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Telephoto Vs Telescope


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I have recently been getting into the camera side of things and stumbled across the similarity between a camera lens and a telescope

i have a astromaster 130, (650mm focal length, F5) and i am getting a 250mm telephoto lens for my 700D, (250mm * 1.6 crop factor = 400mm focal length, F4)

Does this mean that the camera lens is as good as my telescope, yes it has a shorter focal length but better light gathering.

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Hi Alan

I don't own a decent lens camera and I can't answer your question I'm afraid but I have wondered these same things. Also, whether instead of spending best part of a grand on a telephoto, a photographer could buy a half decent telescope which would maybe have more power in some cases?

Just thoughts really.

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I have recently been getting into the camera side of things and stumbled across the similarity between a camera lens and a telescope

i have a astromaster 130, (650mm focal length, F5) and i am getting a 250mm telephoto lens for my 700D, (250mm * 1.6 crop factor = 400mm focal length, F4)

Does this mean that the camera lens is as good as my telescope, yes it has a shorter focal length but better light gathering.

Why do you say the camera lens has better light gathering capabilities? surely the aperture is not 130mm. It will be slightly faster at f4 but you'll probably find you need to wind it back an f/stop or two. It will be a good lens for some of those larger dso's though and great for a portable mount such as an astrotrac etc.

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Does this mean that the camera lens is as good as my telescope, yes it has a shorter focal length but better light gathering.

While superficially similar,  a camera lens is designed to be a general purpose device to get the best out of high-brightness (compared to astronomy) scenes and with the whole scene full of "image". Its meant to operate over a wide range of focal lengths and F-stops / apertures.

A telescope is intended for one, very specific, type of operation. One that is extremely high contrast and it is optimised for one focal length and a single aperture setting.

Having said that, some of the "prime" lenses, such as Canon's EF 200mm F/2.8 are exceptionally good and give excellent astronomical images. However photography of stars is probably the most rigourous test possible of a photographic lens and if the lens has any faults: chromatic aberration, soft focus, flaring, vignetting, diffraction limitations, coma or distortions then using it on astronomical targets will show them all up.

(But, to be fair: the same can be said of a telescope when used for astro-photography :evil:  )

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Hi Alan

I don't own a decent lens camera and I can't answer your question I'm afraid but I have wondered these same things. Also, whether instead of spending best part of a grand on a telephoto, a photographer could buy a half decent telescope which would maybe have more power in some cases?

Just thoughts really.

I'm definately no expert when it comes to camera lens' but I'd imagine that unless you have a selection of stopping down masks, you would be restricted to just one f/ratio with a telescope+camera whereas a camera lens has a wide range of f/stops :). I'll just wait here to be shot down now :D

Pete beat me to the punch ;)

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that was a lot of reply's in a short time :laugh:

i completely forgot about the aperture lol, 35mm camera lens to the 130mm telescope

But you can get larger aperture lens for the camera

in the long run the telescope is obviously better but for us normal (poor) people is it really? say up to £1000, if you mounted a £1000 telephoto lens to your camera to image or you spent £1000 on a telescope what would be better.

I opinion would be the camera and lens as its more usable, lighter, easier to move and you could use it in the daytime for wildlife

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There is no such thing as the crop factor. :evil:  :grin:  In astrophotography it simply perpetuates the myth that you can have a more favourable image scale by reducing your chip size, which is nonsense. (Jupiter looks too small on my DSLR but looks big on my webcam.) Of course, we know why camera makers invented the term; it is intended to allow comparisons in framing between 35mm and APSc formats with the same lens. But if you start applying the term to a camera lens while not applying it to a telescope-as-lens then any attempt to compare them will be meaningless.

The focal length is the focal length. You must compare a camera lens with a focal length of 'x' with a telescope of focal length 'x.'

If you do you'll probably find that the telescope has more aperture, fewer lens elements and a better  corrected field at the edges. Stars are terribly exacting targets, really punishing, and only the very good camera lenses can give a good stellar image to the edge while working at anything like their fastest F ratio. Some do, mind, and there are great astrophotos taken with them. But telescopes are usually cheaper!

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

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Camera lenses can be a nice inexpensive way of getting into AP with sub 100mm focal lengths some of the macro designs have very flat fields too however things start to get expensive after this but up to 200mm lenses still have a slight edge.

