# Lens sharpness and distance from subject



## RAKAMRAK (Apr 26, 2013)

I was wondering whether and how much the distance from the subject affects sharpness of the image taken by different lenses (on same sensor). Is there any website or source where such information may be available for Canon lenses?


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## AlanF (Apr 26, 2013)

RAKAMRAK said:


> I was wondering whether and how much the distance from the subject affects sharpness of the image taken by different lenses (on same sensor). Is there any website or source where such information may be available for Canon lenses?



This isn't a rumor. What do you mean by sharpness? If you mean the ability to resolve two lines a certain distance apart, then, all things being equal (sharp lens, not diffraction limited etc), the answer is simple. If you use lenses of different focal lengths, then the resolving power is directly proportional to focal length - a 400mm lens is twice as good as a 200mm. If you mean the object is at different distances, then the degree of detail resolved is inversely proportional to the distance away. 

You can calculate the resolving power yourself if you know the width of a pixel, _d_. Two lines in the image have to span 2 pixels to be resolved, ie 2_d_. If those two lines are a distance _D_ apart in real life and they are a far-away distance _S_ from the lens, their separation in the image from a lens of focal length _f_ = (_f/S_)_D_. So, those two lines can be resolved if (_f/S_)_D_ ≥ 2_d_.


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## East Wind Photography (Apr 27, 2013)

Distance has nothing to do with sharpness. It's all about having more light to increase the contrast between light and dark edges.


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## Pi (Apr 27, 2013)

It does. Use a lens at 1m, then at infinity - it is a different lens really. I have noticed this with my 17-55 - much better at 2-3m than at infinity. 

Many people test lenses at close distances and make far reaching conclusions; and this is wrong.


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## East Wind Photography (Apr 27, 2013)

???

Maybe you are referring to resolving ability. That is not sharpness. Sharpness is created by having a stronger light and dark along lines in an image. More light means more sharpness. Less light gives it more softness.



Pi said:


> It does. Use a lens at 1m, then at infinity - it is a different lens really. I have noticed this with my 17-55 - much better at 2-3m than at infinity.
> 
> Many people test lenses at close distances and make far reaching conclusions; and this is wrong.


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## meli (Apr 28, 2013)

Pi said:


> It does. Use a lens at 1m, then at infinity - it is a different lens really. I have noticed this with my 17-55 - much better at 2-3m than at infinity.
> 
> Many people test lenses at close distances and make far reaching conclusions; and this is wrong.



I've noticed that too. Some lenses (all?) behave differently depending on distance: min distance to 2-3m to infinity. Havent seen official data on this from any manufacturer, from time to time a reviewer might notice it. (photozone mentions it in a couple of reviews)


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## Delish (Apr 28, 2013)

The longer the distance the more air there will be in between the lens and subject, and that can make a big impact.
Also difference in the temperature of the air in between, can severely reduce sharpness.


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## RAKAMRAK (Apr 28, 2013)

So it is not "sharpness" but resolving ability. So let me rephrase my question - is there any source to learn about canon (many or most) lens's variable "resolving ability" with distance from the subject. And I am talking about different distances like 2-5 meters, 5-10 meters, 10-20 meters etc. (or there counterparts in feet).

To put it in a slightly different language what I am trying to know is if it is true that every prime lens is optimized (in terms of resolving power) for a particular (or small range) of distance from the subject (not sure how to put it in terms of zoom lenses) and if so is there any source (that you know of) to know which canon lens is optimized for which distance (or range of distance) from the subject?

Thanks for the reference to "atmospheric distortion", I just looked it up. No, I am not talking about that.


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## jebrady03 (Apr 28, 2013)

Look into Canon's new DLO tool in DPP. It will help to correct some lens problems associated with distance and aberrations. In fact, you can even adjust the distance info to fine tune the corrections. It's not info like you were requesting, but it could be at least a partial solution if you're experiencing a problem.


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## Pi (Apr 28, 2013)

East Wind Photography said:


> Maybe you are referring to resolving ability. That is not sharpness. Sharpness is created by having a stronger light and dark along lines in an image. More light means more sharpness. Less light gives it more softness.



There is no much difference. Sharpness is a loose term for MTF-50 or so. Resolution is another loose term for lower MTF, like MTF-10 to 20.


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## East Wind Photography (Apr 28, 2013)

Actually it IS all about the differences. You can sharpen an image in Photoshop but you cannot increase the resolution of the image. They are two different things measured in complete different ways.



