On reading Depth of Field . . . . How is that done? at DVinfo.net

Graham Bernard · June 10th, 2004, 11:14 PM

Will somebody either explain and/or point me at a very very simple "unpicking" how this works, "Often you will WANT shallow depth of field and one of several ways to acheive it is to keep the aperature large (the T # small) in order to throw backgrounds out of focus so your subject will separate from an otherwise distracting and confusing background." Yes, I understand WHAT to do - now I want to kow HOW, within the cammie, it is achieved? What is going on with the optics and physics?

I don't want to know THAT it works .. I can work this out for myself . . I want to know why/how, changing the levels of light coming into a camera can affect the depth of field. Sorry if this is so obvious to others, but it aint to me. I've even attend a one-to-one class on this, and I'm still baffled . . Please keep it as simple as you can .. I think I've seen the elegant formula on this - is this at the heart of it? Well, what is going on in this formula?

If somebody could say, "As this raises the . . . then you get . . . so we need to do this . .. to get . .. . THEN we can . . . "

Am I the only one perplexed by this?

Grazie

Ken Tanaka · June 10th, 2004, 11:40 PM

Do you wear glasses, Grazie?

Graham Bernard · June 10th, 2004, 11:48 PM

Er .. yes? Meaning? . . Can I read the posts that have gone on before? You can't mean that? . . .

Are you meaning that my sight is improved by placing glass/plastic in front of my aging eyes? If I take them off things go blurry? Yes, Lens correction I understand .. bringing the "image" back to the retinal plane . . "short sight" . . infront of retinal plane.. "long sight" behind retinal plane . . er yeah? But why changing the Iris/Light levels give changes/qualitites in depth of field?

Ken, I'm serious . . I can't "see" what I'm missing here . .

Thanks for your patience . . I had the same "issue" of understanding over "Manual" white balance .. .

I'm listening ..

Grazie

Ken Tanaka · June 11th, 2004, 12:14 AM

Well, for starters, you can actually demonstrate the effect for yourself. Take off your glasses, curl your index finger inside your curled thumb until you have only a tiny hole inside the curl. Now look though the hole and adjust it until you can see a distant object clearly. Voila. Same thing happens in the camera.

Why does this happen? Well, unfortunately, it turns out that the explanation is rather inescapably technical. (It gets even more technical when you go beyond the boundaries of visible light.) But perhaps the best online explanation of the photographic effect is at Digital Gallery Plus, a site in the UK. It has a good mix of plain English and equations.

Hope this helps.

Graham Bernard · June 11th, 2004, 12:32 AM

.. . sssush! . . I'm reading it .. ( thanks Ken . . . . Grazie . . . )

Graham Bernard · June 11th, 2004, 12:52 AM

Uhuh?! . . I do like the "Now we need to fit the circle of confusion into this equation .. . " . . like it was NOT there already . .

Am I right/correct in thinking that most/majority of cameramen just "Do It!" and don't understand the why/how and just get on with putting the money in the bank? If this is the case then I release all hold on reality .. Ken, do you understand the formula?Guess you do on reading up about you . . weren't you in the optics/science trade?

The pinhole thing? Yes I did know of this. Hell I can save a lot on my optomertists bills .. it's gonna make driving a car a tad difficult .. if not just plain dangerous! "Hey! Who's that maniac driving that Ford!?!? IS he blind?!?!" . ."nah, that'll be Grazie using BOTH hands to pinhole his way down the High Street!"

Father is an Optician, Sister and Brother-in-law too . . that does not give me an open door into the ways and where fors of optics . . oh yeah, I did 2 months of optical calculus at City Uni, here in London .. . found the whole thing "sleep" inducing . . shouldn't have. Then went my own way . . Too much Optics in the family . . Lost sight of what I wanted to do in life . .

Best regards,

Grazie

Ken Tanaka · June 11th, 2004, 01:27 AM

"Am I right/correct in thinking that most/majority of cameramen just "Do It!" and don't understand the why/how and just get on with putting the money in the bank?"

Essentially, yes. It's more important to understand the subject in practical terms when planning your shots and dramatic blocking. I don't think many DP's or camera operators have, or need, a firm grounding in optical physics.

Jeff Donald · June 11th, 2004, 01:55 AM

Light from objects comes to a point (true focus) in front of and behind the film plane. This is referred to as Depth of Focus. The light that comes to focus on the film plane produces a point that appears to be in focus if the size of the point is 1/300 of an inch or less. Light that is slightly out of focus does not produce a point, but rather a disc or circle. The disc or circle is larger than 1/300 of an inch and is sometimes referred to as a Circle of Confusion (CoC). The closing of the diaphragm reduces the diameter of the CoC, thus more light forms points and more objects appear to be in focus (sharper).

