How do cinematographers focus on an object and blur the background?

waloshin

macrumors 68040
Original poster
Oct 9, 2008
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How do cinematographers focus on an object and blur the background especially so quickly?
 

nateo200

macrumors 68030
Feb 4, 2009
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Northern District NY
Lol...sorry not trying to be an ass but its kind of a weird way to ask this. Its called Depth of Field which in turn correlates to the F/stop of the lens but the cinematographer uses his or herselfs skills with a focus puller (both a tool and a position).
 

thejadedmonkey

macrumors G3
May 28, 2005
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Any still camera that costs over $500, or any video camera that costs over $1000 (more or less) will have a lens that doesn't need to focus on everything (infinity) and can produce those results.
 

waloshin

macrumors 68040
Original poster
Oct 9, 2008
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Any still camera that costs over $500, or any video camera that costs over $1000 (more or less) will have a lens that doesn't need to focus on everything (infinity) and can produce those results.
I do own a Nikon D40 so I know that there are focus points in the view finder, but I have never used a professional quality video camera to know how that works.
 

thejadedmonkey

macrumors G3
May 28, 2005
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544
Pennsylvania
I do own a Nikon D40 so I know that there are focus points in the view finder, but I have never used a professional quality video camera to know how that works.
The D40 is a professional quality camera! What happens is the lens focuses on something at a certain distance away, and everything that isn't at that distance (either too close or too far) is out of focus.

It's like an eye. Put your hand in front of your face. Now focus on the computer screen. Notice how your hand isn't really clear. Now focus on your hand. Notice how the computer screen isn't really clear? Expensive lens' do the same thing.
 

waloshin

macrumors 68040
Original poster
Oct 9, 2008
3,171
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The D40 is a professional quality camera! What happens is the lens focuses on something at a certain distance away, and everything that isn't at that distance (either too close or too far) is out of focus.

It's like an eye. Put your hand in front of your face. Now focus on the computer screen. Notice how your hand isn't really clear. Now focus on your hand. Notice how the computer screen isn't really clear? Expensive lens' do the same thing.
Ok makes sense.
 

handsome pete

macrumors 68000
Aug 15, 2008
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Any still camera that costs over $500, or any video camera that costs over $1000 (more or less) will have a lens that doesn't need to focus on everything (infinity) and can produce those results.
It's a little more complicated than that. Depth of field has just as much to do with the imaging sensor as it does with the lens.
 

pemb145

macrumors newbie
Jun 24, 2011
11
0
Depth of Field

Try looking at Vimeo School for some tips:

DSLR Shooting - there is lots of info on depth of field
 

Small White Car

macrumors G4
Aug 29, 2006
10,889
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Washington DC
Things that help you get a shallow depth of field:

* Large aperture (smaller f-stop numbers)
-so if you're in the sun this requires a faster shutter, or filters

* Larger image sensor (depends on your camera type)

* Long distance between background, subject, and camera
- and if you do that, you'll find you'll need a longer zoom lens
 

thejadedmonkey

macrumors G3
May 28, 2005
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544
Pennsylvania
It's a little more complicated than that. Depth of field has just as much to do with the imaging sensor as it does with the lens.
How so? With the exception of 3CCD vs CMOS, and perhaps sensitivity to light, all sensors are about the same. an iPhone sensor with a high-end lens isn't going to be much different (relatively speaking, of course) from a D90.... is it?
 

Nov 28, 2010
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How so? With the exception of 3CCD vs CMOS, and perhaps sensitivity to light, all sensors are about the same. an iPhone sensor with a high-end lens isn't going to be much different (relatively speaking, of course) from a D90.... is it?
It is also the size of the sensor, that is important. The iPhone 4s has a much smaller sensor than the D90:
DEPTH OF FIELD REQUIREMENTS
As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).
from DIGITAL CAMERA SENSOR SIZES
 

thejadedmonkey

macrumors G3
May 28, 2005
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544
Pennsylvania
It is also the size of the sensor, that is important. The iPhone 4s has a much smaller sensor than the D90:

from DIGITAL CAMERA SENSOR SIZES
But my understanding is that the size of the sensor doesn't change whether or not an image is in focus, just whether or not what's cropped into focus. A sensor is, in its most basic sense, just a collection of light sensing receptors...
 

handsome pete

macrumors 68000
Aug 15, 2008
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But my understanding is that the size of the sensor doesn't change whether or not an image is in focus, just whether or not what's cropped into focus. A sensor is, in its most basic sense, just a collection of light sensing receptors...
Of course the lens is the mechanism that focuses the image, but the size of the sensor directly affects the amount of depth of field. Of course achieving that shallow DOF "film look" is possible with smaller sensors, but it is vastly different than from something like a full frame DSLR.

http://news.cnet.com/8301-13580_3-9802758-39.html

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
 

jdavtz

macrumors 6502a
Aug 22, 2005
548
0
Kenya
The size of the sensor does NOT affect the depth of field.

