Sunday, 24 October 2010

Focal length and focus distance

You'll notice that the focal length of most lenses is specified at infinity focus. It turns out that some lens designs imply a change of focal length, as the focus distance changes.

Here's a comparison of two quite different lenses. The Nikkor 200mm f/4 AIS is a traditional fixed focal tele lens. Compact, light, and reasonably fast, it is a classic lens. It is also rather simple from a mechanical point of view. If features "only" five lens elements.

The other lens is the Panasonic Lumix G 45-200mm f/4-5.6. Since it is a zoom lens, and it also has special lens groups for the OIS, it features a whopping 16 lens elements.

They are both shown here, the Nikkor 200mm lens with the Nikon-M43 adapter mounted. The 45-200mm lens is shown zoomed to maximum tele, to be comparable with the Nikon lens:


They are also quite different when it comes to focusing. The Nikkor has a traditional focus mechanism, which simply moves the entire lens assembly forward when going from infinity to close distance focus. In the picture below, it is shown focusing at infinity (left) and just below 2 meters (right).


The Panasonic Lumix G 45-200mm f/4-5.6, on the other hand, features internal focusing. This is very practical, since it means that the length of the lens stays the same regardless of the focus.

Also, the internal lenses moving around are much smaller, and faster to move about. This makes the focus faster, and requires less juice from the batteries. And the 45-200mm does focus very fast indeed!

The negative side of internal focusing, is that it can affect the focal length when changing focus. Let's compare the two lenses when focused far and near. The upper and lower images are taken at exactly the same place, but with the focus placed at the parked car (upper), and the foreground flower (lower).


What we see here, is that when focused far away, the field of view of the lenses are mostly the same. The Lumix has a slightly wider field of view, but there is hardly any significant difference. When focused at 2 meter distance, however, the field of view is significantly wider with the Lumix lens.

Mostly, this is not any problem at all. When recording video, however, it can be annoying if the field of view changes significantly during focus. This feature is usually referred to as "focus breathing", as the objects recorded will pulse in size as the focus moves back and forth.

With contrast detection autofocus (CDAF), this can in fact be a big problem during video recording, as the camera must jog the lens focus back and forth to verify that the focus is correct. You can see these focus movements in a video recorded using the Lumix 20mm f/1.7 pancake lens. The Lumix 20mm f/1.7 lens features a traditional moving lens assembly, and does not suffer from focus breathing. This is good, since the continuous autofocus operation is not very visible in the video, but on the other hand, the lens is probably not as solid and weather resistant with this construction.

I first noticed this change of focal length when examining the bokeh of the 45-200mm at 200mm, compared with the Nikkor 200mm lens.

Lumix G 14-42, new kit zoom

For the introduction of the Panasonic Lumix G2 and G10 cameras, a new kit zoom was launched. Much to the dismay of Micro Four Thirds users, since on first sight it looks like a dumbed down version of the old kit zoom.


First of all, it has slightly worse technical specifications: The long end of the zoom is 42mm, while the old had 45mm. Next, the new kit lens has got a plastic mount, while the older has a metal mount. The new zoom also lost the OIS switch: Switching OIS on or off is now done through the menus.

In terms of ergonomy, the new lens also lost the rubber zoom ring. It now features a plastic zoom ring, which gives somewhat less friction when operating it with your fingers.  Some users of the old lens experienced that the rubber zoom ring came loose.  This will not be a problem with the new lens, since there is no rubber ring.

Now, the change of the long end focal length doesn't bother me. 42mm and 45mm is basically the same field of view, there is no significant difference here. Also, the plastic mount, if done properly with good quality materials, is probably solid enough. After all, this is a very light weight lens, and in normal use, it doesn't need as strong support as larger lenses.

What about other aspects? Some reports indicate that the sharpness of the new lens is not as good as the original Lumix G 14-45mm lens. I cannot comment this, since I haven't used both.

Here's an analysis of the sharpness and bokeh of the lens.

The GH1 kit lens, the Lumix G HD 14-140mm, is specified with an aperture range from f/4 to f/5.8. However, while zooming from wide to tele, it closes down very quickly. So it is fair to say that this is essentially an "around f/5.6" lens, with a bonus brightness in the short end.

What about the other kit lenses? This diagram shows the relationship between the focal length and the maximum aperture for the three kit lenses:


For the 14-42mm and 14-140mm kit lenses, these values were sampled by using the actual lens. For the 14-45mm lens, I took the values from various reviews off the Internet.

It looks like the new kit lens has slightly better speed at f=25mm: f/4.6, compared with f/4.9 for the old kit lens.

I also added the aperture data for the premium Olympus standard zoom, the Olympus M.Zuiko Digital ED 12-50mm F3.5-6.3 EZ power zoom. This lens is comparable with the other kit lenses in the short end, but the aperture closes down very quickly as the focal length increases. I think this is consistent with the Olympus M 4/3 design philosophy, which generally puts compactness ahead of maximum aperture.

The main purpose of the kit lens, is to be cheap and good enough for most beginners. I'm guessing that the size and number of the glass lens elements is an important contributor to the price.

The diameter of the front element of the 14-42mm lens is 13% smaller than that of the 14-45mm lens. And that means the area is 25% smaller:


It is quite remarkable that Panasonic has essentially retained the specifications, while shrinking the front element so much.  Of course, reducing the front lens diameter is not necessarily good for the image quality.  It could lead to more vignetting at max aperture, for example.

