Lenses for Wide Field Imaging

Introduction

Normally the purchase of a digital camera includes a ‘kit’ lens.  This is normally a zoom lens giving a focal length of 28 to 70 mm for a full frame camera or 16 to 50 mm for an APS-C camera.  These tend to have a maximum aperture of f/3.5.  To improve the image quality across the frame it is often best to stop down a lens by one or one and a half stops, and so the use of kit lenses will require longer exposures than if a wider aperture prime lens were used.  In the past, the image quality of zoom lenses was not up to that of a simpler prime lens, but with multi-coating on the lens elements and the use of aspherical and high refractive index glass elements, these are now far better and one of my very best images (described in the chapter ‘Taking a stars trails image’) was taken using an excellent, very wide angle, Sigma zoom lens.  In addition, the latest sensors are more sensitive and have less noise than those employed in the past so higher ISOs can be used, so compensating somewhat for the lower apertures.

But, for serious wide field astrophotography prime, fixed focal length, lenses are likely to be best; they will tend to employ fewer glass elements so there will be less internal scattering so that the contrast should be higher and the optical aberrations less.  There is one great advantage when using prime lenses for astrophotography as they will be used in manual focussing mode so that second hand manual prime lenses can be used which can be bought for far less than new autofocus lenses.  Older lenses were usually made of metal rather than plastic so can last for many years.  Another good feature of these old lenses is that they usually have a ‘hard stop’ at infinity focus.

If using a DSLR, lenses made for a particular make will usually need to be designed specifically for use with it, either by the camera manufacturer itself or by companies such as Sigma or Tamron.  There is one exception in that Canon cameras have a lesser flange to sensor distance and a wider throat than Nikon cameras and so that manual Nikon lenses can be used with an adapter on a Canon body.  [I can thus use Nikon lenses with my H-alpha modified Canon 1100D.]  If, instead, mirrorless cameras are used with the use of adapters, then as well as lenses made specifically for the camera make, lens designed for virtually any DSLR (digital or film) or rangefinder camera systems can be used.  This is why, if one were going to specifically buy a camera where astrophotography was to be part of its use, a mirrorless camera would be a better buy.  I have a wide range of manual prime lenses ranging from Leica and Zeiss rangefinder lenses to Sony and Nikon DSLR lenses with focal lengths ranging from 8mm to 400 mm.  As described below, such a range enables a focal length and hence captured field of view to be chosen suitable for the area of the heavens that is to be imaged.

One of the best manufacturers of these adapters is ‘K&F Concept’ which, when the aperture is only controlled from within the camera, will allow one to control the aperture of the attached lenses. They are beautifully made and cost ~£25.  I have them to mount Nikon, Sony A-mount and Contax G lenses onto the Sony A5000 camera that I use for much of my wide field imaging.  When manual lenses having the iris control on the lens body are to be used, simpler and hence cheaper adapters can be used.  For example, for use with Leica 39mm screw mount lenses and m42 screw mount lenses, these can be bought for less than £10.

Using Full Frame, FX, lenses on APS-C or Micro 4/3, DX, cameras

I suspect that the use of legacy full frame lenses on mirrorless APS-C or Micro 4/3 cameras will be very common − I use both Nikon and Zeiss full frame lenses on my Sony A5000 camera.  There are three advantages: as only the central part of the image circle is being used, vignetting should not be an issue, the sharpness should be uniform across the field of view and the problem of coma that can often be seen in the corners when used with a full frame camera should not be present or certainly far reduced.  Due to the crop factor, the field of view will be smaller than when used on a FX sensor.

There is only one small caveat.  If a lens is designed to give a smaller image circle then it might be that the resolution of the lens can be somewhat higher but I doubt that this would have much effect on the use of a lens when astroimaging.

Using DX lenses on FX cameras

This may sound nonsensical but it is not necessarily so as usually the field of view is increased.  Assume a DX lens can give a perfect image across the Nikon APS-C sensor which is 23.6 x 15.6 mm in size. Then when used on a FX camera, an area of 23.6 x 23.6mm on the sensor should give a perfect image.  This has an area which is 1.5 times greater than when the lens is used on a DX sensor.  Not to be sniffed at.  I tried this out with a kit lens at its maximum aperture of f/4 and found that it performed even better than I had expected, with less cropping required giving an area 1.75 times greater than when used on a DX sensor. Somewhat surprising.

The required crop of a DX f/4 kit lens at full aperture on an FX sensor.

