Determining DOF

Is there a set way to determine DOF?

IE: 3.5 gives a 1 foot of clearly focused subject.

The reason i ask, is because i want to be able to determine what aperture i'll need depending on the situation.

Lets say i have a model on a car, and i need her, and the car completely in focus, but outside of that, as little as possible.

How can i (without taking 10 pictures) determine what the DOF will need to be?

Hi funk, there are a lot of books and charts out there that will list what you need, but DOF will vary depending upon the focal length of the lens you're using and the change in aperture. Most lenses have a scale with some "bracket lines" that will show the focus envelope in which the objects will be in acceptably sharp focus.

For what you want to do with the model, you need a wide aperture.... say around f/2.8 which will give you a fairly shallow DOF on something like an 85mm lens. But that same f/2.8 will give you an even SHALLOWER dof on the 70-200 lens. See the pattern here? - Stu

Looking at your current gear list, if you zoomed out to 55mm (which would be about 88mm on FF) and used your largest aperture, that would give you the narrowest DOF. And of course the closer you got to your subject with those settings, the more the background would be thrown out of focus. - Stu

Go play around with DOFMaster for a while, it will give you a very good idea of what your depth of field will be.

funk1196 wrote in post #5836200
Is there a set way to determine DOF?

IE: 3.5 gives a 1 foot of clearly focused subject.

The reason i ask, is because i want to be able to determine what aperture i'll need depending on the situation.

Lets say i have a model on a car, and i need her, and the car completely in focus, but outside of that, as little as possible.

How can i (without taking 10 pictures) determine what the DOF will need to be?

Depends on your focal length. A 200mm focal length at f/4 will give a lot less DOF then a 50mm will at f/4.

go here

Megapixle wrote in post #5836248
Go play around with DOFMaster for a while, it will give you a very good idea of what your depth of field will be.

This is the correct answer. No modern lenses of which I'm aware have depth-of-field markings on them any more, and they change with focal length and format (or maximum acceptable circle of confusion). Even the old DOF markings were notoriously inaccurate. The web page linked above is excellent, providing easy assumptions of your standard of sharpness or the ability to select that standard (by choosing a smaller or larger circle of confusion).

I usually use the smallest aperture that won't cause noticeable diffraction when I'm trying to get as much as possible sharp. But the question that needs those tables is where to focus when taking that approach. The hyperfocal tables on that website provide that information.

Rick "who chimps to check DOF nowadays" Denney

Mike30D wrote in post #5836283
Depends on your focal length. A 200mm focal length at f/4 will give a lot less DOF then a 50mm will at f/4.

Not if you step back with your 200mm lens to frame your subject as you would with a 50mm lens. A shot from 40 feet away with a 200mm lens will yield about the same DOF as a 50mm lens at 10 ft using the same aperture (framing the subject the same although the perspective will be different). There are several things to keep in mind when it comes to DOF and they are the aperture, distance from subject, focal length, media format, final print size and viewing distance (along with what you would consider acceptable sharpness).

I REALLY miss the DEP mode-- it figured out the proper aperture for the desired DOF beautifully.

A-DEP is pretty useless, but on your XSi, you should be able to get a rough feel for what your aperture be set at. The trick is that it will take a little positioning of the AF points to get them to focus on precisely what you want, but you should at least get a rough idea of what aperture you'll need. If the picture is how you like it in A-DEP mode, then you're all set. If not, you can then recompose in aperture priority mode, using the aperture A-DEP gave you.

ftlum wrote in post #5838511
I REALLY miss the DEP mode-- it figured out the proper aperture for the desired DOF beautifully.

A-DEP is pretty useless, but on your XSi, you should be able to get a rough feel for what your aperture be set at. The trick is that it will take a little positioning of the AF points to get them to focus on precisely what you want, but you should at least get a rough idea of what aperture you'll need. If the picture is how you like it in A-DEP mode, then you're all set. If not, you can then recompose in aperture priority mode, using the aperture A-DEP gave you.

good idea.
a-dep always seems to pick a much smaller aperture than i need, requiring longer shutter times... but i'll give it a try and see how well it goes.

I've always pondered about this topic, am I right when I say:

The wider the aperture is the shallower of DOF from the focus point?
Or is it the DOF from the camera? I always thought it was the distance the subject is from the camera. This confused me once, I think I'm making sence of it.

fubarhouse wrote in post #5841097
I've always pondered about this topic, am I right when I say:

The wider the aperture is the shallower of DOF from the focus point?
Or is it the DOF from the camera? I always thought it was the distance the subject is from the camera. This confused me once, I think I'm making sence of it.

DOF is determined by:

Aperture (As your aperture increases (lower f number), DOF decreases - all else equal)
Focal Length (As your focal length increases, DOF decreases - all else equal)
Distance (As your distance increases, DOF increases - all else equal)

fubarhouse wrote in post #5841097
I've always pondered about this topic, am I right when I say:

The wider the aperture is the shallower of DOF from the focus point?
Or is it the DOF from the camera? I always thought it was the distance the subject is from the camera. This confused me once, I think I'm making sence of it.

There is a distance at which the lens is focused. Items at that distance are sharply focused. Items not at that distance are not. As they move away from that distance, they get fuzzier and fuzzier, until finally they cross a threshold and no longer appear sharp.

That threshold is determined by how much you want to be able to enlarge the image, and how strict your standards are even then. Some people want the image to appear sharp from normal viewing distance of a print. Others want it to appear sharp even when viewed as closely as possible without a magnifying glass. Some want it to appear as sharp as the printer can render. The decision of which of these standards to use is the photographer's alone. The size at which the image is enlarged is the photographer's alone.

Ansel Adams would not enlarge his image of Georgia O'Keeffe beyond 8x10, because at larger print sizes it did not meet his standards. But he never had a problem displaying prints that size and smaller.

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Okay, two points that I didn't notice mentioned above (although I admit I only skimmed through).

1. DoF extends roughly twice as far behind the point of focus as it does in front. So, if you know the near and far points that you want to delineate your DoF, focus approximately one third the way between them. Of course, the only truly sharp point is on the focal plane itself, the rest of the area in the DoF gets gradually softer until it reaches a point at which it becomes unacceptably sharp. At that point you are leaving the DoF. So focusing one third the way in may give you acceptable sharpness over the whole area you want, but not be pin-sharp on the subject itself.

2. I would imagine that your camera has a DoF preview button, I haven't met an SLR which doesn't. Using that will show you, in the viewfinder, what is within the DoF and what isn't. It's a very useful button and is perfect for the situation you described.

sandpiper wrote in post #5843837
Okay, two points that I didn't notice mentioned above (although I admit I only skimmed through).

1. DoF extends roughly twice as far behind the point of focus as it does in front.

From what I've read, this relationship is good until you get to macro distances, then it falls apart (where it's more 50/50).