In-Camera Focus Bracketing Demystified: Part 3, telephoto bracketing

In part one of these posts, directed at demystifying in-camera focus bracketing, I introduced a hyperfocal based model that allows us to reinterpret the camera manufacturer’s focus bracketing 'quality' variable, from 1 to 10, in terms of the overlap circle of confusion (CoC) or optical blur.

In part two I extended the model to estimate the focus bracketing in the macro region, where one needs to account for optical and pupil magnifaction.

In this third part of the focus bracketing story, I'll have a look at telephoto lenses and show why one needs to be wary about focus bracketing at long focal lengths.

As a reminder, ignoring optical and pupil magnification, the basic equation to estimate the number of images to take, to non-macro focus bracket from a near point (x) to infinity, is given by:

Where C is the overlap blur criterion that you wish to use and pragmatically x is measured from the entrance pupil, ie the non parallax point.

An alternative way of looking at the above is to note that the first term is simply H/x, ie the hyperfocal distance divided by the near point of focus distance. Thus the number of brackets you need can be estimated from (H/x + 3)/2 or rounding up as H/(2x) + 2. Putting x = H/k, ie a fraction of the hyperfocal, we get the linear relationship (k + 3)/2, ie the number of lens rotations you need to make between the nearest point of focus and infinity.

Using the above equation, let's look at a 150mm focal length lens (in fact my EFM 18-150mm) at an aperture of f/8 and a CoC of 19 microns, the maximum, ie worst, CoC one should consider for a Canon APS-C sensor.

In the above we see the number of brackets to be captured as we vary the near point of focus between a near focus distance of 0.45m and 5m. Clearly, once the near point of focus becomes much less than, say, 4m, the number of brackets increases rather sharply. In this case, at a near point of focus of 0.45m, the number of required brackets is over 160.

Of course, one could close down the aperture to, say, f/16, but many would not find that an acceptable thing to do because of diffraction, especially on a crop sensor.

The alternative would be to reduce the overlap blur criterion, but as it is already at 19 microns, this, once again, would not likely be an option that many would take, ie introducing 'focus gaps’.

The following graphically shows the impact of going much beyond, say, 50mm and taking a deep focus bracket set. The chart shows two focal lengths: 50mm, the lower, curve, and 150mm, the upper curve. As before we are plotting the number of brackets against the near point of focus, from 0.45m to 5m. Remember, the top curve is just over 160 images at 0.45m:

And the same curves as a log plot:

Plus a linear plot where I’ve extended the near point of focus out to 50m, to further illustrate the sharp increase in the number of focus brackets as you approach the minimum focus distance:

Finally, putting the near point of focus in terms of a fraction of the hyperfocal distance, from 2 to 100 or 20, the number of brackets, ie lens rotations, looks like this:

Thus, we arrive at the following general conclusions:

• In-camera deep focus bracketing is ideally suited for wide angle lenses. In real world space, the focus position move varies at each focus step, but the lens rotation remains the same for each focus position;
• Although macro in-camera bracketing is obviously achievable, you will need to take a large number of brackets if you wish to capture a quality stack, eg low diffraction impact, no focus gaps and over a reasonable total depth of field, ie x to y. Macro focus bracketing is different to deep focus bracketing, as the near and far depth of fields are essentially equal each focus step;
• Deep focus bracketing telephoto lenses beyond, say, a full frame 50mm focal length, will potentially result in large bracket sets, according to the aperture, the overlap blur selected and the position of the near point of focus: so think about settings carefully. As a rule of thumb, if you wish to keep the size of the bracket set low, keep the near point of focus longer than, say, a tenth of the hyperfocal, which will result in no more than, say, seven brackets. Remembering you can calculate the hyperfocal in your head using the Rule of Ten, see the featured post link on the right. 

Finally, this link will allow you to explore your own non-macro lenses. The link will open the equation in Wolfram Alpha, where you can change the equations, ie it is set up to compare two use cases, and set the input variables, as you wish.

As usual I welcome any comments on this post or any of my posts.

­­