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08-01-2012, 08:54 PM   #1
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Relationship between distance and sharpness

Let's assume we have two lenses that have been shown to have identical sharpness (i.e. in all the usual measurement criteria), but where one is five times the focal length of the other. We take a shot with the shorter lens at say, three meters from the subject, and then, with the longer lens, we move away from the subject until we obtain the same image size we had with the shorter lens. Assuming lighting and camera settings are the same for both shots, and ignoring DOF and perspective differences as well as any atmospheric factors, would one expect the same apparent sharpness when viewing the images at actual size?

Why am I asking? Because I'm trying to understand how these things work, and whether or not the subject distance is a significant factor in the apparent differences in sharpness that I see with some of my zooms at different focal lengths. Yes, I know that a zoom's resolution will vary with focal length and aperture, but I'm trying to understand if subject distance is also a factor.

Thanks for any input.

08-01-2012, 08:59 PM   #2
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Subject distance can matter, as focus point changes the focal glass location in the lens
08-01-2012, 09:38 PM   #3
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Well, are we saying then that a given lens will appear to have different sharpness levels between shots taken at different subject distances? I'm trying to focus (sorry) on the question of how lens resolution is affected by subject distance from an "optical science" (if there's such a thing) perspective, not from a lens construction perspective.
08-01-2012, 09:44 PM   #4
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Some lenses exhibit this severely - they shoot perfectly fine at the short range focus, but infinity focus is terrible. Many lenses don't really exhibit it. I don't know the exact reason why, optically - but the only thing that changes as you shoot near or far is the focus lens element moving. So if you really want the optical science behind it, it's that point in the lens.

08-01-2012, 11:40 PM   #5
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Considering the difference that a little vibration makes at 3m versus 30m, no, I cant imagine that distance shots can be as sharp as closeups. Everything from air turbulance, light scattering and vibrations must come into play to make it tougher. I tend to shoot at 500mm far more than with my 15mm glass and well know the difficulty in obtaining distance sharpness.
08-01-2012, 11:52 PM   #6
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QuoteOriginally posted by bxf Quote
Let's assume we have two lenses that have been shown to have identical sharpness (i.e. in all the usual measurement criteria), but where one is five times the focal length of the other. We take a shot with the shorter lens at say, three meters from the subject, and then, with the longer lens, we move away from the subject until we obtain the same image size we had with the shorter lens. Assuming lighting and camera settings are the same for both shots, and ignoring DOF and perspective differences as well as any atmospheric factors, would one expect the same apparent sharpness when viewing the images at actual size?
You answered your own question in the first sentence.

The only empirical way the relative sharpness of lenses can be measured is by shooting the same subject (e.g. a test chart) at the same magnification under controlled settings, i.e. exactly the setup you described. In other words you would have had to already done the test you described to arrive at that initial statement.

Last edited by Cannikin; 08-02-2012 at 12:13 AM.
08-02-2012, 03:04 AM   #7
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QuoteOriginally posted by JinDesu Quote
Some lenses exhibit this severely - they shoot perfectly fine at the short range focus, but infinity focus is terrible. Many lenses don't really exhibit it. I don't know the exact reason why, optically - ..........................
I agree. My Tamron 17-50/2.8 is very sharp for close objects not good for distant ones. Some 85/1.4 portrait lenses do the same. In my experience, it's rare for a lens not to be sharp at distance and less sharp for close subjects. This is why I think sharpness figures on reviews doesn't tell the whole story even about sharpness alone. Then you throw in micro contrast and at times it becomes very murky.
08-02-2012, 03:46 AM   #8
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QuoteOriginally posted by imtheguy Quote
Considering the difference that a little vibration makes at 3m versus 30m, no, I cant imagine that distance shots can be as sharp as closeups. Everything from air turbulance, light scattering and vibrations must come into play to make it tougher. I tend to shoot at 500mm far more than with my 15mm glass and well know the difficulty in obtaining distance sharpness.
Yes, of course. However it was my intention to eliminate these variables from my question. I must admit that I had to pay close attention to the shutter speeds of the various photos when performing the comparisons. As you say, even little vibration makes a noticeable difference.


