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[OM] Re: E-3 focus problems?

Subject: [OM] Re: E-3 focus problems?
From: Moose <olymoose@xxxxxxxxx>
Date: Mon, 31 Dec 2007 14:53:30 -0800
Dan Mitchell wrote:
> One thing that I'd really like would be a numeric display of DOF -- 
> the camera knows what lens I'm using, what the focal length is, what the 
> aperture will be when it stops down to take the photo, so it can do the 
> math to tell me how much DOF I have.
>   
I have that on my modest little A710 IS digicam. However, it comes not 
courtesy of Canyon, but of the unofficial add-on software. so it would 
obviously be easy for the camera makers to add.
> Sure, I can hit the 'stop down' button, but I very rarely find that to 
> be much use -- even on OMs with big bright viewfinders, I can't tell 
> which bits of the image will be in focus in the final thing, or at least 
> not well enough to be useful.
>   
Yeah, it is usually too dark an image stopped down to be clear about 
focus. When there is time, as with flowers, landscapes, buildings, etc., 
I tend to focus on the nearest and farthest points I want in focus and 
note the DOF markings on the lens. An aperture where the two overlap, or 
at least touch, covers that focal depth. Then set focus to the middle.

Of course that requires a DOF scale on the lens.
> (now, this is one place where a lot of AF spots and some whizbang computing 
> could possibly be handy -- while the camera is racking focus back and forth 
> to get things "in focus", it could track the in-focus distance to each focus 
> point.
>
> That then gives it a 3-d model of the world, because it knows how far away 
> the world is at each of the AF points -- and it could then interpolate, and 
> _highlight_ the bit of the image that would be in 
> focus.. This only works as long as the world is 'smooth' between AF spots, of 
> course, but I think otherwise it should theoretically be possible.)
>   
I think it would require a different focusing strategy that would slow 
down focus. As I understand the current strategy, the camera runs the 
lens focus motor until it runs slightly past focus, then backtracks to 
the focus point. I don't think they rack back and forth unless a clear 
focus is missed. Otherwise, I think at least some systems simply go 
directly to the now known focus distance setting. So it could only 
gather focal distance info for one side of the DOF.

The other thing to remember about DOF is that it is all relative. All 
those DOF tables that came with our lenses are predicated on someone 
with 20/20 vision looking at an 8x print from 10 inches. Print bigger or 
smaller or, like me, have different visual acuity, and it all changes. A 
lot of people who crop or print bigger routinely went at least one stop 
further from the DOF tables or markings.

With digital, if I am particularly concerned about DOF, I focus on the 
nearest and farthest points I want in focus, move the focus manually to 
halfway between them and take a series of shots at different apertures. 
Particularly with the smaller 4/3 sensor, diffraction softening of 
sharpness may kick in at apertures we are used to safely using from 35 
mm. Figuring out ahead of time where the balance will be would be beyond 
me. I have even run into that effect on 35mm film with the 135/4.5 
macro, which will stop down to f45.

That's another thing a smart camera could figure out for us someday. In 
the meantime, aperture bracketing for free isn't a bad way to go. It's 
worked for me.

Moose

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