# Function of focus ring



## k.udhay (Mar 29, 2013)

Hi. What exactly happens in the lens barrel when I turn the focus ring of my DSLR (Nikon D3200)? What I see as the effect is the objects at increasing / decreasing distance become sharp as I turn it. I am curious to know the mechanism and physics that does this. Thank you very much!


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## Judobreaker (Mar 29, 2013)

You're probably talking about the focus ring on a lens (as far as I know the camera itself does not have a focus ring).

Basically each camera-lens is made out of a combination of glass/plastic lenses (usually referred to as elements, Lens (optics) - Wikipedia, the free encyclopedia). These elements are grouped together in groups, each with their own function. The amount of element groups varies per lens.
Somewhere among these lens groups there is one group that is used for adjusting focus. This group is moved backward or forwards using the focus ring which results in a change of focus in your camera.


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## bratkinson (Mar 29, 2013)

Disclaimer: I'm no optics engineer nor have I ever done any optical/optitian work.

Back in the early days of photography, cameras had a single lens on them that was moved nearer/closer to the flat focal plane on the back of the camera that the 'old time' photographers would see as an upside down image under the black tent they used.  Acting just like a magnifying glass you likely used as a child, the image would get bigger/smaller and when the lens was at just the right position, the image was clearly focused.  Once focused, they slid in the film in a holder, removed the cover, and exposed the film for a few seconds while the subject held perfectly still.  Then they put the film cover back in its original position to stop the exposure.  If I were to take my glasses off and move them away from my head, the images get blurry, and distorted as well, as they are 'pre-focused' for my eyes.

Modern photography lenses such as those used on DSLRs are far more complex in that they typically contain anywhere from 4 or 5 separate lenses up to as many as 12 or more.  Some of these lenses are convex (bulge out) and some are concave (thinner in the middle).  The reason for multiple internal lenses is to correct some of the normal limitations of the lenses themselves.

Lenses have a number of properties that need to be corrected to varying degrees.  Among them is the angle of light entering each lens (piece of glass or whatever it's made of) gets altered at the air/glass boundary going in and again at the glass/air boundary going out.  That angle change varies from small to large or vice versa depending on the initial angle of the light relative to the front side of the lens.  Additionally, each color of light (wave length) reacts differently at the air/lens boundary as well.  Add to that, what the glass is made of, eg, what KIND of sand, and what chemicals are added, also alter the behavior of a light beam as it is travelling through the lens itself.  Lenses can even alter the color of the light slightly as well.  

The lens makers of the 1800s didn't know about all these issues so they did nothing to correct them.  Only the modern technologies of the last 75 years or so could identify the issues, and hence, they could be corrected.  Think of it like modern prescription medicines.  (Theorectical situation, not real, follows)  If I take pill A, it will solve my stomach acid problem, but gives me constipation.  So, I take pill B.  That solves the constipation, but it gives me headaches.  So, I take aspirin on top of that to take care of the headache, and so on.  The same is true with lenses.  One lens 'corrects' some or all of the 'issues' of the lens or lenses in front of it, but may add a new one to the mix, and on and on.  The end result is that the image coming out the back and onto the sensor (or viewfinder) is a miniature version of what the lens 'sees'.  

But like my glasses if they aren't positioned properly on my nose, things are not quite in focus, so I move them.  The same is true in the camera.  If all the elements of the mounted lens aren't in just the 'right place' for the camera sensor, it will be out of focus.  So, by turning the focusing ring, one or more internal elements in the lens itself will move somewhat until you stop turning the ring...or the auto-focus feature of the camera/lens 'tells it' to stop moving the elements.  Obviously, manufactured lenses can only be focused on one point at a time, so person 15 feet away could be in focus, but someone 20 feet behind them would not be.  If you want to have that person in the rear be in focus, the lens has to be re-focused for that subject. 

