Light Gathering Power vs Area - Which one should I use?

[rnclark]

Camera lenses operate differently to telescopes in some fundamental ways.

No they don't. The physics does not change just because it is a lens versus a telescope. The photons don't say, hey look, it s lens so I'll behave differently, versus hey look I'm going in to a telescope so I'll behave this way.

Actually they very much do. And while the physics of light may not change, how they are manipulated does.

No. This is just bizarre.

The type of lens versus mirror in the optical trains do. The type of glass, element design, selected focal length etc, all have a very different effect in optical trains used for astronomy versus lenses need for photography. Cant say I've ever seen a shutter or IS system in my telescope.

Now you are confusing cameras with telescopes. Cameras have shutters, telescopes do not. Modern cameras lenses do not have shutters, lenses for view cameras have an in-lens leaf shutter because the view camera typically does not have a shutter. This is irrelevant. And whether or not a lens has IS or not is irrelevant. For example, the dragonfly arrays Swims linked to used standard commercial 400 mm f/2.8 lenses with IS.

This is all irrelevant to the question from the OP.

For astronomical purpose it can be said that "big is best but bigger is better". The larger the area of the optics the more light you can gather. So a bigger mirror or lens in a telescope will always yield more light at the focal plain.

OK so far.

Heres an example, you are looking at globular cluster M13 with an 8 inch scope and its a lovely bright object that shows granulation in the star field. Now look at the same object using the same eyepiece in a 24 inch telescope and you see a beam of light from the eyepiece ( visual astronomers know what I refer to ) as you are about to look at the object, which is very bright and may take you several seconds to adjust to. Thats the power of light gathering, which is a direct function of the area of the objective lens or mirror.

OK so far. In fact I have used star light from a single star coming out of the telescope eyepiece to read my charts and notes. (It was an 88-inch diameter aperture telescope.)

Now for camera lenses it doesn't work that way because the descriptive language means something totally different. Camera lenses work on different principles where DOF, fast focal ratios and minimum f values , autofocus, IS are just some of the important factors. Camera lenses are very efficient at what they do and some are very good for astrophotography.

Again, the physics does not change because someone uses descriptive language differently. For example, incoming photons:

Photon 1: "Oh look, I'm going in to a 300 mm f/4 camera lens, so I'm going to behave in camera lens mode."

Photon 2: "Oh look, I'm going in to a 75 mm aperture, 300 mm focal length telescope, so I'm going to behave in telescope mode."

Nope.

Here I feel your are being intentionally sarcastic.

Yes, to show you the absurdity of your position.

The two biggest factors as you well know are aperture and resolution with astronomical telescopes and their fundamentals ( eg: focal length) An 80mm f5.0 ( 400 mm focal length) refractor for example wont necessarily match a 80mm f5.0 camera lens. Theres an awful lot of light bending going on in the camera lens to get the lens to supply an image at the focal plane. All these extra elements degrade the final image. Contrast this to a simple refractor with one front element and a camera attached at the the other end, I know which I would rather use.

As other have pointed out you are confusing nomenclature. First get your nomenclature straight and compare physics consistent and compare similar systems. For example an 80 mm f/5 telescope has 400 mm focal length. If a simple achromat the image quality is not very good. Compare to a modern multi-element 400 mm f/5 telephoto lens and the image quality will be much better, NOT degraded as you say. Multiple elements do not necessarily degrade an image. And with modern coatings, lens transmission remains very high despite many elements.

It is for these very reasons why those building the dragonfly array are using commercial lenses rather than using achromatic or apochromatic telescope refractors--the multi-element camera lenses produce better images over a large field of view.

For a accurate idea of a given lenses performance at a given ISO and f ratio use the Calculator provided by lonely speck. To date this has served very well and I have found that it works very well for any lens and the times are accurate with a little leeway if you want to push it.

This has nothing to do lens performance. That calculator is simply a "350 rule" on an APS-C (He finally has backed off from a 500 rule).

I put in 35 mm focal length, iso 1600 f/0.7 to f/8 and the exposure time does not change: 10 seconds, or 150 arc-second drift at the celestial equator. On a camera with 4-micron pixels, that would result in a 6.4 pixel smear. Not very good at all.

Cant speak to your results . However the exposure times very closely match my results so I have no issue with the calculator.

Heres a little quote that may help to understand this a little more -

Essentially yes, light gathering ability of a lens is determined by its maximum aperture. Transmission rates of the materials used also has an effect but it is very small.

You intuition is correct in that you would expect a large aperture lens to have a large barrel, however the aperture is specified as a ratio of the *apparent** size of lens opening divided by the focal length.