With a few lenses the improvement can be even greater.  For some reason, I mounted the Nikon 35mm f/1.8 DX lens bought for use with my Nikon D7000 (and S-BIG CCD camera) onto my FX Nikon D610 camera and was very surprised with the results.  Yes, when stopped down somewhat, there was still some vignetting in the corners but this was easily correctable in Photoshop.  The image quality did fall off in the corners, but using a crop to slightly reduce the horizontal width gave a very acceptable ~37 mm equivalent lens and an effective sensor area of ~30 x 24 mm which is close on twice the area of the DX sensor.  Looking on the web I found that others had found the same thing.

The required crop of a Nikon DX 35mm f/1.8 prime lens at f/4 on an FX sensor.

This would be an excellent lens to purchase for use on a Nikon DX camera or with an adapter for use with DX mirrorless cameras and could be even more useful if, at sometime in the future, a full frame Nikon or mirrorless FX camera were to be purchased.

I have found one other interesting example.  I have the Samyang 8mm fisheye lens that perfectly covers the Nikon DX sensor giving a rectangular outline.  It is obvious that, if used on a FX sensor, an almost full circular image will result − providing that the lens hood is removed.  So this will be far better for full sky imaging, perhaps to capture meteor trails.  [Coupled with the High Sensitivity Sigma A7s full frame camera this is the perfect (and unbeatable) combination for all sky imaging at night.]

The Samyang 8 mm fisheye lens on an APS-C sensor (above) with its image on a full frame sensor below

Fields of View

The point of having a variety of focal lengths available is that a suitable lens can be chosen to cover the desired field of view.  The program ‘Astronomy Tools Field of View Calculator’ can be used to find the field of view that will be given with a particular lens/camera combination.  In ‘Imaging Mode’, a target object is selected along with the camera and focal length of the lens.  If the camera to be used is not in the wide selection of cameras given in the list, one can simply chose one with a similar sized sensor.  For example, for a FX sensor choose the Nikon D750 and, for a DX sensor, the Nikon D7000.

The images below give the approximate field of view provided by full frame equivalent lenses so, when using an APS-C sensor, one would use a focal length ~2/3 less.

Sirius and Orion with a 45mm lens

Orion with a 55mm lens

Sword of Orion with a 200 mm lens

M42 region with a 400 mm lens

Pleiades and Hyades Clusters with a 75mm lens

Pleiades Cluster with a 135 mm lens

Pleiades Cluster with a 200 mm lens

A bestiary of lenses  (Ok, lenses are not animals, but the word can also be used to refer to a collection.)

Over many years, I have acquired many lenses for use in general photography; including those for my Nikon film camera later followed with my D7000 APS-C and D610 full frame digital cameras.  More recently I have acquired both Sony APS-C and full frame cameras along with two Fuji APS-C cameras.  For astrophotography use these are usually coupled with prime lenses, some modern but often with old manual ‘legacy’ lenses.  Only a few have been bought new. In the list below, I have noted the lenses that I have used for astrophotography which may help to guide you to what is available at reasonable cost, either new or second hand.

Fish Eye

An excellent such lens is the Samyang (Rokinon) 8mm f/3.5 lens which sports a diagonal field of view of 180 degrees.  As one would expect, this lens produces lines that are wildly curved close to the edge of the frame, but as it employs a stereographic projection, images are more natural looking and objects near the edge of the frame are less ‘squashed’ than those when fish eye lenses employ an equal area or orthographic projection.  An AE version is available for Nikon cameras and, at lower cost, a non AE version available for Nikon, Canon, Sony and Micro 4/3 cameras.  This lens would be superb for time lapse imaging of the whole sky to capture meteor trails.  [Particularly when used with a full frame sensor as described above.]

Ultra Wide Angle Lenses with 10 to 20mm effective focal lengths

An excellent zoom lens is the Sigma 10-20mm f/3.5 which, set at 10mm so giving an effective 15mm focal length and mounted on my Nikon D7000 camera, produced one of my very best images as described in the ‘Startrails’ chapter.  It has a constant f/3.5 aperture and is available for Nikon, Canon and Pentax DX cameras.   Its diagonal field of view is 102 degrees at 10 mm and 64 degrees at 20mm.

A widely commended manual, fixed focal length, prime lens is the Samyang (Rokinon) 12mm f/2 lens which is available to fit a wide range of mirrorless cameras such as those made by Sony, Fugi, Samsung, Canon, Panasonic and Olympus.  I have used mine, mounted on a Sony A5000 camera, to produce wide field images of the Milky Way.  This has an 18mm equivalent focal length on APS-C cameras.

