Camera Lenses

When I first decided to buy a DSLR for AP imaging, I was also very attracted by the perspective of being able to use not only telescopes, but also camera lenses, which are usually faster than scopes and easier to manage given their (again, usually) shorter focal length.

I have to say I was a bit naive in the way I thought the fast focal ratio of some camera lenses could allow me to image very faint DSOs with short exposures.

It is all well explained here, here, and also here.

Indeed I had read them all, but sometimes you need to experience things directly to make a real conviction out of rational reasoning.

I cannot hide that when I was planning to buy a 50 mm F1.8 prime lens I was making really optimistic calculation factoring in the very fast focal ratio and the longer exposure time allowed by the short (vs telescopes) focal length.

It does indeed produce very bright images, but, besides the limitation imposed by light pollution, particularly in an urban environment, resolution is quite obviously a limit too.

The great advantage of camera lenses ultimately lies in the wider field they allow to image, when compared with telescopes, essentially because of the shorter focal length.

Camera lenses can be divided in two big families or types: zoom lenses and prime lenses.

Zoom lenses may vary their focal length from a minimum to a maximum value (correspondingly also varying their focal ratio), which may be very handy, but it is obtained through the use of many lenses in many groups and every piece of glass may (and indeed does) reflect or absorb some fraction of the incoming light.

Not just that: these complex optical trains must usually be optimized mainly to work at distances lesser than that of infinity, which is of interest in AP, hence their quality in star imaging is well below that of a telescope of the same price.

Prime lenses have just one focal length and so do not need as many elements as zoom lenses have, but nevertheless they usually have more than telescopes, and again, they are not designed with astrophotography in mind.

This said, the short (relatively to telescopes) focal length of most lenses allows for easier tracking and hence longer exposures (for altaz mounts field rotation is in this case the real limit). Moreover prime lenses do not even really need to be focused (there is a preset position for infinity) and the wider angular field covered by a given sensor makes also subject finding and centering quite simpler.

On the other hand camera lenses can cost a lot, surely more than most telescopes per inch of aperture and not of all the reasons of that cost are really beneficial to astrophotography: we do not need autofocus or stabilization, for instance.

But when I bought my DSLR I was so much in love with the idea of using camera lenses as a shortcut to astrophotography, that I bought a certain number of them. Of course, in accordance with my general philosophy, they are all rather cheap.

The Canon EOS 1300D came with a Canon EF-S 18-55 IS II zoom lens, by many considered as very “introductory” even for daylight photography, and yet none other than Jerry Lodriguss, whose high pass filter for equipment is usually much tighter than mine, has praised this cheap lens.

Canon-EF-S-18-55mm-f-3.5-5.6-IS-II-Lens

Specifications and image from The Digital Picture site .

Lenses / Groups 11/9  11 lenses!
Angle of View: Diagonal 74° 20′ – 27° 50′ wider field at lower FL
Angle of View: Horizontal 64º 30′ – 23º 20′ wider field at lower FL
Angle of View: Vertical 45º 30′- 15º 40′ wider field at lower FL
Aperture
Aperture Range – Wide / Long f/3.5-22 / f/5.6-32
Aperture Max by Focal Length 18-23mm = f/3.5
24-28mm = f/4.0
29-38mm = f/4.5
39-46mm = f/5.0
47-55mm = f/5.6

I also craved for longer focal lengths, something stretching towards the focal lengths of short refractors and I found an acceptable compromise in another zoom lens, the bigger but always cheap Canon EF-S 55-250 IS STM.

Canon-EF-S-55-250mm-f-4-5.6-IS-STM-Lens

Specifications and image from The Digital Picture site .

Lenses / Groups 15/12 as many as 15 lenses!
Angle of View: Diagonal 27° 50′ – 6° 15′ wider field at lower FL
Angle of View: Horizontal 23º 20′ – 5º 20′ wider field at lower FL
Angle of View: Vertical 15º 40′- 3º 30′ wider field at lower FL
Aperture
Aperture Range – Wide / Long f/4.0-22 / f/5.6-32
Aperture Max by Focal Length 55-63mm = f/4.0
64-99mm = f/4.5
100-154mm = f/5.0
155-250mm = f/5.6

After this purchase I was done with zoom lenses and started to look at prime lenses.

First a very fast, cheap, and rather appreciated by the AP community, 50 mm lens, the well known Canon EF 50 F1.8 STM .

Canon-EF-50mm-f-1.8-STM-Lens

Specifications and image from The Digital Picture site .

Lenses / Groups 6/5 six lenses in a prime …
Angle of View: Diagonal 46º on a full frame sensor
Angle of View: Horizontal 40º about 24° on APS-C
Angle of View: Vertical 27º about 16° on APS-C
Aperture
Aperture Range – Wide / Long f/1.8-22

Of course I could not have been really interested in camera lenses if I had not explored the second hand market, so I looked at what was on offer on the shelves of the best shops in town.

In the end I got two vintage telephoto prime lenses: one was a good purchase, the other one less so.