No, it is not the apparent size. Technically, it is the entrance pupil. It is the same definition, whether a lens or a telescope.

Actually it is expressed as max aperture. The optical train can well be stopped down with internal baffles to limit diffraction in some instances, such as you find in refactors. So for example you may have a 100 max aperture scope that is in fact using a 90mm entrance pupil to limit said refraction. There are a number of camera lenses that use this function as well. Note focal length is unchanged. I use to own a catadioptric scope that also employed this principle. I believe that this is what was being alluded to.

No, and you are mixing diffraction and refraction. Decreasing the aperture on a camera lens or telescope INCREASES diffraction. Refraction is not changed.

If you had a telescope with an aperture stop to reduce the maximum entrance pupil, that is a trick use by manufacturers of cheap optics to reduce aberrations. For example, when I was a kid, my parents bought me a 60 mm refractor. I later learned there was an aperture stop just behind the objective with a 24 mm hole. The lens was a single element plate glass, so horrible chromatic aberration. False advertising!

So a 200mm f/2.0 lens must have a front element large enough to see a 200/2.0 = 100mm aperture, so the barrel must be at least 10cm. However a 20mm f/2.0 only appears to have a 10mm aperture, which is small is comparison to most lens sizes.

* note that 100mm f/2.0 doesn't mean the physical opening in the middle of the lens is actually 50mm diameter, only that the image of said opening when viewed through the front of the lens appears to be 50mm in diameter.

It means the entrance pupil is actually 50 mm diameter. It is a precise physical definition, not just something that "appears" a certain size.

See above.

Summary, the responses that you called out as not being accurate really were quite accurate,

see my correction above

and the concept that camera lenses behave differently than telescopes is what is not accurate.

I think we can agree that this statement is not in error but rather there are some very different design elements between the two systems, even though the physical properties of light remain the same.

No, I do not agree. There are dozens and dozens of different optical designs for both telescopes and lenses. So what. The basic results are the same: focus light onto the focal plane. The amount of light collected and delivered to the focal plane is the entrance pupil area times the transmission of the optics, and that was the question of the OP.

Roger

 

[rnclark]

Heres a guide that can help you decide what lens will give the result you want.

You can see the whole article here and its worth reading as it alludes to what I referenced earlier in this post.

Picking a lens for astrophotography

Yeah, he copied that from my table 1:

That could be a dangerous accusation given the apparent lack of proof offered here.

Note: I don't mind copying if it gets the word out. But not when it adds confusion, like that lonelyspec table.

http://www.clarkvision.com/articles/nightscapes/

But note 3rd paragraph below the table:

"There is an additional factor not seen in Table 1. As the camera records fainter stars, more stars appear in greater numbers than the light gathering. For example, double the collections area pr double the exposure time, more than twice as many stars will show. That biases the number os stars factor in favor of the larger aperture lenses. An example of this effect is shown in Figure 5b."

Are you saying that as the exposure progresses the lens somehow magically exceeds it ability to resolve fainter stars? Now to be fair atmospheric seeing can exclude faint stars on a frame by frame basis, but you assertion would seem to suggest that a longer exposure will increase the number of stars to the point of exceeding Dawes Limit?

I will assume here that what you are actually trying to say is that the longer the exposure the greater the chance of gaining increased fine detail up to the limit of the lenses resolving power and the sensors sensitivity.

If you do a simple google search, you will find that there are many more faint stars than bright stars, at least for the brightness range of night sky photography with wide angle camera lenses. For example if you make an exposure and collect 1000 stars, then double the aperture area for the same exposure time so you collect twice as much light, the number of stars, if the number scaled in proportion would be root 2 more (1414 stars), but you will record more than 1414 stars (30 to 100% more depending on brightness range and position in the sky). This is the effect shown in Figures 2a versus 2b at http://www.clarkvision.com/articles...-and-lenses-for-nightscape-astro-photography/

I'm sure that the formula purported by Etendue is important to you, but I wonder as to how many here would have the slightest interest in this. For those of us who actually make optics for telescopes it is just one of several other important formula in the making of said optics, but for the sake of simplicity most people would settle for a simple and concise ( ballpark ) way to calculate which lens is best at what exposure time.

Astronomers in general, including amateur astronomers, do understand aperture rules and that is what Etendue tells you. Photographers seem to have a hard time with this, wedded to the erroneous concept that constant f-ratio is constant total light.

Lenses are not "best at what exposure time." That is not the right way in my opinion to choose a lens, whether for astro or regular photography.

