Q:

How to turn off OpenLayers 4 smooth zooming

After upgrading from OpenLayers3 to OpenLayers4, the zooming has changed. Now the user can zoom to zoomlevels between integers. It does not look okay for some raster layers.
Good (same as source raster map):

Little bit zoomed by OpenLayers (looks blurred):

How can I turn off this zoom option?

A:

Check constrainResolution option on ol.interaction.PinchZoom
http://openlayers.org/en/latest/apidoc/module-ol_interaction_PinchZoom.html
var map = new ol.Map({
    interactions: ol.interaction.defaults({}).extend([
        new ol.interaction.PinchZoom({
            constrainResolution: true
        })
    ]),
    .....
});

If you want to set for mouseWheelZoom, just change ol.interaction.PinchZoom to ol.interaction.MouseWheelZoom.
plus, zoomDuration: 0 makes it no animation.

