Wednesday, 28 November 2012

Fisheye sharpness

When I reviewed the Samyang 7.5mm f/3.5 fisheye, it was natural to compare it against the native Lumix G 8mm f/3.5 fisheye lens. Since the Lumix lens costs about 2-3 times the price of the Samyang lens, I would expect that the Lumix lens comes out as the winner.

However, what I found was that the Samyang lens was sharper, even in the extreme corners, and without the in camera CA correction which is done with the Lumix lens when using it on a Panasonic camera.

Some have doubted this result, and I have also been a bit unsure now and then. So I decided to do another test.

This time, I photographed a couple of trees against the bright sky, a useful test for lens sharpness. The focus was set on the centre of the image. With the Lumix lens, I used autofocus, and set a centre spot. With the Samyang lens, I used the 10x focus assist magnification available with the Panasonic GH2 camera. Here are the two images, both at f/3.5:

Samyang 7.5mm f/3.5 Lumix G 8mm f/3.5

To better evaluate the sharpness performance, I have compiled some 100% crops. From the centre of the image:


And from the top right corner:


Based on this study, it is perhaps not correct to say that the Samyang lens is much better. I think the Samyang lens appears to have slightly better sharpness in the corner, but the differences are fairly subtle.

Keep in mind that the Lumix lensis automatically corrected for CA artifacts, though. By looking at the RAW image files, and converting them to JPEG with third party software, we can see how the images were before the software CA corrections. These 100% crops are from the top left corner:


I used the free program UFRaw to convert the RAW images, but any program could have been used here, as long as it allows for doing the conversion without applying CA correction.

Here we see that without the in camera software CA correction, there are some significant red/green fringing artifacts. These are largely removed in the out of camera JPEG image. In the light of this, the Samyang lens's performance is even more impressive, as no in camera CA correction was done.

Conclusion

I wouldn't say that the Samyang 7.5mm f/3.5 fisheye lens is significantly better than the Lumix G 8mm f/3.5 fisheye lens, even if it appears to perform better in this test. So the image quality should not be an important factor when deciding between the two.

You could also consider the fact that they have slightly different projections. The Samyang lens gives images that are slightly less barrel distorted, and may be easier to defish.

The Samyang lens also handles flare better, see a direct comparison between the two in my review. Flare handling is very important for a fisheye lens, since you are quite likely to find the sun or a strong light source inside the image frame, due to the wide field of view.

When focusing manually with the Samyang lens, it is important to keep in mind that you shouldn't blindly trust the focus distance scale. On my lens, it is slightly off, and I reach infinity focus slightly before the infinity mark. This is not uncommon with manual focus lenses. Exact calibration would be very expensive, and the producers usually leave some slack for themselves by allowing the lens to focus beyond infinity.

The autofocus of the Lumix G 8mm f/3.5 fisheye isn't really needed for landscape pictures, as you easily get infinity into focus manually. But when taking closeup images, the autofocus can come rather handy.



Sunday, 25 November 2012

Prime lens overview

Prime lenses are lenses without a zoom function. They have a constant focal length, and a constant angle of view. When the Micro Four Thirds system was introduced, the very first prime lenses were the Olympus 17mm f/2.8 and the Lumix G 20mm f/1.7. While the Olympus prime lens was generally disliked for the limited maximum aperture, the Lumix lens was an instant classic with the brighter aperture and the perceived very good optical qualities.

Since that time, a lot of prime lenses for the M4/3 system have been introduced. In this article, I present an overview of these lenses.

First, here is an illustration which presents the lenses in terms of the focal length (the X axis) and the aperture (the Y axis). In the picture, the lenses are plotted as dots, where the size illustrate their relative price, and the colour the brand:


In the rest of the article, I illustrate what the focal length and aperture means, before going into details about the individual lenses.

Focal length

The focal length of the lens determines the angle of view: The shorter the focal length, the wider the lens is. This can be illustrated with a series of images all taken from the same point using a tripod, with different focal length lenses:



7.5mm14mm


25mm42mm


75mm100mm

Crop factor

Anyone familiar with photography using negative or positive film, will know that a 35mm lens is a wide angle lens, a 50mm lens is a normal lens, and so on. When comparing with Micro Four Thirds lenses, it is important to note that the Four Thirds sensor has roughly half the diagonal of the traditional 36x24mm negative size. Hence, there is a 2x crop factor associated with the Four Thirds format.