The one drawback with some camera lenses is that they are normally designed for standard camera sensors so can suffer with the extra bandwidth of a modified camera or ccd sensor.

Alan

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Hi

Of course, if you want to do long exposure AP with a camera and lens you still need some means of accurately tracking it across the sky - either something like an Astrotrac or a tracking mount. It seems that even in AP with a basic scope most of the initial expenditure goes on a good quality tracking mount. Either way is quite expensive! Mind you I have contemplated getting an Astrotrac purely for it's portability. At the moment I'm stuck with imaging from indoors :( and something like an Astrotrac would at least let me get outside and give me more scope (sic). I'd be limited to widefield and probably limited exposure times with the basic kit but at least I'd be able to image more of the sky!

Louise

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One of the nicest things about using camera lenses is that the shorter focal length means you don't have to have your mount polar aligned to the same degree of accuracy as for a longer FL (note: we're not talking about focal ratio here, just focal length: the arc-seconds per pixel) telescope. So you can get away with a cheaper / lighter mount or even no mount at all: just a tripod, a wide lens and lots of short exposures stacked.

I like wide, camera + lens images. They are much closer to what you can see with your own eyes (or could: if there wasn't the light pollution and you could keep your eye open for 10 minutes, accumulating photos :smiley: ) and give a sense of perspective that telescopic images don't have. Plus you get a lot more stars per £££ for the equipment you use.

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I was about to go into attack mode and target the crop factor statement, but Olly already did that. Camera lenses can be great but are stopped down by means of non-round devices (diaphragms) with various number of parts which case a lot of different diffraction patterns. Some lenses even has the diaphragm slightly closed even at the lowest f-number, which means it will always give spikes.

Then there's the mechanical stability of a camera lens. Usually not so good unless you go for the expensive and often older designs. Both the Canon and the Nikon couplings are also prone to mechanical instabilities. The camera usually sits on a single screw; another cause of mechanical problems... 

Older lenses (non-AF) usually had a mechanical stop at infinity focus - very good for astro use. Newer lenses can go past infinity (I wonder where you end up then) in order for the AF to be able to pass focus and go back also at targets at infinity. Focusing a lens is, generally speaking, much harder than focusing a telescope, especially in the dark and with the relatively low magnification they sport.

I have used a Nikon 300mm f/4 lens (1980's model) and a Nikon Micro 105mm (late 1970's). Both perform very well, but the hassle with focusing is troublesome with both. I intend to use the 105mm on my D3s with a Vixen Polarie for travel AP.

As Olly stated, a lens that performs well in terrestrial photography may not perform well when used on targets like stars. They also exhibit quite a bit of irritating optical error, especially towards the edges of the field. You'll notice this with DSLRs in particular as they have large sensors.

I guess you can tell that I'd take a telescope any day ;)

/per

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I've used camera lenses quite a bit as telescopes. There was a long period when I was a relapsed astronomer so didn't have a telescope but always had an eyepiece adaptor for my long telephotos. In theory the problem with camera lenses as telescopes is that they have too many glass to air surfaces lowering the contrast. The problem with telescopes as camera lenses is that the close focus is hopeless and as Pete says, there's only one aperture. That aside, my experience is that if you spend a big bucket on money on a top notch camera lens it makes a pretty decent telescope and if you spend a big bucket of money on a top notch telescope it makes a pretty decent camera lens :)

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Most of the basic imaging scopes have quite a few lenses in the optical train my relatively low end scope has 5 if you include the FF/FR but i still use camera lenses too if i am after a sub 300mm image scale.

The mount requirements are also reduced with a lens of modest FL and a recent big bonus is the lens correction data now available to correct a lot of the shortcomings they have.

Alan

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I've tried both approaches and my feelings are:

f/stop isn't the same as absolute-aperture when you're considering light-gathering abilities, and telescopes win hands-down over even the fastest and most expensive telephoto lenses on this count

Telephoto lenses result in horrendous (or artistic if you're that way inclined) diffraction spikes unless you use an aperture-mask instead of the inbuilt iris diaphragm, so another 'scope-win

It's possible to add high focal-length multipliers such as Barlows and Powermates or reducers to a 'scope more easily and less expensively than to a camera lens, another tick for the telescope

A telescope can take a variety of sensors for planetary or deep-sky imaging whereas a DSLR/lens is more limited having a single sensor type and usually no option for uncompressed video

Telephoto lenses are not designed to resolve point-sources such as stars, and tend to introduce significant aberrations on anything even slightly off-axis

A reasonable telescope/mount is cheaper than a similarly specified telephoto lens

For wide-field shots, a camera lens is the only way to shoot without having to resort to mosaic/panoramas

Don't get me wrong I've taken some astro-images I'm more than pleased with using telephoto lenses, it's just that using a telescope is cheaper, easier, more flexible, and better quality!

