Camera lenses vs telescopes

I was wondering how for example a refractor with 400mm focal length would compare to a Canon 100-400mm lens at 400mm when paired with an astro camera. The apertures don't seem very different. I guess the telescope is effectively a prime so is likely to have better image quality than a zoom camera lens.

Hi Ewen, one of the biggest differences would be the quality of star shapes at the edges as well as chromatic aberration. Refractor telescopes usually have field flatters or a Petzval design to correct for star shapes while camera lenses do not. If a small camera sensor is used or the camera lens is stopped down, you might be able to get away with minimal star distortion.

As for chromatic aberration, refractors control those better compared to lenses especially if ED glass with multiple glass elements are utilised. Usually triplets or higher are recommended but there are some pretty good doublets as well. If you can live with colour haloes then a camera lens is acceptable.

The above is true even for camera prime lenses.

Camera zoom lenses do have some advantages, being able to use multiple focal ratios and FOVs. Lenses are also usually smaller and lighter which makes them easier for travel.

Hope this helps,
Joon Ren

Hi Ewen,
i tried to make a photo with Canon 100-400, but ther's a big problem: the focusing operation.
For this lens the focus motor is very accurate and you can't use the autofocus, so you must proceed manually, but the movement that you must do is microscopical, and when you subject rise in the sky or the temperature or humidity change, the focal plane changes slightly and with you lens and he's accurate focal point your object will be out of focus.
The only solution to use camera lenses is they are not zoom, or or you must check focus after some frames.
It's also true that say njr95

Ewen Charlton:
I was wondering how for example a refractor with 400mm focal length would compare to a Canon 100-400mm lens at 400mm when paired with an astro camera. The apertures don't seem very different. I guess the telescope is effectively a prime so is likely to have better image quality than a zoom camera lens.

Zoom lens are notoriously fickle things although very recent ones are a much better bet than the older ones such the Canon 100-400. Prime lens are a much better bet with the very recent ones from the quality manufactuers (say the Nikon FL series) most likely better than anything a standard refractor telescope at the same focal ratio could pull (but you won't find them), both in terms of chromatic errors and field correction. Besides, by default they are essentially yielding a flat field without the need of other correctors.

I would agree with what Andrea said. I've used the Canon 24-70 f/2.8LII, the 70-200 f/2.8LII, the 135mm f/2L, and the 400mm f/5.6L. The 400mm f/5.6L was the best for astro work. With a good focusing system like what exists on the 5D Mark IV, I could actually autofocus on a bright star and switch to MF which would give me very accurate focus. Another advantage is that tilt and spacing will, with good lenses and cameras, not need to be fiddled with.

Hi Ewen, one of the biggest differences would be the quality of star shapes at the edges as well as chromatic aberration. Refractor telescopes usually have field flatters or a Petzval design to correct for star shapes while camera lenses do not. If a small camera sensor is used or the camera lens is stopped down, you might be able to get away with minimal star distortion.

Thanks @njr95 Joon that's really helpful. I did notice that my EOS lenses handle stars much better when stopped down. But of course that is taking me into smaller apertures than a refractor.

The biggest issue I've had with lenses is focus and filters. Focus is much more difficult using a lens than a telescope. Telescopes have a slower, more accurate focus and a much larger range near infinity. This matters when you want a filter. Telescopes almost always have some way to insert a common 2" filter in the chain somewhere and are able to focus with a filter in place. Many of my lenses can not focus past infinity enough to actually work with a filter and that's if I can even figure out how to attach one. For me, wider lenses have worked better than longer lenses.

The exception seems to be lenses design for manual focus. Another trick is adapting DSLR lenses (Canon EF) to cameras with a shorter flange distance like a mirrorless or an astrocam. Then you can shorten the spacing by 1mm to compensate for the filter thickness and often have enough room to put a filter drawer between the camera and the lens.

Thanks so much for all the advice!

I have indeed found that focusing with my EOS lenses is tricky, requiring extremely delicate adjustments. And then I'm there trying to tightly wrap the dew strap around the lens without touching the focus ring 🥴

Currently my setup is limited to my DSLR and a Star Adventurer mount. And a bahtinov mask. I've had mixed results but also been very pleased with a few images.

I guess the limitations of my camera lenses will become more apparent once my ZWO OSC is delivered 😀

The star adventurer can carry a few smaller (also inexpensive) telescopes. Askar FMA180 Pro and ACL200, Redcat 51, most 60mm doublets with reducer like the AT60ed. It might carry a 72mm scope but the focal length is getting a bit long for unguided and you would likely want to upgrade mounts at that point. All of those are true telescopes with a place to mount a filter and the better focusing even if they look like lenses.

