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02-26-2010, 04:32 AM   #1
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A basic question on optics

Many of you know that I use scopes rather than conventional telephotos for my bird shots.

In a conventional camera lens it is generally always better to stop down significantly to gain optimum IQ. It is well understood why this is so and I won't get into that.

However my scopes are fixed aperture and I'm always shooting wide open. In spite of this I think it's fair to say I can get excellent IQ out of my scopes shooting wide open.

What I wonder about is how much the degradation in IQ is due simply to pushing the optics to their limits (wide open) and how much, if any, has to do with the F stop being used?

To put it another way -
ALL ELSE BEING EQUAL would a lens wide open at F/4 have IQ similar to an F/2.8 lens stopped down to F/4?

I think you can see where I'm going with this. Why do my conventional camera lens', with the possible exception of m FA35, seem to suffer so much wide open while my scopes do not?

I have my own ideas on this but I'll let others speak on this first.

560mm taken at F/7 (wide open)


Last edited by wildman; 03-16-2010 at 09:49 PM.
02-26-2010, 07:25 AM   #2
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Well, I'm an optical designer and completing my PhD in optics, so I can help...

You approach this topic in a way that will not allow for an accurate answer. If you prfer, you do not ask quite the right question There are two reasons why lenses wide open are not quite as good as stopped down.

The first relates to optical effects on the sides of a lens. Especially true with spherical lens elements, but applicable in any case, you will get more aberrations from the borders, because the curvature "seen" by light rays will be more important (I'm simplifying, but that's the basic explanation). CA, coma, etc, all these aberrations are more important from the sides.

The second effect is vigneting. What causes vigneting is that, at wider apertures, all the areas of the image frame get different illumination (this is not the same as thinking that you "see" part of the iris, as some think). This effect is mitigated when closing down the aperture because only the center of the lens is used to create the image. The rays are more parallel to the axis of the lens.

Of course, it really depends on how the lens is made, so you cannot just compare lenses by specs.

A prime lens (or a refractive telescope) is designed and optimised for one set of parameters. For a zoom, it's about compromise, you cannot optimise everything for every focal length (you usually pick a focal length somewhere in the middle of the range and start from there). A reflecting telescope (with mirrors) works basically in the same way, but the light rays never enter in any material, they are only reflected, which in theory yields better images (reflecting telescopes do have other flaws, such as bokeh and they have their lot of CA/coma issues too, again because of the border effets).

So there is no physical reason why telescopes should be better, but there are design reasons why they might be. The less compromises you have to make, the better your results.

I had to design a 240x beam expander (a refractive telescope, in fact) for my PhD. It had to be optimized for one single wavelength (or colour, if you prefer). It's really near perfect at that wavelength, but you cannot imagine how crappy it is at other wavelengths! But since there is no need for it to be any good except at the wavelength I wanted, no one cares...

Physics are cool
02-26-2010, 01:42 PM   #3
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Most lenses, no matter what their widest aperture, work best arround f/8. At apertures wider than this, aberations (caused by geometric optics) primerally from the outer zones (outer edges) of the lens cause image degredation and reduce the resolving ability of the lens. At abertures smaller than this, diffraction effects (caused by physical optics) limit the resolving ability of the lens. At f/7 your spotting scope is close to the optimum f number and so workes really well wide open. The only reason to go for wider apertures is to get more light into the lens (and sometimes to get a shallower depth of field) and the only reason to use smaller apertures is to get a wider depth of field.
As bdery said the performance of any particular lens in comparison to any other lens at the same aperture depends on the specific lense and the conditions for which they were optimised.
02-26-2010, 02:15 PM   #4
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Wave theory hypothises that any wave with a periodic frequency that pass through an aperture will set up interference patterns. This applies to ocean waves at a harbor enterance, to radio/radar frequencies, x-rays, light waves, etc. And for every combination of variables there will be one optimum aperture for "best" results as defined by the intended use.

The selection of available apertures for most cameras simply increases the selectable options (like shutter speed, ISO and DOF) within the acceptable limits established for the intended use.

