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Is focus determined by more than just distance?
I have been more than a bit interested in how focus is determined in a camera. I have been reading this forum, other websites and any book that I can get my hands on from local libraries and bookstores for those books that I have liked. That being said, I am new to the SLR world digital or film, but am very excited about the opportunities that I will have going forward..

All that being said, I finally feel that I have an understanding how shutter speed, aperture and ISO impact my image and what each will do for me. Well, at least a technical understanding, the practical understanding will come from continuing to shoot and I realize that. That is a big part of the excitement..

The piece that I am missing is around focusing. I have not been able to find much information about how this works. I think I have finally figured it out, but wanted to post here to gain some feedback from everyone that has been so helpful during the past few months..

Is focus achieved strictly based on the distance that an object is away from the camera? If I focus on a tree that is 30ft away, is the focus determined based on the fact that the tree is 30 ft away or is it more than that? Will anything in the frame that is the 30 ft away be in focus regardless of where in the image it is located? I read about getting focus then recomposing, so that continues to make me think it does not take anything into account other than just the distance from the subject. I have played around with this as well as some manual focusing. It is not sinking in as I am a logical thinker how it works, not just happy that it works..

Thanks in advance for helping me piece together another portion of this puzzle..

Duane..

Comments (18)

For the most part yes, focus is determined by the distance to the sensor or film..

You asked if you focus on a tree 30 feet away if everything 30 feet away will be in focus. To the best ability of the lens yes. Most lenses are not as sharp at their edges and corners as they are in the center. In other words something 30 feet away on the edge of the frame may look softer than something 30 feet away that's in the middle..

Another thing to consider is flatness of field. This mostly comes into play when focusing on things close to the camera. Typically macro lenses are optimized to have a very flat plane of focus. The downside is macro lenses may not be quite as good at infinity. Most non macro lenses in the short to medium focal lengths have a slightly curved field of focus. None of this is probably nothing for you to worry about unless you're doing some specialized photography..

You mentioned you have an understanding of aperture. I assume this means you understand depth of field and hyperfocal distances. I won't go into those..

Enjoy...

Comment #1

Mrxdimension wrote:.

None of this is probably nothing for you toworry about unless you're doing some specialized photography..

Sorry, bad edit..

None of this is probably worth worrying about unless you're doing some specialized photography...

Comment #2

Duane0524 wrote:.

Is focus achieved strictly based on the distance that an object isaway from the camera?.

Yes.. distance only..

If I focus on a tree that is 30ft away, is thefocus determined based on the fact that the tree is 30 ft away or isit more than that?.

The camera Auto Focus systems look for an optical condition that only happens when the image is sharpest. When the image is sharp the systems then stop trying to get the target any sharper..

However, the fact that it happens to be one distance or another is of no particular interest to the focusing systems in most cameras, because the systems themselves are passive. One exception was the 'Sonar' (sound ranging system) of some Polaroid cameras, which measured the actual distance by bouncing sound waves, and then set the lens accordingly..

Will anything in the frame that is the 30 ft awaybe in focus regardless of where in the image it is located?.

Yes, anything in frame and at the same distance as the focus target will be practically equally sharp. Therefore, if you are square on to a brick wall, say, and focus anywhere on that wall, the whole wall surface will be sharp. Indeed, the plane of sharp focus may be considered as like a sheet of glass erected in front of the camera, and at 90 to the axis of the lens..

Thanks in advance for helping me piece together another portion ofthis puzzle..

Stop worrying. It is not complicated.Regards,Baz..

Comment #3

Mrxdimension,.

Thanks for speedy reply. I was thinking that I was on the right track with it being based on distance. I understand that not all lenses will be sharp throughout, but it is the concept of items being in focus at the same distance that I was looking for. Someday I will own lenses that will be sharp throughout. .

As for the Macro stuff, thanks for the extra information. You are right, I am not worried about that..

Thanks again for taking the time to respond..

Duane..

Comment #4

Baz,.

Thank you for your response. How does the AF attempt to make the subject shaper? I am confused by that if the distance is all that is necessary?.

