The yin to aperture’s yang, focusing is the only other stylistic thing I must do consciously now that not everything is always in focus.01
Right now I think of focus as being purely a function of distance from camera to a plane in front of it. I am guessing that the true mechanics are more complex, involving (a) true distance from camera, (b) optical effects that change around the borders, or others.
Case study: Column & Shrubs
I saw some shrubs peeking out from a labyrinth of rock, concrete, and tiles in Osaka. I wanted to take a photo that would emphasize the contrast between them, but I wasn’t sure what to focus on. My main candidates were the shrubs and the column. I took one of each.02
Column focus [f1.7]
Shrub focus [f1.7]
I anticipated liking the column-focused one more, but now that I’ve looked at both, I like the shrub-focused photo better.
Another interesting factor that I hadn’t anticipated is the photo’s crop and display size interacts with the perception of focus.
Case study: Shrub Focus Crop
Displayed at the full size of my laptop monitor,03 I like the shrub photo. Here it is displayed at a large size (depending on your screen).
Large size, shrub focus clearer [f1.7]
However, shrunken down, I like it less. To my eye right now, the focused area seems blurry.
Small size, shrubs seem blurry [f1.7]
I wonder whether, at a smaller resolution, there simply aren’t enough pixels to display the in-focus section crisply.
I explored this exhaustively in Crop: Cropping Limits. Here’s the at-width (2x pixel density) crop for comparison:
At "166mm" crop, this should display pixel-for-pixel on a 2x density screen. [f1.7]
I think this looks crisper. This means crop, focus, and display size do interact. This is something I’ve never had to consider.
One thing I noticed when I was cropping was that the shrubs aren’t in the exact center of the photo. I’m assuming I either manually focused on them, or targeted them with autofocus. But I might not have, in which case one of the walls behind them might be the focus point of the photo. It’s funny I can’t easily tell, and brings up the next section.
Where Did I Focus?
One surprisingly challenging thing to determine after the fact: where is the photo focused? Especially for shallow-DOF photos that are capturing a lot of distance (down a street; city skyline), it’s often challenging to see what range is in-focus and what isn’t. I don’t think this information is displayed in the detailed overlay in Apple Photos or Photomator. I guess because photos lack depth information it’d be nontrivial to render on the photo anyway. I wonder whether the focus setting has a digital representation, and whether it’s saved and ever surfaced. Maybe it wouldn’t be useful because it couldn’t easily be displayed.
Another line of thought: the camera is doing some edge-detection algorithm during manual focus to help show what is in focus (with red overlays). Could this algorithm be run after-the-fact on your photo to try to show what part is focused?
Footnotes
I also now have to constantly twist my exposure knob to make sure bright areas aren’t over-exposed. ↩︎
Both are cropped to nearly the same size. The crops aren’t exactly the same because I haven’t figured out how to easily do that yet. ↩︎
My laptop screen isn’t huge. Apple says it’s 2560 × 1600. For websites, it reports itself as 1680 x 1050 with a pixel ratio of 2, which would imply 3360 x 2100. So IDK really what’s going on. ↩︎