If you want to image both day and night targets, a 100-400mm lens is a reasonable compromise. Also, if you already have the lens, it's also a good a idea to use it before committing you hard earned money into a hooby you may or may not like. That's the way I began, using some lenses and DSLR my father lent me. Once I was hooked, I began spending more and more on specific astro equipment. Notice I don't do regular photography.

If your focus is solely on astrophotography, then go for a dedicated telescope. I would say that the difference in quality is enormous. Telescopes, besides being prime lenses, are optimized (at least in theory) for imaging bright point-like objects (stars).

You can spot the difference in those two images:

M8 and M20
This one was imaged using a zoom lens (a Canon EF 100-400 L II at 200mm if I recall correctly). It's a nice image and I like it very much, but theres a lot of chromatic aberration and field curvature that I had to work around in post to correct.

M8 and M20, two years later
On the other hand, this is one of my most recent images mage with an Esprit 100 (550mm focal length). Not really an apples to apples comparison, but to get the idea, the stars are MUCH better and easier to work with.

Hope this helps a bit

Ewen Charlton:
I have indeed found that focusing with my EOS lenses is tricky, r

With respect to the focusing - it depends entirely on which camera you are using. If you are using an older (made before 2016) Canon camera, you will, as you found, need to make fine adjustments with lenses not really made for manual focus. It is a VERY different experience with a more modern camera which has technology like on sensor phase detect auto focus. In that instance, there is no fiddling involved, just aim the camera at a bright star, focus by touching the rear screen on the star. This is very fast, accurate, and reliable. Then switch to MF. This made a big difference.

In the end, camera lenses are designed for use with cameras. If you take them out of that element, they will lose their advantages.

I started my journey with a Canon 100-400 II and even tho it's a amazing lens for capturing stuff on this planet, it's not as good as even a cheap Refractor.
First Telescope I bought was a doublet and it performed so much better than my canon lens.
Stuff just not made to image the night sky.
Even tho you can get good results with it anyway.

I have Canon 300mm F/2.8L IS mk1, i tried it sometimes for astrophotography, it was close to my ST80 being it was more flatted than ST80, but wasn't any better quality, and that was only for Ha filter, in fact i liked that 300 result more than ST80 with flattener/reducer to make it flatter 320mm scope, but both are like a crap once i started to add ED and APO scopes, i will use Canon lens for sports/action and ST80 for solar, period.

I'm actually using Canon lenses with my Fuji X-T5 cos there were various issues with my 7Dii, which is why I've been relying on manual focus.

I think it's inevitable I will be getting myself a better mount (likely an AM3), a refractor and a guide scope. It's a question of which order to get them. I'm restraining myself from plunging in too fast though. Barely 😀

Ewen Charlton:
I was wondering how for example a refractor with 400mm focal length would compare to a Canon 100-400mm lens at 400mm when paired with an astro camera. The apertures don't seem very different. I guess the telescope is effectively a prime so is likely to have better image quality than a zoom camera lens.

*** assuming the focal ratio is the same, then I suspect the main difference would be how the telescope corrects for coma/astigmatism towards the edge of image circle. Wide angle lenses aren't typically designed for astro imaging and so aren't necessarily design to be pinpoint sharp and artifact free when imaging point sources of light. Its a good question in general; i.e what's the difference between a telescope and DSLR camera lens***

If you look at the spot diagrams of telescopes+reducers/flatteners, you will see most of them are not pinpoint sharp at the edge either. Some can be actually quite bad. Unless we are talking about the top 10% premium ones, I am not convinced that one is clearly optically better than the other for prime-focus wide-field imaging.

Clear lenses (wider than f/2.8) are generally good for astrophotos. Fixed lenses are better than zoom lenses, as the zoom itself can be a headache when stacking, if shifted by the millimeter during capture.

It is important to note nowadays that a 135/200mm lens or Redcat , used with an asi533mc for example, will give a "real" focal length of 600mm (compared to a full frame camera). So buy a lens like that (it is cheaper) or otherwise a bigger refractor if you can 130 or 150mm…

I agree with something said here, with full frame cooled cameras the tilt thing becomes hell, so prefer a samaller cheap cold camera, aps-c or minor…

Best!

ricardo leite:
It is important to note nowadays that a 135/200mm lens or Redcat , used with an asi533mc for example, will give a "real" focal length of 600mm (compared to a full frame camera).

