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Author Topic: Diffraction with telephoto lenses  (Read 994 times)

bjanes

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Diffraction with telephoto lenses
« on: May 06, 2018, 05:23:51 PM »

I have a closeup and macro photography book by well respected authors who apparently are not well versed in physics. They state that diffraction is less problematic with telephoto macro lenses, since the diameter of the aperture in millimeters is greater for a given f/number. However, the Airy disc is dependent on aperture, not focal length and it is the projection of the Airy disc on the individual pixels of the sensor that determine the effects of diffraction. At least, that is my understanding. Any other thoughts?

Thanks,

Bill
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Jim Kasson

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Re: Diffraction with telephoto lenses
« Reply #1 on: May 06, 2018, 06:15:21 PM »

I have a closeup and macro photography book by well respected authors who apparently are not well versed in physics. They state that diffraction is less problematic with telephoto macro lenses, since the diameter of the aperture in millimeters is greater for a given f/number. However, the Airy disc is dependent on aperture, not focal length and it is the projection of the Airy disc on the individual pixels of the sensor that determine the effects of diffraction. At least, that is my understanding. Any other thoughts?

By "telephoto" do they mean lenses with negative rear elements, or just long FL lenses?

Not sure, but I think that diffraction in macro photography works like diffraction anywhere else, except you want to use the effective aperture for the calculation.

Jim

bjanes

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Re: Diffraction with telephoto lenses
« Reply #2 on: May 06, 2018, 06:28:11 PM »

By "telephoto" do they mean lenses with negative rear elements, or just long FL lenses?

Not sure, but I think that diffraction in macro photography works like diffraction anywhere else, except you want to use the effective aperture for the calculation.

I think they mean long focal length lenses such as the Canon 180 mm or the Nikon 200 f/4.

Thanks,
Bill
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Jim Kasson

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Re: Diffraction with telephoto lenses
« Reply #3 on: May 06, 2018, 07:16:20 PM »

I think they mean long focal length lenses such as the Canon 180 mm or the Nikon 200 f/4.

Then I don't get it. I had thought it might be something about the effective aperture on inter-focusing lenses not being what you think it would be.

Jim

EricV

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Re: Diffraction with telephoto lenses
« Reply #4 on: May 10, 2018, 01:13:02 PM »

I have a closeup and macro photography book by well respected authors who apparently are not well versed in physics. They state that diffraction is less problematic with telephoto macro lenses, since the diameter of the aperture in millimeters is greater for a given f/number. However, the Airy disc is dependent on aperture, not focal length and it is the projection of the Airy disc on the individual pixels of the sensor that determine the effects of diffraction. At least, that is my understanding. Any other thoughts?  Thanks, Bill

The diameter of the Airy disk on the sensor depends on f/number, but the size of the object on the sensor depends on focal length, so the relative blur depends on physical aperture as they state.  That is, if you care about diffraction blur reducing your ability to resolve features in the subject, physical lens aperture is the relevant parameter.  That is one reason why astronomers build telescopes with big mirrors, and do not care too much about the focal length or f/number of the mirror.
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Jim Kasson

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Re: Diffraction with telephoto lenses
« Reply #5 on: May 10, 2018, 03:03:33 PM »

The diameter of the Airy disk on the sensor depends on f/number, but the size of the object on the sensor depends on focal length, so the relative blur depends on physical aperture as they state.  That is, if you care about diffraction blur reducing your ability to resolve features in the subject, physical lens aperture is the relevant parameter.  That is one reason why astronomers build telescopes with big mirrors, and do not care too much about the focal length or f/number of the mirror.

If you can change the subject distance, as you are usually free to do in macro photography, then this doesn't make sense. 1:1 is the same magnification independent of FL. But it is true that object field disk of confusion is dependent on the physical aperture.

Jim

Martin Kristiansen

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Re: Diffraction with telephoto lenses
« Reply #6 on: May 10, 2018, 03:25:44 PM »

The smaller the aperture the more diffraction, obviously. In this case I donít mean aperture as defined by the f number but by the physical size of the opening. f22 on a 50mm lens is a much smaller opening than f22 on a 200mm lens. The same amount of light, well almost, with both at the same f stop but not the same physical size opening. The smaller the physical opening the greater the effects of diffraction. Donít see what macro has to do with it specifically.

Standard lens on an 8x10 is 150mm and it wasnít uncommon to see minimum apertures of f92 on those lenses. Of course we couldnít pixel peep in those days and mostly used aperture to determine how much depth of field we wanted and werenít as concerned about diffraction as we are now.
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Jim Kasson

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Re: Diffraction with telephoto lenses
« Reply #7 on: May 10, 2018, 03:59:15 PM »

The smaller the aperture the more diffraction, obviously. In this case I donít mean aperture as defined by the f number but by the physical size of the opening. f22 on a 50mm lens is a much smaller opening than f22 on a 200mm lens. The same amount of light, well almost, with both at the same f stop but not the same physical size opening. The smaller the physical opening the greater the effects of diffraction. Donít see what macro has to do with it specifically.

