There are dozens of 'modifiers' that you can use to modify the selected object in Blender. Modifiers can only be applied to objects, not to parts of objects that you may have selected in edit mode. In other words modifiers can only be applied in object mode (it's possible to 'add' them in edit mode, but not to finalise or 'apply' them there). Opening the modifier panel (click the 'spanner' button as shown in this image) and clicking on 'Add Modifier' brings up a list of available modifiers:

It's possible to add several modifiers to the same object at the same time. in that case the modifiers are applied in the same order that they are stacked in in the modifier panel, starting with the one at the top. The stacking order can make a difference, so there are buttons that allow you to change the stacking order.

We will examine only the most useful modifiers such as Mirror and Subdivision Surface (also called 'subsurf'). Modifiers apply to one object at a time, so if you turn on the mirror modifier for one object in a scene, for example, it will not affect any other objects.

Mirror Modifier

Many modelling tasks involve creating objects that are symmetrical. Using the mirror modifier means you only have to create one half of the model and the other half will be created automatically, saving you a great deal of time.

The Mirror modifier automatically mirrors a mesh along its local X, Y and/or Z axes, which usually pass through the object’s centre. We will examine only mirroring along the X axis, which means mirroring about a YZ (vertical) plane. Here is half of Suzanne's head and its mirror image, mirrored along the X axis. A gap has been left between the two halves to emphasise the mirror process, but normally there would be no gap.

To use the mirror modifier on a selected object, open the modifiers panel and click the 'Add Modifier' button then choose 'mirror'. The following panel will open:

You must be in Edit mode to use the mirror modifier. The mirror modifier can be set to join vertices to their mirror images if they are within a specified 'tolerance distance' (the 'merge limit') of the mirror plane. This is called 'clipping' and is recommended because

  • there is no point in having vertices that are very close together
  • there is no point in having more vertices than needed
  • when vertices are very close together or superimposed you might not even see them as separate, which could lead to confusion

Try it yourself

To experiment with the mirror modifier, start a new project, delete the default cube and add the monkey mesh which by default faces forwards.

Switch to Edit mode and unselect everything (A). Then switch to front ortho view (Numpad 1 and Numpad 5) and vertex select mode. We are about to box select (a.k.a. border select) the vertices left of the centre line but box select won't select surfaces that are hidden round the back so be sure that the 'limit selection to visible' option is turned off (unselected) before you do the box select. Now use box select (B) to carefully select all vertices left of the centre line. When using box select, always be aware that the adds to any existing selection, rather than replacing the existing selection. Delete the selected vertices and you should see half of Suzanne's head like this:

Note that the official object centre (the orange dot) is correctly placed on the plane that we are about to mirror about. Now add the mirror modifier as described above, with clipping turned on, and try moving vertices, edges or faces. See what happens if you move a vertex to the mirror plane - note how the vertex is clipped to its mirror image. Try selecting vertices, edges or faces on the 'mirrored' side - you will find it is impossible, because those vertices, edges or faces are 'virtual' - they only become 'real' when you 'apply' the modifier, something you should only do when and if you are obliged to since when you apply a modifier the modifier disappears and therefore you can no longer change its settings.

If you want to do anything to this object that would make it unsymmetrical then you will first need to click the 'Apply' button - the modifier will be applied and will then close. But if you do not need to make the object asymmetric there is no particular need to 'apply' the modifier - images can be rendered normally even if the modifier has not been 'applied'.

For a fuller description of the mirror modifier, search for 'mirror modifier' in the Blender wiki.

Two other types of mirror (not modifiers)

Do not confuse the mirror modifier with the mirroring (Ctrl+M in Object mode) that simply 'flips' the selected object. Do not confuse the mirror modifier either with the 'X mirror' option in the tool box. If you have an object that is already symmetrical along the X axis and you want to modify it while keeping the symmetry then do not use the mirror modifier, otherwise you will have to carefully delete half the model first. Instead, while in Edit mode for the object, turn on the 'X mirror' mesh option in the tool box:.

To experiment with this, add the Monkey mesh (a.k.a. 'Suzanne') to an empty project, rotate her vertical using the 3D rotation manipulator with the control key held down so as to constrain the rotation to 5° increments (watch the rotation angle at the bottom left of the 3D window until the angle is 90°). Then switch to Edit mode, turn on the X mirror option, select a vertex, edge or face and try moving it.

Subdivision Surface (subsurf) Modifier

The smooth option in the tool shelf adjusts the colors on each face to hide the edges between faces - it makes the object look smoother but does not truly change the shape of the object to make it smoother.

To increase the number of faces and truly get a smoother shape we can use the subdivision surface modifier (also called subsurf). Not only does the subsurf modifier give you a smoother shape but it has another great feature too – it lets you remove the additional faces at a much later time if you want to, or add even more faces if you choose, provided yo have not 'applied' the modifier. This is much better than the usual ‘undo’ option (Ctrl Z) which can only be done immediately after you make a mistake. This ‘anytime modify or undo’ feature comes at a price – there are certain operations that you can’t do on these 'virtual' faces. If you are obliged to ‘APPLY’ the subsurf modifier at some point, the virtual faces will become standard faces just like any others. Once the subsurf modifier has been ‘applied’ the modifier disappears so you can no longer adjust the modifier such as to, for example, reduce or increase the number of faces. However you still have the UNDO option, Ctrl Z, to un-apply the modifier, or ‘bring the modifier back to life’, but you will have to undo every step that you did since 'applying' the modifier.

Because you lose so much when you apply a modifier, 'Blender guru' Andrew Price (see YouTube), who I greatly admire, suggests that before you apply the modifier you should make a duplicate (Shift+D) of the object which you will then immediately move (press M) to a so-called 'trash layer' (for example the very last layer) so that you can retrieve the object and its modifier later if necessary.

According to Blender Guru, the subsurf modifier can badly mess up parts of the model that are not made of quadrilateral faces (quads) so he recommends that you should try to use quads exclusively when making models, since the subsurf model is very commonly used.

When you open the Modifiers panel and add a Subsurf modifier you will notice that you have the possibility of setting the number of levels of subsurf divisions, both for viewing and for rendering:

The higher the level, the more surfaces there will be and thus the smoother the model. Why not set the levels very high then? Because the more surfaces there are the slower your computer will become for it will have to do many more calculations. Also, if you 'apply' the modifier after requesting many subdivisions you will then have to work with a highly complex mesh with many, many faces. 'Viewing' refers to your work within Blender and Rendering refers to the final rendering operation - you normally would set the Viewing subsurf level to a number lower than the rendering level for you want your work in Blender to be quick whereas when making the final render the emphasis is on quality rather than speed. Start with viewing subsurf level 2 and render subsurf level 3.

You’ve just learnt that the subsurf modifier increases the number of surfaces, at least temporarily – you might be wondering what is the difference between subsurf and the subdivide option which you have probably already used and which is available in the tool shelf when in edit mode. The answer is that subdividing gives you more faces but WITHOUT smoothing the shape (if you subdivide a cube, for example, it will still be a perfect cube) and also subdivide does not give you the option of removing the new faces at a much later time, while subsurf does (provided you have not APPLIED the subsurf modifier).