They appear to cheat on the laws of physics, by designing tele lenses much shorter than usual. This is achieved by employing a catadioptric optical system, which uses both reflection and refraction to focus the light onto the sensor. Normal lenses only use refraction.
This illustration of a typical reflex lens was made by Paul Chin, with the film plane (sensor) to the right:
The lens is shown below together with another Micro Four Thirds lens capable of delivering 300mm focal length, the Lumix G 100-300mm f/4-5.6:
It is not entirely fair to compare them like this, though, as the Lumix G lens is shown in the 100mm configuration. When zoomed all the way to 300mm, the comparison is more reasonable. The picture shows just how compact the Tokina is, as a 300mm tele lens:
With these long lenses, I would really recommend using the supplied hoods. This is what they look like with the hoods mounted:
Unlike what you might expect, the hood is not just a molded piece of plastic. It consists of at least six parts, most of them metal. It reverses outside of the focus ring for storage:
The lens barrel and the focus ring are made out of quite smooth metal. This makes the lens look good, but the metal is a bit too slippery for my taste. I would have preferred the lens to have a more matte crinkle finish, which is commonly seen on high end lenses. It appears that consumer electronics is expected to have a smooth metal finish these days, which I think is sad. Often, a more ergonomic material is better to handle.
The aperture is fixed at f/6.3. There is no way to stop down the aperture at all.
Mounting and un-mounting the hood from the reversed position is a bit awkward. The focus ring and the whole front of the lens rotates as one single part. Hence, when un-mounting the hood, you usually end up first rotating the focus ring all the way to macro, until you are able to undo it from the bayonet mount.
And the same goes when mounting it: You must first rotate the focus ring, and the front threads to infinity, until you can engage the hood bayonet mount. I usually leave the hood mounted extended, also when in storage.
To more easily be able to remove the lens cap inside the hood, I have used a DIY trick to glue some plastic grips onto the centre grip sections of the cap:
The focus ring rotation is very well dampened, and is a joy to use. It rotates a full 270°, which is quite a generous. There are plenty of distance markings on the focus scale, and they appear to be accurate. Still, focusing manually is not easy, and you must take great care to get the best focus, at both long and short focus distances alike.
The fact that the front of the lens rotates with the focus ring is a bit unusual these days, and appears a bit retro, even. It is not a problem for me, but if you prefer using polarized filters, it may be a problem, as you must re-rotate the filter every time you change the focus.
When focusing closer, the whole front section of the lens extends, by as much as about 8mm in macro mode.
The lens does have electronic contacts, surprisingly, since you can't control anything from the camera anyway. The electronic contacts just serve to inform the camera of the focal length and the aperture. Further, it also communicates the approximate focus setting (close to infinity, close to macro), so that the camera will discover if you are rotating the focus ring, possibly bringing up the magnified focus assist view, if you have configured the camera to do so.
The fact that the focal length is communicated is also good news for those using Olympus cameras. This enables the camera to use the In-camera image stabilization (IBIS) properly, which is very important for a long lens like this.
Focusing the lens is fairly easy, at least with some experience. But only when mounting it on a tripod. If you try to hold it in your hands while focusing, then you will probably find that very hard. I would say that using this lens without some kind of support, preferably a tripod, is pretty much impossible. This is unlike the Lumix G 100-300mm, which, due to the Optical Image Stabilization (OIS) can be used handheld.
When using mirror reflex teles on SLR cameras, people often complain that the viewfinder is very dark. This is due to the small aperture of reflex teles, typically around f/5.6-f/8.
This is not the case when using this lens on Micro Four Thirds cameras, at least not with recent cameras. The camera will increase the gain, so that the viewfinder image you see is as bright as normal.
However, if it is very dark, then the camera must increase the sensitivity, giving more noise. Thus making it more difficult to focus properly. It must be very dark for this to be a problem, though. I was able to focus manually early in the morning at ISO 12800, 1/320s, with the Panasonic GH3. And this was without the use of a tripod.
So unless it is very dark, it is no problem operating this lens, even with the limited f/6.3 aperture.
