Re: Astrophotography (was Re: *istD EOL...)
Great analogy. Bob Blakely wrote: When you realize that if you create an image of a distant 60 foot tree on your film, develop that film, put it back in the camera (with the back open) and shine a light through it, you will project the 60 foot tree back on itself, then you will understand that it's all about ratios and the direction the light is going.
Re: Astrophotography (was Re: *istD EOL...)
When you realize that if you create an image of a distant 60 foot tree on your film, develop that film, put it back in the camera (with the back open) and shine a light through it, you will project the 60 foot tree back on itself, then you will understand that it's all about ratios and the direction the light is going. - Original Message - From: <[EMAIL PROTECTED]> To: Sent: Thursday, January 13, 2005 4:56 PM Subject: Re: Astrophotography (was Re: *istD EOL...) I find telescope eyepieces work pretty much as intuition suggests; a "stronger" eyepiece increases the magnification of the image. A far more interesting question, to my mind, is why that isn't the case in photography. Bob Blakely mused: Ok, the analogy using "light levers" didn't work. Let's try again... Nothing is working opposite to expectations. One lens, the objective lens, is working in one direction with light coming in from the distant object at the *distant* focal point to the image on the other side of the lens at its *close* focal point. The other lens is being used the other way around with the light from the image going from the *close* focal point to the more distant focal point and eventually to your eye. Regards, Bob... From: "Tom C" <[EMAIL PROTECTED]> > OK, I understand the math and don't disagree, but why does a longer > focal > length eyepiece (a set of glass lenses in a tube) give lower > magification, > when a longer focal length camera lens (a set of glass lenses in a > tube) > yields a higher magnification? > > It would seem at first blush that if you have a telescope with a given > focal length producing x magnification and you then viewed that image > through 2 eyepieces of different focal lengths, that the eyepiece with > the > longer focal length would yield the higher magnification. What makes > it > work opposite of what one (I) would expect? > > I know this is a basic optics question that I'm just not too > embarrassed > to ask.
Re: Astrophotography (was Re: *istD EOL...)
On Thu, 13 Jan 2005 19:56:34 -0500 (EST), [EMAIL PROTECTED] wrote: > > I find telescope eyepieces work pretty much as intuition suggests; > a "stronger" eyepiece increases the magnification of the image. > A far more interesting question, to my mind, is why that isn't the > case in photography. We don't usually use a second (objective) lens in photography. It's sometimes done in astrophotography, though. Usually, though, photographic lenses just use one (compound) lens with a specific focal length. When you mount two lenses nose-to-nose, for extreme macro photography, you're doing something similar to a telescope with an eyepiece (objective) lens. TTYL, DougF KG4LMZ
Re: Astrophotography (was Re: *istD EOL...)
I find telescope eyepieces work pretty much as intuition suggests; a "stronger" eyepiece increases the magnification of the image. A far more interesting question, to my mind, is why that isn't the case in photography. Bob Blakely mused: > > Ok, the analogy using "light levers" didn't work. Let's try again... > > Nothing is working opposite to expectations. One lens, the objective lens, > is working in one direction with light coming in from the distant object at > the *distant* focal point to the image on the other side of the lens at its > *close* focal point. The other lens is being used the other way around with > the light from the image going from the *close* focal point to the more > distant focal point and eventually to your eye. > > Regards, > Bob... > > From: "Tom C" <[EMAIL PROTECTED]> > > > > OK, I understand the math and don't disagree, but why does a longer focal > > length eyepiece (a set of glass lenses in a tube) give lower magification, > > when a longer focal length camera lens (a set of glass lenses in a tube) > > yields a higher magnification? > > > > It would seem at first blush that if you have a telescope with a given > > focal length producing x magnification and you then viewed that image > > through 2 eyepieces of different focal lengths, that the eyepiece with the > > longer focal length would yield the higher magnification. What makes it > > work opposite of what one (I) would expect? > > > > I know this is a basic optics question that I'm just not too embarrassed > > to ask. > >
Re: Astrophotography (was Re: *istD EOL...)
