Depth of field question...

OK, so I understand that all else being equal, the larger the aperture, the shallower the depth of field. Is there anything that comes into play with how aperture affects far away objects? To elaborate, if I photography my daughter at 85mm f/1.8 and get just her head (and maybe a little shoulders) anything 15 ft. or so behind her will be SOLID blur. No definition at all. Can't tell if its trees or grass or how many trees, assuming they're back far enough. Now lets say I add my daughter to the picture and because she isn't exactly on the same plane, I stop down to f/4 to get both faces in focus. Now even trees 50 feet away, I can see. They're blurry, yeah but I don't have that completely blown out background. Is that just a reality of life, too bad so sad?

There are 3 things that control dof:

aperture
camera to subject distance
focal length

jeromego wrote in post #10772659
There are 3 things that control dof:

aperture
camera to subject distance
focal length

This +++

If you google DOF calculator you can see representations of this in action. Just plug in the lens, and your distance to see how things change. I shot a butterfly on a flower today a couple feet away from my 135 f2. The DOF was razor thin. At 15 feet its a couple of feet.

imahawki wrote in post #10772631
OK, so I understand that all else being equal, the larger the aperture, the shallower the depth of field. Is there anything that comes into play with how aperture affects far away objects? To elaborate, if I photography my daughter at 85mm f/1.8 and get just her head (and maybe a little shoulders) anything 15 ft. or so behind her will be SOLID blur. No definition at all. Can't tell if its trees or grass or how many trees, assuming they're back far enough. Now lets say I add my daughter to the picture and because she isn't exactly on the same plane, I stop down to f/4 to get both faces in focus. Now even trees 50 feet away, I can see. They're blurry, yeah but I don't have that completely blown out background. Is that just a reality of life, too bad so sad?

Yes - the laws of physics . But Jerome summed up the contributing factors quite well. However, judging by your description of your needs and circumstances it sounds like you're already on the road to having a good grasp of things. You're getting the basics, and you can play around and observe cause and effect. That's what's really convenient about digital.

After sufficient experimentation you'll be able to decide how close to get to the subject (more resultant blurring of far background) and how much to tweak the aperture in either direction. You can also pose the girls in such a way, mild profile, to get them into the same plane of focus or very close to it. - Stu

Don't forget the distance from the subject to the background also affects how "blurry" the background appears.

You are mainly talking about background blur, not DOF. Background blur is a function of many things including how far away the background is, how long the lens is and yes, aperture.

A long lens can really blur the background even though the DOF will not be thin. For that second example, using a long lens from farther away with a narrower aperture can blur the background and preserve adequate DOF.

As an example, using identical framing 200mm f4 from 20ft will have the same BG blur as 85mm f1.8 from 8.5 feet, but have 2x deeper DOF

Just as proof, here's 200mm f5.6 vs 30mm f2 with similar framing.

Great examples Taylor - the pictures often do explain it better than words .

doesnt pixel density have something to do with it to? like a crop vs. full frame?

tkbslc wrote in post #10772710
You are mainly talking about background blur, not DOF. Background blur is a function of many things including how far away the background is, how long the lens is and yes, aperture.

A long lens can really blur the background even though the DOF will not be thin. For that second example, using a long lens from farther away with a narrower aperture can blur the background and preserve adequate DOF.

As an example, using identical framing 200mm f4 from 20ft will have the same BG blur as 85mm f1.8 from 8.5 feet, but have 2x deeper DOF

Just as proof, here's 200mm f5.6 vs 30mm f2 with similar framing.

THIS is the phenomena I'm getting it. Its "fall off" I guess, not truly depth of field. I'm talking about having 2 situations, 1 where the DOF is 3 feet but the background 30 feet away is completely blurred and a 2nd where the DOF is still the same 3 feet but the background 30 feet away is blurred but still distinct. It SOUNDS like this is a function of focal length. Is that right?

