Problema con indice

Tengo esta consulta

select * from folio f
where f.capturo=3

que al revisarla con explaine analyze sale esto

QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
Seq Scan on folio f (cost=0.00..11272.08 rows=93676 width=52) (actual
time=0.481..3191.285 rows=85950 loops=1)
Filter: (capturo = 3)
Total runtime: 3319.928 ms
(3 rows)

pero la duda es porque NO usa el infice si el campo si lo tiene

Esta es mi tabla
CREATE TABLE public.folio
(
folio varchar(9) NOT NULL,
placa varchar(12) NOT NULL,
fecha date NOT NULL,
articulo int4 NOT NULL,
condonado date,
capturo int4 NOT NULL,
fechacaptura date NOT NULL DEFAULT ('now'::text)::date,
id serial NOT NULL,
tipo int4 NOT NULL,
CONSTRAINT folio_pkey PRIMARY KEY (folio),
CONSTRAINT articulo_folio FOREIGN KEY (articulo) REFERENCES
public.articulo (clave) ON UPDATE NO ACTION ON DELETE NO ACTION,
CONSTRAINT capturista_folio FOREIGN KEY (capturo) REFERENCES
public.capturista (id) ON UPDATE NO ACTION ON DELETE NO ACTION,
CONSTRAINT placa_folio FOREIGN KEY (tipo) REFERENCES public.placa
(orden) ON UPDATE CASCADE ON DELETE RESTRICT,
CONSTRAINT folio_condonado CHECK condonado >= fecha
) WITH OIDS;

y el indice (existe 6 personas que capturan)
CREATE INDEX folio_capturo_index
ON public.folio
USING btree
(capturo);

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