OK, that's what I started to conclude must be the answer. Thank you. Tom C. From: "Bob Blakely" <[EMAIL PROTECTED]> Reply-To: pentax-discuss@pdml.net To: Subject: Re: Astrophotography (was Re: *istD EOL...) Date: Thu, 13 Jan 2005 10:29:13 -0800 Ok, the analogy using "light levers" didn't work. Let's try again... Nothing is working opposite to expectations. One lens, the objective lens, is working in one direction with light coming in from the distant object at the *distant* focal point to the image on the other side of the lens at its *close* focal point. The other lens is being used the other way around with the light from the image going from the *close* focal point to the more distant focal point and eventually to your eye. Regards, Bob... From: "Tom C" <[EMAIL PROTECTED]> OK, I understand the math and don't disagree, but why does a longer focal length eyepiece (a set of glass lenses in a tube) give lower magification, when a longer focal length camera lens (a set of glass lenses in a tube) yields a higher magnification? It would seem at first blush that if you have a telescope with a given focal length producing x magnification and you then viewed that image through 2 eyepieces of different focal lengths, that the eyepiece with the longer focal length would yield the higher magnification. What makes it work opposite of what one (I) would expect? I know this is a basic optics question that I'm just not too embarrassed to ask.
Re: Astrophotography (was Re: *istD EOL...)
Thanks for the replies. I'm still not sure I understand the focal length magnification thingy, so I guess I'll have to draw some ray diagrams. I'll try to "shoot the moon" when I next get a chance. It's a bit chilly and windy at night at the moment. Nick
Re: Astrophotography (was Re: *istD EOL...)
Ok, the analogy using "light levers" didn't work. Let's try again... Nothing is working opposite to expectations. One lens, the objective lens, is working in one direction with light coming in from the distant object at the *distant* focal point to the image on the other side of the lens at its *close* focal point. The other lens is being used the other way around with the light from the image going from the *close* focal point to the more distant focal point and eventually to your eye. Regards, Bob... From: "Tom C" <[EMAIL PROTECTED]> OK, I understand the math and don't disagree, but why does a longer focal length eyepiece (a set of glass lenses in a tube) give lower magification, when a longer focal length camera lens (a set of glass lenses in a tube) yields a higher magnification? It would seem at first blush that if you have a telescope with a given focal length producing x magnification and you then viewed that image through 2 eyepieces of different focal lengths, that the eyepiece with the longer focal length would yield the higher magnification. What makes it work opposite of what one (I) would expect? I know this is a basic optics question that I'm just not too embarrassed to ask.
Re: Astrophotography (was Re: *istD EOL...)
OK, basic optics. You do know that a so called magnifying glass does not magnify, right? What it does is allow your eye to focus closer to the image. The shorter the focal length (hight the diopter) the closer the distance you can focus from, and the larger the image appears. Now the basic telescope produces what is called and arial image. That is a image that is focused at a point in space rather than onto something like a ground glass. Once you have that arial image you can by adjusting your eye to exactly the right point focus on it. But your eye would be about 10 inches away. Got that? Now your eyepiece allow you to move your eye closer to that arial image. The shorter the focal length of the eyepiece the closer you can move your eye, and the larger the image appears. It is as simple as that. graywolf http://www.graywolfphoto.com "Idiot Proof" <==> "Expert Proof" --- Tom C wrote: OK, I understand the math and don't disagree, but why does a longer focal length eyepiece (a set of glass lenses in a tube) give lower magification, when a longer focal length camera lens (a set of glass lenses in a tube) yields a higher magnification? It would seem at first blush that if you have a telescope with a given focal length producing x magnification and you then viewed that image through 2 eyepieces of different focal lengths, that the eyepiece with the longer focal length would yield the higher magnification. What makes it work opposite of what one (I) would expect? I know this is a basic optics question that I'm just not too embarrassed to ask. Tom C. From: "Herb Chong" <[EMAIL PROTECTED]> Reply-To: pentax-discuss@pdml.net To: Subject: Re: Astrophotography (was Re: *istD EOL...) Date: Wed, 12 Jan 2005 20:48:29 -0500 short focal length on an eyepiece gives high magnification. total magnification is the focal length of the objective divided by the focal length of the eyepiece. if 900 is the objective FL, then the 20mm eyepiece gives 45X and the 4mm eyepiece gives 225x. Herb... - Original Message - From: "Nick Clark" <[EMAIL PROTECTED]> To: Sent: Wednesday, January 12, 2005 1:28 PM Subject: Astrophotography (was Re: *istD EOL...) > I got a telescope for Christmas with a camera adaptor. I've not had much chance to play with it yet but was quite impressed with its power the first couple of times I used it. It's a Telstar 900x114 reflector, and fills the eyepiece with the moon with the 20mm objective. Strangely the moon is even larger when using the shorter focal length 4mm eyepiece, which I haven't quite worked out yet. -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.300 / Virus Database: 265.6.11 - Release Date: 1/12/2005
Re: Astrophotography (was Re: *istD EOL...)
