Focus by wire warning: not for manual focus bracketing

In previous posts I've shown how you can 'calibrate' your in-camera focus bracketing, such that you can set the focus step in terms of the overlap optical blur that you wish to use: Part 1.

But what if you don't have a camera with an in-built focus bracketing feature, for example my EOS M3. Can you still focus bracket?

The answer is clearly yes, but we need to be aware of a lens limitation: namely, the lens can't be a pure focus by wire type.

To understand why this is the case, we first need to appreciate how a 'normal' lens works. 

As we know, a modern lens is composed of many lens elements and some of these need to move relative to each other as we focus, say with a prime lens. Things get more complicated if we consider a zoom lens, ie more lens elements need to move relative to each other.

Since the early days of complex lenses, manufacturers have made use of a so-called helicoid mechanism:

In the above, taken from this lens rental post, we see the helicoid component, converts lens barrel rotation (focus or zoom) into axial lens movements. The key take away here is that there is a mechanical, which can be manually or electrically driven when using AF, relationship between lens rotation and axial lens element movement. 

In addition, such a design will exhibit hard stops at the minimum focus distance and at infinity, although on some lenses they may 'focus beyond infinity', ie going out of focus beyond the optical infinity, until the helicoid infinity hard stop is reached.

In a previous post I showed how one can add a depth of field scale to any lens.

An alternative way of looking at this is to use the simple expression (k+3)/2. Where k is the ratio of the short hyperfocal distance, ie (f*f)/(NC), divided by, say, the minimum focus distance, as measured from the front/entrance pupil, ie the no parallax point of the lens; as shown here (from Photons to Photos) for an EFM 11-22mm lens, at 11mm, ie in blue:

The (k+3)/2 expression then gives the number of brackets to take to cover from the (minimum) focus distance to infinity. Once we have k, we can then simply put some gaffers or masking tape around the lens, mark the minimum focus distance and infinity focus points, then divide that distance up, to create k tick marks, eg using Thales Theorem. The tick marks giving us the perfect locations for focus bracketing:

The downside of the above is that it is only good for one focus length, which is not a problem with a prime, or with a zoom, when you focus bracket at, say, the short end. It is also limited to a single aperture value and one circle of confusion value. Once again not a problem if you are shooting landscapes, eg, say, at f/10 and with a CoC of 15 microns on a full frame camera.

But what about if you have a focus by wire lens, such as the EFM 11-22mm (shown above).

Will our gaffers tape hack work?

The answer is no: as such lenses decouple the focus rotation from the lens movement. That is, the lens senses the focus ring being rotated and then electronically instructs the lens mechanism to move.

The issue is not that there are no hard stops, as you can work around this; the issue is that the lens will move focus in a variable way, according to the focus ring rotation speed. Although some focus by wire lenses allow you to switch the lens into, so-called, linear mode, where the lens behaves more like a helicoid lens, where rotation of the focus barrel is speed invariant in moving focus.

Although I don't like to say, never; in this case, from my experience, I can never guarantee to rotate the lens at the same speed each time, thus you may find that rotating the lens barrel does not move the lens elements in the controlled way it does with a helicoid mechanism lens.

So, the bottom line is: forget manual focus bracketing with a pure focus by wire lens, although a focus by wire lens that can be switched to so-called linear mode, to make focusing rotation speed invariant, could still use the masking tape hack.

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

 

 

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.

­­