leaflet_custom_zoom_levels.js

/*
Most map tile providers use 256px square tiles so Leaflet's basic zoom algorithm looks like this:

256 * Math.pow(2, zoom);

If you're working with vector layers you can extend one of leaflet's default CRSs 
and make it return tile sizes in smaller increments. This can be very helpful when using 
fitBounds() with layer groups, so the zoomed group fits better within the map bounds:

E.g., extending L.CRS.EPSG3857 (the default CRS):
*/

L.CRS.CustomZoom = L.extend({}, L.CRS.EPSG3857, {
    scale: function (zoom) {
        return Math.round(256 * Math.pow(1.5, zoom)); // need to round since powers of 1.5 result in non-integers. Non-integers will not work with background tiles
    }
});

var map = new L.Map('map', {
    crs: L.CRS.CustomZoom
});

/* 
It's worth noting you can also swap the CRS on the fly if you need to swap between a vector layer and a tile layer
*/

// swap back to standard zoom
map.options.crs = L.CRS.EPSG3857;

Does it work in leaflet 0.7.x? I found the tiles don't display with the above settings.