I would think the lens would know that if the subject is X ft away, then focus is set to Y. How could it go beyond that type of process?.

Thanks again.Duane..

Comment #5

Duane0524 wrote:.

I would think the lens would know that if the subject is X ft away,then focus is set to Y. How could it go beyond that type of process?.

Thanks again.Duane.

How would the lens know that the subject is x ft away? It wouldn't unless it were a rangefinder camera triangulating from two optical finders. When we manually focus, we turn the ring until it looks sharp. We can then read the distance on the scale. We can also measure the distance and dial it in manually..

Baz was right. Now, given that, I'm personally not sure as to the method the camera uses to determine that the image is sharp. Point and shoots and live view use contrast I believe..

Cheers, Craig..

Comment #6

Duane0524 wrote:.

Baz,.

Thank you for your response. How does the AF attempt to make thesubject shaper? I am confused by that if the distance is all that isnecessary?.

I would think the lens would know that if the subject is X ft away,then focus is set to Y. How could it go beyond that type of process?.

In manual focusing YOU tell the lens how far to focus by either setting the distance on the lens (scale focusing) or looking through the viewfinder and turning the lens until the required zone/distance in the subject is sharp to your eye... (TTL focusing). Having focused visually, you may read off the distance from the lens scale index, if you so wish..

However, with Auto Focus the system itself does the focusing, and it is NOT interested in the distance..... it is only interested in whether the image is sharp, or not. Just like manual focusing by viewing through the lens, when the AF has achieved sharp focus, you can read off the distance from the lens scale index, if you so wish..

Note that in NEITHER case do you OR the camera actually need to know the distance to the subject.... and it is merely incidental that the distance information can be extracted from the camera focusing scale AFTER the operation is complete..

So although distance is all that determines the setting of focus, it is not necessary for AF systems to *know* distance in order to get the subject sharp..

Now, if you are interested in how the various AF systems set about getting the image sharp, (and, as a result, the lens distance setting correct) take a look at this Wikipedia article.....

Http://en.wikipedia.org/wiki/AutofocusRegards,Baz..

Comment #7

Craig,.

Thanks for the response..

How would the lens know that the subject is x ft away? It wouldn'tunless it were a rangefinder camera triangulating from two opticalfinders. When we manually focus, we turn the ring until it lookssharp. We can then read the distance on the scale. We can alsomeasure the distance and dial it in manually..

I guess that is the point of my question. I played around with the manual focus and wanted to understand how focus worked. I assumed it was based on distance because how could the camera know if the image was sharp or needed to be sharper. I assumed that the AF determined the distance, then did whatever process it needed to let the lens to know the setting in order to be in focus. Are you saying the camera actually determines if the subject is sharp as opposed to just how far away it is?.

Thanks,Duane..

Comment #8

Baz,.

Thank you, that is what I needed. Distance is a result of the image being in focus, not the reason that it is in focus. How the auto focus figures out what is sharp will remain a mystery for me as I have the answer to what was bothering me..

Thanks for taking the time to explain it twice, I really do appreciate it..

Duane..

Comment #9

The camera can't just know how far away the subject is without some criteria. How would it know that a tree is 10 meters away just because that's the distance it is?Cheers, Craig..

Comment #10

This was probably the unintentional confusing, but true fact, brought up earlier. Some old Polaroid's used sonar (sound waves, like bats, dolphins, and submarines) to detect the distance to the object. Then it set the lens. I'm not aware of any recent camera doing autofocus this way...

Comment #11

Thank you again for the response, I confused my self by getting hung up on the distance. BAZ's second response made everyone's response make sense..

Thanks again for your assistance.Duane..

Comment #12

It was self inflicted confusion..

Thanks for helping to set me straight..

Duane..

Comment #13

Barrie Davis wrote:.

Yes, anything in frame and at the same distance as the focus targetwill be practically equally sharp. Therefore, if you are square on toa brick wall, say, and focus anywhere on that wall, the whole wallsurface will be sharp. Indeed, the plane of sharp focus may beconsidered as like a sheet of glass erected in front of the camera,and at 90 to the axis of the lens..