The Tamron Adaptall ( Chinon ) 135mm f/2.8 (CT 135)

Tamron135

Model: CT-135
Focal length: 135mm
Open F value: 2.8
Lens construction: 4 elements in 4 groups
Minimum aperture: 22
Weight: 375g
Maximum diameter X Overall length: 64.5mm x 77mm
Built in extensible metallic hood
Release date:: 1976; Production ends: 1979

bought for 80€ (plus 60€ for the Canon EOS adapter: a theft, I know) it has been the camera lens I have used the most for astrophotography and with great pleasure. When fully open at F 2.8 there are noticeable chromatic aberration and field curvature, but its speed coupled with the small pixels of the Canon EOS 1300D allow the imaging of nebulas and some galaxies.

Coupled with a 1.4x telextender it gives a 190 mm F4 lens, but the quality at the borders is noticeably lower so it is necessary to crop the images more than one needs with the fully opened (F 2.8) native 135 mm without the telextender.

To obtain “aberration free” image the lens must be stop down to F5.6 even when an UHC filter is used, while with an H-alpha filter F4 is doable.

The least successful purchase is in itself a rather appreciated telephoto lens, namely a Canon FD 200 F4, which however has an issue I ignored when I bought it.

CanonFD200F4

Specifications: 200 mm F4, 7 elements in 6 group.

The issue is that Canon FD lenses do not reach focus at infinity on Canon EOS DSLR … no little thing for AP use!

Eventually, after some swearing and some research, and after having spent a total sum of close to 170€ (!) I was able to reach focus at infinity with my Canon EOS through the use of an “active adapter”, that is an adapter for EOS cameras containing optical elements that make it a 1.25x telextender (it can be seen in the picture above at the bottom of the lens).

Besides the many doubt about the quality of the image produced when the “active adapter” is added, what I now have is a 250 mm F5 lens, with an angular field not very much wider than that of the Skywatcher 80/400, the same focal ratio and an aperture of 50 mm vs 80 mm.

In other words: what could I use it for?

So far I have just taken a couple of test images with it, just to be sure that it really reaches focus at infinity.

This rather unpleasant experience has so far quenched my thirst for vintage camera lenses, but, who knows, it could reignite in the future.

In December 2019 I did indeed try my luck with camera lenses again and this time it was bingo!

The Neewer 85 mm F1.8 is a cheap (I bought it on Amazon for 90 euro) Chinese prime lens. It is new but it looks “vintage” in being totally manual: no autofocus or any other automatism, which of course is not an issue with astrophotography.

Neewer85

The lens is advertised as containing an “aspherical element” and as having 6 elements in 6 groups.

Its aperture stops are not fully standard (F2.5 and F3.5 instead of 2.8 and 4, for instance), but it works quite well with both my DSLR and the ZWO cameras: it can indeed work without noticeable aberrations at F3.5 with an UHC filter and at F2.5 with an H-alpha filter.

Soon after the Neewer 85 mm and favorably impressed by the latter, for Christmas 2019 I also bought a new 135 mm F2.8 lens, hoping to improve on the performance of my old Tamron 135.

When you buy cheap lenses on the internet you are not always sure what you get, and indeed when I opened the parcel the box containing the lens looked far from pristine and also, in terms of number of number of elements and groups, the lens appeared to be different from the one advertised.

To be honest it looked like someone else’s return and indeed I soon realized it has a defect: it does not focus to infinity with a Canon EOS DSLR.

Normally I would have sent it back, however I already had a 135 mm lens that works with the Canon DSLR, while on the lens’ box there was written something that was not among the product specification on the website where I bought it.

IMG_1027

The camera contains 9 elements in 7 groups, a built-in dew-cap, weighs 478 grams and is totally manual.

As you may see by yourself, the lens sports a low dispersion element, and even if that is second hand, it is hard to get an ED/LD camera lens for 94 euro, so I kept it to use it with dedicated astrophotography cameras, rather than with the DSLR.

The producer is named as Kelda, but the website quoted on the box does not appear to be active anymore, while I found at least one quite positive comment on the internet about this lens by a user.

And indeed I was right to keep it: with just the help of a Fringe Killer filter it works quite well at full aperture (F 2.8), with a star quality similar to that of the Tamron stopped down at F5.6 .

When paired with the ZWO 178MM camera it covers a field of 3×2 degrees, quite similar to that obtained with an APS-C sensor and a 400 mm focal length.

All in all it was a stroke of luck to get this lens instead of the similar one advertised on the site.

M65-66-20200412-146x30s-135mmF28
The Leo Triplet – 146x30s – Kelda 135mm at F2.8
M97-M108-20200525-90x30s-135mm
M97 with M108 and Merak to the right – 90x30s – Kelda 135mm at F2.8
NGC6995-20200923-102x45s-135mm-UHC-lpc-cbg
The Eastern Veil Nebula – 102x45s – Kelda 135mm at F4 and ZWO ASI 385MC

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