[westmill]

he, he, I have not used tubes yet, but I don't see how an extension tube can give the added aberration in his bottom-most image unless it is bent.
He went from an image that is decent edge to edge quality to one that has warped at the edge.

Someone let me know if I'm wrong here (again I don't have tubes to test and can only think logically about their function), but...
An extension tube is in effect doing the same as a crop sensor does when used with a full frame lens (not exactly, but similar principles). It is enlarging the image circle onto your sensor to that the sensor is taking only a portion from the center of your image circle. So if anything, it is magnifying the best part of the image created by the lens -- that which avoids edge deterioration. With a lens tube, the edges should be potentially sharper than with the original lens on its own. Now to note, you are enlarging the image circle, so any imperfections in the glass will be magnified and that might render a softer image. I could see that the result might be a less sharp image, but such distortion simply does not make sense.
Am I off my rock here?

Anyway, I agree with the others... never fight a troll.
No matter how hard you hit them, they will regenerate and are just too stupid to stop fighting.

Any glass you put in front of a lens will and does deter image quality. Even top grade very thin filters has a slight effect, let alone anything else.
Tubes and bellows are just hollow with no glass elements. It is not the same at all as using FF on apsc. Neither do they magnify. There only purpose is to move the lens away from the camera body, thus changing the focus point. Tubes are genraly bought in sets of three which can be used individualy or combined where bellows gives you a more flexible constant shift. There is no cropping of the image. It simply allows you to focus closer.

[amoringello]

If you take any focused light and move it away from an object like a wall, or sensor, its size is going to increase... does it not?
Oddly, when I remove my lens and move it away from the sensor, the object's apparent size increases.

Physics would seem to dictate that the light must occupy a larger surface area as it extends away from the prior point. Also resulting in loss of light for a given fixed area, since the light is now encompassing a larger area. I am fuzzy on how the focus point moves, but I'm sure I'll works that out in my head in a bit.

Additionally, the mere act of focusing (bringing the lens closer or further) changes the image size. So I'm pretty certain the physics must act in this way without glass being in there to re-focus light back to its original area.

[Lowell Goudge]

If you take any focused light and move it away from an object like a wall, or sensor, its size is going to increase... does it not?
Oddly, when I remove my lens and move it away from the sensor, the object's apparent size increases.

Physics would seem to dictate that the light must occupy a larger surface area as it extends away from the prior point. Also resulting in loss of light for a given fixed area, since the light is now encompassing a larger area. I am fuzzy on how the focus point moves, but I'm sure I'll works that out in my head in a bit.

Additionally, the mere act of focusing (bringing the lens closer or further) changes the image size. So I'm pretty certain the physics must act in this way without glass being in there to re-focus light back to its original area.

With lenses the size of the image is equal to the ratio of the subject and the relative distance of subject to lens / lens to focal plane

That's why the lens group moves out to focus closer than infinity. You are correct, as the image gets larger, the light is spread out over a bigger area, therefor reducing the illumination over the frame

The only difference between extension tubes and close up lenses, is that close up lenses shorten the focal length. While effectively putting the lens on an extension tube at the same time. This has the effect of increasing the apparent aperture and making the image brighter.

For example a 50mm F2 lens with a +2 diopter is a 45mm F1.8 lens, extended out 5mm.

[pinholecam]

IMO and from what I've used before, the claim is nonsense.
I'd ignore the fella.
Even if you posted evidence, he'd nik-pik this and that, make you frustrated and do more proofing work, and cycle repeats itself.

[Lowell Goudge]

The only possible argument someone might have with respect to tubes over diopters is the following.

Normal lenses are designed for a curved field which is approximately flat at infinity. The closer you focus, the more curved the plane of focus becomes. It is not so much loss of sharpness at the edge, when put on an extension tube, but curvature of the focusing plane. Macro lenses on the other hand are specifically designed to be used on flat fields at close focus.

Now, let's consider what happens when focusing with a diopter. A diopter lens causes light from a subject at the focal length of the diopter (1/diopter number) to exit the diopter lens parallel to the lens axis. Therefore the result is that light from the subject will be correctly focused by the lens with the lens set to infinity, with the subject at the focal length of the diopter. One might argue this is the optimum arrangement for a normal (non macro) lens. It might be, except that the diopter lens is not, itself, perfect. As I said earlier, unless the diopter lens is specifically designed with the knowledge of the lens it will be used with, it simply cannot result in a better image than the lens alone, it will always produce a less sharp image.

Multi element close ups have some correction, but that is for the close up lens itself, and may approach a lens on a tube, but that is all.

Now for edge sharpness and curvature of field. Unless you are shooting stamps, or other perfectly flat objects, and are at right angles to the object with the optical axis of the lens, a curved field does not really matter. Flowers are not flat, so who really cares if the lens is a flat field or not!

[Stone G.]

I'd like evidence not only because his clever writing made me look silly if not stupid, but also because I really want to know.

I'd hate to have his clever writing mislead a lot of folks. It looks to me like there is no certain answer.

When we talk about IQ degradation as a result of adding extra glass (filters, lenses,converters) we are usually thinking in terms of internal reflections/flare,chromatic aberration and perhaps some distortion, aren't we?

However, your opponent may have a point if we incorporate spherical aberration - or so I think. Because, unless you use modern, aspheric lenses, you have to optimize your lenses for minimizing spherical aberration which grows proportional to the fourth power of aperture diameter and inversely proportional to the third powerof focal length.

Now, I am very far from being well versed in this subject, but allow me to quote from Wikipedia:

In lens systems, the effect can be minimized using special combinations of convexand concave lenses, as well as using aspheric lenses.

