For optimal sharpness: Use the infinity (mountains) symbol for anything 6 metres (20 feet) away or more. The 3 metre (group of people) setting is for anything between that and 2 metres (6.5 feet). Use the 1.5 metre setting below that right down to 1.2 metres (about 4 feet).
A fan wrote:
I don't know if you have thought of this but I was once asked what the point of distance for changing the focus ring over from 3m, (group of people) to infinity (mountains) was on the Trip.
An interesting question, and note that this isn't about depth of field at those focusing distances. It's about at which subject distances the 3 meter (group of people) setting is sharper than the infinity setting and those at which it is not.
The answer to that is relatively simple: the distance at which you will want to switch to infinity, is any subject distance past that at which the front defocus blur circle for a 40mm lens focused at infinity has the same diameter as the rear defocus circle for the lens focused at 3m. Practically speaking, the easiest way to do that is to plot two lines on a graph and see where those lines intersect. (I'm using the unambiguous term "blur circle", rather than the popular "circle of confusion"; the latter has a secondary meaning of "the largest blur spot that is indistinguishable from a point".)
I wrote this Python script to generate a table of blur circles at various focus distances (in 10mm increments) and various subject distances which was then fed into this gnuplot script. Here's what I got:
If the graph above looks screwy, that's probably because you're using Internet Explorer 6. Get a browser that doesn't suck, sorry.
The X axis here is the distance of the thing that you want in focus, tne Y axis shows how far in (or out) that object would be in focus. Further up = more defocused. Simple. This is plotted with an aperture f/2.8, but the optimal focusing distance (rather than that at which you'll get acceptable sharpness) is entirely aperture-independent. All the values for the blur circles more-or-less agreed with what VWDOF had to say, which means I'm right, hooray.
Let's look a bit closer at the infinity and 3 metre lines.
So it seems that the lines intersect at 5.96 metres, which is where you'll want to stick it over to infinity. Call it 6, because that's easier to remember.
We can do this with the Trip's other zones. Here's 1.5 metres vs 3 metres:
There we go: the cut-off is at a subject distance of 1996mm, or 2 metres to normal people. In other words: beyond 2 metres, use the 3 metre setting. Here's 1 metre vs 1.5 metres, because I was in a silly mood:
That's 1198mm, or 1.2 metres, or about four feet. Of course, good luck trying to measure it that accurately!
Back to my Trip 35 review.