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

[Ralph McKenzie]

Hi!

I think I got the following equation for light gathering area from this forum.

pi*(14/2.8)^2 = 78.54

pi*(20/1.4)^2 = 641

300 / 14 = 21.4

300 / 20 = 15

78.54 * 21.4 = 1680

641 * 15 = 9615

When I compare two lens to see how much more light lens A gathers than lens B, should I use light gathering power or area?

Thanks!

Moon

Firstly let me say that you are asking a very complex question, and depending upon outcome has some very different results.

Most of the information mooted in the replies is not strictly accurate ( No offence intended )

Camera lenses operate differently to telescopes in some fundamental ways.

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.

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.

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.

Just remember that small apertures may give you nice wide fields of view but yield little in fine detail when used on the night sky. For better detail you need longer focal lengths to reduce the field of view and increase the resolution.

As to the duration of some of the images seen here, I have to question how good a 30 second image at any focal length would look from and un-driven tripod.

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.

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. 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.

To complicate matters wide angle lenses need larger front elements than dictated by their aperture to prevent vignetting across the frame. For focal lengths shorter than about 50mm lens sizes increase as focal length decreases despite apertures, and thus light gathering ability, also decreasing.

Here's nice example, this Nikon lens is only f/2.8:





* 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. The actually opening is often smaller, but the lens front element has to be large enough to accommodate its theoretical size.

End Quote.

--
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Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[rnclark]

Hi!

I think I got the following equation for light gathering area from this forum.

pi*(14/2.8)^2 = 78.54

pi*(20/1.4)^2 = 641

300 / 14 = 21.4

300 / 20 = 15

78.54 * 21.4 = 1680

641 * 15 = 9615

When I compare two lens to see how much more light lens A gathers than lens B, should I use light gathering power or area?

Thanks!

Moon

Firstly let me say that you are asking a very complex question, and depending upon outcome has some very different results.

Most of the information mooted in the replies is not strictly accurate ( No offence intended )

Actually it is accurate, but your response is not.

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.

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.

only that the image of said opening when viewed through the front of the lens appears to be 50mm in diameter. The actually opening is often smaller, but the lens front element has to be large enough to accommodate its theoretical size.

End Quote.

--
Love dat Fuji
http://akiwiretrospective.wordpress.com/
Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

Just remember that small apertures may give you nice wide fields of view but yield little in fine detail when used on the night sky. For better detail you need longer focal lengths to reduce the field of view and increase the resolution.

Not exactly. Fine detail is a function of optical design and sensor pixel pitch, regardless of whether you called it a lens or telescope.

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.

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.

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.

Summary, the responses that you called out as not being accurate really were quite accurate, and the concept that camera lenses behave differently than telescopes is what is not accurate.

I suggest googling entrance and exit pupils, and Etendue.

Roger

 

[Moon0326]

Hi rnclark,

Thank you for the detailed answers. I think I got interested in these calculation after seeing your post from other threads. I will need to re-read your articles on your site all over again. Thank you so much for the effort you put into your site.

 

[Moon0326]

Thank you Ralph McKenzie,

I really need to swallow your writing and articles from rcclark. I don't think I can understand them now with my limited knowledge.

Thank you for the write up!

 

[Ralph McKenzie]

Hi!

I think I got the following equation for light gathering area from this forum.

pi*(14/2.8)^2 = 78.54

pi*(20/1.4)^2 = 641

300 / 14 = 21.4

300 / 20 = 15

78.54 * 21.4 = 1680

641 * 15 = 9615

When I compare two lens to see how much more light lens A gathers than lens B, should I use light gathering power or area?

Thanks!

Moon

Firstly let me say that you are asking a very complex question, and depending upon outcome has some very different results.

Most of the information mooted in the replies is not strictly accurate ( No offence intended )

Actually it is accurate, but your response is not.

I was not necessarily referring to the above calculation but other claims alluded in the conversation, I should have made that clearer.

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. 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.

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.

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.

only that the image of said opening when viewed through the front of the lens appears to be 50mm in diameter. The actually opening is often smaller, but the lens front element has to be large enough to accommodate its theoretical size.

