Autofocus for macro?

[rene3580]

Hello,

I do a lot of macro (small insects, flowers...),mostly in the range 1:2...1:1...2:1.
I have a nice set of manual prime macro lenses (55 - 90 -100 mm. ...) + macrotubes from my analog period.
They give very good results on my D700.
Getting older manual focussing becomes more and more difficult...

Can autofocus be a solution for me?

Regards,

Rene

[loosecanon]

The small depth of field in macro means too often autofocus will not focus on what you want. OK with some modern cameras you can select the focus point more easily, but with non static subjects this is difficult. Live view with magnified section for manual focus can help, as sadly often the screens of modern slrs are no the best for focusing.
A real problem with auto focus can be the lens hunting for a sharp focus so you miss the critical point as the lens if off looking to see if sharp focus is near infinity rather than 2cm away.

[Geoff F]

I occasionally use auto focus for fast moving subjects who move quicker than I can manually focus; but there are a lot of rejects.

Using one of the 'servo' focusing options can sometimes help but even this isn't totally reliable due to the shallow focus depth, as previously mentioned.

Obviously a tripod will help.

However, just using the centre focusing point and allowing a bit of space around the scene for compositional cropping can be worth trying if manual focusing isn't possible.

[Inkanyezi]

AF does not work well for macro shots. Lens manufacturers have alleviated the problem by making macro lenses inner focusing, but still, there is a lot of uncertainty at very close distances. The reason is that any focusing movement will change the whole slate, by also changing the working distance to the object, so it becomes difficult to find proper focus, and not only difficult, but sometimes impossible, as the inner workings of DSLR cameras don't actually focus the lens in the way we do with the eye, but makes a wild guess of where to stop, and then assumes focus is there.

AF with contrast detection is a tiny bit better in this regard, as it does iterate over the point where focus is again lost after seeing highest contrast, but still it suffers from the uncertainty of the distance that changes with the focusing movements.

So for macro shots, manual focus is king.

But not only that, the easiest way to focus is to first set focus of the lens, then move the whole camera back and forth to find best focus.

DSLR cameras have poor viewfinder aids to accomplish manual focusing, but at the higher end cameras, the focusing screen in the viewfinder can be changed to a better one. It is however not necessary if you have live view and can enlarge a portion of the viewfinder area for precise focusing. Also AF confirmation if available in some way with live view may work well, or even in the viewfinder, as you'll get AF confirmation when you move the camera back and forth.

Good bellows units used to have a second set of rails to move the whole outfit when fixed to a tripod. There are also such aids as micro-adjustment focusing tracks that facilitate fine tuning of focus by moving the camera nearer or away from the target.

When using bellows, I noticed that most of them are assembled backwards, putting the camera and lens at wrong ends. If the mounts for camera and lens can be swapped, it is much easier to focus if the camera is at the moving part of the bellows unit and the lens at the fixed end.

[Glenn NK]

I often shoot multiple images in macro and focus stack them - AF is completely useless in this situation.

I suppose if one has a relatively long focal length (100 mm for APS-C format), uses very small apertures, and is quite a ways from the subject, AF could be used. But considering these points;

1) longer focal lengths reduce depth of field (DOF), and reduce focusing errors using the focus/recompose technique. I think that with AF, this technique is pretty well required in order to focus on something with the centre point that is about mid-dof from the camera.

2) moving back from the subject will help increase DOF, but may result in having to crop the image (loss of resolution),

3) smaller apertures (higher f/numbers) increase DOF, but require longer shutter times, with the potential to induce motion blur. And really small apertures introduce loss of resolution due to diffraction.

So, it comes down to what one is willing to sacrifice, and how critical one's requirements are.

In the long run, every macro is based on a set of compromises anyway, it's just with AF, there are more compromises.

Glenn

[rene3580]

Thanks to all of you for this clear and valuable help!
I'll stay with my current set of lenses.

As an info: for inside "static" macro (mostly flowers...) (Nikon D700 on tripod + macrorail + bellows) I use
a thetering program allowing me to have real live view on my laptopscreen. The only problem with small objects is
the resolution of the live view in the camera.

Regards,

Rene

[loosecanon]

Be careful with bellows on a digital camera. Sadly they can pump dust into a camera body like nothing else. I adapted my old FD bellows by installing the glass from a filter so as to seal off the camera from the rest of the bellows.