Eliminate some of the photographically desireable variables and a telescope can be more economically (repeat 'economically') designed and manufactured to a given standard with a fixed aperture.

H2

02-26-2010, 03:22 PM   #5
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while bdery gave a lot of useful information, I believe he could have answered the question with a lot shorter answer.

Fundamentally, refracting telescopes are very simple optical designs, based upon a minimum number of elements.

Each element causes its own distortion, and problems.

THis may lead you to ask why then, optical designers add more elements and optical groups. the reason is that they offer reduced size lenses. Look at a lot of primes and zooms that have a 400mm focal lenght. they are short, mush shorter than teh 400mm (or 16 inch length) that a simple optic would result in, In fact the true definition of telephoto is not just a long focal length, but an effective focal length achieved in a much shorter physical length.

Optical telescopes are usually the equivelent of very old long lenses. they have minimal need for corrections because the optics are simple,
02-26-2010, 03:28 PM   #6
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QuoteOriginally posted by Lowell Goudge Quote
THis may lead you to ask why then, optical designers add more elements and optical groups. the reason is that they offer reduced size lenses.
You got that backwards. Extra optical elements are there to correct spherical and chromatic aberration and this makes the lens larger, not smaller. The reason for the small number of elements in 100 year old designs is that 100 years ago there were no lens coatings.

Last edited by asdf; 02-26-2010 at 04:50 PM.
02-26-2010, 03:44 PM   #7
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QuoteOriginally posted by wildman Quote
To put it another way -
ALL ELSE BEING EQUAL would a lens wide open at F/4 have IQ similar to an F/2.8 lens stopped down to F/4?
What exactly is equal when you have an f/4 lens and an f/2.8 lens? The material used for the lens mount?

There are many examples of slower lenses that perform even better than faster lenses, at the same apertures and focal lengths, in terms of sharpness. Softness is due to chromatic and spherical aberration. How much correction there is for both doesn't depend on the maximum aperture of the lens.
02-26-2010, 04:04 PM   #8
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QuoteOriginally posted by MattGunn Quote
Most lenses, no matter what their widest aperture, work best arround f/8.
Clarification - even when the widest aperture IS f/8?

02-26-2010, 04:10 PM   #9
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QuoteOriginally posted by wildman Quote
Clarification - even when the widest aperture IS f/8?
If the lens only has f/8 with which to work (i.e., if it's a mirror lens), then that's the best and worst aperture for the lens, by definition. If its maximum aperture is f/8 but you can still stop it down, then you can only degrade the quality of the image in focus, because of diffraction.

Last edited by asdf; 02-26-2010 at 04:22 PM.
02-26-2010, 04:21 PM   #10
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Originally Posted by Lowell Goudge View Post

THis may lead you to ask why then, optical designers add more elements and optical groups. the reason is that they offer reduced size lenses.

ASDF:
You got that backwards. Extra optical elements are there to correct spherical and chromatic aberration and this makes the lens larger, not smaller. The reason for the small number of elements in 100 year old designs is that 100 years ago there were no lens coatings.

You are both right/wrong.
SOME of the elements in a telephoto are present to primarily correct for aberrations AND on the back end is present a "telephoto group" which is a negative focus group the purpose is to increase the apparent FL and thus shorten the over-all physical length of the lens for any given magnification just like an external TC or Barlow does. The telephoto group is essentially a permanent built in internal TC.

As a matter of fact so called "prime" telephotos, because of the telephoto group, do not function at prime focus.

In fact a "telephoto" is so defined by the presence or absence of the telephoto group.

My glass, for instance, is not a telephoto but rather a long lens.