Diffraction is increased as the diaphragm is reduced (closed) and the plane of sharpest focus is shifted back along the optical axis. This is partly due to spherical aberration.

Rob Belics · June 11th, 2004, 07:01 AM

What happens is all points in a scene reflect all the light directed at it. These rays of light are scattered in all directions, some toward the camera lens which tries to bend all of those rays to one point on the film or ccd.

But since all those rays are coming from many different objects and many different angles, it cannot possibly bend them all to the same point. Some make it, some fall short or go too far. Circle of confusion.

Closing the aperture eliminates some of those 'stray' rays which don't bend to the proper point. A pinhold camera has a large dof because it passes the straightest light rays. In a camera lens, widening the aperture passes more rays of light which aren't the straightest.

Nick Jushchyshyn · June 11th, 2004, 08:28 AM

The main controls you have over depth of field with the GL2 are zoom and aperture.

Smaller ("darker") aperture and wider lens angles (shorter lens) have greater depth-of-field (less of a shallow DOF effect) than using the lens zoomed "in" as far as possible with the aperture wide open.

Using the lens zoomed in with a wide ("brighter") aperture will let you make the most of a shallow depth of field. Since you would be opening up the aperture, AND you're likely to want a 1/60s shutter speed for NTSC (or 1/50 for PAL) AND it's best to avoid using gain too much, that leaves you with controlling the lighting of your shot to get proper exposure. :)

Have fun.

June 10th, 2004, 11:14 PM	#1
Graham Bernard Old Boot Join Date: Aug 2002 Location: London UK Posts: 3,633	On reading Depth of Field . . . . How is that done? Will somebody either explain and/or point me at a very very simple "unpicking" how this works, "Often you will WANT shallow depth of field and one of several ways to acheive it is to keep the aperature large (the T # small) in order to throw backgrounds out of focus so your subject will separate from an otherwise distracting and confusing background." Yes, I understand WHAT to do - now I want to kow HOW, within the cammie, it is achieved? What is going on with the optics and physics? I don't want to know THAT it works .. I can work this out for myself . . I want to know why/how, changing the levels of light coming into a camera can affect the depth of field. Sorry if this is so obvious to others, but it aint to me. I've even attend a one-to-one class on this, and I'm still baffled . . Please keep it as simple as you can .. I think I've seen the elegant formula on this - is this at the heart of it? Well, what is going on in this formula? If somebody could say, "As this raises the . . . then you get . . . so we need to do this . .. to get . .. . THEN we can . . . " Am I the only one perplexed by this? Grazie

June 10th, 2004, 11:40 PM	#2
Ken Tanaka Retired DV Info Net Almunus Join Date: Dec 2001 Location: Chicago, IL Posts: 6,943	Do you wear glasses, Grazie? __________________ Lady X Films: A lady with a boring wardrobe...and a global mission. Hey, you don't have enough stuff! Buy with confidence from our sponsors. Hand-picked as the best in the business...Really! See some of my work one frame at a time: www.KenTanaka.com

June 10th, 2004, 11:48 PM	#3
Graham Bernard Old Boot Join Date: Aug 2002 Location: London UK Posts: 3,633	Ah! Is that my problem . .. well .. . Er .. yes? Meaning? . . Can I read the posts that have gone on before? You can't mean that? . . . Are you meaning that my sight is improved by placing glass/plastic in front of my aging eyes? If I take them off things go blurry? Yes, Lens correction I understand .. bringing the "image" back to the retinal plane . . "short sight" . . infront of retinal plane.. "long sight" behind retinal plane . . er yeah? But why changing the Iris/Light levels give changes/qualitites in depth of field? Ken, I'm serious . . I can't "see" what I'm missing here . . Thanks for your patience . . I had the same "issue" of understanding over "Manual" white balance .. . I'm listening .. Grazie

June 11th, 2004, 12:14 AM	#4
Ken Tanaka Retired DV Info Net Almunus Join Date: Dec 2001 Location: Chicago, IL Posts: 6,943	Well, for starters, you can actually demonstrate the effect for yourself. Take off your glasses, curl your index finger inside your curled thumb until you have only a tiny hole inside the curl. Now look though the hole and adjust it until you can see a distant object clearly. Voila. Same thing happens in the camera. Why does this happen? Well, unfortunately, it turns out that the explanation is rather inescapably technical. (It gets even more technical when you go beyond the boundaries of visible light.) But perhaps the best online explanation of the photographic effect is at Digital Gallery Plus, a site in the UK. It has a good mix of plain English and equations. Hope this helps. __________________ Lady X Films: A lady with a boring wardrobe...and a global mission. Hey, you don't have enough stuff! Buy with confidence from our sponsors. Hand-picked as the best in the business...Really! See some of my work one frame at a time: www.KenTanaka.com