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"attack mode"...interesting.

Haha, yes, isn't it. I'm sure everybody realizes that it is all about image scale, not "crop". As Olly correctly stated, the 1.6x FL figure for the smaller DSLR sensors is just for comparison.

/per

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Haha, yes, isn't it. I'm sure everybody realizes that it is all about image scale, not "crop". As Olly correctly stated, the 1.6x FL figure for the smaller DSLR sensors is just for comparison.

/per

...and if you apply a "1.6x" factor to the FL, then you have to do the same to the focal-ratio, so your 200mm f/2.8 becomes a 320mm f/4.5, only it doesn't, it's still a 200mm f/2.8.

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I have telescopes and lenses, hurrah for both. :grin:

Thank goodness someone has said it !

If you're after a sub 300mm ( Focal length ) then a camera lens may / will be the best way. There are other makes than Canon by the way ! Loads of older lenses about.

Also, if you're already getting the 250mm lens then use both types. What is the lens by the way, there aren't many 250mm camera lenses about. Medium format ?

The only real way to tell if a camera lens is any good to you is to try it,

Dave.

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Thank goodness someone has said it !

If you're after a sub 300mm ( Focal length ) then a camera lens may / will be the best way. There are other makes than Canon by the way ! Loads of older lenses about.

Also, if you're already getting the 250mm lens then use both types. What is the lens by the way, there aren't many 250mm camera lenses about. Medium format ?

The only real way to tell if a camera lens is any good to you is to try it,

Dave.

the lens is nothing special but excellent for me, still haven't got it though hopefully tomorrow

the lens

http://www.canon.co.uk/For_Home/Product_Finder/Cameras/EF_Lenses/Telephoto_Zoom/EF-S_5-250mm_f4-5.6_IS/

they are around £200 new but i found a used one for £80 :grin:

This topic has really took off, i seem to have struck a nerve lol, but thanks for all the great reply's i have learned alot

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@SJP ,  "Telephoto lenses result in horrendous (or artistic if you're that way inclined) diffraction spikes unless you use an aperture-mask instead of the inbuilt iris diaphragm, ....."

You must be using the wrong telephoto. Different manufacturers use different blade configurations. Yes lenses can produce "sun stars" which are the same effect, but a result of much higher light levels than any but our local star can produce. I've not seen diffraction spikes produced by my telephotos, even when shooting Vega (Lyra).

"Telephoto lenses are not designed to resolve point-sources such as stars, and tend to introduce significant aberrations on anything even slightly off-axis" This in my opinion a bit of a stretch. Most lenses will have issues if not stopped down one stop or so from wide open. The degree is largely a result of the quality of the optics of the lens, just as in EPs.

Having said that in defense of camera lenses, I think debating a comparison of a  purpose built tool being used properly within it's design parameters and a more general purpose tool, being used at the extreme of it's capabilities is a bit...silly? A saloon racing a race car?

I use camera lenses because that's what I already have. I'll have a telescope one day. Right now I'm having fun. Cost? I recently picked up a nice used 400 mm prime f/5.6 for $105 USD. As stacking 1/2 second subs is a bit much, an EQ mount is in the works.--Jack

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...and if you apply a "1.6x" factor to the FL, then you have to do the same to the focal-ratio, so your 200mm f/2.8 becomes a 320mm f/4.5, only it doesn't, it's still a 200mm f/2.8.

No, that makes no sense at all. As Olly said focal length is focal length. The crop factor only applies to field of view compared with 35 mm film, it can't be used for anyting else.

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I noticed that my modded DSLR doesn't like standard camera lenses so much anymore.

It can even be impossible to focus as the lens just doesn't have enough focuser travel to reach infinity focus with longer wavelength light.

The lenses are also not designed to compensate for aberrations in IR. Hot spots and field curvature are more apparent.

Of course this isn't an issue if you don't modify the camera ;)

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