**not a problem**

The combined focal length does not matter. This is where the combined lenses would focus a bundle of parallel rays (ie. from the sun). You never use the fresnels this way in a projector.

Let's look at an example where you have a 220 mm fl condensor fresnel and a 330 mm fl field fresnel. If you put the lamp arc 220 mm from the condensor fresnel, then it will refract the rays to be parallel on the other side. If those parallel rays enter the field fresnel, it will refract the rays to focus on a point 330 mm from the fresnel. Since the rays between the two fresnels are parallel, it does not matter how far apart they are from each other.

If you put the lamp arc only 200 mm from the condensor fresnel, then the rays coming out the other side will be diverging instead of parallel. (If they travel a long distance to the field fresnel, then you would lose some light.) When they go through the field fresnel, they will be focussed at a distance longer than 330 mm. To calculate that distance, you can the use duplet equation (which includes the distance between the fresnels). But you don't need to do that! The duplet equation calculates the focal length on each side of the combined lens system, but that focal length has to be measured from the principal planes of the two lenses. If the lenses are not together, then those two principal planes are far apart. But this is an unnecessary complication.

Calculate the field fresnel to focus point distance as if the two fresnels were together:

1/EFL = 1/220 + 1/330 so EFL = 132 mm combined fl

1/132 = 1/distance A + 1/distance B so distance B = 388 mm

So with the lamp arc 200 mm from the condensor fresnel, the field fresnel will focus the light to a point 388 mm FROM THE FIELD FRESNEL. If the two fresnels are 0 mm apart or 50 mm apart, it will not matter: The light will still be focussed 388 mm from the field fresnel.