Last edited by bxf; 08-02-2012 at 09:18 AM.
08-02-2012, 03:58 AM   #9
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QuoteOriginally posted by Cannikin Quote
You answered your own question in the first sentence.

The only empirical way the relative sharpness of lenses can be measured is by shooting the same subject (e.g. a test chart) at the same magnification under controlled settings, i.e. exactly the setup you described. In other words you would have had to already done the test you described to arrive at that initial statement.
Haha, yes, I see what you mean. Perhaps I would not have had to ask this question if I had paid more attention to the specific details involved in testing/measuring lenses. BUT, are lens tests typically performed at a given consistent magnification, regardless of focal length?

Let's look at it another way: if lens with FL=f is able to JUST resolve a rope at some specific distance=d, would I necessarily expect a lens with FL=5f to show similar resolution at a distance of 5d, assuming these lenses have been tested and shown to be "equally good"??
08-02-2012, 05:14 AM   #10
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QuoteOriginally posted by pcarfan Quote
I agree. My Tamron 17-50/2.8 is very sharp for close objects not good for distant ones. Some 85/1.4 portrait lenses do the same. In my experience, it's rare for a lens not to be sharp at distance and less sharp for close subjects. This is why I think sharpness figures on reviews doesn't tell the whole story even about sharpness alone. Then you throw in micro contrast and at times it becomes very murky.
This is the thing that prompted this particular line of thought on my part. I just put my new Pentax 55-300mm lens through some initial field tests (aka Snapshots), and I found that I get remarkably good (relatively speaking) photos at the low FL and short distances, and much lower IQ at larger (but not necessarily great) distances and longer FL. Sure, this is just a modest lens, but the IQ at short FL and small distances is so surprisingly good, that it made me start to wonder if the relatively poor showing at the longer FL/distances is just a lens limitation, or if there are some other contributing factors.

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?
08-02-2012, 05:25 AM   #11
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QuoteOriginally posted by bxf Quote
This is the thing that prompted this particular line of thought on my part. I just put my new Pentax 55-300mm lens through some initial field tests (aka Snapshots), and I found that I get remarkably good (relatively speaking) photos at the low FL and short distances, and much lower IQ at larger (but not necessarily great) distances and longer FL. Sure, this is just a modest lens, but the IQ at short FL and small distances is so surprisingly good, that it made me start to wonder if the relatively poor showing at the longer FL/distances is just a lens limitation, or if there are some other contributing factors.

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?
the sad reality is that you are dealing here with a zoom, that is full of compromises. many zooms are softer at maximum focal length, so this comes into the issue here, we are not dealing prime against prime, but the compromises made at 55mm and 300mm on one zoom design.
08-02-2012, 06:35 AM   #12
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QuoteOriginally posted by pcarfan Quote
In my experience, it's rare for a lens not to be sharp at distance and less sharp for close subjects.
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.

QuoteOriginally posted by pcarfan Quote
This is why I think sharpness figures on reviews doesn't tell the whole story even about sharpness alone.
The only data set I've seen that really addresses the question of sharpness at different distance ranges
are the MTF charts that Schneider-Kreuznach (not Samsung) provide.
They give separate charts for MFD, mid-range, and infinity:
http://www.schneiderkreuznach.com/foto/an_su_pc/pdf/AN_PCSU_28_28_R43757_2AD.PDF
08-02-2012, 09:23 AM   #13
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QuoteOriginally posted by Lowell Goudge Quote
the sad reality is that you are dealing here with a zoom, that is full of compromises. many zooms are softer at maximum focal length, so this comes into the issue here, we are not dealing prime against prime, but the compromises made at 55mm and 300mm on one zoom design.
Just reading some test reports before buying the lens prepared me for such compromises. However, I was not expecting to be able to so easily distinguish between low VERY GOOD and a high VERY GOOD, for example
08-02-2012, 09:25 AM   #14
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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.



The only data set I've seen that really addresses the question of sharpness at different distance ranges
are the MTF charts that Schneider-Kreuznach (not Samsung) provide.
They give separate charts for MFD, mid-range, and infinity:
http://www.schneiderkreuznach.com/foto/an_su_pc/pdf/AN_PCSU_28_28_R43757_2AD.PDF
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.
08-02-2012, 09:45 AM   #15
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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.
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