Learning DOF

[Helen B]

Not much reaches beyond infinity.

Most threads about DoF go way beyond infinity.

 

[crotonmark]

Helen B

HA

mark

 

[crotonmark]

Thanks Big Mike

 

[unpopular]

when we're talking about circles of confusion, to infinity and beyond.

 

[timor]

Unpopular, please, explain to me one thing. I didn't study photography in school so I am confused. What is the difference between focus and apparent sharpness. Every time people are saying "to get everything in focus". HOW this is possible ? Some new optical technology I am not aware of ? I always new, that any optical system can focus one and only one distance, everything else is out of focus, so how comes ?

 

[unpopular]

Oh boy.

Helen I am sure could do a better job at this. The physics has been a while, but I will try my best.

A lens can focus only on one, two dimensional plane in space. When I say two dimensional, mathematically that is accurate. A perfect lens of infinite aperture would not be sharp, because the sliver of space which would be in focus wouldn't amount to any depth. So as you approach this lens of infinite aperture area, the sliver of sharpness becomes smaller and smaller. This is why we have DOF in the first place, as the aperture becomes larger, the depth of field becomes smaller.

Lenses essentially take light and bundle them into "packets" called circles of confusion. The further in space away from the focus point, the large the circles become at a rate relative to the aperture of the lens. I can't really explain the mechanism here, I can kind of visualize it, but I've never fully understood this.

At any rate, when we're talking about focus we're talking about a singular two dimensional plane from the lens. When we're talking about sharpness, we're talking about the region where the size of the circles of confusion, as defined by aperture, are small enough so that they appear "sharp", giving the appearance of focus in three dimensional space.

Now, infinity has never been fully explained to me ... what I do know is that everything further from "infinity" appears sharp, and if you try to focus beyond "infinity" you don't get anything sharper - instead it falls out of focus. So no matter how much you try, you cannot make distant objects any more sharp than if focused at infinity, as it seems, doing so would cause the CoC to enlarge again.

So, one way to make everything appear sharp is to ensure that nothing in the frame falls forward the region in space which no longer appears sharp, and the far end of the area which appears sharp is at infinity. By setting the lens such that the far field distance is at infinity but not beyond also ensures that the most depth on near field will also appear sharp.

IN OTHER WORDS if you think of the depth of field as an area in space that appears sharp, which changes it's position according to where you focus, you can place the far boundary at infinity. Now, everything from the near boundary and beyond will appear sharp, because everything beyond infinity is sharp to start with.

If you need more detail, you'll need to speak with someone other than me. I've always been more interested in what happens after exposure than before.

 

[timor]

I've always been more interested in what happens after exposure than before.

(Always thought DoF is an effect of an exposure.)
Thanks man but I didn't really ask for physical workings of DoF. The phenomenon I am asking about is why is everybody using term: "get everything in focus", which, being physical impossibility is a plain nonsense.

 

[Frequency]

Every thing in focus must be meaning acceptable sharpness with fine depth of field using a narrow aperture?

 

[timor]

Every thing in focus must be meaning acceptable sharpness

Yeah.. That's my sister in law talk. I forgive her.

 

[unpopular]

I've always been more interested in what happens after exposure than before.

(Always thought DoF is an effect of an exposure.)
Thanks man but I didn't really ask for physical workings of DoF. The phenomenon I am asking about is why is everybody using term: "get everything in focus", which, being physical impossibility is a plain nonsense.

(kind of. DOF exists even when exposure doesn't, it's an optical function, not a chemical or electrical reaction to light.)
Well, sure. On the most technical level, but if the eye cannot determine the focus point in an image because it's within an area of apparent sharpness; such that the recoding medium cannot even resolve it, then it effectively doesn't much matter. I tend to think of objects in field as being "in focus" even though I know that they aren't.

 

[Derrel]

yes thank you
But there is not MUCH difference between the two photos even tho the F stops are widely different
that is what I was asking about
According to Peterson there should be a very crisp photo and one that has a shallow blurred DOF
that I didnt get and was wondering why

Mark

When using a small-sized capture format, like APS-C digital, with the angle of view shown, and at the DISTANCE the photo was made at, and with the focal length used...there really is a pretty fair amount of depth of field, even at a mid-sized f/stop like f/4.5!!!!

If you look at the image with a critical eye, you can see that the distance from the front bottle to the back bottle on that stone wall is not "that long" of a distance, all told.

In the f/4.5 photo, the background flower bed/lawn area is OUT of focus. In fact, the blurred background in the f/4.5 shot begins on the LAWN's shadowed area, which peeks through on the right hand side, between the yellow-labeled Schwepp's bottle and the right-most bottle...the lawn at that distance is sharp, and then right behind that zone, things start to go out of focus quite rapidly. In the f/22 shot, the depth of field extends well,well back, clear to the flower bed and lawn at the back distance in the far zone of the photo.

To get shallower DOF, and less in-focus stuff, the easiest thing to do would be to go to a LARGER FORMAT camera. At the same angle of view, the larger the capture format, the less depth of field there is, given equivalent angles of view. The APS-C format has a nice compromise between deep DOF and shallow DOF, at "normal" shooting apertures in normal conditions using wide-angle to normal focal length lenses, at "normal" shooting distances. And, this is important: with a small-format camera, like an APS-C d-slr, with a shortish focal length lens of say 50mm or shorter, an f/stop of f/4.5 is NOT THAT WIDE .ie. is NOT that "large" of an aperture!!!!! it's just....not!!! f/1.4 is wide! F/1.8 is wide! F/2 is wide....but f/4.5.....uh...not so much... Your premise is flawed: when yo stated that the two f/stops are "widely different"...uh...sorry, but no, they are not all that widely different...they are BOTH trending toward f/stops that under those conditions, provide the user with an ample DOF zone and a bit of focusing "cushion" for focus placement errors.

 

[timor]

I tend to think of objects in field as being "in focus" even though I know that they aren't.

In your case I would consider it as OK. You know how this happened. But look, how much confusion it makes for new adepts of the photography. Digital is only adding to it with its artificial sharpening in PP. Do they developed already software to make circles of confusion smaller ?