[Alex645]

The effective aperture is what will determine the amount of diffraction, so the diffraction of your lens set to f/8 (via the aperture ring) with a 2xTC behind it will have the same diffraction as the lens on its own set to f/16.

I'd be interested in seeing anything you have to the contrary.

Here's one reference to the contrary. The OP in the link writes "diffusion" but meant "diffraction" and #2 and a few following concur with this:

Effect of x2 teleconverter. | Amateur Photographer

I can understand the theoretical arguments both ways. Ultimately it is splitting-hairs because there will be so much more than diffraction influencing IQ with a TC.

[palikrovol]

I was hoping this could be resolved without going that far, it was a pretty simple question... but for those who want a deeper understanding it's good stuff.

Interesting topic for a poll.
Can you use polls to determine the correct answers to technical questions?

I thought it was going to be far more simple than that (the reason for the poll), but as you say it is a good thread to understand a TC behavior.

Thanks a lot everyone

Regards

[BrianR]

Here's one reference to the contrary. The OP in the link writes "diffusion" but meant "diffraction" and #2 and a few following concur with this:

Effect of x2 teleconverter. | Amateur Photographer

I can understand the theoretical arguments both ways. Ultimately it is splitting-hairs because there will be so much more than diffraction influencing IQ with a TC.

Are 'zx9' and 'beejaybee' reliable sources? The theoretical arguments in that link make no sense to me. I understand that the amount the light is messed up will be the same at the physical aperture if you have a TC on or not, but what I don't understand is why the size of the airy disc hitting the sensor (the important measurement of diffraction for a photographer) would be the same in both cases. The TC has changed the optical path of the light after it passes the aperture, so why can we ignore what it does to the airy disc? beejaybee doesn't ignore this optical path change for extension tubes, but does for a TC. Help my understand how this can make sense.

I am in firm agreement with your earlier suggestion that practical tests will be for the win in answering the OP's question. But I do think it's worth sorting out what a TC does to diffraction (we could move to another thread, but I think it's relevant here?). Both versions can't be right. So please, let's not dismiss this as splitting hairs, think of me as one of your curious photography students who wants to make sure he has a sound understanding of the concepts.

[normhead]

Are 'zx9' and 'beejaybee' reliable sources? The theoretical arguments in that link make no sense to me. I understand that the amount the light is messed up will be the same at the physical aperture if you have a TC on or not, but what I don't understand is why the size of the airy disc hitting the sensor (the important measurement of diffraction for a photographer) would be the same in both cases. The TC has changed the optical path of the light after it passes the aperture, so why can we ignore what it does to the airy disc? beejaybee doesn't ignore this optical path change for extension tubes, but does for a TC. Help my understand how this can make sense.

I am in firm agreement with your earlier suggestion that practical tests will be for the win in answering the OP's question. But I do think it's worth sorting out what a TC does to diffraction (we could move to another thread, but I think it's relevant here?). Both versions can't be right. So please, let's not dismiss this as splitting hairs, think of me as one of your curious photography students who wants to make sure he has a sound understanding of the concepts.

What he said ^.... there are times when you really need to just listen to the folks who have studied lenses technically. You refine your knowledge in part by cutting down the number of amateurs you listen to. If you get anything out of this thread , one of the important points should be, the only opinions worth listening to are from those who actually understand the science or at least some of it. Personally, I would have had to do some research, but because I took a course in lens design, I don't know enough to make this judgement on my own, but I know enough that when I read someone else's post I can tell if they know what they are talking about. The behaviour of physical properties of lenses cannot be determined by amateurs giving their opinions on a forum. This poll being a case in point.

I can understand the theoretical arguments both ways. Ultimately it is splitting-hairs because there will be so much more than diffraction influencing IQ with a TC.