Last edited by wildman; 02-26-2010 at 05:00 PM.
02-26-2010, 04:40 PM   #11
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QuoteOriginally posted by wildman Quote
SOME of the elements in a telephoto are present to primarily correct for aberrations AND on the back end is present a "telephoto group" which is a negative focus group the purpose is to increase the apparent FL and thus shorten the over all-all physical length of the lens for any given magnification just like an external TC or Barlow does.
Then the telephoto group's elements aren't the "extra" elements I was talking about.
03-02-2010, 07:17 AM   #12
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QuoteQuote:
Most lenses, no matter what their widest aperture, work best arround f/8.
There is no physical reason why this should be so. Based on experience with lenses designed for photography, it's a good rule of thumb, but that's all there is to it.
03-02-2010, 11:41 AM   #13
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I agree that there is no physical reason why this should be the case. Some cheap lenses are still poor at f/8 and the compromise between aberations and diffraction means they are better at smaller apertures, and some top end lenses are better at wider apertures. However it is generally found that most SLR lenses do work at their best around f/8 which is why it has become a good rule of thumb. Even rules of thumb have some basis to them.
03-02-2010, 02:54 PM   #14
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It has been my experience that SLR lenses work their best one or two stops less than their max aperture. Since so many lenses are f/4 or f/5.6 the f/8 rule has an obvious origin but it is far from absolute. For example, playing around with a test chart and the the 50/1.4 that I use on my Canon cameras I found that f/2.5 gave the sharpest results with that particular lens.
03-02-2010, 04:34 PM   #15
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QuoteOriginally posted by wildman Quote
Many of you know that I use scopes rather than conventional telephotos for my bird shots.

In a conventional camera lens it is generally always better to stop down significantly to gain optimum IQ. It is well understood why this is so and I won't get into that.

However my scopes are fixed aperture and I'm always shooting wide open. In spite of this I think it's fair to say I can get excellent IQ out of my scopes shooting wide open.

What I wonder about is how much the degradation in IQ is due simply to pushing the optics to their limits (wide open) and how much, if any, has to do with the F stop being used?

To put it another way -
ALL ELSE BEING EQUAL would a lens wide open at F/4 have IQ similar to an F/2.8 lens stopped down to F/4?

I think you can see where I'm going with this. Why do my conventional camera lens', with the possible exception of m FA35, seem to suffer so much wide open while my scopes do not?

I have my own ideas on this but I'll let others speak on this first.

560mm taken at F/7 (wide open)
You simply cannot compare a photographic lens with an astronomical telescope. First of all, most photographic lenses are faster (wider aperture) than most telescopes. So all the classical aberrations will be much more prominent in most photographic lenses.

I don't go into the lens correction thing here, because you've already got good answers. But I want to emphasize the fundamental difference between a photographic lens and a telescope:

The photographic lens is optimized for photography. Sounds stupid, doesn't it? But it means, it needs no correction beyond the film/sensor capabilities in terms of resolution and contrast.

An astronomical telescope is different in so far, as it only constitutes the first part of the imaging train and then follows the eyepiece which allows a variable after-magnification of the telecsopic image. This means, that a telescope needs to perform to higher standards, i.e. to its theoretical limits (it is diffraction limited by its open diameter), because the image can be viewed at very high magnifications.

On the other hand several properties, which are necessary for photographic lenses, are usually neglected in scopes: the focal plain needs to be flat for a lens, but often is curved in a scope (the eye can adapt to that). The area of highest optical performance can usually be much smaller in a scope (often only 10mm across), than in a photographic lens, which needs somewhat even performance over a much larger area to cover the film/sensor.

If a scope performs then much better on a camera, than a cheap photographic lens, it is due to its usually diffraction limited resolution and the simple fact, that in these tele images we usually do not scrutinize the corners of an image for the tiniest detail (as here the simple scope often gets much worse, than in the center). If you want even performance over the whole photographic field, you need a photo optimized telescope or an additional optical device to correct the residual aberrations over a larger field (like a coma corrector or field flattener).

Photographically optimized telescopes are usually more expensive than their visual counterparts AND they usually perform somewhat poorer visually, as they forgo the last bit of center resolution to gain more even performance over the whole field.

One could write a lot more, but I simply wanted to add some telescope-specific things in a brief (well, somewhat) way here.

Ben
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