stupid question about lenses. at DVinfo.net

Go Back   DV Info Net > The Tools of DV and HD Production > Open DV Discussion

Open DV Discussion
For topics which don't fit into any of the other categories.


Reply
 
Thread Tools Search this Thread
Old January 9th, 2007, 08:09 PM   #1
Regular Crew
 
Join Date: Nov 2005
Posts: 32
stupid question about lenses.

ok, so i guess I'll post this here, since there isn't a forum for lenses.

second, this is probably a really stupid question, so I'm going to ask for your forgiveness beforehand.

so I've been spending quite a bit of time researching lenses: wide, zoom, telephoto, fixed focal length. so my question is, when i'm "focusing" using a fixed focal length lens, what's happening under the covers?

so I know the focal length of the lens determines (along with the fstop) the depth of field. so with a fixed focal length lens, without touching the aperture, how am I able to use the focus ring to slide my focus backwards and forwards?

am I actually adjusting the focus length, but only in very small increments? meaning that a "fixed" 50mm lens actually has a small amount of variability (like 48mm to 52mm?) or is there another lens inside that i'm moving that has the same effect?

i'm assuming that DOF is actually a range -- there is a specific plane in space that is in "ideal focus" (not sure if there is a term for that) and that as you move an object away from that plane, the more "out of focus" it's going to become. i'm also assuming that it isn't a linear curve; that there is a dramatic falloff in both directions that causes a sort of "shelf" on both sides of the curve.

and, of course, i could be completely wrong on all of that and be basing all of my guesses on fault assumptions.

anyway, i don't really have anyone to ask this stuff, i've never taken a class and i'm only learning this stuff from reading ... books can be great teachers, but they aren't great on feedback :)

thanks in advance !!

JT Coleman
Austin, Texas
JT Coleman is offline   Reply With Quote
Old January 9th, 2007, 11:17 PM   #2
Major Player
 
Join Date: Aug 2004
Location: Durango, Colorado, USA
Posts: 711
When a lens is focused at infinity, the distance from the point within the lens where the image inverts to the plane where the imaging sensors are located within the body of the camera is called the focal length.

"f-stops" are a mathematical relationship between the aperture of a lens and its focal length. The aperture, or iris, is located at the precise point within the body of a lens where the image turns upside down. An f-stop is simply the focal length divided by the aperture. Say a lens has an aperture of 25mm diameter. If the focal length of that lens is 50mm, then the f stop is 2.0. If the focal length is 100 mm, then the same aperture of 25mm only yields an f stop of 4.0. With a 200 mm focal length the same 25mm aperture has an f stop value of 8.0. f-stop values either halve or double the amount of light passing throught the lens. The following basic f-stop scale reduces the quantity of light by half with each new number:

1.2, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0

The smaller the number, the more light is capable of being passed through the lens. Lenses are ususally rated by the greatest amount of light thay can pass. Apertures are also a relationship to the physical diameter of the glass. In the example above, a 200 mm lens with a max aperture of 2.0 would have to be so huge you probably couldn't carry it. So most zoom telephoto lenses list a f-stop range from wide angle to full telephoto.

Depth-of-field: When a lens is focused upon an object there will be some distance in front and in back of that object that will APPEAR to be ACCEPTABLY SHARP. It is nothing more than an illusion of apparent sharpness. With a fixed focal length lens, you focus upon your subject. If the subject moves closer to you or farther away, you adjust the focus of your lens to keep your subject looking sharp. No lens made can ever focus upon more than one single point, or plane. However, the choice of a smaller (larger number) aperture can create an illusion of more objects in APPARENT focus along the line of the camera's view. Generally speaking, depth-of-field will extend forward 1/3 of its distance from the point of focus and 2/3 back of this same point of focus. However, be aware that depth of focus is also very much dependent upon the eyes of the beholder. I will see much more depth of field in a movie from the back row than i will see from the front row closest to the screen.

It is common in video and in portrait photography to use wide apertures to keep the depth of focus shallow and thus separate the subject from both the foreground and background. Check out a full page head shot in a fashion magazine. The eyes are tack sharp. The tip of the nose and the ears are a bit fuzzy!

Video camera lenses have a "back-focus" feature which allows a focus point to be held through a zoom out, but does not work in reverse.

