Tuesday, September 5, 2017

Revisiting some basics

In previous posts I have discussed the ‘normal’ hyperfocal distance (HFD = H) and why it is not (sic) the optimum focus strategy for landscape and architectural photographers trying to get high quality depth of field images, from near to far.

Assuming a ‘simple lens’, and ignoring diffraction, the HFD may be estimated from the following:

Where f is the focal length, N the aperture number and B the defocus blur at infinity, eg what some call the Circle of Confusion, eg for a full frame camera this is often quoted as 0.03mm (30 microns).

If we assume that f term is (very) small relative to the other term and that the blur at infinity can be less than the minimum acceptable blur, then the above simplifies to:

Where Q is a focus quality term, relative to the normal, minimally acceptable blur. Q will typically be between unity and, say, 2ish. For example, if Q is 2, then the blur at infinity would be 15 microns, rather than the ‘normal’ 30 microns.

Understanding this very simple equation is the starting point for maximising the focus quality in your (landscape) images.

Before moving on, it is worth remembering that (defocus) blurs less that two sensor pixels are ‘meaningless’. Thus if we assume, for illustration, a sensor pixel size of, say, 7 microns, then the sensible defocus blur range is between 14 microns (2 x 7) and 30 microns: which leads to Q being between 1 and 2ish.

To emphasis this point, for on-screen work you will ‘get away’ with blurs of 30 microns, or more. For high quality printing, at close scrutiny, eg by judges or in a gallery, you should use defocus blurs of, say, half of this, say 15 microns on a full frame.

Remember that diffraction will always add additional blurring to your image, and that diffraction blurring varies linearly with aperture, ie shooting at F/16 will double the diffraction blur compared to an image captured at F/8.

For high quality work, it is not advisable to push the aperture too much: unless you have to. Most would suggest a ‘sweet spot’ for a full frame DSLR of between F/8-F/11.

The key take away from the above is that, for full depth of field photography, you should focus beyond the (minimally acceptable) HFD, but not more than twice that HFD.

But what if you want to really maximise your depth of field, ie from infinity to near. Well we know the ‘secret’ here is to focus bracket, ie recover the defocus in the near field.

If you are a Canon shooter you can make use of the various Magic Lantern tools at your disposal, including my auto landscape bracketing script and my focus bar script. But what if you don’t have Magic Lantern. Is there a simple focus bracketing approach?

Without proof, the optimum (high quality) two-bracket focus stacking approach is to take the first image at Q times the (normal) HFD and the second one at Q times the HFD divided by 3.

Thus if Q is unity and HFD is the ‘just acceptable’ HFD, ie a defocus blur of the minimum quality, that is often used, then focus your two focus brackets at the (usual) HFD and at HFD/3.

If you wish to capture a higher (optimally focused) quality bracket set, then focus at 2xHFD and 2xHFD/3.

The resulting blur (defocus) field for a high quality (single) capture (Q = 2) looks like this – compared to the defocus field for a (normal) HFD shot, say a B of 0.03mm:

Here we see the real advantage of focusing at 2xHFD, ie the far field, beyond H never gets ‘worse’ than a blur of B/2, ie you have doubled the far field focus quality.

Once again, without proof, taking HFD (or H) as that given by the minimal acceptable blur (say 0.03mm), we can take a high quality focus bracket set at 2xHFD and 2xHFD/3, that results in the near field depth of field being better than if we had focused a single image at the (normal) HFD.

What has been gained is quality of focus in the near field, to complement the far field, as illustrated below, ie for a Q of 2, the near DoF at a blur of B is 2*H/5, ie Q.H/(3+Q).

Finally, I appreciate that not everyone likes reading posts with equations or numbers: in fact some get ‘really turned off’. However, the fact is, to get the most out of your cameras you do need to understand some of the (basic) ‘science and maths’ that is going on behind the scenes.