The Nikon 180mm f/2.8 ED lens is a legendary lens, and with good reason. It's performance for normal daytime photography is simply outstanding. And it is pretty darn good for astrophotography also.
A Little History
The Nikkor 180mm lens line has a long history. It has been re-designed and improved along the way and there are several versions. The first was a Nikkor-H 180mm f/2.5 made for Nikon rangefinder cameras in 1953. It had to be focused manually, and the diaphragm also had to be manually stopped down before the shutter was opened to take the picture. In 1971 the lens was re-designed for Nikon's Single Lens Reflex (SLR) camera bodies. The Nikkor-P Auto was a manual focus lens that incorporated 5 elements in 4 groups with a maximum aperture of f/2.8. Auto meant it had an automatic lens diaphragm. This was my favorite lens for sports photography when I was shooting high school sports for $5 per picture for a small weekly newspaper in New Orleans when I first started my professional photography career.
In 1981, the manual focus Nikkor 180mm ED AI-S design was the first to use ED glass, which then used 5 elements in 5 groups, again with a maximum aperture of f/2.8. This lens and its elements and groupings are seen above. In 1986 the lens was re-designed for autofocus in the AF Nikkor 180mm f/2.8s IF ED MK I version. This lens had a mostly plastic lens body. In 1988, the AF Nikkor 180mm f/2.8s IF ED MK III went to an all metal lens body, and used 8 lens elements in 6 groups. Several other minor design changes were made after that, including the AF-n and AF-D versions. The current version of this lens is called the Nikkor 180mm f/2.8D AF ED-IF Autofocus Lens and was introduced in 1994. The lens has 180mm of focal length, and its maximum aperture when used wide open is f/2.8. It has a CPU, a small electronic chip, built into the lens that sends information about the lens' settings to the camera. The "D" designation in the name means the CPU also sends focus distance information. The "AF" means it is an auto-focus lens. "ED-IF" means the lens incorporated ED (extra-low dispersion) glass, and the IF means it uses internal focus. A group of elements inside the lens move to focus it, instead of the entire lens barrel moving. An Outstanding Bargain Lens for Astrophotography The latest Nikkor 180mm f/2.8D AF ED-IF Autofocus Lens costs about $900 new. A Canon EF 200mm f/2.8L II USM Autofocus costs $769 new. Used late-model AF lenses in good condition cost almost as much as new lenses. However there are real bargains to be had by purchasing a used version of the manual focus Nikkor 180mm f/2.8 ED AI-S lens. It uses ED glass, so its optical performance is excellent. It also is all-metal in the lens barrel, so it's mechanical construction is also excellent. A used Nikkor 180mm f/2.8 ED AI-S manual focus lens in excellent condition from KEH costs about $300. These lenses are wonderful bargains for astrophotography. A Nikkor 180mm ED lens is like buying a 64mm-aperture f/2.8 wide-field Apo refractor with 180mm of focal length. Try finding one of those for $300. The closest would be a Mini Borg 60ED F/5.8 with 348mm of focal length for $569. With the Borg 0.66x DGT focal reducer ($449) you would have a 230mm focal length scope that was f/3.8 for about $1,000 plus $100 for rings ($1,100 total). Another option would be the $845 Takahashi FS-60CB Fluorite apochromatic refractor with 60mm of aperture and 355mm of focal length at f/5.9. With their $425 triplet focal reducer, you would have 264mm of focal length at f/4.4 for $1,270 total. These are about four times the cost of a used Nikkor 180mm ED AI-S, but naturally perform better because they are optimized to work at infinity. However, you have a wider field of view at 180mm with the Nikkor, and f/2.8 is still almost a stop faster than f/3.8, and a stop and a quarter faster than f/4.4. So, if you feel the need for speed, the Nikkor 180mm ED AI-S is a good way to go, for a very reasonable price. Remember that in camera lenses, a 180mm lens refers to the focal length, but in telescopes, a 64mm scope refers to the aperture. Camera lenses don't mention aperture because the aperture changes as you stop the lens down to different focal ratios. Yes, it is kind of confusing. Note that Nikon also made an older model of the 180mm f/2.8, that was manual focus, but did not use ED glass. So when you look for a used 180mm Nikkor lens, make sure you get the ED version. You can distinguish the ED version by the gold ring around the lens just next to the sliding lens shade. Also note that when you buy a lens for astrophotography, whether new or used, you really have to test that individual lens on an actual star field with a long exposure. Used lenses may have been knocked around, but not show any damage. Lens element centering and collimation are critical to performance, and can sometimes be off in used lenses. KEH and most other used lens suppliers usually have a return period during which you can test the lens and send it back for a refund or replacement if it does not satisfy you. But be sure to check the specifics of this policy. Even new lenses have unit-to-unit variations in production and should be tested. Shooting an actual star field is the hardest, but best, test for a lens. It will reveal any problems that a lens might have. Be sure to use long exposures on your test shots because some aberrations and problems can be hidden in short exposures, even on a star field. Cats and Dogs Living Together in Sin I use Canon DSLRs for astrophotography and have several old Nikon manual focus lenses that I use with them. What? Using a Nikon lens on a Canon camera body? Isn't that against one of the natural laws of the universe? Nope, they don't apply to me!