End Quote.

--
Love dat Fuji
http://akiwiretrospective.wordpress.com/
Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

Just remember that small apertures may give you nice wide fields of view but yield little in fine detail when used on the night sky. For better detail you need longer focal lengths to reduce the field of view and increase the resolution.

Not exactly. Fine detail is a function of optical design and sensor pixel pitch, regardless of whether you called it a lens or telescope.

Fine detail is primarily a function of optical design. In terms of the recorded image, then yes sensors play a roll as a secondary component just a film does as well.

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.

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.

I suggest googling entrance and exit pupils, and Etendue.

Been there , done that. When I grind my telescope mirrors, the aperture and exit pupil are taken into account, especially if I intend to use 1.25 or 2 inch lenses. Just one factor in mirror correction when fine grinding and polishing.

Roger

--
Love dat Fuji
http://akiwiretrospective.wordpress.com/
Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[Moon0326]

Not that I understand what you guys are talking about...but that calculator never worked for me.

For example, it says that my exposure should be 25 seconds and ISO 800 if I input

Full Frame, 6400 ISO, 20mm, f/1.4.

25 seconds with 20mm? I'm pretty sure I will get pretty bad startrail.

 

[Ralph McKenzie]

Not that I understand what you guys are talking about...but that calculator never worked for me.

For example, it says that my exposure should be 25 seconds and ISO 800 if I input

Full Frame, 6400 ISO, 20mm, f/1.4.

25 seconds with 20mm? I'm pretty sure I will get pretty bad startrail.

Yep just had another look at it. It seems that it isn't recognising some lenses. As far as I can tell it seems to be only using two of the parameters, namely f ratio and focal length, which it wasn't doing previously and it used to change if iso and frame sizes were changed. I've emailed the lads at Lonely Speck to see whats up. Hopefully I will get a reply.


Love dat Fuji
http://akiwiretrospective.wordpress.com/
Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[Ralph McKenzie]

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




This taken from PetsPixel

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

--
Love dat Fuji

A-KiwiRetrospective

Capturing Moments In Time.

akiwiretrospective.wordpress.com

Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[rnclark]

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


This taken from PetsPixel

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:

Nightscape Photography with Digital Cameras, Clarkvision.com

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."

So look at Figure 5b. Do you really think the images are equal in visual impact? See also http://www.clarkvision.com/articles...-and-lenses-for-nightscape-astro-photography/

The idea that you get the same total amount of light with different focal length lenses at the same f-ratio applies to a uniform subject, like a blank wall. It does not apply to objects within the field of view. For example, look at Figures 2a and 2b on the second link above "characteristics-of-best..." Which image captured more light (should be obvious)? despite both are at f/2.8, Figure 2b is brighter. Sum all the pixels and you would see.

Fortunately the real world is not blank walls and test charts. In the real world the best explanation of what we measure and observe is Etendue of the subjects in the image or that we view with our eyes.

Roger

 

[Ralph McKenzie]

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


This taken from PetsPixel

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.

I checked your tables and the only table that truly seems similar is table 4 of your first section. There are similarities to some of the figures but they differ markedly in the exposure time recommendations. On this basis alone I would under no circumstances recommend either of your tables and certainly wont be on my blog.

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.

So look at Figure 5b. Do you really think the images are equal in visual impact? See also http://www.clarkvision.com/articles...-and-lenses-for-nightscape-astro-photography/

The idea that you get the same total amount of light with different focal length lenses at the same f-ratio applies to a uniform subject, like a blank wall. It does not apply to objects within the field of view. For example, look at Figures 2a and 2b on the second link above "characteristics-of-best..." Which image captured more light (should be obvious)? despite both are at f/2.8, Figure 2b is brighter. Sum all the pixels and youwould see.

What you are describing here is the difference in light gathering ( lightgrasp ) ability of a given lens for a given diameter regardless of its focal length, which I might add is what I pointed out earlier.

Fortunately the real world is not blank walls and test charts. In the real world the best explanation of what we measure and observe is Etendue of the subjects in the image or that we view with our eyes.

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.



