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08-02-2012, 09:51 AM   #16
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QuoteOriginally posted by bxf Quote
Assuming this phenomenon (i.e. the inconsistent relationship between focusing distance and sharpness) is reasonably common, this statement actually answers much of my question. i.e. I was trying to understand whether such an inconsistency is a simple reality of lens design limitations, or an optical/visual fact. I trust I'm making sense here. Perhaps it is clearer if I put it like this: I was wondering if more distant subjects will necessarily always appear less sharp, even if the lens IQ is a prime, and is reported as being consistent, though I suppose this would be determined by the lens testing methodology.

If the above is still somewhat incoherent (and I concede that it may be), consider this: your own eyes will resolve closer objects better than more distant objects, even though it's the same pair of eyes. It occurred to me that similar limitations may exist with lenses.
Most lens tests are conducted at a distance of a few meters, basically mid-range,
so as pcarfan noted, those tests don't tell the full story.

Lens designers consider optimizing their lenses for different distances,
sometimes depending on the intended use of the lens.
A "landscape" lens, for example, would be optimized for infinity,
while a "macro" lens would be optimized for close-up.
Tricks like "floating elements" can help make a lens good
over a whole range of distances.

When thinking about how healthy eyes see, near versus far,
it's better to think in terms of resolving angles,
rather than resolving on some linear scale like millimeters.
That way, resolving a millimeter at a distance of one meter
is the equivalent of resolving one meter at a distance of a kilometer.

08-02-2012, 09:52 AM - 1 Like   #17
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Most lenses (both primes and zooms) will move their elements (or blocks of elements) relatively to each other during focusing. As such, you get essentially a different optical formula at any given distance. Even the focal length of the lens changes (the nominal focal length is at infinity) with distance. F-stop is non constant either.

Since you are dealing with a "different" lens at different distances, you can not generalize lens sharpness without taking into account several variables including subject distance, f-stop, scene luminance, etc.
08-02-2012, 10:41 AM   #18
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QuoteOriginally posted by vonBaloney Quote
No, it is not an optical fact that more distant subjects will be less sharp with the "same" quality lens AT ITS TESTED DISTANCE AND APERTURE. As someone else pointed out, this is just a logical impossibility -- if the lens is just as good, it is just as good since the only way to call it "just as good" is to test it under those equivalent conditions, which include picking a particular focus distance and aperture.

But if we are talking about major distances, atmosphere is a big factor so it doesn't make much sense to remove it as variable unless you are shooting on the moon. So a longer lens will have to be that much better to get equivalent sharpness, and of course atmosphere is never constant (on Earth). And it is a fact that lenses are optimized for certain distances (even primes) -- usually infinity, but a macro might be optimized at close distances. Zooms will be optimized for a certain distance or disances in their range (I've read that the distance markings on the zoom barrel can be a guide to that for some lenses -- best results will be had at the points where distances are marked.) And it is a near universal fact that zooms will be at their worst at the long end. And many telephotos will suffer at close-focusing. Etc.
I completely understand that. I just don't know (never bothered to absorb the fine print) how tests are actually performed, which is why I said "I suppose this would be determined by the lens testing methodology". If tests are routinely done using a specific target magnification, then as you say, the question is illogical.

Further scrutiny of my images indicates that it's the zoom's FL, rather than the subject distance, that is really the problem area, although the latter is contributing substantially to poor IQ due to the greater effect of any camera movement. And then, of course, there are the atmospheric factors, and whereas I know that there is no consistency in this regard, I do wonder how significant these are likely to be at nominal distances e.g. 50m.
08-02-2012, 10:53 AM   #19
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QuoteOriginally posted by bxf Quote
I completely understand that. I just don't know (never bothered to absorb the fine print) how tests are actually performed, which is why I said "I suppose this would be determined by the lens testing methodology". If tests are routinely done using a specific target magnification, then as you say, the question is illogical.
There is no consistency in testing these days. They used to do tests with the lens only on an optical bench, now most tests come from a particular camera/lens combo, where the sensor is a huge factor in the results and it can't be compared to another system.