For simple designs one can sometimes calculate parameters that minimize spherical aberration. For example, in a design consisting of a single lens with spherical surfaces and a given object distance o, image distance i, and refractive indexn, one can minimize spherical aberration by adjusting the radii of curvature and of the front and back surfaces of the lens such that






So, there just might be some truth in the claim that ordinary lenses are not (all and equally well) designed for the close focus distances and the correspondingly long lens-sensor registration distances achieved with extension tubes.

Just a speculation - - - what do you think?!?

[newarts]

When we talk about IQ degradation as a result of adding extra glass (filters, lenses,converters) we are usually thinking in terms of internal reflections/flare,chromatic aberration and perhaps some distortion, aren't we?

However, your opponent may have a point if we incorporate spherical aberration - or so I think. Because, unless you use modern, aspheric lenses, you have to optimize your lenses for minimizing spherical aberration which grows proportional to the fourth power of aperture diameter and inversely proportional to the third powerof focal length.

Now, I am very far from being well versed in this subject, but allow me to quote from Wikipedia:

In lens systems, the effect can be minimized using special combinations of convexand concave lenses, as well as using aspheric lenses.

For simple designs one can sometimes calculate parameters that minimize spherical aberration. For example, in a design consisting of a single lens with spherical surfaces and a given object distance o, image distance i, and refractive indexn, one can minimize spherical aberration by adjusting the radii of curvature and of the front and back surfaces of the lens such that






So, there just might be some truth in the claim that ordinary lenses are not (all and equally well) designed for the close focus distances and the correspondingly long lens-sensor registration distances achieved with extension tubes.

Just a speculation - - - what do you think?!?

I think what you and Lowell say are both true. I have seen cases where tubes are better at the edges and where diopters are better at the edges.

However, I'm still seeking example photos of either case to discern which is more common and how to predict.

At this point I don't know what to tell someone who asks "Which is best; Tubes or Achromats?" Especially if edges are a concern.

Dave in Iowa

[audiobomber]

Hi Dave. I can do the test I outlined this weekend. Do you have any directions other than what I listed above?

[newarts]

The only possible argument someone might have with respect to tubes over diopters is the following.

Normal lenses are designed for a curved field which is approximately flat at infinity. The closer you focus, the more curved the plane of focus becomes. It is not so much loss of sharpness at the edge, when put on an extension tube, but curvature of the focusing plane. Macro lenses on the other hand are specifically designed to be used on flat fields at close focus.

Now, let's consider what happens when focusing with a diopter. A diopter lens causes light from a subject at the focal length of the diopter (1/diopter number) to exit the diopter lens parallel to the lens axis. Therefore the result is that light from the subject will be correctly focused by the lens with the lens set to infinity, with the subject at the focal length of the diopter. One might argue this is the optimum arrangement for a normal (non macro) lens. It might be, except that the diopter lens is not, itself, perfect. As I said earlier, unless the diopter lens is specifically designed with the knowledge of the lens it will be used with, it simply cannot result in a better image than the lens alone, it will always produce a less sharp image.

Multi element close ups have some correction, but that is for the close up lens itself, and may approach a lens on a tube, but that is all.

Now for edge sharpness and curvature of field. Unless you are shooting stamps, or other perfectly flat objects, and are at right angles to the object with the optical axis of the lens, a curved field does not really matter. Flowers are not flat, so who really cares if the lens is a flat field or not!

Thanks Lowell, you are right & the situation is clearly going to vary lens type to lens type.

Here's an image comparing the corners of good diopter lenses on the end of extended bellows. They looknabout the same to me and pretty good!


Dave in Iowa

[newarts]

Hi Dave. I can do the test I outlined this weekend. Do you have any directions other than what I listed above?

Cropping the original image may be a good idea for comparison.

The best way to make sure the effective f-stop is the same is to use the same lighting, ISO, and exposure time - adjust the f-stop (not the time) so the exposures are the same. At first this seems to make no sense, but if +/-EV, ISO and exposure time, lighting, etc are the same, effective f-stop must be the same.

This method is better than calculating effective f-number I think.

Dave

[audiobomber]

I can do the test I outlined this weekend.

Sorry Dave, I didn't get the test done. It's hard to find the time and I'm not sufficiently invested.

[newarts]

Sorry Dave, I didn't get the test done. It's hard to find the time and I'm not sufficiently invested.

No problem! Someday maybe.

Dave in Iowa

[Digitalis]

I concur with Lowell Goudge, Tubes are the safest option if you are aiming for the best image quality - also you won't have to be encumbered by having to buy a new set of close- up dioptre filters if you acquire a new lens that has a bigger filter thread - however it is possible to enhance the amount of corner image quality by buying using close up filters that are larger than the filter thread diameter*. We see a similar effect on APS-C where we use the central portion of the image circle projected by a lens - we will see superior image quality in the corners when compared to a traditional full frame camera.

However be warned extension tubes can reveal some rather unfortunate characteristics of some non-macro lenses - and in some cases even macro lenses can suffer from the unanticipated increase of the flange distance. However close up filters - even the multi-coated ones reduce transmission of a lens** and also can make dealing with flare problematic.


In the above image you can see my Sigma 180mm f/3.5 APO EX Macro suffered from this rather annoying hot spot that was rather significant at the 1:1 setting - it turned out to be internal reflections caused by the increased flange caused by the addition of a 25mm extension tube.

*for example if your lens has a 52mm filter thread buy 58mm filters and use a step-down ring.

**though the don't reduce it anywhere near as much as extension tubes do.