I use Fotodiox adapters to mate the Nikon lenses to the Canon bodies. This is possible because the Nikon register distance, the distance from the lens mount flange to the focal plane, is 46.5mm, whereas Canon's is 44mm. This means you can use Nikon lenses on Canon bodies, but not vice versa. When you use a lens like this, you lose all auto functions - you have to manually focus and manually stop the lens down before you take the picture. But this is not a big deal for astrophotography. The lens that I tested here is the manual focus Nikkor 180mm ED AI-S. Since I use it on Canon DSLR cameras, autofocus won't work anyway, so it is not necessary.
Originally I bought Fotodiox's inexpensive Nikon F to Canon EOS adapter. It cost $28. It worked fine, but the thickness of the adapter was not quite perfect, and my Nikon manual focus lenses, most of which had a hard stop at the infinity focus, all focused just a little bit before the stop, so each had to be physically focused for infinity. I have since bought Fotodiox's Nikon F to Canon EOS Pro adapter for $80. The spacing on this adapter seems perfect, so I don't have to focus any of the Nikon lenses with a hard stop at infinity. This is extremely nice and saves a lot of time under the stars, especially with wide-angle lenses, which are the hardest to focus. How does the Nikkor 180mm f/2.8 ED AI-S perform for long-exposure deep-sky astrophotography? Let's take a look. Where is the Best Focus? Unlike other short focal length manual-focus Nikon lenses, the Nikkor 180mm ED AI-S lens focuses past infinity by design. This is done because longer focal length lenses will focus at a slightly different critical focus point for infinity with changes in temperature. So, I do have to manually focus this lens when I use it for astrophotography. This is not a big deal with Live View focus, and I would have to refocus anyway when an in-camera filter is used. However, on my modified camera, the point of best apparent visual focus, where the red, green and blue colors are at the best average focus and there are no color halos around stars, is not the actual point of best focus in terms of the size of the core of the stars in the image. The actual point of best focus is when the core of the star is smallest but has a red / magenta halo around it. This can be seen in Comparison 1 below. Notice in particular how much sharper the stars in globular cluster M4 are when the lens is focused so they have slight red / magenta halos. This is because different wavelengths, or colors of light, focus at different points in a refractive optical system. Complex lens designs and exotic glass help with this problem, but usually cannot eliminate it completely, especially when a lens is used wide open at its maximum aperture. This is also made worse because my modified digital camera sensor is very sensitive to the long red wavelengths which are normally filtered out.
This image shows crops from the four corners and center of a modified Canon 1000D (Digital Rebel XS) with an APS-C sized CMOS sensor at 100 percent enlargement. Brilliant red-giant star Antares and globular cluster M4 are in the center of the frame. Refractors and camera lenses, with glass lens elements, focus light by bending it when the light passes through the glass. Unfortunately, different wavelengths bend a different amount. This means that different wavelengths focus in a different spot. Optical designers can improve on this problem by using multiple glass elements with different indexes of refraction that bend colors differently and try to make them all focus in the same place. This is where ED, or extra-low dispersion, glass is extremely helpful. Lens and telescope designers are usually fairly successful at focusing most of the visual wavelengths at the same place. But for the short blue wavelengths, and longer far red wavelengths, this is more of a problem. In most inexpensive refractors, it is blue light that is most troublesome. But in a camera that is modified for enhanced red sensitivity to record the red hydrogen-alpha light of emission nebulae, focusing red light at the same place as the rest of the visual spectrum can be a problem also. Most modern DSLR cameras have a long-wavelength filter built in that reduces this problem. But a modified camera, like mine, has this filter replaced with one that passes the longer red wavelengths. Unfortunately, these red wavelengths don't come to focus at the same point as the rest of the visual spectrum. Likewise for short blue wavelengths. When a lens is focused for green light in the middle of the visual spectrum where our eye is most sensitive, the out-of-focus blue light combines with the out-of-focus red light to create a red / magenta halo around stars. In Comparison 2 below, we can see the different star sizes caused by this problem in the red channel, which records the red wavelengths, and the blue channel, which records the blue wavelengths. The red channel and the blue channel are both out of focus when the green channel is in focus. Because of the camera's enhanced red sensitivity, the red channel is even more out of focus than the blue channel. Remember that a color image is made up of red, green and blue channels that combine to produce full color.