At 6:28 PM + 1/12/05, Nick Clark wrote: I got a telescope for Christmas with a camera adaptor. I've not had much chance to play with it yet but was quite impressed with its power the first couple of times I used it. It's a Telstar 900 I assume that this is the focal length of the mirror? x114 ...and this is the mirror diameter? reflector, and fills the eyepiece with the moon with the 20mm objective. Strangely the moon is even larger when using the shorter focal length 4mm eyepiece, which I haven't quite worked out yet. The magnification is calculated by dividing the focal length of the mirror by the focal length of the eyepiece, so the 20mm = 45 power and the 4mm = 225 power (assuming the FL is 900mm). When looking at a group of stars (Seven Sisters) there are many more visible than with the naked eye, even here in light polluted London. That's because the 114mm mirror is a lot bigger than your 7mm pupil diameter so it collects more light. Unfortunately when I put the camera adaptor on with the *istD I couldn't see anything - it was far too dark to focus. A bit disappointing. Did you have an eyepiece in, or a lens on the camera? If so, try it without them...then the telescope acts as a 900mm lens. I haven't tried the camera with the moon yet, but will do next time it makes an appearance, and it's not raining, or cloudy. -- Steve [EMAIL PROTECTED] [EMAIL PROTECTED]
Re: Astrophotography (was Re: *istD EOL...)
the eyepiece is magnifying a fixed location virtual image. take a look at a loupe and see how higher magnification ones have shorted focal length. the objective is focusing an image onto a fixed plane inside the body of the scope for that situation, a longer focal length gives higher magnification. the eyepiece then magnifies that fixed plane. Herb - Original Message - From: "Tom C" <[EMAIL PROTECTED]> To: Sent: Wednesday, January 12, 2005 9:09 PM Subject: Re: Astrophotography (was Re: *istD EOL...) > OK, I understand the math and don't disagree, but why does a longer focal > length eyepiece (a set of glass lenses in a tube) give lower magification, > when a longer focal length camera lens (a set of glass lenses in a tube) > yields a higher magnification?
Re: Astrophotography (was Re: *istD EOL...)
OK, I understand the math and don't disagree, but why does a longer focal length eyepiece (a set of glass lenses in a tube) give lower magification, when a longer focal length camera lens (a set of glass lenses in a tube) yields a higher magnification? It would seem at first blush that if you have a telescope with a given focal length producing x magnification and you then viewed that image through 2 eyepieces of different focal lengths, that the eyepiece with the longer focal length would yield the higher magnification. What makes it work opposite of what one (I) would expect? I know this is a basic optics question that I'm just not too embarrassed to ask. Tom C. From: "Herb Chong" <[EMAIL PROTECTED]> Reply-To: pentax-discuss@pdml.net To: Subject: Re: Astrophotography (was Re: *istD EOL...) Date: Wed, 12 Jan 2005 20:48:29 -0500 short focal length on an eyepiece gives high magnification. total magnification is the focal length of the objective divided by the focal length of the eyepiece. if 900 is the objective FL, then the 20mm eyepiece gives 45X and the 4mm eyepiece gives 225x. Herb... - Original Message - From: "Nick Clark" <[EMAIL PROTECTED]> To: Sent: Wednesday, January 12, 2005 1:28 PM Subject: Astrophotography (was Re: *istD EOL...) > I got a telescope for Christmas with a camera adaptor. I've not had much chance to play with it yet but was quite impressed with its power the first couple of times I used it. It's a Telstar 900x114 reflector, and fills the eyepiece with the moon with the 20mm objective. Strangely the moon is even larger when using the shorter focal length 4mm eyepiece, which I haven't quite worked out yet.
