Preset Focus via Distance settings

[anerjee]

Hi All,

I am reading "Understanding Exposure" by Bryan Peterson. I came across this sentence that I cannot understand -
"...you do not focus the lens at all; but rather preset the focus via the distance setting".

Bryan is talking about focusing for landscape shots using small apertures.

I own a sony a200, and am wondering if this is something I should know about (or just is possible in my camera).

Thanks

[HockeyFan]

Frequently with landscapes, this distance to the "subject" is further than any of the focusing except for the infinity setting. So he's suggesting to make sure you get the focus right, to set to manual focus, set the lense to infinity, and then take your shot. At least this way, with the small aperture, the auto focusing mechanism wont get confused and try to focus on something closer, which isn't what you want.

[wulf]

When was Understanding Exposure originally written? I read it a while ago and suspect it was back in the day when autofocus was not something readily available on low-end cameras but lenses came with helpful markings and usable focus rings.

If the kit lens on the A200 is anything like the Nikon D40, it will have no focus markings. I've found manual focusing much easier with older, second-hand lenses that won't autofocus on my camera and, because of the way they are designed, rarely miss that facility.

Wulf

[corry]

oh no, its first publication was 2004 so its not old. This makes sense though.....i've had the odd time with my zoom shooting landscape where it doesn't want to focus autmatically, now i realize i should have put it on manual....thanks.

[wulf]

Only 2004? That surprises me! Mind you, the market develops so quickly. I could probably get a new D40 now for not much more than I paid for my first digital camera (a Fujifilm Finepix 1400 Zoom, back in 2001)!

Wulf

[ni2sml]

The book's probably referring to setting the hyperfocal distance here. One of the Ansel Adams books covers this too. This is something you can do with any camera that allows manual focusing, though how well it can be done is another thing entirely!

The basic idea is to get landscape shots with as much in focus as possible, you don't focus to infinity, but to some point nearer the camera. How near depends on the aperture, focal length and size of the sensor/film.

The acceptable focus range extends in front and behind the focal point, the key here is the extension behind is considerably further than in front, so the actual focal distance could be pretty close, yet everything out to infinity is still in focus.

Back "in the day", the manual focus lenses had markings which indicated where the hyperfocal distance was for each aperture setting, but with most autofocus lenses now you're lucky if you have any distance scale at all, and if you do it's almost certainly not going to have many useful markings on it.

The good news is you *can* calculate the focal point. I have a piece of free software called CDoF which runs on a Palm Pilot to do this. There are simlar tools for other platforms, and online too. Search for "depth of focus calculator".

As an example, if I have my small-sensor P&S set to 50mm equivalent and f/5.6 I would set the focal distance to about 10.5 feet. This gets everything "acceptably" focused from about 5 feet onward (though in my case the teeny-weeny little sensor can't really resolve fine detail well enough for this task anyway. )

That still doesn't solve the problem with lousy distance markings though. The only way to fix that would be to use an old manual focus lens which has a decent scale on it.

[anerjee]

Thanks for all the replies.

Reading ahead, I realized that Bryan Peterson himself gives a rule of thumb for situations where markings do not exist.

Paraphrasing - "With your lens set to f/8 or f/11 and at the widest focal length, focus on something in the scene that is 5 ft from the camera. Then adjust your shutter speed till a correct exposure is indicated and simply shoot!"

Would anyone like to add any caveats or more details? Thanks a lot!

[lputman]

Originally Posted by corry

oh no, its first publication was 2004 so its not old.

The first edition, which is based strictly on film was done in the 90's if I'm not mistaken. The update, to include digital, was done in 2004.

[wulf]

That sounds more like I expected. Librarything says 1991 on a quick check.

Wulf

[HockeyFan]

I wonder if I can find something like CDOF that'll run on my WindowsMobile-ized phone.