Una dioptría más afecta a la visión.

Because dioptría is a feminine noun in Spanish, so it takes the feminine article una.

• una dioptría
• dos dioptrías

This is just grammatical gender; it does not imply anything about real-world gender.

In Spanish, when más means an additional amount, it often comes after the noun or quantity:

• un minuto más = one more minute
• dos euros más = two more euros
• una dioptría más = one more diopter

So una dioptría más means one additional diopter.

If you said más dioptrías, that would mean more diopters in a more general sense, not specifically one more.

Because the subject is una dioptría más, which is singular.

So the verb must also be singular:

• Una dioptría más afecta...
• Dos dioptrías más afectan...

Spanish verbs agree with the subject in number.

With afectar, Spanish very often uses the pattern afectar a + noun to indicate what is affected:

• afecta a la visión
• afecta al corazón
• afecta a los niños

So here a la visión means vision/eyesight is what is being affected.

You may sometimes also see afecta la visión, but afectar a is a very common and natural pattern.

Spanish often uses the definite article with general concepts, body functions, and abstract nouns where English may use no article.

So la visión here means vision/eyesight in general.

Compare:

• afecta a la visión
• mejorar la memoria
• proteger la salud

If you wanted to make it specific, you could say:

• afecta a tu visión
• afecta a su visión

They overlap, but they are not always used in exactly the same way.

• visión often sounds a bit more technical, medical, or abstract
• vista is often more everyday

In an eye/optometry context, both can be possible, but la visión fits well in a more clinical or formal sentence.

Examples:

• problemas de visión
• tiene buena vista

Also, vista can mean view as well:

• una vista preciosa del mar

This is a very useful distinction.

• una dioptría más = one more diopter, an additional diopter
• una dioptría de más = one diopter too many, one in excess

So:

• Necesitas una dioptría más = You need one more diopter
• Llevas una dioptría de más = You are wearing one diopter too much

The second one suggests excess or overcorrection.

The present tense here expresses a general fact or general effect.

Spanish uses the present for things that are generally true, just like English:

• El tabaco afecta a la salud.
• La falta de sueño afecta a la concentración.
• Una dioptría más afecta a la visión.

So it is not necessarily about only this exact moment; it can mean this has that effect in general.

A simple English-friendly guide:

• dioptría ≈ dyop-TREE-ah
• visión ≈ bee-SYON

About the accent marks:

• In dioptría, the stress falls on trí
• In visión, the stress falls on the last syllable -sión

The written accents show where the stress goes.

For a learner, the important thing is:

• dio-ptRÍ-a
• vi-SIÓN

Yes. That would be grammatically correct.

You just need to change:

• una dioptría → dos dioptrías
• afecta → afectan

So:

• Una dioptría más afecta a la visión.
• Dos dioptrías más afectan a la visión.

This is a good example of both noun and verb agreement in Spanish.