Question about Focus Physics

rapsucks

rapsucks

Well-known member
I'm not exactly new at this photography thing, but pulling focus on a moving image is new to me, and prompted me to realize the things that I don't fully understand about lens physics.

Lenses (nice ones, at least) have distance measurements on them to tell you approximately how far away from the lens the focal plane would be. Now, we often call this the focal plane, but is it a flat plane? Will everything that is perpendicular from the lens and intersecting with the hypothetical point that is directly in front of the lens at the focal distance be in focus? Or will everything that is the focal distance from the lens be in focus--causing the "focal plane" to really be more of an arc?

Upon rereading the preceding paragraph, I'm afraid that it makes no sense at all. Hang on, let me draw a picture.....

1167372612.jpg


I hope that helps explain things better. If anybody has any experience with this, it would be helpful to those of us who just realized that we don't know as much as we thought we did (me).
 
The aim of lens designers is to project a perfect picture of a plane (subject) to a plane (sensor, used to be film). Thus #1 (plane to plane) is defenetly the correct answer. Exept with cheap lenses and even them are not as bad as #2.

It is one of the standard lens tests to measure the flaness of the focal plane. The flatter the better.
 
So the idea is to achieve a flat plane, as per drawing #1. But in reality, what does an average lens of the type that you or I can afford achieve? I know the #2 drawing is exaggerated, but for practical purposes, I need to know if I need someone to pull focus when an actor is walking across frame perpendicular to the lens--even if they don't move laterally towards or away from the lens.

If there is a standard lens test, are lenses given quotients that measure how good they are at achieving planar focus?

These are just the random things I obsess over when I should be worried about lighting, continuity, characterization, editing, composition, delivery, theme, mood, coherence...

edit:
Hey, Karl, I just noticed that you were here with Unaccompanied Minors. How'd you like Salt Lake (he asks, already knowing the common response)?
 
I liked Salt Lake better than I expected to, really. The last time I was there, was pre-Olympics, and the city (at least, around downtown) has changed a lot, since then. We were staying within a few blocks of Gateway Mall, so movies and restaurants and etc were all on hand. We got a lot of runaround and hassle with some of our locations, but in the end we made it work. But gods, it's dead on Sundays.

You don't need to pull focus on a subject moving horizontally through your frame - unless you do something silly like I did once, and shoot with an Eclair 16NPR (wherein the bayonet mount will lock on a lens even if it's hanging cockeyed - so your focus shifts from the left of frame to the right. Not that you can see it, in the viewfinder.
 
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