Pi said:


> East Wind Photography said:
> 
> 
> > Maybe you are referring to resolving ability. That is not sharpness. Sharpness is created by having a stronger light and dark along lines in an image. More light means more sharpness. Less light gives it more softness.
> ...


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## Pi (Apr 28, 2013)

East Wind Photography said:


> Actually it IS all about the differences. You can sharpen an image in Photoshop but you cannot increase the resolution of the image. They are two different things measured in complete different ways.



All of this is unrelated to the OP - the question was clear enough.

We are not talking what you can create in PS - we are talking about the IQ of the lens and the sensor. And no, they are not measured in completely different ways - all that is measured is the MTF curve, usually by the slanted edge method (which actually computes the MTF using the Fourier transform). The rest is words.


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## East Wind Photography (Apr 28, 2013)

With zooms yes you can get difference it resolving ability. You can also see the difference in resolution if you are not obtaining precise focus. Some lenses focus better closer than at infinity. It's not supposed to be like that and usually indicates a bad design, a defect, or simply needs to be calibrated at the factory.

Resolution is measured in lines/mm at a certain distance. If you extend the same chart further out you will resolve less lines and that's because the resolution of the lens is fixed whereby you are changing other constants in the measurement. Maybe a better way to explain it is lines/mm can also be measured as an angle...a very small angle... if you move your subject further away, that angle becomes smaller. The lines/mm resolution of the lens does not change, it's that the subject now fits in fewer lines because the angle is smaller...All of that in a world where lenses are perfectly machined and assembled.

That being said, most of these lenses are quite complex with lots of moving parts, lenses move closer and further away during AF and zoom functions. It is possible that due to lack of tolerances in the machined parts that move that one or more lenses my not be in the same alignment from one focus point to another. most people wouldn't notice it but if you are pixel peeping then you might notice some differences...and it could get worse as the lens is used and wears out.



RAKAMRAK said:


> So it is not "sharpness" but resolving ability. So let me rephrase my question - is there any source to learn about canon (many or most) lens's variable "resolving ability" with distance from the subject. And I am talking about different distances like 2-5 meters, 5-10 meters, 10-20 meters etc. (or there counterparts in feet).
> 
> To put it in a slightly different language what I am trying to know is if it is true that every prime lens is optimized (in terms of resolving power) for a particular (or small range) of distance from the subject (not sure how to put it in terms of zoom lenses) and if so is there any source (that you know of) to know which canon lens is optimized for which distance (or range of distance) from the subject?
> 
> Thanks for the reference to "atmospheric distortion", I just looked it up. No, I am not talking about that.


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## East Wind Photography (Apr 28, 2013)

No actually the original question was about distance affecting sharpness of a lens...and the answer is quite clear. Distance does not affect sharpness. The amount of light affects sharpness. Quality of optics and alignment affects resolution of a lens. What the camera can resolve is up to the sensor. The lens resolution should be relatively fixed..but that is not always the case because no one can make a perfect lens.



Pi said:


> East Wind Photography said:
> 
> 
> > Actually it IS all about the differences. You can sharpen an image in Photoshop but you cannot increase the resolution of the image. They are two different things measured in complete different ways.
> ...


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## Mr Bean (Apr 28, 2013)

I suspect distance can have an effect on sharpness of a lens (or, in reality, aberration control). For example, my 300mm f4 is probably optimised (from an aberration perspective) at distant focal lengths. While it still works well at it's minimum focusing distance, the moving of the elements and groups will have an effect on the balance of aberrations. On the other hand, my 100mm macro is better suited at close work.


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## Lawliet (Apr 28, 2013)

Mr Bean said:


> On the other hand, my 100mm macro is better suited at close work.


If one goes through the detailed lens descriptions the term "floating elements" crops up. Thats a way to deal with the different needs of extreme close focus and infinity - move some elements around as if it where a zoom lens, but instead of changing the focal length its the aberration control that gets adapted. (Well, IF/RF adds another layer of complexity and there may be partial overlap, but the idea stays). Most of the time you find that in macros.


