n, s, or a vowel are normally stressed on the second-to-last syllable unless there is a written accent. The accent mark shows that the stress is irregular here.

So:

• visión → stress on the last syllable
• without the accent, Spanish speakers would tend to read it differently

In everyday Spanish, aumento often means increase. But in optics, it commonly means magnification.

So in this sentence, el aumento is best understood as:

• the magnification
• or the level of magnification

This is a good example of a word whose technical meaning is more specific than its general meaning.

Spanish often repeats the article with coordinated nouns, especially when they are understood as two separate concepts.

So:

• el campo de visión y el aumento

sounds natural because it treats them as two distinct things:

1. the field of view
2. the magnification

In some contexts, Spanish can omit the second article, but here repeating el is the most natural and clear option.

Yes, that would also be grammatical.

There can be a slight nuance:

• campo de visión = field of view / field of vision
• campo visual = visual field

In many practical contexts, especially optics, they may overlap a lot. But campo de visión is very straightforward and common when talking about what an eyepiece changes.

Yes, and that can confuse learners.

Here, cambia means changes/alters.

• El ocular cambia el campo de visión. = The eyepiece changes the field of view.

But cambiar can also mean to switch or to replace, depending on context:

• Cambia de canal. = Switch the channel.
• Cambia la bombilla. = Replace the light bulb.

So the exact meaning depends on what follows and the situation.

Yes. This is a very normal Spanish word order:

• Subject + verb + direct object
• El ocular
  • cambia
    • el campo de visión y el aumento

Spanish is flexible with word order, but this neutral order is the most common and easiest to understand.

Often, yes, especially in technical or descriptive language. But depending on context, a speaker might also say:

• el ocular
• la lente ocular
• el eyepiece (less standard, more influenced by English, and not ideal in careful Spanish)

In standard Spanish, especially in technical writing, el ocular is the normal choice.