La lente está limpia.

Because estar is normally used for a current condition or state, and clean here is a state.

• La lente está limpia = the lens is clean right now.
• La lente es limpia would sound like you are describing it as generally clean by nature, which is not the usual idea for an object like a lens.

So for cleanliness, estar is the natural choice.

Because the adjective has to agree with the noun in gender and number.

Here, lente is being treated as feminine singular, so the adjective is also feminine singular:

• la lente
• limpia

If it were plural, you would say:

• Las lentes están limpias

If a speaker used the masculine form of the noun, then the adjective would change too:

• El lente está limpio

They can. A noun ending in -e does not tell you automatically whether it is masculine or feminine.

So the best habit is to learn nouns together with their article:

• la lente
• el puente
• la clase
• el coche

In this sentence, the noun is feminine, so you get la and limpia.

Yes, in some varieties of Spanish, el lente is used. But for Spanish from Spain, la lente is a normal and natural choice.

The important thing is agreement:

• La lente está limpia
• El lente está limpio

So if you change the gender of the noun, the adjective has to match.

Yes. In standard Spanish, a singular countable noun usually needs some kind of determiner, such as:

• la lente
• una lente
• mi lente
• esa lente

So Lente está limpia would not sound correct in normal Spanish.

In Spain, lente usually means an optical lens.

A few useful related words:

• lente = lens
• objetivo = camera lens / camera objective, often more natural for photography
• lentilla = contact lens, very common in Spain

So if you specifically mean a contact lens, people in Spain will often prefer:

• La lentilla está limpia

It could be understood that way from context, but in Spain lentilla is usually the more everyday word for a contact lens.

So:

• La lente está limpia = the lens is clean
• La lentilla está limpia = the contact lens is clean

If you want to be especially clear, you can also say:

• La lente de contacto está limpia

The accent mark shows the correct stress: es-TÁ.

Without the accent, esta is a different word, such as:

• esta lente = this lens

So the accent helps both with pronunciation and with distinguishing the verb form está from other words.

A simple pronunciation guide is:

la LEN-te es-TÁ LIM-pia

A few notes:

• lente has stress on the first syllable: LEN
• está has stress on the last syllable: TÁ
• limpia is usually pronounced as two syllables after the l sound joins smoothly: LIM-pia

In this sentence, limpia most naturally means clean, as in free from dirt, grease, or marks.

For a lens, that usually implies that you can see through it properly, but the core idea is still clean, not just clear.

So if the lens has no smudges, limpia is exactly the right word.