I’ve been reading and watching tutorials on focusing to try and understand how best to avoid the disappointing results I’ve all to often experienced. I’ve found these tutorials mostly offer too many approaches when it comes to maximizing depth of focus in landscape photography. Rules of thumb about focusing one third into the composition, using focus stacking etc. I’ve tried to distill all this down to a simple, science based, method. So I’m offering my thoughts on this in hopes of learning from readers' reactions and improving my knowledge.
A lot of articles and on line apps offer up hyperfocal length tables where knowing the lens focal length and the aperture the hyperfocal distance can be looked up. But this doesn’t get directly to what the photographer needs when setting up a shot. In landscapes, more often than not, the desired composition dictates the focal length setting (once the camera position is chosen). The foreground elements in the composition which are desired to be in focus determine the focus distance. By the rules of hyperfocal distance the point of focus should be twice the distance to the nearest object that is required to be in focus. [Because the rule for hyperfocal distance is that objects from half the hyperfocal distance to infinity are in focus.] With these two parameters established - focal length and hyperfocal distance - we can readily find the maximum aperture able to achieve this. Smaller apertures will work even better in depth of field terms but may not improve the end result due to other limitations of very small apertures.
So here’s some mathematics. The hyperfocal distance formula is:-
Hyperfocal Distance (in millimetres**) = Focal Length^2/(Sensor Factor x Aperture)
** because focal lengths are in millimetres
Sensor Factor here relates to CoC or circle of confusion in lens theory. Sufficient for current purposes the numbers are:-
Full Frame 35mm use .03,
for Nikon APS-C use .02 and
for Canon APS-C use .018
If we know focal length and the distance to the nearest object we want in focus we can rearrange the formula to compute the largest aperture we can use.
Maximum Aperture = Focal Length^2/(Sensor Factor x Hyperfocal Distance [in millimetres])
Nobody thinks distance in millimetres. We are used to metres or feet. So if we want to use metres the formula becomes:-
Maximum Aperture = Focal Length^2/(Sensor Factor x Hyperfocal Distance(metres) x 1,000)
And for feet it becomes:-
Maximum Aperture = Focal Length^2/(Sensor Factor x Hyperfocal Distance(feet) x 304.8)
I imagine there are apps out there which provide just this calculation. I haven’t found one but no doubt a reader can correct this omission on my part.
For spreadsheet savvy readers used to working in feet who shoot with a Canon APS-C camera here’s an option. There should be a graphic of a spreadsheet attached to this post. Referring to that-
In box B3 the formula is =2*B2
In box B4 the formula is =0.4248*B3
In box B5 the formula is =B1^2/(0.018*B4*1000)
In the example below F15 or smaller aperture is required.
I’ve done this in Google Sheets on my phone so I have this calculation on hand.
If the required aperture calculates out to be impractically small, for example if we have a long focal length to crop our image but want objects near and far to be in focus, some rethinking of the approach is needed. Focus stacking is likely the only option if the composition, as conceived, must be achieved. The furthest focus point would be the hyperfocal distance then a couple of closer focused shots.
And, of course, the calculated maximum aperture then dictates the shutter speed at which point getting to the desired shutter speed may require deployment of ND filtering for slower shutter speed situations or higher ISO setting for faster shutter speed demands.
I'd love to hear reader's thoughts on this approach.