QuoteQuote:
And then, of course, there are the atmospheric factors, and whereas I know that there is no consistency in this regard, I do wonder how significant these are likely to be at nominal distances e.g. 50m.
On a hot day in the desert (the real or the concrete variety), it is huge. In moderate weather, not so much. Unless there is fog. Etc.

08-03-2012, 01:31 AM   #20
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Some lenses are designed/optimized for a specific distance to give maximum sharpness and/or micro contrast and/or bokeh.

Example I found my FA*84/1.4 perfect in portrait distances, very sharp and flare resistent, but not as good in infinity or group shots.
My FA50/1.4 at larger apertures does have it's own fav distance, which is about 2-5 meters.
Macro lenses are for close ranges. At minimum focus distance, there are compromises - DOF, aperture etc.

Lens test charts are pretty borring. It is better that you go out and do the test yourself.
Many lenses with bad reviews turn out to be very good at certain conditions - use them at their sweet spots.
But there are some real lemon too, that my kids are playing as toys.
08-03-2012, 04:11 AM   #21
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QuoteOriginally posted by bxf Quote
...............
BTW, if I read you correctly (and I'm not sure I do), your statement "it's rare for a lens..." is the opposite of what your first sentence says. Am I misunderstanding something?
What I meant is, I've never encountered a lens that was sharp for distant objects and then being not sharp for close subjects, only the reverse.
08-03-2012, 04:13 AM   #22
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QuoteOriginally posted by lytrytyr Quote
I'm not sure if I've understood your syntax here, so let me give two examples:
My Voigtlaender 90/3.5 is sharp at closer distances, not so sharp near infinity.
My ZK 85/1.4 shows the reverse: sharp at infinity, less so close up.



]
Wow! good to know, an 85/1.4 being sharper at infinity? I would think a portrait lens would be the reverse.
08-03-2012, 07:57 AM   #23
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QuoteOriginally posted by pcarfan Quote
Wow! good to know, an 85/1.4 being sharper at infinity? I would think a portrait lens would be the reverse.
There are very few subjects who can model for a lens that's supersharp in the portrait range!

08-03-2012, 11:54 AM - 1 Like   #24
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I wondered about why a lens might vary its performance with distance to the subject. What's different about light entering the lens from a subject 2 focal lengths away and a subject 200 focal lengths away? The answer of course is the curvature of the light's wavefront.

Consider an f2 lens; the aperture is 1/2 the focal length. Here's a diagram of the geometry in this case:


It is reasonable to expect that designing a lens for optimum focus on a flat image plane depends on the curvature of the wavefront entering the lens.

From this it follows that good macro lenses are designed to meet this condition - hence may not be so good for subjects at long distances from the lens.

It also implies that reversing a normal lens stacked on a primary lens may be helpful when using them for close-up photography. If a subject is placed at the focal point of a reversed lens, light reflected from the subject will exit the reversed lens as parallel illumination (light does not know which direction it is traveling through a lens). This light then enters the primary lens as if it were originating at infinity - if the primary lens was a normal lens it probably does a good job at creating an image.

Dave in Iowa
08-03-2012, 06:04 PM   #25
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Thanks to all for the all useful info. It has helped.

Bill
08-03-2012, 10:45 PM   #26
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Distance theoretically does matter. No lens can take clear photos of a small object on a planet in another solar system ... thus distance does indeed make resolution more difficult.

A $300 50mm lenses will have better resolution (close up) than a $300 500 mm lens far away. But a 'good' 500 mm lens, costing a LOT more than $300, perhaps might resolve just as well. Is it fair to compare a 'good' 50mm lens to a good 500 mm lens when the 500mm costs multiples more? ... only you can answer depending upon the context you intended the question.

I'm not a physicist, but I imagine as an object gets further away, (and becomes smaller and smaller relative to a fixed focal length) greater and greater optical precision is required where impurities in the glass, relections within the lens, etc, etc, result in greater and greater deterioration on resolution. To what extent these factors have practical influence a 50 vs 135mm lens... I have no idea ... lol!



Interesting question!