This means that the Lumix G 14mm f/2.5 corresponds to 28mm in terms of a traditional film camera. We say that the focal length of the Lumix G 14mm f/2.5 is 28mm equivalent, i.e., it is a wide angle lens.

Aperture

The aperture is defined as the ratio of the focal length over the diameter of the lens opening, and is given as an f-number. A lens with a maximum aperture of f/1.4 is usually considered as a large aperture lens, often called bright or fast. Fast refers to the fact that it lets through a lot of light, hence you can use a fast shutter speed.

Most lenses feature an aperture mechanism, which allows you to stop down the lens to a smaller aperture than the maximum. With most Micro Four Thirds lenses, this is controlled electronically through the camera, however, older lenses typically have an aperture ring which stops down the lens. This video illustration shows the classic Pentax FA 50mm f/1.4 lens, as it is stopped down from f/1.4 to f/22:



The Pentax FD50 has eight aperture diaphragm blades. Mostly, the more blades, the more smooth out of focus rendering you get. However, an even number of aperture blades also gives you fever spikes in the flare when you have a strong light source in the frame. Most Micro Four Thirds lenses have an odd number of aperture blades.

A large aperture (small f-number) obviously lets through more light, hence, it is better suited for photography indoors without a flash, or when it is dark.

Selective focus

In addition to letting through more light and allowing for faster shutter speeds, a larger aperture (smaller f-number) also gives you more selective focus. Selective focus means that whatever is further away from the focus distance, or closer, is rendered blurry. The quality of the out of focus rendering is called bokeh.

How the aperture controls the out of focus rendering can be illustrated with this series of pictures. All the images were taken with the Lumix G 20mm f/1.7 pancake lens. The focus was set on the brake lever in the centre of the image frame. The focus distance was 0.2m, the minimum possible for this lens.



f/1.7f/2.0


f/2.8f/4.0


f/5.6f/8


f/11f/16

As you can see from the image series, the amount of out of focus rendering is related to the aperture: The larger aperture, the more blurred background. This is one of the reason why some people desire large aperture lenses.

However, the blurring of the background is also related to the sensor size: The larger the sensor, the more blurred background. The Four Thirds sensor size is smaller than the traditional film cameras, and it turns out that the 2x crop factor applies here as well: All else being equal, an f/1.4 lens on the Four Thirds sensor size behaves like an f/2.8 lens on the 36x24mm sensor size (full frame), in terms of selective focus.

So for the ultimate in selective focus, it is not enough to buy a fast lens for the Micro Four Thirds format, you would need to go for one of the full frame cameras.

Micro Four Thirds prime lenses

Having explored what the focal length and aperture properties mean, let's go to the lenses. Here are some details about the prime lenses in various categories.

Pancake lenses

One of the key benefits of the Micro Four Thirds system is the size: Both the cameras and the lenses can be made smaller than DSLR systems. With this in mind, I think they pancake lenses are important. Pancake lenses are designed to be as compact as possible. There are three available: Lumix G 14mm f/2.5, Olympus 17mm f/2.8, and Lumix G 20mm f/1.7:



Lumix 14mm, Olympus 17mm, Lumix 20mm (not to scale)

I have written a very detailed comparison between the Lumix 14mm and Lumix 20mm lenses.

When deciding between the lenses, it is of course important to note the differences in field of view: The 14mm lens is a wide angle lens, the 17mm lens is a moderate wide angle lens, and the 20mm is closer to a normal lens. Hence, one could say that the Lumix 20mm lens can be used for, e.g, environmental portraits indoor or in dim light, the Lumix 14mm lens can be used for pictures of groups of people, landscapes, or other cases where you need to capture a wider view. The 17mm lens sits between them.

In terms of image quality, the Olympus 17mm f/2.8 generally gets bland reviews, with some people being disappointed with the sharpness. For most uses, though, I think the image quality is certainly good enough.

Both the Lumix 14mm and Lumix 20mm lenses are quite good in terms of image quality. If you search the internet, you will generally find that people are very pleased with the 20mm lens, while people are more unsure about the 14mm lens. In my experience, they are both quite good, and I think the some people dislike the 14mm lens, not so much because of the image quality, but more because it does not have an equally impressive maximum aperture.

