Sunday, December 30, 2018

Some thoughts on focus bracketing with the G7X

Having had a few days to 'play around' with my 'new' G7X, I thought I would start talking about the power that's built into this little marvel, especially when running CHDK.

The basic G7X has limited exposure and focus bracketing and an inbuilt ND filter capability. So, on its own, it's pretty powerful. But add on CHDK and we can take it to to another level.

Canon bracketing is restricted to three brackets: one at the set exposure or focus and one either side of these points. Although you can set the Ev offset for exposure bracketing, with focus bracketing there is no user information to indicate what the focus deltas mean.

Using CHDK, in 'enhanced photo operations', we have access to a great feature called 'Bracketing in Continuous Mode'; which results in CHDK adjusting exposure or focus between the Canon controlled multiple shooting that is accessible via the custom timer. 

Thus we can control up to 10 brackets with CHDK and whether the bracketing sequence is +,++,++... or -,--, ---... or i,+,--,++....

As we know, focus bracketing for landscapes is not the same as that for macro shooting. In macro shooting the depth of field ether side of the point of focus is essential the same and small. In landscape focus bracketing this is not the case. For example, if we take an image at the hyperfocal, H, then if we wish to extend the focus quality either side, we would take a bracket at H/3 and, say, at 2H. See previous posts that illustrate the non-symmeteric nature of landscape focus bracketing, eg:

The above chart hints at a way of focus bracketing without having to write a script. The objective being to achieve the 'best' focus from near to infinity. 

As we know, focusing at the hyperfocal is far from optimal, as, by definition, the hyperfocal is where we just about achieve acceptable focus quality at infinity. Namely, by defining a blur criterion, also called the circle of confusion, we can work out the focus distance that gives this blur at infinity, ie this is our hyperfocal.

The CHDK inbult depth of field and hyperfocal calculations appear to be based on an acceptable CoC that is far from optimal, ie about 30 microns x the G7X crop of 2.7: or about 11 microns. 

We also know that focusing beyond H, although it will give us better focus quality at infinity, will result in loss of near depth of field. In the limit, if we focus at infinity, our near depth of field will be at H; and the blur at H will be the CoC, whereas the blur at infinity will be zero, as it always is at the point of focus.

So if 11 microns is the 'just good enough' value for the G7X, what is the best we can do? Well the G7X pixel pitch is about 2.4microns and for a line pair we need two lines, so the sensible, smallest CoC would be about 5 microns, ie 2 x sensor pitch.

All this ignores diffraction blur, which on the G7X means we shouldn't really go beyond F/5.6. So focus backeting around F/4 to F/5.6 is a sweet spot.

Bringing all this together means that, after experimentation, I ended up with the following approach to achieving landscape focus bracketing on the G7X, without resorting to scripting.

Using the chart above we can see that a four bracket set up, around, H, will gives a quick and simple way of enhancing focus quality. The usual caveat being that our subject shouldn't be moving, well at least not wildly.

I selected F/4 as my working aperture, ie for diffraction reasons, and, using the inbuilt CHDK depth of field on-screen feedback, at the widest focal length, the hyperfocal is indicated to be 1.87m. Giving a near DoF of about 0.9, ie H/2.

But, as we know, the blur at the near and far limits will be about 11 microns, ie only just OK. We also know we can easily half that blur at infinity,bringing it down to about 5 microns by simply focusing at 2H.

For the near field we need to do better than H/2, which is what we get if we focus at H alone. The next bracket down from focusing at H is H/3, which extends the near DoF to H/4.

So now we have the two extremes of our bracket set: one at H/3 and the other at 2H. The difference between these two is 5H/3, which we can divide by 3 to give an estimate of the inter bracket distance, ie 5H/9, just under 2/3 of H. Resulting in the following four focus bracket schema:

So what's the bottom line?

For landscape focus bracketing I recommend focusing on the nearest object of interest, assuming it is less than H, then set the camera to manual focus mode and look at the CHDK DoF info to get the value for H (make sure you do a half shutter press to ensue CHDK refreshes things), take about 2/3 of H and use this as the subject distance in the CHDK focus bracketing variable.

Then in the Canon side set up for 4 continuous images in the custom self timer.

Here are the four test images I just took in our back garden:

And here is the focus stacked image after a round trip from LR to Helicon Focus, plus a little LR tweaking:
So there you have it, landscape focus bracketing informed by CHDK and taken with the help of CHDK, but without any scripting.

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