Last edited by stills999; 08-03-2012 at 10:58 PM.
08-04-2012, 02:24 AM   #27
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Interesting thread and loved Newarts explanation.

re. your 55-300mm Do note that this lens, when used at telephoto ranges, seems to be sharpest at around 240mm and if using at 300mm it is noticeably inferior. It is better with this lens to shoot at 240-260mm and crop to a 300mm FoV, this will yield a sharper image.
08-05-2012, 08:23 PM   #28
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QuoteOriginally posted by stills999 Quote
Distance theoretically does matter. No lens can take clear photos of a small object on a planet in another solar system ... thus distance does indeed make resolution more difficult.

A $300 50mm lenses will have better resolution (close up) than a $300 500 mm lens far away. But a 'good' 500 mm lens, costing a LOT more than $300, perhaps might resolve just as well. Is it fair to compare a 'good' 50mm lens to a good 500 mm lens when the 500mm costs multiples more? ... only you can answer depending upon the context you intended the question.

I'm not a physicist, but I imagine as an object gets further away, (and becomes smaller and smaller relative to a fixed focal length) greater and greater optical precision is required where impurities in the glass, relections within the lens, etc, etc, result in greater and greater deterioration on resolution. To what extent these factors have practical influence a 50 vs 135mm lens... I have no idea ... lol!



Interesting question!
To a large extent, your comment is the crux of my thinking, hence my reference to the resolution achievable by a pair of eyes at different distances. I was sufficiently knowledgeable, however, to relate this somewhat obvious fact to lens test results.
08-05-2012, 08:48 PM   #29
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QuoteOriginally posted by Frogfish Quote
Interesting thread and loved Newarts explanation.

re. your 55-300mm Do note that this lens, when used at telephoto ranges, seems to be sharpest at around 240mm and if using at 300mm it is noticeably inferior. It is better with this lens to shoot at 240-260mm and crop to a 300mm FoV, this will yield a sharper image.
I don't want to give the impression that I'm losing any sleep over the performance of this 55-500mm lens, especially since it is more than adequate for my purposes, and could be said to be a bargain at its price. As long as it's not malfunctioning, it is what it is, and I accept it as such. In fact, it is doing very well under some conditions.

At the risk of appearing to be flogging a dead horse, I'll add a comment, because I find the observation interesting: I hadn't performed any exhaustive tests, but shots at FL up to roughly 150mm look good, and as has been said, shots approaching the highest FL (270-300mm in my case) look less sharp (I subsequently realized that vibrations played a part in SOME of the long FL shots, even at reasonable shutter speeds). Now, I was curious to see how the "small" and "large" images compared when re-sized to the same magnification, and I got quite a surprise: the 300mm shots actually appear noticeably sharper than the 150mm shots of the same subject. This appears to be the case regardless of which direction the re-sizing is performed. Admittedly, this is not a very scientific test, and I have yet to determine if this observation will turn out to be consistent, but for now, it makes me think that we are simply seeing more imperfections with the higher magnification. Sure, one could say this is rather obvious, and this is where the question of how to relate test results to the actual images comes in.
08-06-2012, 09:08 AM   #30
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QuoteOriginally posted by stills999;2049646.:
...

I'm not a physicist, but I imagine as an object gets further away, (and becomes smaller and smaller relative to a fixed focal length) greater and greater optical precision is required where impurities in the glass, relections within the lens, etc, etc, result in greater and greater deterioration on resolution. To what extent these factors have practical influence a 50 vs 135mm lens... I have no idea ... lol!
It isn't defects in the lens so much as basic physics that limits how far away an object can be and still be resolved.

Because of diffraction effects the smallest sized spot a perfect lens can make (the Airy Disk) is about equal to its f-number/1,000,000, in meters with blue light.

Image.size/object.size = focal.length/distance & since the smallest image possible is N/1,000,000 in meters,

Distance.max = focal.length*object.size*1,000,000/N = aperture.size*object.size*1,000,000

a 1meter telescope can resolve a 1meter asteroid a million meters away.

So the maximum distance at which an object can be resolved is proportional to the aperture diameter of the telescope - if it were more distant it would appear to be the same size as the Airy Disk - that's why astronomers want big telescopes -

Dave in Ames

Last edited by newarts; 08-06-2012 at 09:15 AM.
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