The extent of this problem depends on how well corrected the lens is, and whether the camera is modified. Using certain filters, such as light pollution filters, can also make the problem worse for astronomical photography with a modified DSLR camera. This problem is not unique to camera lenses however. It can also occur in all but the absolute best-corrected, most-expensive, apochromatic refractors. It can be partially corrected by using a minus-violet filter to filter out most of the out-of-focus blue light, but there is not much you can do for the red if you want to record the red light of emission nebulae, except use star reduction techniques in the red channel during later image processing. Stopping down the lens will also improve these color halos, but this improvement comes at the cost of light gathering ability. To focus, I examine a star at 10x magnification on the LCD on the back of the camera with Live View. The point when the star does not have a blue or red fringe around it, is not, in fact, the point of best focus. At this point, the green and blue channels are actually out of focus to about the same average as the red channel, and all of the stars are bloated. The point of best focus, when the star has the smallest core, seems to be when the star also has a red / magenta fringe around it. At least this is so with my modified camera. This makes sense in terms of how a DSLR sensor works. It has a Bayer array of red, green and blue filters over each individual pixel. One color filter per pixel. An array of four neighboring pixels has two green filtered pixels, and one red and one blue filtered pixel. So most of the luminance information in an image comes from the green pixels. This is where we focus, in the green, and the blue light is out of focus on one side, and the red light is out of focus on the other side. Stopping Down to Improve Lens Performance Almost every camera lens will improve in performance when it is stopped down. Most lenses perform best when stopped down about 3-4 stops from their maximum aperture. For normal daytime photography, where you have plenty of light, this is not much of a problem. But for astrophotography of faint objects where very photon is precious, it is an important consideration. Normally, for long-exposure deep-sky astrophotography, aperture is the most important thing. More aperture gathers more photons and improves the signal in the most-important signal-to-noise ratio in the final image. However, some lenses perform so poorly on star fields when used wide open at their maximum aperture, light gathering ability must be sacrificed and they must be used stopped down. Luckily, one of the Nikkor 180mm ED lens' greatest attributes is its excellent performance when used wide open. The lens is not perfect wide open, and it does have some aberrations. These improve when the lens is stopped down. But ultimately you have to choose between better optical performance and more signal in the final image. For brighter deep-sky objects, stopping down one stop is a good compromise. For the really faint objects, the Nikkor 180mm f/2.8 ED AI-S lens is good enough to use wide open, as can be seen in the images below.
In Comparison 3, we can see how the stars improve when the lens is stopped down one stop to f/4. It shows crops from the four corners and center of a modified Canon 1000D (Digital Rebel XS) with an APS-C sized CMOS sensor at 100 percent enlargement. Brilliant red-giant star Antares and globular cluster M4 are in the center of the frame. The lens has little if any coma, astigmatism or distortion when used wide open. It does have a little bit of lateral chromatic aberration that is most prominent in the corners on bright stars. This can be improved by stopping down. But if you examine this comparison closely, you will see that while the chromatic aberration is improved at f/4, it is not completely removed, especially not on a modified camera. You would have to stop down to f/5.6 or f/8 to see a vast improvement, and the reduced aperture at these f/stops is too high a price to pay for astrophotography, at least in my opinion. Honestly, to me, the improvement is so slight that I would rather put up with this particular aberration to gain the additional photon gathering ability realized by using the lens wide open. It would come down to the question of whether you want to waste clear dark-sky time shooting more frames stopped down to improve the signal-to-noise ratio, or spend the time in the electronic darkroom fixing the halos on a cloudy night. Personally, I would prefer an image with a higher signal-to-noise ratio because there is no substitute for this in terms of image quality, and clear dark-sky time is precious, whereas I know I can improve the halo problem later in processing. Some people find the diffraction spikes from the lens diaphragm distracting when the lens is stopped down. If optical performance improvement is desired by stopping down, it is possible to simply make a circular aperture stop that goes in front of the lens. Constructed to the proper size, a circular aperture can stop the lens down by any required amount without diffraction spikes. On the other hand, some people like diffraction spikes, and will even add crossed wires in front of the aperture, even when used wide open, to create them. If you have a 48mm or 52mm light pollution filter, you can also use it with a 72mm to 48mm or 72mm to 52mm step-down adapter ring. This will also effectively stop down the lens. For example, a 52mm filter will give an effective focal ratio of f/3.5, stopping the lens down a bit and improving performance at a small cost of light gathering ability.
In Comparison 4 above, we can see how the on-axis chromatic aberration in the center of the field is improved by stopping down to f/4. Again, remember that this is made worse because this is a modified camera that is unusually red sensitive. On an unmodified camera, this would probably be much better, and the lens could easily be used wide open. Vignetting and Field Illumination Vignetting can be produced by several different causes. Optical vignetting is caused by the complex arrangement of optical elements in a camera lens where front elements can block part of the off-axis light reaching rear elements. This type of vignetting in common in camera lenses when they are used wide open. Optical vignetting can be improved by stopping the lens down. But again, this is at the cost of photon-gathering ability. As we can see in Comparison 5 below, vignetting is considerably improved in the Nikkor 180mm f/2.8 lens by stopping it down to f/4. It would improve further by stopping down more.
