How far away is infinity? (optically speaking, of course)

Rob-F

Rob-F

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A better way to put it, is "how far away from a camera/lens is far enough that it can be considered infinity?"

I feel the distance should be expressed in focal lengths. There is a precedent for defining where the macro range starts--that's ten focal lengths or less (eleven as measured from the film plane). The image has to be at least one-tenth the size of the object. But has the distance to "practical infinity" ever been defined?

I have some NBS (National Bureau of Standards) test targets that are supposed to be photographed at 26 focal lengths, for a 25:1 reproduction ratio. I'm sure NBS didn't think that was far enough away to call it infinity. They must have considered it a good average distance: not macro, but not infinity either. A practical distance for actual photography.

It seems to me that infinity could be defined as that distance, compared to which the focal length is negligible. So all we have to do is define "negligible" and we are home free. So for instance, maybe a focal length that is only 1/100 of the camera-to-subject distance would qualify. Then for a 100mm lens, infinity would be at 10,000mm; 10 meters, or 32.8 feet.

Hmm, that might seem a bit too close. OK, how about an order of magnitude greater than the NBS target distance? That's 250 focal lengths, or 25,000mm for our 100mm lens. So 25 meters, or 82 feet. Not sure I could tell the difference between that and infinity on the scale of a 100mm lens.

Or maybe 1000 focal lengths. Just to be on the safe side. Surely the focal length of the lens is negligible when it's only 1/1000 of the camera to subject distance? Now we are at 328 feet.

So where is this going? People advise checking infinity focus on a target some 2 or more miles away. Except for a very long telephoto lens, I question if that's necessary.

Agree or disagree?
 
I read somewhere that infinity is 4000*focal length. So for 50mm lens that'll be 4000*50=200.000mm = 200m. A safer bet would be to aim for very distant objects found in nature, or tall building many miles away.
 
In practical terms 'infinity' is far enough away that everything beyond (ie to infinity) will be in focus. So, a matter of depth of field; not only focal length but aperture and resolution will matter. So at what distance —measure in focal lengths if you like— will any resolvable detail still be either 'in-focus' or too small to be visible?
 
Further than you think... Or. sometimes, closer.

For long, fast teles, probably the best part of a kilometre: call it 1000 yards. For slow wide-angles, as little as 10 metres. The thing is, what criteria are you using for sharpness? How accurate is your focusing mechanism? The whole question strikes me as a common photographic error: trying to quantify the unquantifiable.

Cheers,

R.
 
In practical terms?
Well, when I want to see if my cheap 500mm f8 lens is focusing past infinity I look at the railing on a water tower at 2000m. But I still have to wait for just the right atmospheric conditions. You can really see the air boiling when the sun starts to heat the ground. But for most of my lenses a couple of blocks is fine. Especially for the moderate enlargements I do. I only print full frame 35mm to 6X9 in. and half frame 35 to 6X8 in. So that is about 6X and 8X respectively.
 
I recall somewhere a Zeiss lens tester (Mr. Cornelius Fleischer I think) said, he was testing at 300 x focal length for infinity sharpness.
 
I remember seeing this question raised in a magazine, a very long time ago. If I recall correctly, the consensus opinion, from a number of optical repairers, was 1000 times the focal length. That would work out at 50 metres for a 'standard' 24x36 lens and a Kilometre for what I would consider the longest practical focal length for the 35mm format.
 
Roger Hicks said:
Further than you think... Or. sometimes, closer.

For long, fast teles, probably the best part of a kilometre: call it 1000 yards. For slow wide-angles, as little as 10 metres. The thing is, what criteria are you using for sharpness? How accurate is your focusing mechanism? The whole question strikes me as a common photographic error: trying to quantify the unquantifiable.

Cheers,

R.
Click to expand...

Roger, optical phenomena are anything but unquantifiable. Lenses are designed mathematically. The reason for expressing the distance in focal lengths is to identify that distance, for any given focal length, for which there is no practical difference between it and infinity.

The focusing mechanism doesn't enter into it. I'm assuming that the lens is being focused perfectly. DOF does not enter into it.

As to aperture: the effect of a small aperture would be only to mask that difference, via DOF.

Something that does need to enter into it would be the size of the blur circle. I wasn't thinking about that, initially. I think it's fair to say that infinity is farther away for an apochromat than for a Cooke triplet! Maybe 10,000 focal lengths? Still, that's only 3,280 feet for my hypothetical 100mm lens. Only half that, 1640 feet, for a 50mm. That's a lot closer than the figures of two miles, or "use the moon" that are often suggested.

See my point?
 
Can this be infinity?:

Infinitely small until there is nothing?

Is nothing (0) infinity?

Infinitely large, example the universe? Perhaps not a good example if it isn't infinitely large!

Could there be multiple Universes?

How about this undetectable stuff, we have labeled "dark" matter & "dark" energy?

What is infinity as we get closer then inside a black hole?

How does God fit into this? Some have said we have found the God particle! Is it true?

Remember, people thought and even put into their theology that the Earth was the center of the Universe and everything went around and around our Earth. They were correct with only one, our moon!

New discoveries are being made. This is an exciting time to be alive as I have been an amateur astronomer for a long time.

Have you looked at some of the recent exciting discoveries? How about exo-planets?

Lots of exciting photographs are being made! Check the Hubble & Kepler web sites. Just curious, check our Curiosity!

Photography is important to the discoveries and the discoveries are important to photography. There is an interesting book titled, "Leica M - Advanced Photo School," by Gunter Osterloh. Check it out if you get a chance to peruse a copy and look at all the mathemetics involved in lens design. Leica M lenses starts on page 99.

Did you hear about the astronomer who went to a party but only stayed for a short amount of time? S(he) didn't like the atmosphere! Ha!
At any rate, here is a spot to check out on infinity:

http://scidiv.bellevuecollege.edu/math/infinity.html

Thanks for strarting this thread!
 
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Somewhere on this forum a member pointed out the best target for calibrating a rangefinder camera at infinity: The moon.

Wayne
 
infinity, optically speaking, is the object plane when you put your film into the focal plane of your lens, LOL







OK. So seriously. 25m for a 100mm lens is far from infinity. I mean much closer, than infinity. Of course matematically this doesnt make sense, "much closer than infinity" since infinity minus a large number is still infinity.
:p







Anyway. Seriously, now. The moon is at infinity. The sun as well, but it's rather difficult to focus on it. Alternatively, you can pick any other celestial or terrestrial object on the horizon, if your horizon is not limited by the neighbor's cat on the roof.
 
OK. Photographically speaking, i think the correct way to calculate is the following:
Take the highest number from these three: [thickness of emulsion/sensor, flatness of emulsion/sensor, Planck's constant expressed in meters].
Double that value. That will be your starting value below which, no further accuracy is necessary.
use this number and simple Snel's optics laws to calculate how much it corresponds to on the objects' side of the lens. You shall get a quite large number of meters.
Now subtract this number from infinity, and that will be your practical infinity, LOL





No- seriously. Compare the accuracy number (the one you chose and doubled) to the focal length, you get a ratio. Use the same ratio with the other number, the one on the object plane side, and you will get a practically-infinity number for your object distance.
I think this makes sense. Kind of.
 
Bill Clark said:
Infinitely large, example the universe? Perhaps not a good example if it isn't infinitely large!

Could there be multiple Universes?
Click to expand...

I think you need to qualify that as the "visible universe" which is very far from being infinitely large unless you define infinity as being limited by visibility. i.e. you can't focus on anything which is invisible, which is that which isn't sending you light waves to focus. So as far as lenses go there is no such thing as infinity.
 
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