Wide Angle Lenses with 24 to 35mm effective focal lengths

 For use on full frame cameras I have Tamron Adaptall 24mm f2.5, Nikkor 28 mm f/3.5 and Nikkor 35mm f/1.8 lenses. The first of these was used with a Nikon D610 to make a vertical panorama of the northern Milky Way as imaged from the Kerry Dark sky reserve and described in the chapter ‘Imaging the Milky Way’.  For use on my Fugi mirrorless APS-C camera I have a 7Artisans 25mm f/1.8 manual lens that is available at very low cost for APS-C and Micro 4/3 mirrorless cameras.  It is a perfect lens for use with a Micro 4/3 camera but images will need some slight cropping when used with APS-C sensors.  I would highly recommend this lens as a first prime lens to acquire for such cameras.  Its image quality across virtually all the frame is superb.  For my Sony A5000 camera I can also use a Sigma 19mm f/2.8 lens giving an effective focal length of 28.5mm.

Normal’ Lenses with 40 to 55mm effective focal lengths

These are called ‘normal’ as they have a similar perspective as viewed by our eyes.  The nominal focal length of such lens is the 50mm as, for example, the Nikkor 50mm f1/8.  Another very sharp lens for full frame cameras is the Micro-Nikkor 55mm f/2.8 Macro lens.  Such lenses are produced by every camera manufacturer and companies such as Sigma and Tamron.  A superb ‘legacy lens is the Zeiss Contax-G 40 mm f1/8 Planar which, sadly, has now become quite expensive second hand.  Used to take the 45 mm Sirius and Orion image above) and I have used one to make panoramas of the southern Milky Way.  A more humble lens that can be bought for around 1/10th the price is the Russian Jupiter 8 50mm f/2 but which would need to be stopped down to ~f/5.6 for astronomical imaging.

For Olympus and Panasonic Micro 4/3 cameras, the tiny pancake Panasonic 20mm f/1.7 gives an effective focal length of 40 mm – apart from astrophotography it would make a superb ‘walk about’ lens.  It is sharp from corner to corner and, coupled with a Panasonic GX1, has produced beautiful images of the southern Milky Way.

For APS-C cameras having a crop factor of 1.5 or 1.6 (Canon), focal lengths of ~35 mm give an effective focal length of 52.5.  I have a Sony 35mm f/1.8 prime designed for their DSLR’s and the very neat and well regarded Sigma 30 mm f/2.8 (so 45mm effective focal length)

Short prime telephoto lenses with 60-90mm effective focal lengths

I have three prime lenses for a full frame camera in this range, a Tamron f2.5 90mm macro lens, a Leica 90mm f/4 Elmar (this has, perhaps, rather a small aperture for astrophotography use, but gives surprisingly good images at full aperture) and a beautiful Zeiss Contax-G 90mm f/2.8  Sonnar which is really excellent at f4. 

There are many lenses that can be used on APS-C cameras as the ubiquitous 50mm lens translates to 75 mm.  The Nikon 50mm f1.8 lens gives excellent performance when using an adapter as do those produced by virtually all camera manufacturers.  I also have an A-mount Sony 50mm f1.8 lens that can, as for their similar 35mm f/1.8 lens, be bought very cheaply second hand as few are currently buying the Sony DSLRs.  

Medium to Long Focal length prime lenses 100 – 300mm focal length

There are many ‘legacy’ lenses that can be bought in this focal length range.  At 135mm, I have a Carl Zeiss Jena 135mm f/3.5 lens which I can use with an m42 to Sony e-mount adapter on my Sony DX and FX cameras. The Nikkor 200mm f/4 Ai which is a beautiful, all metal, lens that can now be bought quite cheaply second hand.  It is very sharp even at full aperture and near perfect at f/5.6.  At the top end of this range is the Nikkor 300mm f/4.5.  Again, it has a beautiful, all metal, construction and excellent optics which are near perfect when stopped down to f/5.6.  Not surprisingly, it is somewhat more expensive second hand.

For use with an adapter on a mirrorless APS-C camera, the Tamron 90mm f2.5 lens gives an effective focal length of 135mm, the Zeiss 135mm f/4 lens gives an equivalent focal length of ~200mm and the Nikon 200mm f/4, 300mm.

Very long prime and zoom focal length lenses > 300mm focal length.

I only have one prime lens in this category, an old Sigma APO 400mm f5.6 lens bought for just £40.  It is not highly regarded by some reviewers but I have found it to be quite satisfactory in use.

Recently I acquired an optically stabilised zoom lens − the Sigma 150-500mm f/5 to f/6.3.  This was bought primarily for nature photography, but it has been used successfully for imaging the Moon and planets (in skyscapes not in detail!) as described in the chapter ‘Imaging with a fixed tripod’.