All in all, I think this will be a pretty successful lens. Some early reports indicate slightly worse sharpness, however, for the target audience that may not be a problem. The cheaper construction means that Panasonic can sell them in kits at a lower price point, which they will need now that the competition has gotten their systems launched.

The autofocus speed of the newer 14-42mm lens is very good.

Sunday, 17 October 2010

Lumix 20mm compared with Sigma 30mm

A lot of people have complained that the Panasonic Lumix 20mm f/1.7 pancake lens is overpriced. To have a look at this statement, let's compare it with a lens in the same price range, the Sigma 30mm f/1.4.

The lenses have a lot in common. They share the same price tag in my market, and they do essentially the same job. The Lumix 20mm lens has a slightly wider field of view, and the Sigma 30mm has half a stop larger aperture. But these differences are not very significant. When I compare their fields of view, I refer to the Sigma 30mm being used on an APS-C camera, for which is was designed.

Here they are both:


The Sigma 30mm lens is shown with the supplied lens hood, which is very nicely designed. Sadly, the Lumix 20mm does not come with a hood, but I have put a step down ring on it, which acts as a compact hood.

As is apparent from the image, these lenses are very different in size. The Sigma (left) is 77x59mm, 430g. To the right, the Lumix is 25.5x63mm, 100g. Adding the supplied hood to the Sigma lens will make the difference even larger, of course.

When mounting the lenses to cameras, they look like this:


To the left is the Panasonic Lumix GH1 with the Lumix 20mm lens, and to the right is the Pentax K10D with the Sigma 30mm lens.

Image quality

What about the image quality? I tried to take the same picture with both setups, to see how they compare.


GH1 + Lumix 20mm @ f/1.7, 1/60 second, ISO 100 (click for larger image)


K10D + Sigma 30mm @ f/1.7, 1/45 second, ISO 100 (click for larger image)

Note that I used the same aperture on both lenses. The Sigma lens was stopped down from f/1.4 to f/1.7 to be comparable with the Lumix 20mm, which was used at the maximum aperture.

What we see straight away, is that the Lumix provides a wider field of view. I also think that the Pentax colours are more pleasing straight from the camera. Of course, using the RAW files you are free to adjust the colours of either images as you want.

To make the images easier to compare, I also added 100% crops from two sections of the images (click for larger image):


Here it is quite apparent that the Lumix lens is the sharpest. Also, there are less purple fringing artifacts in the Lumix image.

Now, we know that Chromatic Apperation (CA) artifacts are corrected for in software in the Panasonic Lumix G-series cameras. So the Lumix lens has an advantage here, in that these artifacts are automatically removed. Still, as a user of the systems, I care about the end result, not how it was achieved. And the end result is most certainly a lot better using the Lumix lens.

The Pentax setup uses phase detection autofocus (PDAF). With a large aperture lens like the Sigma 30mm f/1.4, this means that you can worry about the precision of the autofocus. Some camera body/lens combinations suffer from front-focus or back-focus. The setup might need expensive calibration to avoid these problems and achieve the best focus.

With the GH1 and Lumix 20mm lens, though, you get contrast detection autofocus (CDAF). With this system, you are ensured the best focus every time, as long as you set the focus region to suit your needs.

Size

The Lumix lens is a lot smaller and lighter. For me personally, that is a huge advantage. It could be a disadvantage for some users, though. Some customers might not take you seriously if you show up at a photography job with a small lens like this. When they pay for a photographer's services, some expect to get a person with a big camera and lens. For most users, though, this is not an issue.

Conclusion

From my point of view, these lenses, which do more or less the same job and are priced similarly, do not have an equal value. I much prefer the Lumix 20mm lens, which gives me better images, and is easier to lug around. I like the hood supplied with the Sigma 30mm lens. And having the option of using half a stop larger aperture is nice. However, you're not very likely to use the Sigma 30mm lens at f/1.4, since it is not very sharp wide open.

One could argue that my example image is not the most relevant for this type of lenses. These lenses are made for low light images of people, in which the corner sharpness doesn't matter too much. Also, while I haven't studied it carefully, I have a feeling that the bokeh from the Sigma 30mm lens is better. The Lumix 20mm bokeh is certainly very adequate, though.

I have also tested the autofocus of these two camera/lens combinations. The Pentax/Sigma combination is pretty fast in terms of autofocus, but not as fast as the Lumix. Also, the Pentax/Sigma makes much more noise when focusing.

Wednesday, 13 October 2010

Firmware v1.3 for Lumix 14-140 and AF speed

In the beginning of October 2010, a new firmware version 1.3 became available for the Panasonic Lumix 14-140mm f/4-5.8 superzoom lens. The new version is said to give faster AF speed, as well as faster startup time.

I have previously tested the AF speed of the Lumix 14-140mm. My conclusion was that the AF speed was very fast, albeit with some hunting in the longer end of the zoom, at close to the minimum focus distance. So I wanted to see if the new version improved the focus speed.

Again I used the same setup, with a LEGO figure as object to be photographed. The LEGO figure was placed at a distance of 0.5 meters, which is the minimum focus distance for the lens. After turning on the camera, I pressed the shutter release button, and measured the time until focus was achieved. The lightning was dull, about 6 EV.

I used the Panasonic Lumix GH1. Here's the experiment using firmware version 1.2:

video

And using the firmware version 1.3:

video

The time until focus was 0.76 seconds with firmware version 1.2, and 0.68 seconds with the latest firmware version. This is hardly any significant difference, but there is still some improvement.

I was not able to reproduce the focus hunting at 140mm that I experienced in my first comparison. The first test was done using firmware 1.1, so I guess this was fixed already in the version 1.2.