Hope this helps. Other readers, please add your opinions.
__________________
Waldemar
Waldemar Winkler is offline   Reply With Quote
Old January 10th, 2007, 12:04 AM   #3
Regular Crew
 
Join Date: Nov 2005
Posts: 32
Quote:
Originally Posted by Waldemar Winkler
When a lens is focused at infinity, the distance from the point within the lens where the image inverts to the plane where the imaging sensors are located within the body of the camera is called the focal length.

"f-stops" are a mathematical relationship between the aperture of a lens and its focal length. The aperture, or iris, is located at the precise point within the body of a lens where the image turns upside down. An f-stop is simply the focal length divided by the aperture. Say a lens has an aperture of 25mm diameter. If the focal length of that lens is 50mm, then the f stop is 2.0. If the focal length is 100 mm, then the same aperture of 25mm only yields an f stop of 4.0. With a 200 mm focal length the same 25mm aperture has an f stop value of 8.0. f-stop values either halve or double the amount of light passing throught the lens. The following basic f-stop scale reduces the quantity of light by half with each new number:

1.2, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0

The smaller the number, the more light is capable of being passed through the lens. Lenses are ususally rated by the greatest amount of light thay can pass. Apertures are also a relationship to the physical diameter of the glass. In the example above, a 200 mm lens with a max aperture of 2.0 would have to be so huge you probably couldn't carry it. So most zoom telephoto lenses list a f-stop range from wide angle to full telephoto.

Depth-of-field: When a lens is focused upon an object there will be some distance in front and in back of that object that will APPEAR to be ACCEPTABLY SHARP. It is nothing more than an illusion of apparent sharpness. With a fixed focal length lens, you focus upon your subject. If the subject moves closer to you or farther away, you adjust the focus of your lens to keep your subject looking sharp. No lens made can ever focus upon more than one single point, or plane. However, the choice of a smaller (larger number) aperture can create an illusion of more objects in APPARENT focus along the line of the camera's view. Generally speaking, depth-of-field will extend forward 1/3 of its distance from the point of focus and 2/3 back of this same point of focus. However, be aware that depth of focus is also very much dependent upon the eyes of the beholder. I will see much more depth of field in a movie from the back row than i will see from the front row closest to the screen.

It is common in video and in portrait photography to use wide apertures to keep the depth of focus shallow and thus separate the subject from both the foreground and background. Check out a full page head shot in a fashion magazine. The eyes are tack sharp. The tip of the nose and the ears are a bit fuzzy!

Video camera lenses have a "back-focus" feature which allows a focus point to be held through a zoom out, but does not work in reverse.

Hope this helps. Other readers, please add your opinions.
So first off, thanks for the answer.... what's you've written is a really solid, detailed explanation of what I picked up from reading ... but unfortunately it doesn't quite answer my question.

so, given all of that, what happens -- inside the lens -- when you "focus." when you turn that focus ring, what are you doing? are you actualling changing the focal length of the lens in small degrees, varying around the "mean" of (say) 50mm? are you moving another smaller lens around? i mean, the lens itself is the same regardless of the clockwise or counterclockwise orientation, yes? spinning it in place shouldn't do anything (assuming the glass isn't warped or anything.) So what are you doing when you turn "focus" the lens?

that's what i don't understand.

again, though, thanks for the clarification !

JT
Austin, Tx
JT Coleman is offline   Reply With Quote
Old January 10th, 2007, 10:33 AM   #4
Regular Crew
 
Join Date: Sep 2006
Location: Albany Oregon
Posts: 173
Couple of minor corrections - first, 1.2 is not a full f-stop - 1.0 would be the full stop below f1.4. When you increase the diameter of a circle (the aperture) by the square root of 2 (actual value is 1.414) you double the area of that circle, and as stated doubling the amount of light reaching the "film" = 1 full f-stop.

Second, on the subject of back-focus - this term is usually applied to positioning the "film", or image sensor, in a plane whose distance from the aperture of the lens is such that the lens can be zoomed full range and the image will stay in focus, only changing size. If you want to "rack" the focus during a shot, the proper way (assuming back focus has been set properly) is to zoom to full telephoto, focus on the desired target, then back out to wide angle. By doing this, it's then possible to zoom In to a tight shot and not have focus be out (all this is of course assuming that you do NOT have autofocus enabled on the camera)

The way to tell if your camera is properly set up for back focus adjustment is to zoom in tight on a target that's near infinity focus, adjust focus for clearest picture of the target, then zoom out to wide angle and see if that target is STILL in focus. If it's not, then your backfocus (position of the sensor in relation to the lens) is NOT properly set.