When it comes to autofocus performance, the Olympus 17mm f/2.8 and the Lumix G 20mm f/1.7, being the very first M4/3 prime lenses, feature a quite old fashioned focus mechanism. The whole array of lens elements move back and forth during focusing, which is slow and noisy.

The Lumix G 14mm f/2.5 lens, on the other hand, has the more modern solution with internal focus, in which only some lens group inside the lens moves, which is much faster and virtually inaudible. With most use, this is not an issue, but when the autofocus speed is important, you may want to go for the Lumix G 14mm lens.

All the lenses rely on in camera geometric distortion correction. This means that the lenses are not perfectly rectilinear: They give a quite substantial barrel distortion which is corrected inside the camera by software. The uses will never notice this, and it allows the manufacturer to make smaller, more compact lenses. Some purists will dislike this feature, but I think it is a sensible compromise.

My favourite pancake lens is the Lumix G 14mm f/2.5, as it is very compact (the smallest of the three), focuses very quickly and noiselessly, has good image quality, and is well suited for video use.





Fisheye lenses

With the short register distance of the Micro Four Thirds system, it is possible to design very compact wide angle lenses, e.g., fisheye lenses. So far, we have two full frame fisheye lenses to choose from, the Samyang 7.5mm f/3.5 and the Lumix G 8mm f/3.5:



Samyang 7.5mm f/3.5 and Lumix 8mm f/3.5

While the lenses have a slightly different focal length, they both cover an 180° diagonal field of view, just like a full frame fisheye lens should:



The Samyang costs about 60-70% less than the Lumix fisheye lens. Still, according to my tests, the Samyang lens has the best overall image quality. However, I also find that the Lumix lens generally gives the best colours.

The Samyang lens is manual focus, and has no electronic contact at all. Both the focus and the aperture must be set manually with rings on the lens, just like traditional lenses are designed. With a fisheye lens, I'm sure many would argue that you don't need autofocus. Just set the focus to about 1-2m, and virtually everything is in focus.

When photographing reasonably far objects, I agree with that. However, both fisheye lenses can focus very close, down to about 1 inch from the front of the lenses. This allows for some very interesting macro use, see an example here and here.

And when focusing down to about one feet distance and below, the autofocus of the Lumix G 8mm f/3.5 fisheye lens comes very handy, and can allow you to get more correctly focused shots, especially with moving subjects, like pets.

Both lenses of course give you quite distorted image. That is the nature of fisheye lenses. However, the Lumix lens gives you slightly more barrel distortion, and for that reason, the Samyang lens is probably easier to use if you plan to defish the images.




Premium lenses

Olympus has their line of "High Grade" prime lenses, in silver metal finish, and Panasonic has some Leica branded lenses. These make up the line of premium prime lenses:



Olympus 12mm f/2 and Olympus 17mm f/1.8



Lumix-Leica 25mm f/1.4 and Olympus 75mm f/1.8

The Olympus 12mm f/2 and 17mm f/1.8 are among the very few Micro Four Thirds lenses to feature a focus scale, and a depth of field scale. They also have an ingenuitive focus ring, which can be moved back and forth so choose between manual and automatic focus.

The Olympus 12mm f/2 borders on the ultra wide focal ranges, useful for, e.g., dramatic landscape photos.

The Olympus 17mm f/1.8 is their take on the traditional wide angle lens. The first reviews so far indicate some disappointment with the image quality, but then again, the expectations were probably very high.

The Leica branded Lumix 25mm f/1.4 corresponds to a traditional normal lens in terms of field of view, and with the f/1.4 aperture length, is the fastest native prime lens at the time of writing this. Contrary to the previously fastest M4/3 lens, the Lumix G 20mm f/1.7, the 25mm f/1.4 lens features internal focus, and focuses much faster.

The Olympus 75mm f/1.8 is a high end medium length tele lens, and can be used, e.g., for portrait photography.

Personally, I dislike the silver metal finish of the Olympus premium lenses, but I am sure many will like their retro look.






Portrait lenses

Traditionally, a portrait lens is a lens with a focal length of about 85mm (equivalent), and a fast aperture. This corresponds to about 42mm in on the Four Thirds sensor size.