In the comparison images, the center of the field is set to the same RGB brightness, 80,80,80 in both images. The center may seem to get darker at f/4 for a moment after you do the mouse over, but this is an illusion caused by your eye / brain / mind visual system. It is actually the corners that get darker at f/2.8, caused by vignetting. In image frame 7704, the lower left corner's RGB brightness values are 35,35,35 at f/2.8. This is about one stop darker from the optical vignetting. In image frame 7705, the lower left corner has RGB values of 60,60,60 at f/4. There is still some vignetting, but it is much improved. Note that the exposure time had to be doubled in the f/4 frame to keep the image brightness the same in the center of the frame. This is the price you pay when you stop down one f/stop. This amount of vignetting present when the lens is used wide open is easily corrected with flat-field frames, or later during image processing with software like the GradientXTerminator Photoshop plug-in. Mounting the Lens One serious consideration of using the Nikkor 180mm lens is mounting it on top of your telescope for piggyback astrophotography. It does not come with a tripod collar. You can not just attach it to your scope by the tripod socket on the bottom of the camera. This lens weighs a lot compared to the camera body, and you risk damaging the lens mount on the camera if you attach it only by the camera's tripod socket. The lens itself really needs to be supported. The best way to do this is with a set of guidescope rings on a dovetail bar. I use a set of ADM 100mm guidescope rings that cost $100. They have substantial 3/8th inch bolts with Delrin tips so they won't mar the lens.
The latest version of the Nikkor 180mm f/2.8D AF ED-IF Autofocus Lens has internal focus. Most of the body of the lens is fixed and does not move as the lens is focused manually with the focus ring. This version of the lens can be mounted with the rings in a variety of positions - pretty much anywhere except for on the focusing ring. However, on the old manual focus Nikkor 180mm AI-S ED lens that is tested here, the entire lens barrel moves when the focus ring is turned. This is a difficult complication. It is not trivial to mount this lens. There is really only one very thin metal collar with grooves in it between the focusing ring and beveled area that you can anchor it by, which can be seen in the image above. The front set of lock downs can't be tightened until the focusing is complete because the barrel of the lens moves when it is focused. After the lens is focused, then the front ring supports can be tightened. Note that the lock downs on the front end of the lens should only by loosened a very small amount - just enough to let the lens move to focus it. This way, the lens shade will act as a sort-of safety stop and prevent the lens from slipping out of the rings completely if the rear lock downs accidentally come loose. You don't want your nice ED lens crashing down to the ground. It might also be a good idea to tie the camera's strap around the rings or something on the scope or mount so that if the camera and lens does slip out of the rings, it will be saved by the camera strap. Using guidescope rings to mount the 180mm lens also allows you to rotate the lens for the best framing of the deep-sky object you are shooting. An alternative is to buy a Manfrotto 293 Telephoto Lens Support. But this device does not allow the camera to be rotated for vertical framing of deep-sky objects, which is a serious drawback. Also, at a price of $71, it is not much less expensive than a good set of ADM guidescope rings. Summary For $300 for a used Nikkor 180mm AI-S ED lens in excellent condition, $28 for the Fotodiox adapter, and $100 for some rings, you have quite an awesome f/2.8 telephoto astrograph for about $428. If cost is not a consideration, the latest autofocus Canon EF 200mm f/2.8L II USM Lens is another excellent lens. If you use Canon DSLRs, you can even use autofocus with it on a bright star, planet, or the Moon. This lens costs about $775 new. The tripod collar is an additional $200, bringing the total to almost $1,000. The latest autofocus Nikkor 180mm f/2.8D AF ED-IF Lens costs about $900 new. Add another $100 for the guidescope rings to hold it. Nikon does not make a tripod collar for any of their 180mm lenses. Your total then is about $1,000, the same as for the Canon 200mm f/2.8. The performance of the latest versions of the autofocus Nikon and Canon 180mm lenses is probably a little bit better than the older manual focus Nikon 180mm ED lens. But the performance of the manual focus Nikon 180mm ED AI-S lens is excellent for astrophotography, even when used wide open. At less than half the price with the Nikon 180mm ED AI-S, you get about 95 percent of the optical performance of the latest autofocus versions. This manual focus lens also has outstanding mechanical construction, even better than the autofocus versions, in my opinion. The optical performance and rugged construction of the manual focus Nikon 180mm ED AI-S lens are what make it such a great bargain for astrophotography. |
||||||||||||||||||||||||
|
Back | Up | Next |