Now, to the original question - when you adjust the focus ring on a lens, you are twisting an internal helical ring with small pins of some sort riding in its grooves (think of coarse screw threads here) - you are actually moving one or more of the individual elements of the lens forward or back to change the distance that lens becomes in focus. It's also true that you are slightly changing the effective focal length of the lens, but not by much. Most lenses these days have built-in compensation mechanics that, when you focus, these also slightly change the aperture diameter in order to maintain a constant f-stop -

When you zoom in or out, you are also moving parts of the lens internally - but these DO change the focal length (that's the point of zooming) - a similar compensation ring causes the aperture to open more as you zoom in (longer focal length means that in order to maintain that f-4 setting you need a larger aperture diameter)

To better visualize the mechanics of this, find a large, coarse threaded lag or wood screw - insert two fingernails on the same hand into opposing sides of the threads of the screw, then turn the head of the screw. Think of your fingernails as the pins I mentioned earlier - notice that as you turn the screw, your fingers change longitudinal position along the screw. This is basically how the internal components of a lens work during zoom and focus, each with it's own particular set of internal elements.

More on back focus -

http://videouniversity.com/backfoc.htm

Hope this helps... Steve
Steve Leverich is offline   Reply With Quote
Old January 10th, 2007, 10:43 AM   #5
Inner Circle
 
Join Date: Sep 2002
Location: San Mateo, CA
Posts: 3,840
JT
To answer your question about what happens "INSIDE A FIXED LENS" when focussing.

There are a number of 'elements' whithin any fixed lens. This will vary depending on the quality of the lens. Each 'element' is actually another glass 'lens'. When you 'focus' by turning the barrel of the fixed lens, you are moving one or more of these elements forward or backward within the lens. This allows FOCAL POINT to travel forward and backward WITHIN the defined field of view of the lens, whether it's 28mm or 50mm or 100mm... whatever. No, ideally there is no change of field of view of the fixed lens.

Think of your own eyeball as a fixed lens. (In 35mm photography, the 48-50mm lens is called 'normal' because it approximates the normal field of view for human vision). Your field of view does not change when you change 'focus' on what you are looking at. But your CORNEA does change shape. Muscles in the eye flex and shape the cornea to alter the focal point. (Unless of course, the shape of your cornea is flawed, in which case another 'element' in the form of a glass lens is added to the front of your instrument) But the field of view does not change.

Make sense?

Richard

( I will add, that in zoom and some primes, when you 'rack' focus back and forth, IF you see a slight change in the field of view, this is referred to as 'breathing'. IT does happen. More in zooms than in primes. )




One more coda. A lens designated as 'internal' focusing moves ONLY internal elements. A lens designated as external, or more likely NOT designated as internal, will move the front or rear elements.
Richard Alvarez is offline   Reply With Quote
Old January 10th, 2007, 05:47 PM   #6
Major Player
 
Join Date: Aug 2004
Location: Durango, Colorado, USA
Posts: 711
Thanks for the corrections. I knew I was fumbling abit.

by the way, your explanation is one of the best I have read.
__________________
Waldemar
Waldemar Winkler is offline   Reply With Quote
Old January 10th, 2007, 11:08 PM   #7
Regular Crew
 
Join Date: Sep 2006
Location: Albany Oregon
Posts: 173
Yeah, among all of us I think we covered everything BUT JT's original question :=) (just kidding) Steve
Steve Leverich is offline   Reply With Quote
Old January 11th, 2007, 02:30 AM   #8
Regular Crew
 
Join Date: Jul 2006
Location: Tampa, Florida
Posts: 261
Okay Iím gunna keep this trailer park simple cause whatís written above makes my head hurt. Lol. Basically every lens has 2 lenses (two curved pieces of glass) inside of it.

http://i32.photobucket.com/albums/d1...lberg/lens.jpg

This is my bad drawing of the inner workings of a lens. The blue thing to the left labeled 1 is the front. The filmstrip labeled 3 is the back. Okay soÖ

Light comes in the first one (which in a prime lens doesnít move) and then goes to the next one (number 2). That one moves forward and backwards to focus the image on whatever you want to.

