Infinity

[julie32]

Can someone please briefly explain the concept of infinity to me? Of course, as it relates to photography! (otherwise we'd be here all day....though I may be here all day anyway.)

thanks,
J

 

[RKW3]

"Infinity focusing locks the focus at the far end of the standard autofocus range so that objects at a distance appear in sharp focus. Sometimes labeled with an infinity symbol, this setting is great for landscape photos.
Because the focusing ranges for each mode vary from camera to camera, check your digital camera's manual to find out what mode is appropriate given the distance between you and your subject."

that's from this link I found in a google search: http://tech.yahoo.com/gd/working-with-your-digital-camera-s-autofocus-mode/153054

 

[julie32]

Thanks Robbie....

Are you able to explain it in layman's terms? Like.. what's the purpose of it, why can't we "focus on infinity" in AF mode? or how it relates to everything else in the photo etc...

thanks

 

[fatsheep]

I'd like to hear a good explanation on this as well. *subscribes*

 

[Ls3D]

Well since no pros have chimed in, yet - here is my stab:

why can't we "focus on infinity" in AF mode?

Because your focusing engine will find objects closer that the furthest point and is designed for sub infinite focus. Besides it is just so natural to manual focus the distant objects in your FOV. Or sometimes just set it for X number of feet from where your shooting. I'm still experimenting with this myself.

Are you able to explain it in layman's terms

Being a layman, perhaps. Infinite focus is for times when you just want to capture the whole scene as is with everything, (or nearly everything in focus). A field of flowers that also has an interesting landscape and mountains in the distance for example. Conversely DOF shots are more creative in nature as you have explicitly sought out to soften backgrounds or to isolate an subject.

how it relates to everything else in the photo etc...

Loosing me now, but be aware large apertures (small f/stops) will allow too much light in (scatters) and therefore blur objects outside the focal point. Small apertures (large f/stops) will limit the amount of light and are key to creating images with near infinite focus.

Now that I've spoken up I'm sure some more people will have something to say, so I stand corrected in advanced.

-Shea :mrgreen:

 

[Josh66]

Being a layman, perhaps. Infinite focus is for times when you just want to capture the whole scene as is with everything, (or nearly everything in focus). A field of flowers that also has an interesting landscape and mountains in the distance for example.

I'm not a pro either, but...

I think this is a situation where you would want to focus to the hyperfocal distance of your lens. Google "hyperfocal distance" if that term is unfamiliar to you.

 

[yeti]

Here is my two cents:

juli32, I want you to look out your window on an object fairly close. You have two eyes (I assume) and as your eyes focus on an object, they no longer look straight ahead, but rather slightly cross-eyed.

Looking at an object further away, your eyes get less cross-eyed to give you depth perception again.

Looking at things even further away, your eyes start making smaller and smaller adjustments until you finally set your two eyes to watch straight ahead. Your eyes stop being cross-eyed entirely. Your brain cannot make so fine adjustments to your eye location and it doesn't need to: something so far away is unlikely to be a danger to you. To be exact, you don't have depth-perception for objects more than 40m away. No human does.

Well your camera doesn't get cross-eyed but it has a much larger front element. As you focus, you select light coming from certain angle to be in focus. As you focus to infinity, you receive light coming from very distant objects. Your lens receives them all at angles so similar that your lens can't tell the difference. This means that they are all in focus, regardless of their distance from you. This is focusing at "infinity".

I tried to explain it in layman's terms. But at the end of the day this is nothing more than the concept of limit, and as far as focal distances go, the limit is finite.

 

[Saint-Brown]

Even though I know what it is, it is alway good to hear it explained

 

[Alex_B]

if you focus on the moon, then this in first approximation is focus to infinity. This also works with AF.

Just most objects are not infinitely away, not even approximately! They are closer.

Infinity means in a simplified explanation, that light which comes from an object at infinite distance is parallel. hence to focus, the lens has to converge this parallel light in one spot on the film/sensor.

i think some illustrations might help here

 

[julie32]

thank you for these helpful explanations! YETI--- I laughed so hard when you said "you have 2 eyes, (I assume) ......"
I did exactly what you told me to do ...and I totally understand what you're saying, so thank you very much!!

 

[fatsheep]

Infinity means in a simplified explanation, that light which comes from an object at infinite distance is parallel. hence to focus, the lens has to converge this parallel light in one spot on the film/sensor.

That makes sense. However, is "infinity" (in photography terms) a certain distance away? Or does it depend on the lens? Is there a way to find out how far away infinity is for a given lens?

 

[chris]

Infinity is a number greater than any assignable quantity, simply, it is an unimaginably large number.

Light from an object an infinite distance away would arrive at a positive lens as parallel rays and would be focussed behind the lens at a distance equal to the focal length measured from the rear nodal point of the lens. Objects nearer to the lens would be focussed farther behind the lens.

 

[yeti]

That makes sense. However, is "infinity" (in photography terms) a certain distance away? Or does it depend on the lens? Is there a way to find out how far away infinity is for a given lens?

As I said, infinity is just a limit. You don't have objects at an infinite distance, even the Universe itself has a limit beyond which you can't see anything (an object approaching the rim of the Universe gets more and more red-shifted until you loose it).

When focusing at "infinity" you tell the lens to a select parallel light beams. Strictly speaking if two light beams are parallel, they cannot come from a single point in space and therefore cannot be coming from any single object, but this is often used to approximate light coming from an object a long distance away, so long that the light beams coming from it *APPEAR* parallel.

What lens consider "infinity", of course, would differ from lens to lens. "Infinity" for a 28mm lens would be different from that of a 200mm lens, as the 200mm lens is built to zoom into and focus objects much further away.

Please don't get stuck on the term used here and instead simply consider its purpose. It's a limit the way your precalculus textbook defines it, nothing more: you can only approach it, you can't become it as it is not a well-defined number.

 

[chris]

What lens consider "infinity", of course, would differ from lens to lens. "Infinity" for a 28mm lens would be different from that of a 200mm lens, as the 200mm lens is built to zoom into and focus objects much further away.

The infinity setting on any lens should be the position at which parallel light rays are focussed on the sensor/film and this should be so for any lens. A 200 mm lens is not designed to focus on objects farther away than a 28 mm lens but it does have a magnifying effect compared to a 28 mm lens if an object is viewed from the same position due to the narrower angle of view.

There is usually a difference in the close focus range with a 28 mm lens being able to focus on closer objects than a 200 mm lens but this is due to the practicalities of producing a telephoto lens with sufficient extension to focus on close objects - though Nikon (and probably other manufacturers) produce a 200 mm macro lens that will focus down to 0.5 m without the need for extension rings. If you put sufficient extension rings on any 200 mm lens you can get it to focus just as closely as a 28 mm lens.

In practical terms anything more than a few hundred metres away could be treated as being at infinity for all but the longest lenses. For subjects such as landscapes it is usually best not to focus at infinity but to focus at the hyperfocal distance to maximise depth of field.

 

[Helen B]

In terms of photographic optics, it's generally safe to assume that infinity can be considered as anything that is more than 1000 times the focal length away.

Some of us, in certain circumstances, prefer to focus on infinity or near infinity instead of the hyperfocal distance to give preference to critical sharpness at the horizon.

Current AF Micro-Nikkors achieve their remarkable close-focusing ability because they shorten their focal length as they focus closer. At its closest focus the current '200 mm' AF has a focal length significantly less than 200 mm.

Sorry for the rushed contribution.

Best,
Helen