There are many ways to make photographic lenses as we all know, some more efficient than others, and some better suited to particular use cases. One of the greater challenges in creating longer focal length telephoto lenses has always been balancing focal length with physical size and weight. Canon embarked on a limited series of lenses several years ago using a technology that they call Diffractive Optics. You can recognize a Canon DO lens by the distinct green line. Recently, the folks at Nikon have done something similar and released their own design using a new element mode called Phase Fresnel or PF. At the outset, it looks and sounds like DO. Since I am a huge fan of DO when the use case fits, I wanted to check out Nikon's Phase Fresnel system and they graciously loaned me one to try.
Traditional lenses focus light onto the sensor plane via the photo refractive process. You may recall from your high school physics that different wavelengths (colours) of light bend at different angles when passing through a medium, and in the case of lenses that medium has been glass. If you don't remember the refractive process, google the cover of Pink Floyd's Dark Side of the Moon to see it in action.
In photo refraction through a traditional design, light is refracted into focus points starting with blue, then green, then red. That the different wavelengths form focus points in different places is the root of chromatic aberration and less than sharp images. Lens designers add elements with different curvatures and use special design elements and special coatings to try to bring these focus points together. They do a superb job in this but it results in lenses, particularly telephoto lenses, that are large and heavy.
In a Phase Fresnel lens, photo refraction still occurs but the light refraction starts with red, through green and then blue. When the lens designer combines this with a traditional refractive element or element group the aberrations cancel each other out. This creates a smaller and lighter weight design that delivers the same effective focal length and optical acuity as a wholly refractive lens.
If you've ever seen a fresnel lens, perhaps in a live theatre, you will have noticed that the lens has a series of concentric rings carved into it. You would look at such a lens and say to yourself, "wow, that looks like it would really distort things and there's no way a clean image could be made." On its own, you would be right but photographic lens makers combine fresnel lenses with refractive lenses to deliver crisp and sharp images. Moreover, if you look into a PF lens, the fresnel is not visible the way it is visible and tactile in a lighting implement.
On occasions where there is a very bright point source of light in the frame, you may see an outer glowing ring around the source caused by the fresnel. This is mostly well accounted for by lens makers however. Many microscope lenses, including those built by Nikon, have used phase fresnel lenses for years. It's not a new technology, this particular lens is a new implementation.
The lens came with a D4s to shoot it on. Given that I shoot a 300/2.8 and a 120-300/2.8 Sigma on Canon bodies, I know what to expect in terms of weight. I also still have a Minolta Auto-Rokkor X MD 300/5.6 and can say that the Nikon 300/4 PF is smaller and lighter than all of them. It feels like less weight than a 24-70 but I didn't weigh them both. The D4s has an excellent AF system and the lens, even with a max aperture of f/4 focused pretty readily down to EV -1 (pretty dark). In brighter situations, the AF was blazing fast, faster I would say than Nikon's prior 300/4.
The light weight is awesome. I own Canon's 400/4 DO IS L and have used it at night for High School Football and love both the optical speed and the fact that I don't have physical pain after a game. The 300/4 is a bit short for football but is a great focal length for amateur hockey shooting from the top row in the towns near me and would not require a monopod. The big f/2.8s are really tiring without a monopod, so photographers who can handle a one stop ISO push in their shooting scenario will find the 300/4 PF a viable alternative to a larger and heavier f/2.8
I had read in a couple of places that other shooters felt that the PF lens didn't deliver the same contrast or colour pop as a traditional lens. I don't have another Nikon 300 but I just recently completed an eval of the very impressive 200-500/5.6 and I could not find indications of quality reduction with the PF lens. I understand that there will always be fear of the unknown, and it is important to realize that PF is not a new idea, this is simply the first go from Nikon in a camera lens.
As I have found from time to time when using CFL constant lights, the D4s test camera tends to be a bit cool in AWB. Flipping to Daylight provides a nicer colour temperature guidance, even with the RAW file. The 300 f/4E PF focuses quite closely so one might be inclined to use it for close up work when you cannot get too close. I did a couple of experiments in this regard in the studio with regular model Sondra. Shooting off a tripod at ISO 400 gave me decent enough shutter speeds, with a +1 EV the most pleasing OOC result. I tried at f/4 but the depth of field was so shallow that if I set the focus point on the rear eye, the front eye was completely soft. Nikon does not capture the subject focus distance in their RAW files so RAW Digger was not able to be explicit in the focus distance, thus I used the not so exacting tape measure method and found the sensor plane to eye distance to be about 68 inches.