--
Love dat Fuji

A-KiwiRetrospective

Capturing Moments In Time.

akiwiretrospective.wordpress.com

Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[Ralph McKenzie]

As I noted in an earlier post in this thread I contacted Lonely speck and got a very fast reply. Kudos to Lonely Speck for that.

Here my question to them and their reply.



So it seems it aint broke but it does have some limitations. Got some random typos in there to I see

I'm not sure that this answer directly addresses what seems to be happening however it does explain some of the results and if its working as intended then use it if you want to. I do and to be fair to date its worked well enough for what I do.

Love dat Fuji
http://akiwiretrospective.wordpress.com/
Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[swimswithtrout]

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.

If you look at my responce from 5 days ago, I explicitly mentioned Etendue , I am interested, as well as many others here ! it it's a very important tool.

I find your "logic" and "facts" confusing and completely erroneous. At one point you said

"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."


Well of course it won't ! That's comparing an 80mm f5.0 "Telescope" to a 16mm f5 "Telescope" aka "camera lens". A more useful comparison would have been to compare an 80mm f5.0 "Telescope", to a 400mm f5.0 "lens".


I personally like my 65mm f2.8 refractor telescope. It's flat from corner, at least on my APS-c camera and only cost $375 USD. You can find them listed as a "Nikon 180mm f2.8 ED" lens.

 

[Ralph McKenzie]

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.

If you look at my responce from 5 days ago, I explicitly mentioned Etendue

Yes you did and I quote " Etendue....learn it....faster wider is not always correct.

Also, to get an acceptable 30" untracked exposure requires at maximum, a 9mm lens on a FF camera." Unquote

Rather a pompous and arrogant reply to say the least. Perhaps you should expand on the subject to which you so reverently purport, and while you are at it break it down into a more simplified form so that lay people can get to grips with what is being said, because as you say there a lots of folks with a interest in the subject.

You also arrogantly assume that the vast majority of readers here have a sufficient grasp of mathematics and physics to be able to competently follow your assertion.

, I am interested, as well as many others here ! it it's a very important tool.

Well good for you. I would wager that a much larger majority are simply looking for a clear, quick and concise method for determining the best exposure time for a given lens from a fixed tripod. Quite frankly we dont need to know how a lens is constructed to use one, we do need to know its basic operational parameters so that we can get the best exposure for a given subject.

I find your "logic" and "facts" confusing and completely erroneous.At one point you said

"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."

Well of course it won't ! That's comparing an 80mm f5.0 "Telescope" to a 16mm f5 "Telescope" aka "camera lens". A more useful comparison would have been to compare an 80mm f5.0 "Telescope", to a 400mm f5.0 "lens".

No it wouldn't because the comparisons being used here are how camera lenses work as opposed to telescope optics which are two very different animals. Comparisons have been made here to how much light grasp a given lens has and in some cases the results can be quite misleading. A person may well think that their 85mm lens for example would perform the same as an 85mm telescope when in fact there are significant differences.

I personally like my 65mm f2.8 refractor telescope. It's flat from corner, at least on my APS-c camera and only cost $375 USD. You can find them listed as a "Nikon 180mm f2.8 ED" lens.

Indeed while they both employ the properties of a refractor, they are designed for completely different purposes. And while you will get good performance from your camera lens the the refractor at prime focus will give better performance per mm of aperture. Thats one reason why astronomers use large telescopes, rather than large camera lenses.

 

[Moon0326]

Thank you for your effort Ralph McKenzie, but that chart does not work at all if you try.

36 seconds exposure with 14mm f/2.8? No way I will use that. 500 rule simply does not work at all. I really have no idea how people still use 500 rule. May be those people never tried it, but have some background knowledge.

 

[swimswithtrout]

because the comparisons being used here are how camera lenses work as opposed to telescope optics which are two very different animals. Comparisons have been made here to how much light grasp a given lens has and in some cases the results can be quite misleading. A person may well think that their 85mm lens for example would perform the same as an 85mm telescope when in fact there are significant differences.

Who would assume that ? This is a forum devoted to astronomical photography and the VAST majority of us know the difference between 85mm and 85mm.