Re: Astrophotography (was Re: *istD EOL...)
short focal length on an eyepiece gives high magnification. total magnification is the focal length of the objective divided by the focal length of the eyepiece. if 900 is the objective FL, then the 20mm eyepiece gives 45X and the 4mm eyepiece gives 225x. Herb... - Original Message - From: "Nick Clark" <[EMAIL PROTECTED]> To: Sent: Wednesday, January 12, 2005 1:28 PM Subject: Astrophotography (was Re: *istD EOL...) > I got a telescope for Christmas with a camera adaptor. I've not had much chance to play with it yet but was quite impressed with its power the first couple of times I used it. It's a Telstar 900x114 reflector, and fills the eyepiece with the moon with the 20mm objective. Strangely the moon is even larger when using the shorter focal length 4mm eyepiece, which I haven't quite worked out yet.
Re: Astrophotography (was Re: *istD EOL...)
On Wed, 12 Jan 2005 18:28:39 -, Nick Clark wrote: > Strangely the moon is even larger when using the shorter focal > length 4mm eyepiece, which I haven't quite worked out yet. If I understand it correctly, the magnification of the image you see will be the focal length of the scope divided by the focal length of the eyepiece. So, with the 20mm eyepiece you'd get 45X magnification, but with the 4mm eyepiece you'd get 225X magnification. TTYL, DougF KG4LMZ
Re: Astrophotography (was Re: *istD EOL...)
Think of the lenses as simple, one element lenses. Think of the center of the lenses as fulcrums (pivot points) of a "light lever" too. If the eyepiece has a short focal length, the distance from the image in the telescope to the lens "fulcrum" is short. The distance from the lens "fulcrum" to your retina is longer. Small image in the tube, big image on your retina. Shorter focal length on the lens even larger image on your retina. Yes, I know there is another lens involved, your eye's lens, but this is sufficient to demonstrate the principle. Regards, Bob... From: "Tom C" <[EMAIL PROTECTED]> Bob wrote: Think of it like a lever. The objective is focusing the image in the air inside the telescope's tube. The longer the focal length, the larger this image (like a camera lens). The eyepiece is used like a magnifying glass to view this image "in the ether". The shorter the focal length of the magnifying glass, the larger the image to your eye. There's something fundamental I'm missing, maybe you can help. I've been wrestling with the idea for a while... why, on let's say a camera lens or optical tube, does longer focal length = larger image, and on an eyepiece longer focal length = smaller image (less magnification). In my mind, it seems that an eyepiece is a lens with an optical tube and therefore it should work reverse of what you've stated regardless of whether it's focusing on he object itself or an image of the object "in the ether". I realize your statement is quite correct. What am I not getting? I'm sure I need to dig out a basic optics book.
Re: Astrophotography (was Re: *istD EOL...)
Bob wrote: Think of it like a lever. The objective is focusing the image in the air inside the telescope's tube. The longer the focal length, the larger this image (like a camera lens). The eyepiece is used like a magnifying glass to view this image "in the ether". The shorter the focal length of the magnifying glass, the larger the image to your eye. There's something fundamental I'm missing, maybe you can help. I've been wrestling with the idea for a while... why, on let's say a camera lens or optical tube, does longer focal length = larger image, and on an eyepiece longer focal length = smaller image (less magnification). In my mind, it seems that an eyepiece is a lens with an optical tube and therefore it should work reverse of what you've stated regardless of whether it's focusing on he object itself or an image of the object "in the ether". I realize your statement is quite correct. What am I not getting? I'm sure I need to dig out a basic optics book. Tom C.
