Bokeh comparison

With kit zoom lenses, the bokeh, the rendering of out of focus areas, is not too important. This is because they feature a small maximum aperture, leaving quite a lot in focus anyway. So that the rendering of out of focus objects is less of an issue, with most of the image in focus.

When using prime lenses with a larger maximum aperture, though, the bokeh becomes more important. With a larger aperture, you're more likely to find the background or foreground out of focus, and the quality of this rendering matters.

I've looked a three common prime lenses. The Lumix G 20mm f/1.7, the Sigma 30mm f/2.8, and the Olympus 45mm f/1.8. Out of these three, the Sigma lens is the odd man out, with a not too impressive max aperture of f/2.8. On the other hand, it is also the cheaper of the three lenses.

From my previous experiments, I know that both the 20mm lens and the 45mm lens have quite ok bokeh. The 20mm lens can give out of focus highlights that are elliptically shaped outside the centre of the frame. The 45mm lens is pretty much unproblematic. But the 30mm lens is a bit of an unknown to me at this time.


First test

My first comparison is done in the morning, with a low sun behind some foliage. I focused on the LEGO figure in the centre. I moved closer with shorter lenses, so that the LEGO figure fills out about the same space in the frame. The distance is about 0.9m with the longest lens, and around 0.4m. The distance is slightly shorter than what you would use when photographing the head of a person, given the same lens.

This situation will typically reveal lenses that render the out of focus background in a "busy" way. Ideally, we'd like to see the lenses blur the background so that it does not distract the viewer from the main subject.

Here are the full images, with all three lenses. They are all taken using a tripod, at ISO 160 on a GH2, and the exposures are generally between 1/60s and 1/5s. I used autofocus, and set a small centre spot area. Click for larger images:

Lumix G 20mm f/1.7	Sigma 30mm f/2.8	Olympus 45mm f/1.8

To look at how the centre, which was intended to be in focus, was rendered, here are some 100% crops from the centre area:

And from the top right corner:

Second test

My first comparison is done at night time, with high contrasts. I have focused on the basket of a bicycle, and there are some lights in the background. With the shorter focal length, I moved closer to the basket, so that the framing is pretty much the same. The distance is about what you would use when photographing the head of a person, given the same lens.

Here are the full images, with all three lenses. They are all taken using a tripod, at ISO 160 on a GH2, and the exposures are generally between 1/2s and 4s. I used autofocus, and set a small centre spot area. Click for larger images:

Lumix G 20mm f/1.7	Sigma 30mm f/2.8	Olympus 45mm f/1.8

To look at how the centre, which was intended to be in focus, was rendered, here are some 100% crops from the centre area:

I wouldn't use these to evaluate the sharpness, as I am not 100% sure that the camera focused at the very centre each time.

And a look at the out of focus rendering of the background highlight:

Conclusion

This comparison shows us that the bokeh of all these lenses is quite good. The Lumix G 20mm f/1.7 has elliptical out of focus highlights outside of the centre, as we have seen previously.

On the other hand, both the Sigma 30mm f/2.8 and the Olympus 45mm f/1.8 display slightly "busy" out of focus rendering when wide open. These two lenses could have blurred the background in a more pleasing way, but this is not a big deal. I don't think the background is very distracting in my test.

My perception is also that the Sigma 30mm f/2.8 lens handles high contrast situations the best. It appears to retain the contrast well, and flare/ghosting is not a problem. For a night exposure, I would use this lens when possible. This property may be due to the relatively simple construction of the 30mm lens: It does not have a large maximum aperture, not a wide angle, and is not a pancake. Hence, the optical construction is more trivial.

Another conclusion to be made, is that a longer lens gives more out of focus images, given the same framing and aperture. If you're after this effect in your images, it may pay to look for lenses that are long.