Standard lens on an 8x10 is 150mm and it wasnít uncommon to see minimum apertures of f92 on those lenses. Of course we couldnít pixel peep in those days and mostly used aperture to determine how much depth of field we wanted and werenít as concerned about diffraction as we are now.

You have added a variable that was not in the original post: format size. I don't think that complication helps us with the original question.

Jim

Martin Kristiansen

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Re: Diffraction with telephoto lenses
« Reply #8 on: May 10, 2018, 04:15:01 PM »

You have added a variable that was not in the original post: format size. I don't think that complication helps us with the original question.

Jim

I added the piece about the 8x10 merely to illustrate the point that longer lenses allow for smaller apertures as longer lenses suffer less from diffraction than short lenses at the same f number.

The point is f22 on a 50mm lens is a smaller physical opening than f22 on a 200mm lens and so suffers worse diffraction. The smaller the physical opening the greater the diffraction. Simple really.
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Jim Kasson

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Re: Diffraction with telephoto lenses
« Reply #9 on: May 10, 2018, 04:18:39 PM »

I added the piece about the 8x10 merely to illustrate the point that longer lenses allow for smaller apertures as longer lenses suffer less from diffraction than short lenses at the same f number.

The point is f22 on a 50mm lens is a smaller physical opening than f22 on a 200mm lens and so suffers worse diffraction. The smaller the physical opening the greater the diffraction. Simple really.

Simple, but wrong. The Airy disk size on the sensor is dependent on f-stop, and not on focal length except as used to calculate the f-stop.

Jim

EricV

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Re: Diffraction with telephoto lenses
« Reply #10 on: May 10, 2018, 04:55:23 PM »

If you can change the subject distance, as you are usually free to do in macro photography, then this doesn't make sense. 1:1 is the same magnification independent of FL.... Jim

You are correct, I was thinking of long distances and low magnifications.  For high magnification macro photography, it is convenient to express everything in terms of magnification, and focal length then becomes irrelevant, as you point out.  The size of the object on the sensor is of course proportional to magnification.  Diffraction blur at the sensor is proportional to the effective f#, which is nominal f# * (1+magnification).  So the relative effect of diffraction is given by f#(1+m)/m.

All of these formulas are approximations, whose validity may depend on lens design.  In particular, there are corrections for asymmetric lenses, where the entrance and exit pupils are different.  I think the effective f# gets multiplied by (1+m/P), where P is the pupil magnification ratio.  Maybe this is what the authors of the book in the OP were referring to?

I just found a nice set of articles on all of this: http://coinimaging.com/photo_articles.html.
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EricV

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Re: Diffraction with telephoto lenses
« Reply #11 on: May 10, 2018, 06:13:56 PM »

I added the piece about the 8x10 merely to illustrate the point that longer lenses allow for smaller apertures as longer lenses suffer less from diffraction than short lenses at the same f number. The point is f22 on a 50mm lens is a smaller physical opening than f22 on a 200mm lens and so suffers worse diffraction. The smaller the physical opening the greater the diffraction. Simple really.

Simple, but wrong. The Airy disk size on the sensor is dependent on f-stop, and not on focal length except as used to calculate the f-stop.  Jim

You are both right.  If you care about diffraction blur on the sensor, irrespective of object size on the sensor, then f-stop is all that matters.  (At f/8 the diffraction spot is roughly 10um, for any focal length.)  If you care about diffraction blur relative to the size of the object being imaged, then physical aperture is all that matters.  The first quantity might loosely be called image resolution, while the second quantity might loosely be called angular resolution.  If I take a picture of a distant tree, and I want diffraction to be small, compared to the size of a leaf on that tree in the final print, then I care about angular resolution.
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Jim Kasson

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Re: Diffraction with telephoto lenses
« Reply #12 on: May 10, 2018, 08:12:30 PM »

You are both right.  If you care about diffraction blur on the sensor, irrespective of object size on the sensor, then f-stop is all that matters.  (At f/8 the diffraction spot is roughly 10um, for any focal length.)  If you care about diffraction blur relative to the size of the object being imaged, then physical aperture is all that matters.  The first quantity might loosely be called image resolution, while the second quantity might loosely be called angular resolution.  If I take a picture of a distant tree, and I want diffraction to be small, compared to the size of a leaf on that tree in the final print, then I care about angular resolution.

Up above I said, "object field disk of confusion is dependent on the physical aperture." So I agree on that point. But I thought that part was settled.

BJL

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If you care about diffraction blur relative to the size of the object being imaged, then physical aperture is all that matters. [This] quantity might loosely be called angular resolution.  If I take a picture of a distant tree, and I want diffraction to be small, compared to the size of a leaf on that tree in the final print, then I care about angular resolution.
Agreed to this point: the "physical aperture"* (a.k.a. entrance pupil size, a.k.a. effective aperture diameter) effects the "angular smearing" of the light.