It crops the image. That's all.

So to compensate for this cropping effect you, for example, use a 20mm lens instead of a 40mm lens. Using the shorter-focal-length lens at the same subject-object distance is what will affect your depth of field.
 

handsome pete

macrumors 68000
Aug 15, 2008
1,719
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The size of the sensor does NOT affect the depth of field.

It crops the image. That's all.

So to compensate for this cropping effect you, for example, use a 20mm lens instead of a 40mm lens. Using the shorter-focal-length lens at the same subject-object distance is what will affect your depth of field.
Poor choice of words on my part, but sensor size does affect it indirectly as Keith pointed out.
 

acearchie

macrumors 68040
Jan 15, 2006
3,276
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Really don't see why this is down voted. It provides links to relevant material?

Of course achieving that shallow DOF "film look" is possible with smaller sensors, but it is vastly different than from something like a full frame DSLR.
Don't forget that most motion pictures are made using a super35mm sized film or chip therefore the film look is technically a super35 crop not full frame.
 
Nov 28, 2010
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Really don't see why this is down voted. It provides links to relevant material?
Don't forget, that links with additional information are not liked by many people, it needs extra effort to click them and actually read and understand the context presented there.
Those many people like to be spoonfed.

Waiting for another ten down votes. If you can manage clicking that button, you can actually click a link or two. But reading them, hmm, yeah, I see the fallacy.

Onto FacialBukake, crying about internet cruelty.
 

acearchie

macrumors 68040
Jan 15, 2006
3,276
103
Don't forget, that links with additional information are not liked by many people, it needs extra effort to click them and actually read and understand the context presented there.
Those many people like to be spoonfed.

Waiting for another ten down votes. If you can manage clicking that button, you can actually click a link or two. But reading them, hmm, yeah, I see the fallacy.

Onto FacialBukake, crying about internet cruelty.
What happened to the days of having to find information! I'm only young but mostly everything I have learnt I have had to find. Spoilt for choice on here!

It's funny that people don't like the extra effort yet I doubt they consider the effort that you have had to put in to go to three different sources and get the links!
 
Nov 28, 2010
22,668
27
located
What happened to the days of having to find information! I'm only young but mostly everything I have learnt I have had to find. Spoilt for choice on here!

It's funny that people don't like the extra effort yet I doubt they consider the effort that you have had to put in to go to three different sources and get the links!
Maybe it is my old age, I just become more cynical every day, often when visiting this forum. I guess it is a love-hate relationship, or some kind of masochism to stay here, especially as many questions asked can be answered via searching.
I mean, we live in the digital age and have a wealth of information at our fingertips, and still people haven't figured out, how to look for it.

When we still had libraries, and looked for an answer in some of those ancient books they had, there was no one reading the correct answers back to me. Or am I just ludicrous?

Anyway, back to the thread: Focus pullers will be a thing of the past with this.

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depth of focus, wonder how useful it is in porn.. interesting really
It might not be useful, but it can produce artsy erotica.
 

acearchie

macrumors 68040
Jan 15, 2006
3,276
103
Anyway, back to the thread: Focus pullers will be a thing of the past with this.

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Seen the Lytro but not convinced at the moment. You may lost a production focus puller but maybe a new digital focus puller will be created because someone would still have to decide what area is in focus and it could be just as complicated if when mixed with video matching the focus to a moving object!
 
Nov 28, 2010
22,668
27
located
Seen the Lytro but not convinced at the moment. You may lost a production focus puller but maybe a new digital focus puller will be created because someone would still have to decide what area is in focus and it could be just as complicated if when mixed with video matching the focus to a moving object!
I meant it more as a gist. The focus puller will hopefully stay, why complicate things? Though if the technology matures, maybe the viewer can pull the focus, if the audience is still there for longer films.