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/technical/dofcalc.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

[ic-racer]

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.

Another way to think of it is that there is a non-linear relationship between the displacement along the rail and the focal distance. So, you are right to think that you need to focus a little more closer than the middle of the objects, and, as pointed out above, just splitting the distance on the focusing rail automatically does this for you because of the rail's non-linear scale.

[steve simmons]

What focal length lens will you be using. This makes a big difference for DOF?

Your question is so badly asked that you don't know enough to do this job in lf.

Take more time to become familiar with lf before trying this.

sorry to be so blunt.

steve immons

[Struan Gray]

Marcus: the problem is that the position of exact focus moves with respect to the near and far limits of acceptability as you change the subject distance. For close ups it's about half way between the two, for the middle distance it's about a third of the way beyond the near limit, and for the horizon the far limit can be beyond infinity so you can't define a ratio at all.

Of course, with a view camera you can put the near limit, the far limit and the plane of best focus all beyond infinity. At which point you are making art, literally in the case of Sugimoto.

For the sorts of photos you have on your site the one-third ratio works well enough. If you get closer for headshots you can easily get close enough to the macro regime with LF that the split becomes 50:50.

[Marcus Carlsson]

Steve: It's ok. I won't hate you for saying that :)

I use an 180 mm lens and I usually only take photos of one persons (and I don't use advanced focus with shift tilt and such), but I was more interested in the theory than real life and I may agree with you that my question was a bit bad in the first place, but if I could ask you it in Swedish it would have been better but you would probably not understand it then too :)

And yes, I will try to learn more about lf before doing advanced stuff like that, but on the other hand one has to ask to learn (at least the learning curve will be steeper).

/ Marcus

[Ron Marshall]

Marcus, in his above post Struan has explained it well. For the close distances you will use focus one third of the distance from the near point you want to be in focus to the far point.

[ic-racer]

For the sorts of photos you have on your site the one-third ratio works well enough.

Are we all describing the same thing? I am describing where to put the standard that is being focused, which (except for infinity and macro) should be HALF the distance the standard travels. (or 2/3s the standard travels for infinity as the far point).

[Struan Gray]

Are we all describing the same thing?

I don't think so :-)

Marcus is concerned with where to place the plane of best focus in object space. In image space (where the standards reside) the problem is simpler: as you said, you just split the distance and stop down using Hansma's table.

Object space rules make sense if you're photographing quickly, or with a rangefinder or box camera. I use them myself if I have no time to fine-tune the focus. But the Hansma technique is so simple and quick that I have to be chasing some really fleeting light before I just wing it on ground glass and experience. It helps that I use a monorail with a built in millimeter scale.

[Leonard Evens]

I think Struan and Brian have given you good advice.

But if you want to know the general rule, here it is.

The correct place to focus in the subject is the so-called harmonic mean of the near and far distances you want in focus. The harmonic mean is found by take the sum of the reciprocals of the two distances, dividing by two, and then taking the reciprocal of the result. For example, if the near and far distances are 2.5 and 3.5 meters, the harmonic mean is found by first forming 1/2.5 + 1/2.35 which is about 0.68571, dividing that by 2 to get 0.34285, and then taking the riciprocal of that, which is about 2.92 meters. Notice this is a little closer to 2.5 meters than to 3.5 meters. That is a little further into the scene than one third of the way, but using one third wouldn't put you too far off, and if you are judging it all visually without measuring, which is likely to be the case, it is certainly as close as you are going to get.

Unfortunately, once you have specified the near and far distances, the proper f-stop to use will be determined (subject only to the focal length of the lens and your criterion for sharpness). There is a minimal f-stop that will work, and if you also take diffraction into account, there is an optimal one, given approximately by Hansma's table. That can be determined from the near and far distances, but the calculations would be too involved to use easily in the field. It is simpler if you do it from the focus spread on the rail (or using the Sinar's focusing knob), but as you point out, that may not be practical in your circumstances. In the field, you can focus at the desired position, then stop down to see when objects at the near and far distances come into focus. If you don't have to go below f/16, and there is enough light, this may work pretty well. Unfortunately, if you have to stop down further, you may not be able to see clearly enough to decide. Alternately, you can use a DOF calculator at home, put in your focal length, an assumed circle of confusion, and the distance you focus at (chosen as above) and see if any aperture available to you will work for that combination. Once you've done that for a variety of near and far distances that you commonly use, you can put the information in a crib sheet to take with you when you shoot.

I