How to choose a lens:

1) For a given sensor size, what field of view do I want and what detail? That determines focal length. At 35 mm on a camera with 4 to 5 micron pixels, the resolution is similar to that of the human eye.

2a) Is mosaicking considered? if no, then choose the lens that covers the desired field of view.

2b) Is mosaicking considered? If yes, then choose a longer focal length for the desired field of view and number of frames in the mosaic. For example, a 15 mm f/2.8 single frame versus a 24 mm f/2 2x2 mosaic with 30% overlap: the mosaic will produce a far superior image with fainter detail and smaller stars.

3) How much light to collect in a given time? That sets the desired aperture.

4a) Time may be limited by fixed tripod, but can be extended with multiple short exposures. Exposure time limit for sharp stars with modern digital cameras is simple: 200 /focal length in mm; no table or web site needed. And it is not related to sensor size.

4b) Tracking removes the limitation of Earth's rotation. If you can't afford a sophisticated tracker, build a barn door hand cranked tracker for a few $.

5) To collect the most light in the shortest time, use the maximum aperture you can afford in quality optics.

6) The most complex issue in choosing settings has nothing to do with the above. I has to do with the noise pattern performance of the camera you are using. The ISO needs to be set high enough to digitize fine tonality at the low end, and high enough to boost those tiny signals above the pattern noise from camera electronics, as illustrated in Figure 2 here: http://www.clarkvision.com/articles/nightscapes/

The tables at loneleyspec and other web sites are making things really simple, and not necessarily optimum. And the extremely low light night sky photography needs to be optimized for best results.

Or just be simple: choose a very wide angle lens, like 15 mm f/2.8, use the 500 rule (30 second exposures), ISO 6400 making white stars , push the white balance to unnatural blue which boosts noise, and make images that are common on the internet. People will OOH and AAAHHH over the beautiful blue you faked.

 

[Ralph McKenzie]

It seems we are going at this from two different angles and I can only concluded from your reply that you have not necessarily read some of my previous post to some of the other readers.

Clearly we will never agree on this subject so from this point on I suggest we leave it, as things stand we are likely confusing some people.

I had a reply from Lonely Speck which I posted here and they stand by there website. Which is fine by me as to date I and many others have gained good results from this. It is clear that you wish to use a different and in your view more accurate method and have gone to great lengths to say so, and if this works well for you then that too is a good result.

 

[Moon0326]

Hi Ralph,

So..what is your conclusion? Is that 500 works? Even Ian Norman from Lonely Speck left comments that 500 rules does not always work. He only recommend it for the beginners.

 

[RustierOne]

The two biggest factors as you well know are aperture and resolution with astronomical telescopes and their fundamentals ( eg: focal length) An 80mm f5.0 ( 400 mm focal length) refractor for example wont necessarily match a 80mm f5.0 camera lens.

I assume that all gentlemen taking part in this discussion can agree that the lenses mentioned above are vastly different:

1. An 80mm F/5.0 refractor has an aperture of 80mm and a 400mm focal length
2. An 80mm F/5.0 camera lens has an aperture of 16mm and a 80mm focal length

This is because telescopes use the millimeter specification to primarily refer to aperture. Camera lenses use the millimeter specification to primarily refer to focal length. So it is not surprising that these lenses "won't necessarily match".

Thank you for your reply, its a good example of how to make things simplistic for others to understand, without getting bogged down in the technical aspects.

I was hoping that my reply would help clarify things a bit. So often when we can't agree on some subject it is because we don't understand the other person's point of view. It can be beneficial considering things from a different angle, even if perhaps our own understanding is different. All of our contributions are of value to this forum.

 

[Ralph McKenzie]

Hi Ralph,

So..what is your conclusion? Is that 500 works? Even Ian Norman from Lonely Speck left comments that 500 rules does not always work. He only recommend it for the beginners.

I was quite surprised to get such a speedy reply from Lonely Speck, to my enquiry. I think its a case of what works for each of us. I use it as a point of reference thats fast and easy to use and adjust the exposure times depending on the results I get.

So as a fast reference it does what Ian designed it to do and will help many folks who want to just try out astrophotography and use it as a starting point.

As with everything we do we tend to refine things as we gain experience.

 

[chris gunn]

Hi Ralph,

So..what is your conclusion? Is that 500 works? Even Ian Norman from Lonely Speck left comments that 500 rules does not always work. He only recommend it for the beginners.

It depends on the lowest declination in the FOV. The stars move fastest at declination 0 deg, such as at around Orion. They move slowest at the poles.

I usually use 15s for 24mm APSC for declination >20deg or so. Rule 360?