Guesstimate places the total depth of field at this distance at 0.44 inches which in theory should have been enough. But, it did not work out that way so back to the drawing board. I was using the circle of confusion calculations for the D4 instead of for the D4s but do not see why the series would have been as soft as it was. Reshooting everything exactly the same from a lighting and distance perspective but now at f/8.0 delivered the tack sharpness I was expecting. This change should alter the depth of field to 0.63 inches and it shows. More testing wide open is needed, but to be fair, at greater distances as wide open at minimum focus distance pushes the boundaries of any lens' design.
Adobe has profile corrections already in place for the lens, which is expected given that it was announced about a year ago. Engaging them shows a visible difference with some clear vignetting adjustments and distortion corrections. The unadjusted images look just fine, but you will appreciate the difference when working with the adjusted version.
I opened the same RAW file in DXO Optics Pro 10 and downloaded the DXO profile for that body and lens pair. It provided about the same level of adjustment as the Lightroom profile, but it was clearer that DXO was definitely fixing some pincushion distortion. I will say that I much preferred the colour rendition that DXO delivered to the OOC RAW files, the second series having been shot with the in camera white balance set to Daylight. Regular readers know that this doesn't get cooked into the RAW file but is provided to the RAW converter as a hint for display. I used the white balance eyedropper on the same image in both Lightroom and DXO Optics Pro and for this lens and camera combination preferred the DXO conversion which resulted in a white balance of 100K warmer. This is always subjective, and I colour calibrate my displays every two weeks. I just seem to find LR a bit cooler in its RAW converter when working with NEF files from the D4s. That of course has nothing to do with the lens, but I share the experience so readers are cognizant that different RAW converters produce different results.
Capture One 9 was similarly warmer than LR, by about 170K. This is less a complaint about the Nikon products and more an emphasis that the RAW converter you choose will produce subtly different results. My preference in this case is Capture One because it offers multi-point white balance and I am shooting against a neutral grey background. In the end, I can achieve a pleasing image with any of the tools.
When I got outside to shoot, I did not run into the same exposure or white balance challenges I encountered when working under CFLs. As Shot colour was accurate to the lighting of the shot. The D4s has excellent dynamic range so even if I pulled or pushed a shot, recovery and adjustment worked out fine. I found the 300PF to be very fast to focus at distance, a bit slower for closer work. I am not fond of telephotos that have limited close focus capability. The 300PF does not have this problem as you will see in the sample images, you can get very close to your subjects when you need to. Focus and depth of field appeared right on the money in the outdoor shots. I encountered an issue where focus would not happen. Performing a factory reset on the D4s resolved the issue, so I have no concerns with the lens in that regard.
Outdoor Image Gallery
Sharpness and colour rendition from the 300PF is superb. While I shot the lens a lot, I did not encounter the glowing ring scenario at all, even when I had a specular highlight in frame.
While it is most common to use telephotos for distant subjects, I like to shoot close to medium distances as a big part of my evaluation process to reduce the potential impact of atmospheric effects that could dissuade a viewer from considering a lens, specifically when the lens is not at fault.
I am very impressed by the 300mm PF lens. It is smaller and lighter than a traditional design and when used on a body with a decent performing sensor, the one stop difference between this lens and the much heavier and much more expensive 300mm f/2.8 is not apparent other than slightly more depth of field wide open (f/4 vs f/2.8) and of course, the need for one more stop of light in a given shooting situation.
At the time of writing (March 2016) the lens retails in Canada for $2340 or so, but I expect prices to rise April 1st as a result of the falling Canadian dollar. You can certainly get a zoom lens covering this range for less than the price of the 300/4 PF, but you will not get that level of lens speed and incredible light weight. While I own and use zooms, I am very comfortable shooting primes because in most cases the benefits outweigh the cons of a fixed focal length. If you are a Nikon shooter and are looking for a mid-length telephoto that is optically fast, and light enough to carry around all day long without a support mechanism, this lens is well worth your time to investigate.