Where will it be sharp?

[Marcus Carlsson]

Hi,

I wonder if I focus on an object three meters away will the DOF be the same for the same distance?

However it shouldn't be since I can only move three meters towards me and beyond the lens I can go on forever.

The reason for me asking is that if I have a couple of objects that I want to have in focus and I don't have time to do the calculations on what should be in focus, where should I focus. In the "middle" or?

/ Marcus

[ic-racer]

Hi,

I wonder if I focus on an object three meters away will the DOF be the same for the same distance?

However it shouldn't be since I can only move three meters towards me and beyond the lens I can go on forever.

The reason for me asking is that if I have a couple of objects that I want to have in focus and I don't have time to do the calculations on what should be in focus, where should I focus. In the "middle" or?

/ Marcus

You can focus in the middle.

This is cut and pasted right from the "Articles" section of this forum, good stuff! You may have missed this practical paragraph among all the technical justification.

"There is a very simple and practical way to find which f-stop you need to use, taking both point 1 and 2 into account. It is due to Paul Hansma. Make your movements first. Then focus on far, focus on near, read the distance "D" in millimiters between the two positions on your rail, refocus so as to split the distance on the rail, and use the following table that I recommend you carry with you all the time. "F" is given in decimal f-stops, as on a hand-held meter, for example 16.6 is 16 and 0.6 (aka between 1/2 and 1/3) of a f-stop. See here for details.
D(mm) F
1 16.6
2 22.6
3 32.2
4 32.6
5 32.9
6 45.2
7 45.4
8 45.6
9 45.8
10 64 (end quote)

[Ron Marshall]

Here is a link to DOF articles and a calulator:

http://bobatkins.com/photography/tec...calc.html#calc

[Marcus Carlsson]

When I have looked at the numbers provided by the DOF calculator it seems that the DOF is a bit longer in the far distance than near distance, so in my example shouldn't I focus a bit closer than the middle?

/ Marcus

[Alan Davenport]

This is large format: look at the groundglass. If it's sharp there, it will be sharp on the film.

[Struan Gray]

I focus a bit closer than the middle?

You focus so that the standard is at the midpoint between the near and far focus positions on the rail. That automatically puts the best focus out in front of the camrea at the correct position between the near and far focus points. The best thing is, the same trick works for distant imaging and macro and near-macro.

[Marcus Carlsson]

What I ment with my question is this:

Say that you are in a hurry (you will photograph a group of kids playing and you don't want to interfere with them.
I will not have time to measure my movement of standards and even though I have a Sinar F2 I will not have time to see what my near and far focus will be. Naturally I understand that in real life I should have time for this or change camera, but just imagine a bit.

Let say I stand three meters from my kids and they are placed like 2.5 - 3.5 meter away total. Where should I put the focus to be "sure" that I get the best focus. 3.0 or 2.8 or something else? Naturally I know everything about changing aperture, but let's just play a bit with the idea.

Sometimes one should be able to do some real fast point and shoot with a LF too and I don't want to use hyperfocal because I want a shallow DOF at the same time.

/ Marcus

[Joanna Carter]

For a more technical discussion of DoF :

http://www.trenholm.org/hmmerk/HMbook14.html



Joanna

[Brian Ellis]

I didn't understand the queston as originally phrased. As amplified by your second post, and the example of the children playing, assuming they all stay within the 2.5 - 3.5 meter area but it isn't feasible to determine near-far distances or focus on any particular child, you probably would do best by pre-focusing at a point about 2.8 meters away (roughly a third of the way into the scene) and stopping down as much as possible consistent with your desired shutter speed. The idea of focusing a third of the way into the scene to maximize depth of field isn't a "rule," among other problems it varies with the focal length of the lens you're using and the distance from lens to subject, but with a 4x5 camera and children playing it's about as good as you can get. The suggestion to judge depth of field by looking at the ground glass is certainly a good one with static subjects in bright light and with apertures wider than about f22 but often not very practical otherwise.

[Marcus Carlsson]

Thanks Brian (and rest)

/ Marcus