Focal Point

[Sid Morvan]

I've been shooting for a few years now but recently started using a 4 x 5 Land Camera for landscapes. I was advised by a pro to focus at 1/4 the distance between my lense and the horizon. I was a bit shy to ask him to explain because I had presented myself as sort of a pro as well, which isn't true. Can anyone make sense of this?

[John Cook]

Generally speaking, as you stop down the plane of acceptably sharp focus increases more beyond the point of your focus more than in front of it.

Just exactly how much is a matter for the optical engineers among us.

Focusing about halfway between near and far is usually enough for me. Or perhaps 1/3 into the scene from the front is slightly more accurate. 1/4 sounds a bit short.

It is possible to figure it out to the fraction of an inch. But by that time the clouds will often have moved. ;0)

[Sid Morvan]

Thanks John. That brings up another question. If I focus at infinity will that then cause the forground to go soft? Even at, say f90?

[Ole Tjugen]

If you focus at infinity, the resolution of foreground details will be identical to the aperture opening - excepting tilts and swings, of course.

Sometimes it's better to have a sharp horizon and a slightly blurry foreground than having both equally soft.

For a more thorough discussion of this see Harold Merklinger's "The Ins and Outs of Focus".

[Ed Richards]

As the aperature gets smaller, the diffusion get worse as the depth of field gets deeper. At some point (F 512?) these converge and you get a pinhole camera where everything is in focus and nothing is sharp.

[Jorge Gasteazoro]

The advice the pro gave is what is called using the hyperfocal distance. This has two advantages, as it was stated above it allows to keep everything in focus and it gives you a tiny bit more coverage by increasing the image circle, this could be important if you have a lens that just covers your format.

It is good advice...

[jj_4045]

Ed: "As the aperature gets smaller, the diffusion get worse as the depth of field gets deeper."

I don't know too many LF photographers who worry about diffusion between wide open and F64.

[John Cook]

Sid, the matter of the foreground going soft is determined by the closeness of the foreground, the format of the film, the f/stop, etc.

There is a dandy little program you can download onto your computer to quickly figure these calculations:

http://tangentsoft.net/fcalc/

However, with a little practice, a bright groundglass and a good Schneider 4X loupe you can learn to see when something is in acceptable focus.

[robc]

if you have a 35mm or medium format camera with a prime lens, then on the lens barrel you should have a depth of field scale.

focus on any distance and then compare the distance between, for example, the f11 marks on the scale and the distance the lens is focussed on. You will find that the distance of the foreground f11 mark to the focus point is approx 1/4 the distance from the focus point to the distance against the far f11 mark. Works for any lens on any format.

to set the actual focus point on a large format camera you can simply focus on the nearest point in your subject that you want in focus and make a note of the position on the focus rail. Then focus on the far point in the subject you want in focus and note the position on the focus rail. Then set the focus to the exact midpoint between the two positions on the focus rail. Assuming your aperture is small enough then you will have good sharpness between foreground and background.

[Paul Fitzgerald]

Hi Sid,

If you do as 'rob' suggested, you can stop the lens down until the forground is sharp and the background will be sharp, depth of field extends further back than forward. If your camera doesn't have a scale, a piece of masking tape and a sharp pencil works well. This does assume that your ground glass and film holders match well.

Have fun with it, it works every time.

[Ed Richards]

> I don't know too many LF photographers who worry about diffusion between wide open and F64.

Then they should spend a lot less time picking nits about the performace of one lens versus another. This is esp. true for wider lenses. I am sure there is some interesting formula like:

every 1/2 stop past f22 is like stepping back 25 years in lens design.:-)

[Leonard Evens]

I am rather puzzled by the suggestion that one focus one fourth of the distance between the lens and the horizon. In the normal sense, the horizon is at infinity, and one fourth of infinity is infinity. In addition, I can't see any way to to interpret the statement in terms of movements along the rail. Perhaps someone can explain to me how to do it.

The only way I've been able to interpret the usual one third rule is that the rear depth of field (in back of the plane of exact focus) should be twice the front depth of field. Then the exact plane of focus will be one third of the way into the depth of field. This of course also doesn't make sense when the rear depth of field is infinite. And it holds just when the exact plane of focus is at one distance, one third the hyperfocal distance. So it might be a reasonable rule of thumb for relatively nearby subjects, but would generally not work for landscapes.

The best rule for focusing is discussed elsewhere at this website. Find the positions on the rail (or bed) for the nearest and furthest points you want in focus and then put the standard half way in-between. That will make the circles of confusion equal at the extreme points. (Another way to use this rule is to focus so that the near and far points appear on the gg to be equally out of focus.) That has the advantage that you don't have to note marks on the rail. in some cases, one may want to favor the foreground or background for appropriate reasons by moving towards one extreme point or the other.