Sunday, 2 January 2011

AF comparison: Lumix 20mm vs Sigma 30mm

I previously compared the Lumix 20mm f/1.7 pancake lens with the Sigma 30mm f/1.4. The Sigma lens was put on a Pentax K10D. While the comparison is not entirely fair, since the K10D is an older camera, my conclusion was that the Lumix 20mm lens was a better performer.

In this article, I am comparing the lenses again. I set the Lumix 20mm pancake lens on a Panasonic GH2 camera, and the Sigma 30mm f/1.4 lens goes on a Pentax K10D camera.

The Sigma 30mm f/1.4 (left), and the Lumix G 20mm f/1.7

Here you can see both cameras set on tripods, pointing towards a LEGO figure on a table. Both cameras are set to f/2.8, and base ISO. I raised the built in flash on both cameras, and set the initial focus to infinity. The focus mode was centre point.

Here's a video showing both cameras' shutter releases being pushed fully at the same time:

video

Not surprisingly, the Panasonic setup fires off first. It has the fastest autofocus.

With the Panasonic camera, focus was reached after 0.4 seconds, and the pre-flash was fired after an additional 0.17 seconds. There is a pre-flash to measure the required intensity of the main flash, since the TTL flash metering does not work in the same way as with older SLR film cameras. The main flash was fired after a total of 0.77 seconds since the shutter release button was pressed the first time.

With the Pentax camera, I don't have the measurement of the time until focus was reached, since this is not displayed. But the main flash went off after 1.10 seconds. The pre-flash was fired around 0.1 seconds before the main flash.

Now, let me be the first to say that this comparison is unfair. The Pentax K10D camera is from 2006, while the Panasonic GH2 was launched in 2010. Also, I'm using a third party lens on the Pentax camera, which is known to not always give the best AF performance.

The Sigma lens is operating closer to it's minimum focus distance, which is 40cm. The Lumix 20mm lens, on the other hand, has a more generous minimum focus distance of 20cm.

Let's look at the images. Here are the full images, scaled down and resharpened a bit:

Lumix 20mm, f/2.8, base ISOSigma 30mm, f/2.8, base ISO

To better compare the images, here are 100% views from the centre, directly from the out of camera JPEG files. These are unsharpened. Click for a larger view.


It is apparent that the the Lumix 20mm lens is much sharper. Also, the Panasonic exposure is better. This is a tricky situation for a DSLR to expose, anyway. The camera has no way of knowing that the white surface is supposed to be white, and not grey. So in this case, I should have helped the camera by saying that it should over expose. The Panasonic camera sees the whole picture prior to making the exposure, and the automatic exposure is clever enough to see that the surface should in fact be white.

When looking at the 100% crops from the two different cameras, one could conclude that the Pentax/Sigma image is unsharp due to front-focusing or back-focusing. I.e., that the AF sensor or lens is not calibrated correctly. Since I am stopping down to f/2.8, though, I doubt that poorly calibrated focus is the reason for the unsharp image. But perhaps it is, and this kind of uncertainty is common when using DSLRs. Thankfully you don't need to worry about it with Micro Four Thirds.

Micro Four Thirds cameras use Contrast Detection Autofocus (CDAF) to verify the correctness of the focus, and it is generally more accurate than Phase Detection Autofocus (PDAF), used on DSLR cameras.

I am confident that the Pentax/Sigma combination focused on the LEGO figure, and not on the background. If I set the camera with the figure slightly off-centre, it would not focus at all, since the white table is too even and glossy.

Conclusions

This study reveals several interesting issues. First of all, we see again that the the Lumix G 20mm f/1.7 pancake lens is very sharp. It's autofocus is not quite as fast as other Micro Four Thirds lenses, however, it is still fast enough for most uses. All in all, it is a very good lens, especially for environmental portraits.

Again, I see that the Sigma 30mm f/1.4 is not very sharp. This disappoints me, since it is otherwise a very interesting combination of focal length and aperture for use on an APS-C DSLR. Some might suggest that my camera/lens combination requires calibration. Perhaps they do, but again, this just illustrates one of the drawbacks with DSLR cameras and PDAF technology.

Some would say that the Sigma 30mm f/1.4 lens is not intended to be very sharp anyway. Rather, it is designed for available light photography of people, to be used with apertures around f/1.4-f/2, say. In these settings, the quality of the bokeh (out of focus rendering) is also important. One could say that the Lumix 20mm lens is more of a generalist lens. Still, I think it's bokeh is usually adequate.

The autofocus performance of the Pentax/Sigma combo was better than I had expected. On the other hand, it is also significantly more noisy. Keep in mind that the Lumix 20mm lens is one of the most noisy Micro Four Thirds lenses I have tried, but the Pentax/Sigma still makes much more noise.

The GH2 camera has a flash lag of about 1/4 to 1/3 second. This is due to the fact that the TTL flash metering does not work during the main flash, like with older SLR film cameras. So to figure out how large the main flash exposure needs to be, the camera does a pre-flash. After having evaluated the pre-flash exposure, the main flash is fired.

1/3 second flash delay is in fact significant, and this is one of the drawbacks of Micro Four Thirds. Note that this is not just related to the built in flash. An external TTL flash, like the Panasonic FL360 also has the flash delay in TTL mode. To avoid the flash delay, you will need a flash that can do traditional flash auto mode. This is a bit more cumbersome to use, and may require some calibration from situation to situation. But an advantage is that you can use just about any flash for this purpose, even if it is an older flash from a different system. Just be sure the flash trigger voltage is not so high that it fries your camera.

Note that the flash delay is not exclusive to Micro Four Thirds. DSLRs also have it, like the Pentax K10D in my example above. However, since DSLRs will use the exposure sensor for the flash evaluation, rather than the main imaging sensor, the pre flash delay will usually be quite short.

2 comments:

  1. TTL flash metering for digital SLRs doesn't work during the main flash either. There's a separate preflash (or preflashes) to measure the required flash power.

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