Lens sharpness and distance from subject

[RAKAMRAK]

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?

[AlanF]

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 2d. 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 ≥ 2d.

[East Wind Photography]

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

[Pi]

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.

[East Wind Photography]

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.

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.

[meli]

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)

[Delish]

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.

[RAKAMRAK]

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.

[jebrady03]

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.

[Pi]

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.

[East Wind Photography]

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.

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.

[Pi]

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.

[East Wind Photography]

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.

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.

[East Wind Photography]

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.

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.

[Mr Bean]

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.