Pentax FA 100mm Macro Lens

Hi everyone

just purchased the above lens as my 1st macro lens after reading all the good reviews about it. being my 1st i'm not sure if it is focusing correctly. When i manaul focus it it seems to only focus dead centre regardless of what focus points are set, ie centre or 11 point. and if moved it refocus on the new point and blurs everything else. does this sound right or should it focus all over. Meant to say this is when i am very close to the subject. say 10-15cm away.

The closer you get with macro lenses the narrower the depth of field, are you shooting at F16 and above?

When you focus, either with AF or MF, you are looking through the lens at full aperture. Especially at close distances this means very little depth of field. This aids the focusing process.

When you take the picture, the lens stops down to the selected aperture. Thus you will then have more DOF. Use a small aperture, focus one third of the way into a scene and you will get the maximum possible DOF. For critical work close up, where there is very little DOF at best, focus very carefully on the critical point.

Other essentials are a solid tripod and using the 2 second delay, which also switches off SR. By using the delay any vibration from operating the shutter release will have dampened down by the time the picture is exposed.

John,
How do you focus one third of the way into a scene?

You can actually forget that because it's wrong.
The focus does not always extend 2/3th to the back and 1/3th to the front.
With 1:1 magnification so macro it's actually very close to 1/2 and 1/2 and when you focus near the horizon it can be 1/4th to the front and 3/4th to the back how the focus lays.
Sure there might be a point that focus is 1/3th 2/3th but it's mostly not the case.

here is a good read about "focus" and everything else around it, it can be a bit technical though...
http://toothwalker.org/optics/dof.html

Don't forget you can use optical preview (beyond "On" on the main switch) to judge depth of field for the selected lens aperture although with the lens well stopped down the image in the view-finder will probably be too dark to be useful.

Glyn

Focusing one third of the way into a scene is a "rule of thumb" that will in most cases secure the maximum DOF without recourse to tables or scales.

This is useful in practice.

You focus one third of the way in by deciding where your closest focusing point should be and where the furthest should be. One thirdof the way along the distance involved gives an approximation of the "hyperfocal point" the point which will give maximum DOF.

Of course in reality the point of focus is the only plane absolutely in focus, but there are areas in front of and behind that are acceptably sharp. This is the DOF.

As GlynM points out you can preview DOF, although I have always found the focusing screen to be too small to accurately judge what is and isn't sharp.

At macro scale (1:1) DOF is razor-thin, essentially zero. Plug numbers into an online DOF calculator; read-em and weep. (200mm lens; f/32; subject 100mm away; DOF is 0.1mm on either size of the focus point. Ouch.) Focusing rails can be used, and focus-stacking software, to make the non-flat viewable. That's the grim reality.

Focus stacking is an excellent way of achieving excellent results, the shot below is only me second or third attempt at it.

Beware the circles of confusion, eh John. First time I've thought of that term in a while.

We can define the circle of confusion and make it a more demanding standard for larger prints and other critical requirements.

It's like a lot of things - the more we know the more there is to know.