Focus offsets for a filter of thickness t can be calculated as follows:
Consider the light from a converging beam falling on a translucent
plate
of thickness t and refractive index n as shown in Figure 
.
[ TIFF ]
Figure: The light path of INT or WHT prime focus near the filter
Now the change in focus 
For the glass broad-band filters, the refractive index is 1.6,
so the focus offset = 0.38
For the interference filters, the values for the refractive indices are
approximately 1.45 for the OII(3727Å), SII(4070Å), HeII, H
, OIII,
HeI(5876Å), OI and NII filters, and 1.48 for the H
, SII(6724Å),
OII(7325Å), SIII(9069Å and 9532Å) and HeI(10830Å) filters.
(N.B. Do not use the refractive index of the interference coating,
which, although of large refractive index (
2.1), is thin and enclosed
in a glass substrate.
In practise you have to multiply
the correction calulated above by (f/f')
, where f is the focal
length without corrector lens, and f' the focal length of the whole system.
For INT prime these values are 2.94 and 3.29, and the
change in focus is 
.
For WHT prime these values are 2.498 and 2.81, and the
change in focus is 
.