In a zoom lens itís a bit different. The front lens moves forward and backwards and the number 2 lens also moves. If you moved the blue drawing to the left you would see that the top and bottom of the lens would ďhitĒ the yellow ray of light coming in. Well obviously light wont restrict a lens so the light is bent, making your field of view narrower. That will also affect the point that the light crosses the second lens so you will have to move it front to back to focus it. I hope that helps.
Alan James is offline   Reply With Quote
Old January 11th, 2007, 09:31 AM   #9
Regular Crew
 
Join Date: Nov 2005
Posts: 32
Quote:
Originally Posted by Alan James
Okay Iím gunna keep this trailer park simple cause whatís written above makes my head hurt. Lol. Basically every lens has 2 lenses (two curved pieces of glass) inside of it.

http://i32.photobucket.com/albums/d1...lberg/lens.jpg

This is my bad drawing of the inner workings of a lens. The blue thing to the left labeled 1 is the front. The filmstrip labeled 3 is the back. Okay soÖ

Light comes in the first one (which in a prime lens doesnít move) and then goes to the next one (number 2). That one moves forward and backwards to focus the image on whatever you want to.

In a zoom lens itís a bit different. The front lens moves forward and backwards and the number 2 lens also moves. If you moved the blue drawing to the left you would see that the top and bottom of the lens would ďhitĒ the yellow ray of light coming in. Well obviously light wont restrict a lens so the light is bent, making your field of view narrower. That will also affect the point that the light crosses the second lens so you will have to move it front to back to focus it. I hope that helps.
yeah, it does, in fact this whole discussion has gone a long way to helping me puzzle it out.

except one line you just said; "Well obviously light wont restrict a lens so the light is bent, making your field of view narrower."

i get that the field of view narrows, but whats that about light not restricting a lens?

JT
Austin, Tx
JT Coleman is offline   Reply With Quote
Old January 11th, 2007, 01:38 PM   #10
Regular Crew
 
Join Date: Jul 2006
Location: Tampa, Florida
Posts: 261
Sorry about that. I think of everything as physical things. Light isn’t like a physical thing but a lens is so one of the two has to give when they all start moving. It will obviously be the light. In school I was shown this with six strings tied to a single point that all went out to form what looked like a cone. We then put a ring around the strings and pulled it forward, the ring hit the strings and then string cone narrowed. The strings were rays of light and the ring was a lens. Its easier explaining it in person.

This is an off topic question but where in Austin is Trouble Maker studio? I might be in Austin soon and I want to drive by it just to take a look.
Alan James is offline   Reply With Quote
Old January 11th, 2007, 04:47 PM   #11
Regular Crew
 
Join Date: Nov 2005
Posts: 32
Quote:
Originally Posted by Alan James
Sorry about that. I think of everything as physical things. Light isnít like a physical thing but a lens is so one of the two has to give when they all start moving. It will obviously be the light. In school I was shown this with six strings tied to a single point that all went out to form what looked like a cone. We then put a ring around the strings and pulled it forward, the ring hit the strings and then string cone narrowed. The strings were rays of light and the ring was a lens. Its easier explaining it in person.
yea, ok, thanks ... i got your meaning, the words just through me off a bit.

oh, here's another question:

>> (In 35mm photography, the 48-50mm lens is called 'normal' because it approximates the normal field of view for human vision).

..is this the FOV of one eye? or both eyes?

Quote:
Originally Posted by Alan James
This is an off topic question but where in Austin is Trouble Maker studio? I might be in Austin soon and I want to drive by it just to take a look.
i believe they're behind the gate at Austin Studios, which is at the old Austin airport off of east 51st street. that said, i'm pretty sure they don't have a public office facility or reception area or anything ... haven't seen it myself, the only time i go in there is for odds and ends (delivering something to AFS or renting a dolly from chapman, who has one of the hangars.)