Re: Astrophotography (was Re: *istD EOL...)
The "power" of an astronomical telescope is computed thusly: (Focal length of the objective) / (Focal length of the eyepiece). For example, I have a Meade ETX 90. The focal length of the objective (consisting of the front meniscus, primary mirror and secondary mirror) is 1250mm. With a 17mm secondary eyepiece, the power is 1250/17 = 73.5. With a 4mm secondary eyepiece, the power is 1250/4 = 312.5. Think of it like a lever. The objective is focusing the image in the air inside the telescope's tube. The longer the focal length, the larger this image (like a camera lens). The eyepiece is used like a magnifying glass to view this image "in the ether". The shorter the focal length of the magnifying glass, the larger the image to your eye. Note: usually, the eyepiece is removed to attach the camera adapter. Focus will be in a markedly different place than with the eyepiece in place. In fact, the image must be moved from inside the telescope to outside it and onto your film or CCD. This is usually several inches. When *severely* out of focus, you'll see nothing but black. Try focusing on a brighter object, such as a planet, first. Saturn is overhead now, and it's rings are tilted so as to be quite spectacular. Regards, Bob... - Original Message - From: "Nick Clark" <[EMAIL PROTECTED]> To: Sent: Wednesday, January 12, 2005 10:28 AM Subject: Astrophotography (was Re: *istD EOL...) I got a telescope for Christmas with a camera adaptor. I've not had much chance to play with it yet but was quite impressed with its power the first couple of times I used it. It's a Telstar 900x114 reflector, and fills the eyepiece with the moon with the 20mm objective. Strangely the moon is even larger when using the shorter focal length 4mm eyepiece, which I haven't quite worked out yet. When looking at a group of stars (Seven Sisters) there are many more visible than with the naked eye, even here in light polluted London. Unfortunately when I put the camera adaptor on with the *istD I couldn't see anything - it was far too dark to focus. A bit disappointing. I haven't tried the camera with the moon yet, but will do next time it makes an appearance, and it's not raining, or cloudy. Nick -Original Message- From: "Tom C"<[EMAIL PROTECTED]> Sent: 11/01/05 23:45:05 To: "pentax-discuss@pdml.net" Subject: Re: *istD EOL... Bruce, I'm curious why the shop people thought the Digital Rebel was junk. Was it based on look and feel ? What about image quality? I'm asking because right now the Rebel is the top selling DLSR for astrophotography. I haven't had a chance to try the *ist D yet with my telescope, but depending on results I get with the *ist D I wouldn't mind trying the Rebel. Tom C. >From: Bruce Dayton <[EMAIL PROTECTED]> >Reply-To: pentax-discuss@pdml.net >To: Nick Clark >Subject: Re: *istD EOL... >Date: Tue, 11 Jan 2005 15:33:08 -0800 > >I was in my local shop yesterday - mostly Nikon and Pentax, some >Canon. They are selling about five D70's everyday. People just keep >walking in a buying them. They think highly of the *istDS, but it >doesn't sell anywhere like the D70. Pretty much the word is out in >all media that photographers and would be photographers that the D70 >is THE camera to buy. Pop Photography proclaimed it Camera of the >Year. Size doesn't matter to most people at the time of purchase. >Later on, when having to carry the extra they might care, but it is >too late. The best Pentax can do with the *istDS is be respected. My >shop thinks the DRebel is junk next to the DS and they are very >willing to tell any potential customers. But they are really making >their money on the D70. It could well be the camera that saved >Nikon's hide. > >-- >Best regards, >Bruce > > >Tuesday, January 11, 2005, 3:18:50 PM, you wrote: > >NC> My local dedicated Camera shop says the *istDs isn't selling. >NC> They have it and the 300d and D70 (and the Minolta which is >NC> humungous) on display alongside each other. Even though the Pentax >NC> is smaller, they say the reasons people don't go for it are partly >NC> the SD card but mostly the difference in price. The 300d is >NC> something like GBP200 cheaper. > >NC> Nick > > >NC> -Original Message- >NC> From: "Pål Jensen"<[EMAIL PROTECTED]> > >NC> I think it will be more than enough. As high-end DSLR are >NC> larger than medium format cameras, and consequently suffers from >NC> the same lack of portability, the marke
Astrophotography (was Re: *istD EOL...)