I do understand the basics for the record. At f/1.8 and 30mm, the closer you are to the subject, the thinner the DOF. At 30mm and a fixed distance, the larger the aperture, the thinner the the depth of field, and finally, at a fixed distance fixed aperture, the longer the focal length, the thinner the DOF. But as I said, it doesn't seem like DOF is exactly what I was referring to, its more like fall off after you get past the back edge of the sharp part of DOF.

ilumo wrote in post #10772846
doesnt pixel density have something to do with it to? like a crop vs. full frame?

No - I don't know the math involved, but on a crop you are literally cropping the center circle/field of view of the lens. Pixel density does not really figure into it..... rather the size of the sensor does.

It appears that the DOF is greater but there's really more going on here. Closer objects will inherently appear more isolated from the background, giving an apparent shallower DOF

ilumo wrote in post #10772846
doesnt pixel density have something to do with it to?

No! Don't even start down that rathole.

That's a difference of format and ultimately a difference of enlargement to the final display dimensions--which is another factor of Depth of Field.

Depth of field at its base is a matter of how big a blur can be before the eye discerns it's a blur rather than a point. Below the limits of the eye's acuity, a very tiny blur looks like a sharp point. If you keep enlarging that blur, the eye will eventually discern that it's a blur (a disk), not a point. Where focal length makes a difference is that the "rate of increasing blur" from the focal plane will appear greater with a higher focal length.

In a photographic image, there are, of course, no true "points." They're all blurs of varying sizes. On the plane of focus will be the smallest blurs. Farther from the plane of focus, the blurs grow larger and larger.

If the image is displayed very small, even the larger blurs are still too small for the eye to discern that they're really blurs, so they look sharp. But if you keep enlarging the image, you begin to see first the larger blurs as blurs and then increasingly smaller blurs also as blurs they truly are.

I hope that's clear enough...or else I'll have to get into Circles of Confusion.

sapearl wrote in post #10772935
No - I don't know the math involved, but on a crop you are literally cropping the center circle/field of view of the lens. Pixel density does not really figure into it..... rather the size of the sensor does.

It appears that the DOF is greater but there's really more going on here. Closer objects will inherently appear more isolated from the background, giving an apparent shallower DOF

Right. but doesn't the fact that you are magnifying a portion of the picture more make the blurry parts even blurrier? And when you have more MP, you can magnify the picture more, which is why I brought up that point.

I'm not sure if this calculates into DOF or making it blurrier, etc etc. i dont know the proper terminology.

ilumo wrote in post #10772953
Right. but doesn't the fact that you are magnifying a portion of the picture more make the blurry parts even blurrier? And when you have more MP, you can magnify the picture more, which is why I brought up that point.

I'm not sure if this calculates into DOF or making it blurrier, etc etc. i dont know the proper terminology.

Pixels are irrelevant. The initial size of the image is what matters, as in the sensor or film size.

To answer the OP: if 85mm f/4 doesn't obliterate the background, use a longer lens.

ilumo wrote in post #10772953
Right. but doesn't the fact that you are magnifying a portion of the picture more make the blurry parts even blurrier? And when you have more MP, you can magnify the picture more, which is why I brought up that point.

I'm not sure if this calculates into DOF or making it blurrier, etc etc. i dont know the proper terminology.

Not megapixel related, no, but sensor size does come in to play. That is a different topic IMO, though. Using a larger sensor does generally require using longer lenses from the same subject distance, which does alter DOF and bg blur appearances.

imahawki wrote in post #10772889
THIS is the phenomena I'm getting it. Its "fall off" I guess, not truly depth of field. I'm talking about having 2 situations, 1 where the DOF is 3 feet but the background 30 feet away is completely blurred and a 2nd where the DOF is still the same 3 feet but the background 30 feet away is blurred but still distinct. It SOUNDS like this is a function of focal length. Is that right?
.

Technically it is a function of perspective, which is controlled by how far away to the subject you are. But in practice, yes it is easily controlled by focal length. Try the same shot at 500mm f6.3 on your bigmos and you will see it in action. You'll be half way down the street, but it will work!