June 11th, 2004, 12:32 AM	#5
Graham Bernard Old Boot Join Date: Aug 2002 Location: London UK Posts: 3,633	That'll be the ole pinhole effect! .. . sssush! . . I'm reading it .. ( thanks Ken . . . . Grazie . . . )

June 11th, 2004, 12:52 AM	#6
Graham Bernard Old Boot Join Date: Aug 2002 Location: London UK Posts: 3,633	Uhuh?! . . I do like the "Now we need to fit the circle of confusion into this equation .. . " . . like it was NOT there already . . Am I right/correct in thinking that most/majority of cameramen just "Do It!" and don't understand the why/how and just get on with putting the money in the bank? If this is the case then I release all hold on reality .. Ken, do you understand the formula?Guess you do on reading up about you . . weren't you in the optics/science trade? The pinhole thing? Yes I did know of this. Hell I can save a lot on my optomertists bills .. it's gonna make driving a car a tad difficult .. if not just plain dangerous! "Hey! Who's that maniac driving that Ford!?!? IS he blind?!?!" . ."nah, that'll be Grazie using BOTH hands to pinhole his way down the High Street!" Father is an Optician, Sister and Brother-in-law too . . that does not give me an open door into the ways and where fors of optics . . oh yeah, I did 2 months of optical calculus at City Uni, here in London .. . found the whole thing "sleep" inducing . . shouldn't have. Then went my own way . . Too much Optics in the family . . Lost sight of what I wanted to do in life . . Best regards, Grazie

June 11th, 2004, 01:27 AM	#7
Ken Tanaka Retired DV Info Net Almunus Join Date: Dec 2001 Location: Chicago, IL Posts: 6,943	"Am I right/correct in thinking that most/majority of cameramen just "Do It!" and don't understand the why/how and just get on with putting the money in the bank?" Essentially, yes. It's more important to understand the subject in practical terms when planning your shots and dramatic blocking. I don't think many DP's or camera operators have, or need, a firm grounding in optical physics. __________________ Lady X Films: A lady with a boring wardrobe...and a global mission. Hey, you don't have enough stuff! Buy with confidence from our sponsors. Hand-picked as the best in the business...Really! See some of my work one frame at a time: www.KenTanaka.com

June 11th, 2004, 01:55 AM	#8
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	Light from objects comes to a point (true focus) in front of and behind the film plane. This is referred to as Depth of Focus. The light that comes to focus on the film plane produces a point that appears to be in focus if the size of the point is 1/300 of an inch or less. Light that is slightly out of focus does not produce a point, but rather a disc or circle. The disc or circle is larger than 1/300 of an inch and is sometimes referred to as a Circle of Confusion (CoC). The closing of the diaphragm reduces the diameter of the CoC, thus more light forms points and more objects appear to be in focus (sharper). Diffraction is increased as the diaphragm is reduced (closed) and the plane of sharpest focus is shifted back along the optical axis. This is partly due to spherical aberration. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

June 11th, 2004, 07:01 AM	#9
Rob Belics Major Player Join Date: Apr 2003 Location: St. Louis, MO Posts: 581	What happens is all points in a scene reflect all the light directed at it. These rays of light are scattered in all directions, some toward the camera lens which tries to bend all of those rays to one point on the film or ccd. But since all those rays are coming from many different objects and many different angles, it cannot possibly bend them all to the same point. Some make it, some fall short or go too far. Circle of confusion. Closing the aperture eliminates some of those 'stray' rays which don't bend to the proper point. A pinhold camera has a large dof because it passes the straightest light rays. In a camera lens, widening the aperture passes more rays of light which aren't the straightest.

June 11th, 2004, 08:28 AM	#10
Nick Jushchyshyn Major Player Join Date: Feb 2004 Location: Philadelphia, PA, USA Posts: 548	The main controls you have over depth of field with the GL2 are zoom and aperture. Smaller ("darker") aperture and wider lens angles (shorter lens) have greater depth-of-field (less of a shallow DOF effect) than using the lens zoomed "in" as far as possible with the aperture wide open. Using the lens zoomed in with a wide ("brighter") aperture will let you make the most of a shallow depth of field. Since you would be opening up the aperture, AND you're likely to want a 1/60s shutter speed for NTSC (or 1/50 for PAL) AND it's best to avoid using gain too much, that leaves you with controlling the lighting of your shot to get proper exposure. :) Have fun.