The next question is how DOF comes out, particularly in this special case of macro photography. I know that at normal distances, the circle of confusion size relative to image size also depends on the effective aperture diameter when subject distance is equal, but in addition it varies inversely with the [square of the] subject distance, though these rules vary at very close range. If I have remembered that right, then using a longer focal length at equal effective aperture size and also using a greater subject distance to cover the same field of view will give equal "angular" diffraction effects but also more DOF, which sounds like an advantage for macro photography. (This is to be balanced against the need for a longer exposure time and/or a higher exposure index.)



* Aside. AFAIK (please correct me if I am mis-rembering any part of this!) it is not quite the actual physical diameter of the aperture opening that counts, since its position along the optical path also matters: the same sized physical opening closer to the focal plane produce the same angular smearing leaving the aperture but therefore less smearing at the focal plane. This also corresponds to a larger effective aperture diameter and so a lower aperture ratio.
    aperture ratio = (distance from aperture to focal plane)/(diameter of the aperture opening)
    effective aperture diameter = entrance pupil diameter = (focal length)/(aperture ratio)
with some possible corrections in the macro regime due to the focal length shift in that situation.

The various measure of aperture size agree for the simplistic case of an objective [so-called "lens"] with a single lens [so-called "lens element"] with the aperture size being the diameter of the allowable light path through that lens itself.
« Last Edit: May 11, 2018, 08:37:41 AM by BJL »
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ErikKaffehr

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Re: Diffraction with telephoto lenses
« Reply #14 on: May 13, 2018, 01:25:51 AM »

Hi,

The angle of the first minimum is given as 1.22 lambda / d. The radius of the airy ring depends on the projection of that angle, so a longer lens will project a larger disc.

https://en.wikipedia.org/wiki/Airy_disk

So, the radius of the first airy ring is approximately 1.22 lambda * f-number.

Best regards
Erik







I have a closeup and macro photography book by well respected authors who apparently are not well versed in physics. They state that diffraction is less problematic with telephoto macro lenses, since the diameter of the aperture in millimeters is greater for a given f/number. However, the Airy disc is dependent on aperture, not focal length and it is the projection of the Airy disc on the individual pixels of the sensor that determine the effects of diffraction. At least, that is my understanding. Any other thoughts?

Thanks,

Bill
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Erik Kaffehr
 

bjanes

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Re: Diffraction with telephoto lenses
« Reply #15 on: May 13, 2018, 06:59:30 AM »

Standard lens on an 8x10 is 150mm and it wasnít uncommon to see minimum apertures of f92 on those lenses. Of course we couldnít pixel peep in those days and mostly used aperture to determine how much depth of field we wanted and werenít as concerned about diffraction as we are now.

Usually, a standard lens focal length is approximately equal to the diagonal of the format of the sensor. For 35 mm cameras, the diagonal of a 24 mm *36 mm image sensor is 43 mm and the standard lens is often taken as 50 mm. For an 8 * 10 inch film, the diagonal is 12.8 inches or 325 mm. 150 mm would be a standard lens for a 4 * 5 inch camera.

At f/92, the Airy disc with green light would be about 100 microns or 0.1 mm. This 0.1 mm blur circle would be about 1/2000 of the picture height of the sensor and the resulting image might be acceptable for a contact print, but a 35 mm image would have to be magnified by 8 x for this size print and the print would be quite blurred if one used an aperture of f/92. A 0.1 mm blur would take up about 1/240 of the picture height of the 35 mm sensor. The acceptable blur circle has to be correlated with the sensor size.

Regards,

Bill
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Martin Kristiansen

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Re: Diffraction with telephoto lenses
« Reply #16 on: May 13, 2018, 09:53:36 AM »

Quite right and my apologies. No idea why I said 8x10 has a standard lens of 150mm. My error.
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BJL

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At f/92, the Airy disc with green light would be about 100 microns or 0.1 mm. This 0.1 mm blur circle would be about 1/2000 of the picture height of the sensor and the resulting image might be acceptable for a contact print, but a 35 mm image would have to be magnified by 8 x for this size print and the print would be quite blurred if one used an aperture of f/92. A 0.1 mm blur would take up about 1/240 of the picture height of the 35 mm sensor. The acceptable blur circle has to be correlated with the sensor size.

Regards,

Bill
Yes; another way to put it is that for both DOF and diffraction comparisons, scaling the focal length and aperture ratio in the same proportion also scales image size, circle of confusion sizes and Airy disk size in the same proportion, so both those source of blurring look about same on same-sized prints.

By the way, this means that for OOF and diffraction effects, the f/64 movement in 8"x10" format was on a par with "f/8 and be there" in 24x36mm format.


... BUT this only applies if subject distance stays the same. Some posts in this thread ignore the greater subject distance make possible by using a longe focal length in the same format, which improves the trade-offs between OOF and diffraction effects.
 
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