I got a telescope for Christmas with a camera adaptor. I've not had much chance to play with it yet but was quite impressed with its power the first couple of times I used it. It's a Telstar 900x114 reflector, and fills the eyepiece with the moon with the 20mm objective. Strangely the moon is even larger when using the shorter focal length 4mm eyepiece, which I haven't quite worked out yet. When looking at a group of stars (Seven Sisters) there are many more visible than with the naked eye, even here in light polluted London. Unfortunately when I put the camera adaptor on with the *istD I couldn't see anything - it was far too dark to focus. A bit disappointing. I haven't tried the camera with the moon yet, but will do next time it makes an appearance, and it's not raining, or cloudy. Nick -Original Message- From: "Tom C"<[EMAIL PROTECTED]> Sent: 11/01/05 23:45:05 To: "pentax-discuss@pdml.net" Subject: Re: *istD EOL... Bruce, I'm curious why the shop people thought the Digital Rebel was junk. Was it based on look and feel ? What about image quality? I'm asking because right now the Rebel is the top selling DLSR for astrophotography. I haven't had a chance to try the *ist D yet with my telescope, but depending on results I get with the *ist D I wouldn't mind trying the Rebel. Tom C. >From: Bruce Dayton <[EMAIL PROTECTED]> >Reply-To: pentax-discuss@pdml.net >To: Nick Clark >Subject: Re: *istD EOL... >Date: Tue, 11 Jan 2005 15:33:08 -0800 > >I was in my local shop yesterday - mostly Nikon and Pentax, some >Canon. They are selling about five D70's everyday. People just keep >walking in a buying them. They think highly of the *istDS, but it >doesn't sell anywhere like the D70. Pretty much the word is out in >all media that photographers and would be photographers that the D70 >is THE camera to buy. Pop Photography proclaimed it Camera of the >Year. Size doesn't matter to most people at the time of purchase. >Later on, when having to carry the extra they might care, but it is >too late. The best Pentax can do with the *istDS is be respected. My >shop thinks the DRebel is junk next to the DS and they are very >willing to tell any potential customers. But they are really making >their money on the D70. It could well be the camera that saved >Nikon's hide. > >-- >Best regards, >Bruce > > >Tuesday, January 11, 2005, 3:18:50 PM, you wrote: > >NC> My local dedicated Camera shop says the *istDs isn't selling. >NC> They have it and the 300d and D70 (and the Minolta which is >NC> humungous) on display alongside each other. Even though the Pentax >NC> is smaller, they say the reasons people don't go for it are partly >NC> the SD card but mostly the difference in price. The 300d is >NC> something like GBP200 cheaper. > >NC> Nick > > >NC> -Original Message- >NC> From: "Pål Jensen"<[EMAIL PROTECTED]> > >NC> I think it will be more than enough. As high-end DSLR are >NC> larger than medium format cameras, and consequently suffers from >NC> the same lack of portability, the market niche is definitely >NC> there. Mind you, Pentax need to design cameras that look small, >NC> not only are small. The problem with the *istD(S) is that they >NC> look big. Products that are going to sell on their smallness need >NC> to comunicate their size through design. Small cars don't look >NC> like shrinked big ones. If they did they wouldn't sell. The small >NC> SLR's of the past looked small without any reference. Pentax M >NC> series and Olympus OM's had a slim smallish look whereas the >NC> *istD(S) look big and fat until you actually handle one or see a >NC> photo of it next to the competition. Since most people never see >NC> or handle a Pentax theres nothing telling them how small they are >NC> unless they do a lot of homework. >NC> The Pentax 40mm pancake lens is agreat idea. It is a pity >NC> though that Pentax haven't made a pancake camera. > > >NC> Pål > > > > > > > >