There's nothing that will have as much effect as diffraction with a good TC. The rated sharpness loss from the Pentax 1.4 is 3% with a 40% increase in subject resolution for a 37% net gain. On a DA 70 2.4 on a K-5 and measured at photozone between ƒ5.6 and ƒ8 you lose (2698-2577) or 4.5% resolution. One stop of the effect of diffraction degrades the image 50% more than the effect of adding a TC. Especially true since theoretically, since without diffraction the circles of confusion become smaller as the aperture gets smaller meaning more of the image should be in acceptable focus. So diffraction both wipes out the expected gain in sharpness and then further reduces it.

Pentax SMC DA 70mm f/2.4 Limited - Review / Lab Test - Analysis

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Last edited by normhead; 03-02-2019 at 07:45 AM.

[Alex645]

Are 'zx9' and 'beejaybee' reliable sources? Help my understand how this can make sense.

I am in firm agreement with your earlier suggestion that practical tests will be for the win in answering the OP's question. But I do think it's worth sorting out what a TC does to diffraction (we could move to another thread, but I think it's relevant here?). Both versions can't be right. So please, let's not dismiss this as splitting hairs, think of me as one of your curious photography students who wants to make sure he has a sound understanding of the concepts.

Reliable sources is a whole other thread, and although that particular thread came from Amateur Photographer, I don't think you have to be either an amateur or a photographer to post comments in their threads or ours. And even the most professional publications and sources are not 100% reliable, albeit more credible than, for example, wiki sites.

Personally, I am not an expert in knowing the factual truth, I was just sharing what I had been taught and had observed from anecdotal experience over the years. But I, like most of us in the forum, share what we believe is correct. My own limited theoretical understanding of optics in this regard prevents me from explaining further why this does or does not make sense. That is why I revert to practical tests to see the results.

Would results from different combinations of lenses, focal lengths, apertures, teleconverters create a repeatable trend to prove or disprove either theory? Probably. But if it didn't, it would show that the cause and effect is more complicated than a simple always or never answer.

I did not mean to belittle this thread as "splitting hairs" and am engaged in reading everyone's responses. Good teachers are students themselves, and good students have their own role as teachers too.

[Lowell Goudge]

i think one needs to consider diffraction quite differently, Diffraction is always present, even wide open, diffraction is the bending of light around an edge, in this case the aperture. The issue with diffraction is that as you stop down the lens, the amount of light that is projected to the image which is distorted by difraction, increases as a percentage of the total light emitted from the lens, diffraction is much worse, on shorter lenses because the aperture is relatively small to begin with, and therefore plays a much more important role than on a long lens.

as a result i tend to discount it, for me there are a lot of other causes of lack of image sharpness that occur with very long lenses, especially due to vibration when on a tripod, shake when hand held, image movement due to low shutter speed, distortion due to air currents when shooting over long distances, and focus inaccuracy due to perhaps camera shake or movement to and away from the subject when hand held combined with narrow depth of field.

using long lenses, (beyond 300mm) requires a lot of work, the learning curve for 500mm+ lenses can be quite steep, and there are a lot of people out there who lack the patience and persistence to work outy all the issues. We should not get bent out of shape here discussing the theoretical optimum capabilities of a lens, before we tackle all the practical limits of its use. i for one, dont shoot MTF charts for a hobby

This is also the opposite of what I've understood. Though perhaps calrrification is needed. Are you saying diffraction on a 20mm lens set to f/16 would be worse than the diffraction of a 200mm lens set to f/16? I have understood diffraction to be the same on a 20mm lens and a 200mm if both were set to the same f/16 aperture.

.

Consider a simple lens, the longer the focal length the bigger in diameter it needs to be to capture the same total light to make the exposure ,

If you block out part of the lens, which is what the aperture does, the diameter gets smaller. Now consider the ratio of area to circumference.

Compare , a 200 mm f2.8 lens and a 20mm f2.8 lens. You will see the difference directly.

What may be more of an issue is the interference patterns that appear due to the cusps in a non round aperture.

As I said, before pure diffraction there are a lot of other issues to deal with

[BrianR]

Personally, I am not an expert in knowing the factual truth, I was just sharing what I had been taught and had observed from anecdotal experience over the years. But I, like most of us in the forum, share what we believe is correct.