Lastly, a warning...depending on the specific lens model and manufacturer, turning the focus ring on a lens while the auto-focus function is enabled MAY have no effect AND/OR MAY cause physical damage to the mechanism within the lens.  Not all lenses have the ability to be manually focused after the autofocus mechanism has completed its task.  Check the user instructions for each of your lenses regarding adjusting the focusing ring while AF is active.


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## KmH (Mar 29, 2013)

bratkinson said:


> Back in the early days of photography, .......... the 'old time' photographers would see as an upside down image under the black tent they used.


This has not changed, in so far as the lens still inverts the image onto the camera image sensor (or film).

In the 'old days', to focus the camera the photographer was putting his eyes where the film was going to be placed to make the image.


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## vintagesnaps (Mar 29, 2013)

When you turn your focus ring, that moves the lens forward and back - you can see that the lens moves farther away or in closer. It's a different way of doing the same thing that older cameras with bellows do - as you slide the bellows forward on a track the lens moves farther away from the surface where the image will be recorded. Prior to bellows older cameras were more or less two boxes that slid in and out to move the lens forward and back.


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## AaronLLockhart (Mar 29, 2013)

On a D3200 with a kit lens, you can actually see the front lens element moving back and forth when focusing, can you not? Doesn't it come with the same 18-55 VR that all Nikon entry level dSLR's come with? If so, that lens technically doesn't have a "focus ring." The entire front lens element assembly rotates and moves back and forth. 

Anyway, to answer your question, the focus "ring" moves the front lens element system (there's actually more than one piece of glass in the front lens element) in relation to the rear lens element, depending on how far your distance is to the subject. Your focal length changes the distance of the back lens element in relation to the sensor. For an example, at 18mm on a kit lens, the rear element is 18mm away from the sensor. at 55mm, it's 55mm away from the sensor. (Not sure if you even wanted to know that, I just thought I'd throw it in there since we were on the topic of lens elements and how they move.)


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## TCampbell (Mar 29, 2013)

In a very simple camera "lens" there are two elements (in reality there are more).  

There's a notion called "back-focus" which can mean two entirely different things in photography, but in lenses and bodies there's a distance from the rear element to the focus plane inside the camera (your camera body has a mark on it somewhere... it looks like a circle with a flat line drawn through it.  That line on the body marks the position of the focus plane inside the camera.)  "Back-focus" in this context is the distance from the rear-most element (which does not move) to the focusing plane (which also does not move.)

To focus the lens, the front-most element is moved nearer or farther -- you're changing the spacing between the front and rear elements.  That alters the focus distance in front of the camera.

Ok... so that's the "simple" version.  And for conceptual purposes, that's all you need to know.

The truth is, there are many more elements inside the lens and they move as groups.  If you really had just ONE lens element, the image quality would be awful.  Take a simple magnifying glass (single lens design) and hold it over a newspaper.  Notice the text in the center is clear and magnified, but near the edges the text is distorted.  Worse still... even though the newsprint is just "black and white" if you look at the text near the edges, you'll notice red and blue color 'fringing' on your text.  Red on one side, blue on the other (if it's black text on a white page then I think you get blue fringing on the "inside" edges of the letters (closest to the center of the lens) and red on the "outside" edges (farthest from center).  But if you had white text on a black page you'd get the opposite.)

That color fringing is called "chromatic aberration" (or "CA" for short).  It's caused by the fact that different wavelengths of light bend by different amounts as they go through glass at an angle.  The edges of the glass lens behave like a prism... splitting light into a rainbow.  Great for pretty effects in the sky... really bad for photos.  The CA technically means that the different wavelengths are coming to focus at a different distance.  Blue is easiest to bend ... so it wants to focus nearer.  Long waves (reds) bend less... so they come to focus farther away.  You get the colors (even from black & white newsprint) because "white" is really all the other colors combined.

So you know when they say "you can't unscramble an egg"?  Well... it may not work with eggs, but you CAN unscramble light (pull it apart again even though it was previously blended together.)  For photographic purposes, we don't want to pull it apart.