I would have thought that a spherical surface would more accurately describe the area of perfect focus. If we postulate a long wall and a wide lens, parts of that wall that are included in the image will be several times as distant from the image plane as the central portion on the wall. I've done small group shots with a fast lens and found that the folks on the ends aren't sharp though everyone is on the same plane. I tell myself, "well, what did you expect, when your DOF is only a couple of inches? Stop down or arrange the people in a shallow arc."Regards, John...

Comment #14

I don't think the field of focus is quite spherical, but less radical. I have no hard date, just an empirical gut feel. More like shallow concave arc..

That's the difference between a planar lens and a "normal" lens. At wide aperture I'd consider your case a special situation. I'd use my "normal" lenses, stop down and be done with it...

Comment #15

Obie1 wrote:.

Barrie Davis wrote:.

Yes, anything in frame and at the same distance as the focus targetwill be practically equally sharp. Therefore, if you are square on toa brick wall, say, and focus anywhere on that wall, the whole wallsurface will be sharp. Indeed, the plane of sharp focus may beconsidered as like a sheet of glass erected in front of the camera,and at 90 to the axis of the lens..

I would have thought that a spherical surface would more accuratelydescribe the area of perfect focus..

Well, that depends on the quality of the lens in question, doesn't it?.

Even with wide angle designs, the AIM of the designers is to get a flat object field falling on a flat image field....

.... although with super-wides and fisheyes they may stretch their parameters and allow the exceptionally deep DoF characteristic of such lenses cover any "curvy" shortcomings.... just as those lenses are not rectilinear drawing, either.Regards,Baz..

Comment #16

Auto focus on a camera does not depend on the distance. the user selects a house as the subject. then autofocus then looks for contrast differences in adjoining parts of the subject. then tries to maximize the contrast separation of the 2 adjoining sections. when it does, it is in focus..

BUT, it works, the problem is in lowlight conditions or any other that have little contrast. the camera will serch for the focus because it cannot find enough contrast change to lock on to. a ver7y bad condition for auto focus is to try to shoot during a fog. the auto focus will have a fit..

See the cambridge article just above for more...

Comment #17

Baz pretty much covered everything. Let me add a few notes:.

Duane0524 wrote:.

I have not been able to find much information about how this works..

There are two very different autofocus systems used in digital cameras..

"Contrast detect" autofocus relies on data from the main sensor, and "hunts" the lens to achieve the maximum contrast within the designated focus point area(s). This form of autofocus is generally used in digicams, although a few DSLRs offer it as an option in Live View..

"Phase detect" autofocus relies on separate autofocus sensors, and is used on every DSLR that I'm aware of. It's based on the same concept that split-image viewfinders use, and measures how far out of focus (and in which direction) a selected contrasting edge is. This allows direct correction of focus without hunting. More detail can be found here:http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf.

Is focus achieved strictly based on the distance that an object isaway from the camera? If I focus on a tree that is 30ft away, is thefocus determined based on the fact that the tree is 30 ft away or isit more than that?.

If by "focus" you mean the camera's choice of focus distance, it's not achieved by measuring distance but rather by measuring the visible effects of focus vs. out of focus. Those visible effects are contrast for contrast detect systems, and relative positions of projected image for phase detect systems..

Will anything in the frame that is the 30 ft away be in focus regardlessof where in the image it is located?.

I want to add to Baz's explanation that becausewith rare exceptionslenses for still photography are designed to have a flat focus field, the "30 feet away" applies only directly in front of the lens. As you go off at various angles, the focused distance increases. Focus is not an arc or hemisphere at a constant distanceit's a flat plane..

As noted, if you point your camera directly at a brick wall and properly focus at that wall, the entire wall should be in focus (plus or minus the effects of imperfections). This is even though the bricks toward the edge are actually much farther away from the camera than the brick in the center..

This is why "focus and recompose" can be problematic at close distances with narrow depth of field...

Comment #18

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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