To evaluate the image quality, I compared it with the Lumix G 100-300mm f/4-5.6 at 300mm. I chose a situation where the background is relatively dark, where the reflex lens performs the best, more about this later. Here are some leaves with a wall in the background:
|Lumix G 100-300mm @ 300mm f/6.3||Tokina 300mm f/6.3|
To better evaluate the image quality, here are some 100% crops from the centre:
And from the top left corner:
I think that the sharpness of the Tokina 300mm f/6.3 is actually quite good, compared with the Lumix G 100-300mm at 300mm. We already know that the Lumix G 100-300mm lens can be somewhat unsharp at maximum tele, and according to this test, the Tokina reflex lens beats it.
Where the reflex lens suffers heavily, though, is when the background is bright. Taking a picture against a bright sky reduces the contrast for both lenses, but by far the most for the mirror reflex lens. Here is one example. I focused on the centre leaf, and the background is a very bright sky, to be more precise, the sun is in fact pretty much in the front of the lens:
|Lumix G 100-300mm @ 300mm f/6.3||Tokina 300mm f/6.3|
As you see, the Tokina image is pretty much impossible to use, due to the reduced contrast.
Keep in mind that here I generated the most extreme example I could find, by pointing the camera towards the sun. Normally, you wouldn't do this, so you should not get as extreme loss of contrast.
The video example below contains some more normal real life examples where the background is bright. They could have been edited to look more normal, by stretching the histogram, but I left them as is.
Another special property of a reflex lens is the donut bokeh. Any highlights out of focus will be rendered as donuts, not blobs, as we are used to. This is due to the centre of the lens being blocked: That is where the centre mirror is. Here is one typical example:
First of all, to be able to use the lens for video, you must have a tripod. There is no way you can hold the lens stably enough without support.
Apart from that, it is as usable for video as for images. You have the same challenges, i.e., a bright background reduces the contrast, and out of focus highlights will become donuts. Here are some example video clips:
The lens focuses quite close, down to just below 0.8m. While this may not sound very impressive, keep in mind that the lens has a very narrow field of view. So at the closest focus, it can capture objects 35mm long, corresponding to a 1:2 enlargement. Which is certainly not bad. The Panasonic and Olympus macro lenses enlarge twice as much at their maximum, 1:1.
This picture of a ruler was taken at maximum enlargement.
The working distance is about 0.7m at 1:2 macro, which is very generous. Hence, this lens will be quite good for photographing insects, which tend to be camera shy. However, you cannot stop down the aperture, so you have to live with a somewhat small depth of focus (DoF). It is still quite usable for insect macro, though.
Here are a couple of example macro images taken at 1:2:
The lens is quite sharp in macro mode, in my opinion.
Here is a photo of a flower and a bee, at maximum magnification. As you see, f/6.3 is not sufficient to bring it all into focus. The picture was taken with the Panasonic GH3 at ISO 4000, 1/500s:
And another macro example taken at the same settings:
The Tokina 300mm f/6.3 is an interesting lens, but has quite limited usage areas. First of all, there is no zoom, and you only have an extreme tele. Which is not something that most people use a lot.
Second, there is no autofocus, and no OIS. Without this, you pretty much need to have a tripod, or some other similar support, to be able to use this lens.
Third, the lens is useless against a bright sky due to loss of contrast, so it can be difficult to use it for photographing birds, as they tend to be topside.
Fourth, the donut shaped bokeh is not going to be everybody's cup of tea, and many would say they are very distracting.
Fifth, there is no aperture mechanism, you are left with using f/6.3 all the time
With that said, the lens is still interesting, due to being extremely compact. And the price is quite reasonable.
Most of the time, the Lumix G 100-300mm f/4-5.6 will do the job better, and it is much more versatile, with a zoom, autofocus, and aperture iris mechanism, and OIS. And it "only" costs 60% more.
But if you value the compactness, plan to use a tripod anyway, and can live with the poor contrast against a bright sky, then this lens might be for you. It is also the only Micro Four Thirds lens which can do 1:2 macro with a 0.7m working distance. This is good for photographing (fairly large) insects.
The lens is also rather exotic, which can be fun. As for the donut bokeh, I actually think it is a bit charming.