Fair enough. I think far too often people end up sharing what they think is correct as fact, when it's in fact not a fact, but one of the often repeated photography misconceptions. It gets frustrating trying to suss out the accurate information. I always try to keep an open mind about my own understanding as I'm utterly loath to unwittingly spread misinformation.

ps. I consider falconeye to be in the 'reliable' category.

Consider a simple lens, the longer the focal length the bigger in diameter it needs to be to capture the same total light to make the exposure ,

If you block out part of the lens, which is what the aperture does, the diameter gets smaller. Now consider the ratio of area to circumference.

Compare , a 200 mm f2.8 lens and a 20mm f2.8 lens. You will see the difference directly.

What may be more of an issue is the interference patterns that appear due to the cusps in a non round aperture.

As I said, before pure diffraction there are a lot of other issues to deal with

So you do think the focal length matters, and that Cambridge in Colour is wrong when they say diffraction only depends on f-number*? (let's ignore for the moment possibly differently shaped apertures on different lenses)

*and on wavelength of the light, but that's not an issue for a focal length comparison

---------- Post added 03-02-19 at 03:23 PM ----------

The behaviour of physical properties of lenses cannot be determined by amateurs giving their opinions on a forum. This poll being a case in point.

How fun the world could be if its physical laws were dictated by cartoon-physics determined by a popular vote

[normhead]

Fair enough.

So you do think the focal length matters, and that Cambridge in Colour is wrong when they say diffraction only depends on f-number*? (let's ignore for the moment possibly differently shaped apertures on different lenses)

What you might be missing is that ƒ5.6 on my 135mm lens is about 4 times the size as the ƒ5.6 aperture on a 35mm lens. The same ƒ number, does not mean the same size aperture. But I'm just guessing at what the source of confusion might be.

How fun the world could be if its physical laws were dictated by cartoon-physics determined by a popular vote

I bet we'd all have time machines, and be perennially 26 years old.

[BrianR]

What you might be missing is that ƒ5.6 on my 135mm lens is about 4 times the size as the ƒ5.6 aperture on a 35mm lens. The same ƒ number, does not mean the same size aperture. But I'm just guessing at what the source of confusion might be.

Nope, I get that. The larger physical aperture of the 135mm is also further away from the sensor than the smaller physical aperture of the 35mm lens. The size of the airy disc on the sensor depends not just on the physical size of the aperture, but also on how far the apertuure is from sensor. My physics text agrees with wikipedia, so I'll just link to it, see the "camera" section : Airy disk - Wikipedia

Are you also asserting that the focal length influences diffraction, and that it doesn't just depend on the f-number? i.e. a 200mm lens @ f/16 will have more diffraction than a 20mm lens also at f/16?

[normhead]

Nope, I get that. The larger physical aperture of the 135mm is also further away from the sensor than the smaller physical aperture of the 35mm lens. The size of the airy disc on the sensor depends not just on the physical size of the aperture, but also on how far the apertuure is from sensor. My physics text agrees with wikipedia, so I'll just link to it, see the "camera" section : Airy disk - Wikipedia

Are you also asserting that the focal length influences diffraction, and that it doesn't just depend on the f-number? i.e. a 200mm lens @ f/16 will have more diffraction than a 20mm lens also at f/16?

No not at all. My notion that the ƒ-stop is constant in it's characteristics across various focal lengths, that being based in the resolution charts of many lenses. The point at which diffraction starts reducing the resolution is pretty much the same, ƒ 5.6, on the majority of lenses. As your aperture gets smaller lens resolution should increase. Regardless of the lens used 5.6 is probably the point where your lens is limited by diffraction, regardless of focal length. (Although Photozone has listed both the DA 55 1.4 and FA 31 as sharpest at ƒ4. As I said before, the more resolution produced by the lens, the sooner the diffraction limit sets in.) The focal length of the lens would seem to have nothing to do with it.