mobile AF testing and fine tune

Started Jan 26, 2011 | Discussions thread
OP Kerusker Senior Member • Posts: 1,319
My generic model (GM)

As I said in my post above:

Since I don't want to bore a human model with test shootings, I made a simple generic model (GM) which is portable - sitting on a tripod. For my AF testings, the latest addition is a kind of DoF meter (DoFM).

Here it is:

a view from top:

and a view from back (for those who want to set up a sibling):

Some remarks:

  • This is not intended to be a high precision instrument (nor is it quick and dirty

  • The DoFM on top can easily be mounted or removed. It's light and is available as heating insulation.

  • The DoFM is 1m in length and the markings are 10cm apart; the double line in the middle is above the focus point in the middle of the model. It goes across at an angle of appr.25° (75°) and is tilted a bit downwards at the front such that the wide angle lenses can see the end of the DoFM.

  • The 'head of the model' (rectangular foam) is 3D with some extras for testing image renditions.

  • For AF testing I fit the 'head of the model' into the inner rectangular AF-Frame of the K-5 view finder screen (read below 'AF Testing').

These are the main reasons why I prefer my model with it's DoFM:

  • Small printouts of line pattern shot at a 45° angle at a short distance are very artificial and my be valid only at short distances.

  • AF can be fine tuned in the field, by reading the DoFM on the camera monitor (using magnification; K-5 monitor: up to 16x is sufficient)

  • Natural objects aren't suited so much for comparisons over a wide focal range and may be hard to measure.

  • I can use it nearly everywhere - inside, outside, for lighting conditions, resolution, color rendition, close, far, ....

AF Testing

For AF testing I'm changing the distance to the GM for each focal length such that the GM image size stays the same. Since this is much below the hyper focal distance, the DOF is approx. the same for the same aperture (see Wikipedia DoF formulas) i.e. with zoom lenses having different wide open apertures at both ends, the DoF is different only for this reason.

Consequently, the DoF is inside the length of the DOFM for all such lens tests! - And most importantly, the actual focus point/distance (resp. FF, BF) can be read/guessed very accurately.

In my next post I will show how I use my GM with it's DoFM for AF testing and AF fine adjustments.
we don't see that we don't see (eye's blind spot)

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