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## LetTheRightLensIn (Apr 28, 2013)

East Wind Photography said:


> No actually the original question was about distance affecting sharpness of a lens...and the answer is quite clear. Distance does not affect sharpness. The amount of light affects sharpness. Quality of optics and alignment affects resolution of a lens. What the camera can resolve is up to the sensor. The lens resolution should be relatively fixed..but that is not always the case because no one can make a perfect lens.



nah some lenses go to mush near MFD (say 70-200 f/4 IS at 200mm, it's RADICALLY insanely sharper at infinity than near MFD) and some do better at say 4' than near infinity and some, say some tamron standard zooms do worse at 3' than at infinity for sure.


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## Aglet (Apr 28, 2013)

A couple posters here have hit the answer to your question.
The answer is YES, lenses may have different abilities to resolve detail (defined as lines on the sensor at a given contrast ratio), depending on the focus distance.

As of yet, I know of no site that actually measures this characteristic of Canon or any other lenses.

I've certainly noticed this with a few lenses, typically zooms, where they will do a better job at close distances than at or near infinity. I had a 17-40/4L that comes to mind first.
With the minute inconsistencies of manufacturing tolerances, various copies of a given lens model may exhibit more or less of this tendency as well with further variation possible at different focal lengths.
It sometimes seems that more low cost zoom lenses perform this way than others, likely because they're compromised to work better at the closer "people-are-the-subject" distances they're used with when kitted with consumer bodies.
Pro level lenses aren't exempt from this effect either, many workhorse zooms are a bit less sharp near minimum focus distance.


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## RAKAMRAK (Apr 28, 2013)

Thank you guys for all the replies and discussions.


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## Plainsman (Apr 28, 2013)

RAKAMRAK said:


> I was wondering whether and how much the distance from the subject affects sharpness of the image taken by different lenses (on same sensor). Is there any website or source where such information may be available for Canon lenses?



The lens designer has to have a reference point when designing his lens eg 50xfocal length.

The big telephoto lenses are really birder lenses and as such have max sharpness at relatively close range ie if you want really sharp shots of the moon you need a reflector telescope.

Read Thom Hogan's Nikon long lens reviews where he points out the lenses which aren't "distance" lenses like the 200-400/4 or even the 400/2.8. The same is very likely true for the Canon equivalents.


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## insanitybeard (Apr 29, 2013)

Pi said:


> It does. Use a lens at 1m, then at infinity - it is a different lens really. I have noticed this with my 17-55 - much better at 2-3m than at infinity.
> 
> Many people test lenses at close distances and make far reaching conclusions; and this is wrong.



This is a very good point, and not one I've seen covered in any depth on most review sites I've come across- anybody know any review sites where this is considered with more than a passing comment?


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## Pi (Apr 29, 2013)

insanitybeard said:


> This is a very good point, and not one I've seen covered in any depth on most review sites I've come across- anybody know any review sites where this is considered with more than a passing comment?



I have not seen any. Maybe the reason is that every such site has one (expensive) chart only and this pretty much determines the distance. I believe that you can still test at different distances even with the same chart, in the center at least; but they do not do that.


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## East Wind Photography (Apr 29, 2013)

That's because the test chart is designed to reveal the limits of the lens at the test distance. Doesnt matter if you move the chart further away. Likely the test chart wont show anything. In order for the distance test to work you would need a chart that is proportionately larger to show the same pattern but larger scale.

For example moving the same chart out to twice the distance will double the number of lines shown per mm...so you are effectively trying to test the lens at an even finer resolution which most sensors would likely not be able to resolve.



Pi said:


> insanitybeard said:
> 
> 
> > This is a very good point, and not one I've seen covered in any depth on most review sites I've come across- anybody know any review sites where this is considered with more than a passing comment?
> ...


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## Pi (Apr 29, 2013)

East Wind Photography said:


> That's because the test chart is designed to reveal the limits of the lens at the test distance. Doesnt matter if you move the chart further away. Likely the test chart wont show anything.


It would but you need software which would still analyze it. The slanted edge test allows you to vary the distance to some degree. An edge is an edge at any distance but you do not want the whole chart to be viewed at an extremely narrow AOV, so there is some practical limit how far you can get. This requires a curious tester, not the typical businesses type.


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## East Wind Photography (Apr 29, 2013)

And likely a test of the lens OFF of a camera body since the limiting factor of most decent lenses is the chip.



Pi said:


> East Wind Photography said:
> 
> 
> > That's because the test chart is designed to reveal the limits of the lens at the test distance. Doesnt matter if you move the chart further away. Likely the test chart wont show anything.
> ...


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