JT
Austin, Texas
JT Coleman is offline   Reply With Quote
Old January 11th, 2007, 05:16 PM   #12
Inner Circle
 
Join Date: Sep 2002
Location: San Mateo, CA
Posts: 3,840
If you look through a 48-50mm lens on a 35mm camera, things will look 'normal' that is, not larger or smaller than normal vision. YMMV but that's the closest comparison.
Richard Alvarez is offline   Reply With Quote
Old January 12th, 2007, 10:17 AM   #13
Regular Crew
 
Join Date: Nov 2005
Posts: 32
Quote:
Originally Posted by Richard Alvarez
If you look through a 48-50mm lens on a 35mm camera, things will look 'normal' that is, not larger or smaller than normal vision. YMMV but that's the closest comparison.
so ... one eye?

JT
Austin, Tx
JT Coleman is offline   Reply With Quote
Old January 12th, 2007, 11:53 AM   #14
Regular Crew
 
Join Date: Jul 2006
Location: Tampa, Florida
Posts: 261
If you put a a camera with a 50mm lens on it up to one of your eyes and open both eyes there isnt and zooming. Everything will look the same size in the lens as it does in you open eye. Try it out and you will see what I mean. 50mm is only normal on 35mm film because of the frame size. A 50mm SLR lens would be a longer lens on 35mm film and even more zoomed in on super 35, becuase each format is smaller then the one before. 35mm SLRs film runs sideways and 35mm movie cameras run up and down. So the frame size on a 35mm SLR is almost 4 times bigger then on a movie camera (aside from vista vision).
Alan James is offline   Reply With Quote
Old January 12th, 2007, 05:17 PM   #15
Regular Crew
 
Join Date: Sep 2006
Location: Albany Oregon
Posts: 173
Alan, you're not quite right on this - original 35mm movie film, IIRC, had a frame size of 18 x 24mm with the 24mm dimension running across the film - so when shown in a normal projector, the image was wider than it was tall (as you said).

Some of the first 35mm still cameras, including later Olympus, STILL used this format which at that time was called "single frame", sometimes "half-frame.) - but most 35mm still cameras used what was then termed "double frame", and later (in still cameras, anyway) became known as "full frame" - this frame size was 24 x 36 mm, exactly TWICE the surface area of a movie film's 35mm format.

In any case, the lens that is considered "normal" for ANY format up to the newer "wide" formats will be the same, or slightly longer than, the diagonal of the frame. With "full frame" 35mm, the diagonal is 43.27mm and in the 35mm still camera world, lenses from 45mm to 58mm were sold as "normal" - the longer ones came a bit later and I think it was maybe because of portraiture work - a longer than normal lens "flattens" details (less bulbous noses, etc) and for that reason a good lens for portraiture in 35mm is somewhere between 85mm and 135mm; one of my favorite Nikon lenses for that purpose is the 105mm.

I've yet to dig quite that deeply into sizes/shapes of CCD chips, don't even know if a "1/3" chip is called that because the sensor area is that size diagonally, or something different - but it's definite that our old "Normal" 50mm lenses sure wouldn't be "normal" on one of these, since I see lenses on DV cams that go from 4-80mm and are only SLIGHTLY wide angle at the 4mm setting -

In my own DV work (mostly industrial video) I've had to almost always use as much as a 0.5 x wide angle lens in order to get large industrial equipment even partially in the frame...

So, maybe someone who IS familiar with the newer (DV) 16:9 formats can tell us what a so-called "normal" focal length would be for THAT format - I'm wondering if it's still gonna be just a bit longer than the diagonal, or if that changes due to the different ratios... Steve
Steve Leverich is offline   Reply
Reply

DV Info Net refers all where-to-buy and where-to-rent questions exclusively to these trusted full line dealers and rental houses...

Professional Video
(800) 833-4801
Portland, OR

B&H Photo Video
(866) 521-7381
New York, NY

Z.G.C.
(973) 335-4460
Mountain Lakes, NJ

Abel Cine Tech
(888) 700-4416
N.Y. NY & L.A. CA

Precision Camera
(800) 677-1023
Austin, TX

DV Info Net also encourages you to support local businesses and buy from an authorized dealer in your neighborhood.
  You are here: DV Info Net > The Tools of DV and HD Production > Open DV Discussion

Thread Tools Search this Thread
Search this Thread:

Advanced Search

 



Google
 

All times are GMT -6. The time now is 03:58 PM.


DV Info Net -- Real Names, Real People, Real Info!
1998-2017 The Digital Video Information Network