The focal length of a lens is an optical property that is independent of the camera that is connected to it. I suspect you are referring to the field of view, which will, of course, depend on the sensor size.

Yes Arun, the field of view…
but the new sigmas, or a zeiss lens, for example, have the same quality that the best refractors. So, something to think before you buy a good refractor.

Best!

I use camera lenses to get a very wide FOV and gather light at a fast rate without having to worry about guiding. At 3/4 of my dark site visits since last summer, I've used either my Rokinon 135mm or Sigma Art 105mm because I wanted to make sure I could get as much data as possible in one night. I like the wide FOVs and ability to capture really faint signal in a short amount of time. Even if the aperture of a similarly priced refractor (say, a Redcat 71) is similar, it doesn't scratch the same itch. I have my Takahashi Epsilon for when I want more light gathering ability, but then I have to worry more about tilt, guiding, and other headaches.

Ewen Charlton:
I was wondering how for example a refractor with 400mm focal length would compare to a Canon 100-400mm lens at 400mm when paired with an astro camera. The apertures don't seem very different. I guess the telescope is effectively a prime so is likely to have better image quality than a zoom camera lens.

I would not buy the Canon 100-400 if i had only astrophotography in mind. But if you have it already, it is a great way to start. You will soon realize that camera lenses are not optimized for astro. A good APO refractor is almost always better than a good DSLR/DSLM lens when it comes to astro. But there are camera lenses optimized for astro / astro landscapes like the Rokinon 135mm or the Sigma 14mm etc. I had the Sigma Art 28mm/1.4 which is an excellent astro landscape lens (when not used wide open).
CS, John

What make a scope better than a lens if say having same focal length?

I have a prime lens, is it is worse than a smaller aperture scope?

Let's say you image with the lens to remove stars and compare the nebulosity between the two, is the scope giving better signal or quality than a lens?

I have Canon 300mm f2.8 lens, i measured the front lens diameter and i assume it i 107mm aperture lens, so it is larger than my 90mm APO triplet and even faster, so is the scope 90mm F/6 triplet better than my 300mm [107] f2.8 lens for nebulae without stars? Regardless that the scope has 3 optics while the lens maybe 100o optics and hundreds groups.

Tareq Abdulla:
I have Canon 300mm f2.8 lens, i measured the front lens diameter and i assume it i 107mm aperture lens, so it is larger than my 90mm APO triplet and even faster, so is the scope 90mm F/6 triplet better than my 300mm [107] f2.8 lens for nebulae without stars?

The two are not comparable. It is much harder to design a f/2.8 lens versus an f/6 scope of the same aperture as the off axis aberrations are much harder to control due to the much wider field of view. And yes, this will be apparent in the nebulosity as the point spread function will dictate the resolution and contrast of larger features as well. There is a reason why premium telescope manufacturers like AP don’t go faster than say, f/5, when the objective is to cover a full frame sensor with high quality.

Arun H:
Tareq Abdulla:
I have Canon 300mm f2.8 lens, i measured the front lens diameter and i assume it i 107mm aperture lens, so it is larger than my 90mm APO triplet and even faster, so is the scope 90mm F/6 triplet better than my 300mm [107] f2.8 lens for nebulae without stars?

The two are not comparable. It is much harder to design a f/2.8 lens versus an f/6 scope of the same aperture as the off axis aberrations are much harder to control due to the much wider field of view. And yes, this will be apparent in the nebulosity as the point spread function will dictate the resolution and contrast of larger features as well. There is a reason why premium telescope manufacturers like AP don’t go faster than say, f/5, when the objective is to cover a full frame sensor with high quality.

Ok, and i still don't understand what is the answer, which is better?

Tareq Abdulla:
Ok, and i still don't understand what is the answer, which is better?

Better for what? Is a sedan better than an SUV? It depends on what you want to do with it. A 300 mm f/2.8 lens is designed mainly for terrestrial photography. For its focal length, it covers a very wide FOV. Off axis aberrations, while still important, matter somewhat less in terrestrial photography than in astrophotography where you are basically examining the PSF at all points along the focal plane of interest in every image. If you are a pixel peeper, the f/2.8 will likely disappoint, but if you own one and are interested in capturing a wide FOV without too much pixel peeping, you'll be happy. A premium f/6 or f/5 APO of the same aperture will probably give you much better star quality across a common FOV, but of course will cover a much smaller FOV.

Related discussions