One day, some folks realized that various configurations of extra elements could correct (or at least partially correct) for these problems.  Lenses evolved.  Modern lenses may even have multiple focusing groups and it gets more complicated if it's a zoom lens.

More elements are "better" in that they can more finely tweak the light to produce an optimum image, but "worse" in that each time light has to pass through a glass-to-air or air-to-glass transition there is yet another opportunity for reflection, loss of light, ghosting, flaring, and other horrors.  So the number of elements is a trade-off game.

Oh... and the other definition of "back focus"... if you focus your camera on your subject's eyes (at least you *think* you focused on your subject's eyes) but in reality their ears are more sharply focused and their eyes are a bit soft... that's also called "back focus".  It is, of course, completely different and unrelated to the distance from the rear-most element to the focus plane so you have to watch the context when you see people use that term.

You can read some history here:  LensRentals.com - From Petzval?s Sum to Abbe?s Number

(Roger Cicala at LensRentals writes some really great blog articles.)


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## KmH (Mar 29, 2013)

Many DSLR lenses don't move the the front/group lens elements when they focus.
Some of the lenses that do move the the front/group lens elements, also rotate the front/group lens elements, which causes problems if you want to use a GND filter, CPL filter, or a petal type lens hood.

Note than Nikon has many IF -Internal Focusing - lenses.



> *nikonusa.com*
> *IF Lens
> *A NIKKOR lens in which only the internal lens group shifts during focusing. Thus, *IF* NIKKORS *do not change in size during AF operation*, allowing for compact, lightweight lenses capable of closer focusing distances. These lenses will be designated with the abbreviation IF on the lens barrel.


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## 2WheelPhoto (Mar 29, 2013)

KmH said:


> Many DSLR lenses don't move the the front/group lens elements when they focus.
> Some of the lenses that do move the the front/group lens elements, also rotate the front/group lens elements, *which causes problems if you want to use a GND filter, CPL filter, or a petal type lens hood*.
> 
> Note than Nikon has many IF -Internal Focusing - lenses.
> ...



+1

another reason to use only top-shelf glass


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## AaronLLockhart (Mar 29, 2013)

2WheelPhoto said:


> another reason to use only top-shelf glass



I agree with that statement over all. However, top end Nikkor glass isn't the only glass of Nikon's that are IF-ED. My 18-70mm f/3.5-4.5 AF-S DX that came on my D70 (Still have the lens) is an IF-ED lens.


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## 2WheelPhoto (Mar 29, 2013)

Yes sir i agree 100%. I should've said "67th reason to use top shelf glass". The 18-70 has other limitations whether IF or not =)


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## k.udhay (Mar 30, 2013)

TCampbell said:


> In a very simple camera "lens" there are two elements (in reality there are more).
> 
> There's a notion called "back-focus" which can mean two entirely different things in photography, but in lenses and bodies there's a distance from the rear element to the focus plane inside the camera (your camera body has a mark on it somewhere... it looks like a circle with a flat line drawn through it.  That line on the body marks the position of the focus plane inside the camera.)  "Back-focus" in this context is the distance from the rear-most element (which does not move) to the focusing plane (which also does not move.)
> 
> ...



Thanks, Tim. Sorry for extending this question. What happens when I 'zoom' by turning the lens barrel?


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## BrianV (Mar 30, 2013)

You have two optical mechanisms at work with a zoom lens: changing distance between lens groups to change the effective focal length (EFL), and compensating the focus as the EFL changes. Focus compensation can be done mechanically with cams to change the back-focus, or optically by moving elements. The equation for focus length gives back-focus distance, and the "lens makers formula" gives effective focal length as a function of distance between groups of lenses.

 Back Focus: 1/fl= 1/dist+1/back


EFL=   (FL1*FL2)/ (FL1+FL2-separation)


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