This what my 85mm Samyang camera lens can do at f4, ISO 400, from my backyard that is so light polluted that I can barely see Cygnus

Indeed while they both employ the properties of a refractor, they are designed for completely different purposes. And while you will get good performance from your camera lens the the refractor at prime focus will give better performance per mm of aperture. Thats one reason why astronomers use large telescopes, rather than large camera lenses.

BS !


Of course, when it comes to cost of scale, an 8" f4 reflector is going to be less expensive than the equivalent camera optic, but the notion that the only real AP uses telescopes, is so false, it doesn't require another comment.


Currently the ground based search for the faintest surface area objects in the sky , over the past several years, has been not using some mountain top 10M telescope, but rather a set of ordinary Canon 400mm f2.8 II lens'.


Before you respond back, read THIS PAPER and then argue against it

 

[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".

--
Best Regards,
Russ

 

[swimswithtrout]

Thank you for your effort Ralph McKenzie, but that chart does not work at all if you try.

36 seconds exposure with 14mm f/2.8? No way I will use that. 500 rule simply does not work at all. I really have no idea how people still use 500 rule.

Because it gets blog hits when displayed at 800px wide and feeds the companies selling "fast" lens.

Static tripod is SOOOoooo totally "Old School" !

For only $299, far less cost than buying any f1.4 "fast" lens or a real "telescope" you can buy a tracking mount, that sets up in less than 5 min and lets you shoot up to 5 min exposures at 100mm fl or less. You can shoot up to 60-90 sec at 200-300mm and do "real" astrophoto's using consumer grade kit lens' .

These were all taken using cheap camera lens', none faster than f4, none longer than 300mm, all less than $400, using ISO 400-800, no telescope needed, mounted on a $299 tracker, taken from a "White Zone" area where barely any stars with my the naked eye.

And this was taken using a kit zoom WA lens at f4, ISO 1600, 30 sec., from a dark site

 

[Ralph McKenzie]

Thank you for your effort Ralph McKenzie, but that chart does not work at all if you try.

36 seconds exposure with 14mm f/2.8? No way I will use that. 500 rule simply does not work at all. I really have no idea how people still use 500 rule.

Because it gets blog hits when displayed at 800px wide and feeds the companies selling "fast" lens.

Static tripod is SOOOoooo totally "Old School" !

Yes it is, but it is all some of us can afford.

For only $299, far less cost than buying any f1.4 "fast" lens or a real "telescope" you can buy a tracking mount, that sets up in less than 5 min and lets you shoot up to 5 min exposures at 100mm fl or less. You can shoot up to 60-90 sec at 200-300mm and do "real" astrophoto's using consumer grade kit lens' .

Maybe where you live, but the cheapest tracking platform sold here is around $600 unless on sale as it is at the moment, which is of course still cheaper than a fast camera lens and around the same price as a short tube Orion scope .
These were all taken using cheap camera lens', none faster than f4, none longer than 300mm, all less than $400, using ISO 400-800, no telescope needed, mounted on a $299 tracker, taken from a "White Zone" area where barely any stars with my the naked eye.

And this was taken using a kit zoom WA lens at f4, ISO 1600, 30 sec., from a dark site

Nice images btw.

--
Love dat Fuji

A-KiwiRetrospective

Capturing Moments In Time.

akiwiretrospective.wordpress.com

Fuji HS20EXR,S5700
Fujifilm XA2, XC16-55, XC50-230, XF27 f2.8

 

[Ralph McKenzie]

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".

 

[Tristimulus]

A quick formula for the lazy mind:

- Double the diameter of the aperture and the gain is 1.5 stellar magnitudes.

Doubling the diameter of the aperture will increase the area four times.

...

And a quick formula for the calculator:

Log D x 5 + C

Where D is the aperture expressed in cm and C is a constant.

- For visual magnitude limits telescope manufacturers usually use C = 7.0

- Most observeres find the visual limiting magnitude is closer to C = 7.7

In my case photographically C is typically somewhere between 13 and 14.5 depending upon the gear used and the local conditions. Height above the horizon matter as well.