Currently, there are two lenses that can be classified as portrait lenses, the Olympus 45mm f/1.8 and the Lumix 42.5mm f/1.2:



Olympus 45mm f/1.8 and Lumix 42.5mm f/1.2 (mockup)

The Olympus 45mm f/1.8 is a good and reasonably priced portrait lens. I would recommend it for anyone who is interested in portrait photography, or low light photography with a short tele lens.

The Lumix 42.5mm f/1.2 is not yet released. Only a mockup has been shown so far. It will probably be very expensive, and might be Leica branded. The lens is expected to feature OIS.



Sigma lenses

Sigma joined the Micro Four Thirds lineup with two prime lenses in 2012: The Sigma 19mm f/2.8 EX DN and the Sigma 30mm f/2.8 EX DN:



Sigma 19mm f/2.8 and 30mm f/2.8 (not to scale)

These lenses were designed by Sigma to be used on several mirrorless formats, including the Sony NEX series, Sony E-mount. The NEX cameras use an APS-C sized sensor, slightly larger than the Four Thirds sensor, with an 1.6x crop factor.

On this crop factor, the 19mm lens corresponds to a traditional wide angle lens, while on Micro Four Thirds it is not as interesting. It is very close to the Lumix G 20mm f/1.7. As the Lumix G 20mm lens is more compact, and brighter, I don't see the need for the Sigma 19mm lens. On the other hand, the Sigma 19mm lens focuses much quicker and more silently, and is cheaper. If this is important to you, it may be an interesting lens. See my review here, which compares the focus speed and image quality with the Lumix G 20mm f/1.7 lens.

The Sigma 30mm f/2.8 EX DN lens, on the other hand, is more interesting in my opinion. As I see it, it has quite good image quality, and can be used as a portrait lens. If you want good image quality on a budget, this might be a lens to consider.

These lenses are in fact already discontinued. Replacing them in 2013 are new versions, called Sigma 19mm f/2.8 DN and Sigma 30mm f/2.8 DN. They are part of Sigma's "Art" line of lenses. They have the same optical designs, but a different exterior. They are available in a silver metal finish, and a black metal finish. The focus ring is now made out of smooth metal:



Sigma 19mm f/2.8 DN (2013 version, "Art lens")

From an ergonomic point of view, this is a strange choice. It looks like a smooth metal focus ring will be less convenient to use. In practice, the ring rotates quite easily, so this is no real issue, though. It looks like the remake is all about a new styling, while retaining the original optical properties.

In 2013, Sigma also released their third lens for the Micro Four Thirds system, the Sigma 60mm f/2.8 DN. It corresponds to a 120mm tele prime lens in 35mm equivalent terms, and also gets the new metal finish.


Sigma 60mm f/2.8 DN




Macro lenses

Macro lenses are used to photograph small items, e.g., insects, flowers. For the Micro Four Thirds format, we have two options, the Lumix-Leica 45mm f/2.8 1:1 macro and the Olympus 60mm f/2.8 1:1 macro:



Lumix-Leica 45mm and Olympus 60mm macro lenses

Both lenses are rated as 1:1, meaning that they can photograph items as small as the sensor. The Four Thirds size sensor is 17.3mm x 13mm, hence, you can photograph very small items.

The Lumix 45mm f/2.8 macro lens has good image quality, but is expensive, and focuses a tad bit slow.

The Olympus 60mm f/2.8 macro lens has a longer focal length, giving you more working distance, the distance between the front of the lens and the object you are photographing. This is better for insect photography, in which case you need to keep some distance to avoid scaring the insects off. The Olympus lens also adds weather resistance, which is quite useful for outdoor use.

Generally, it could be wise to pair the lens off with the camera of the same brand. The Olympus lens does not have OIS, but Olympus cameras have built in image stabilization. With Panasonic, it is the other way around. So for these lenses, it makes most sense to buy the same brand as your camera.

Cosina Voigtländer Nokton lenses

These are quite similar in terms of appearance and functionality:



Cosina Voigtländer Nokton 17.5mm f/0.95 and 25mm f/0.95

The first has a focal length of 17.5mm, corresponding to a traditional wide angle lens. The second is more of a traditional normal lens, at 25mm.

Both have an ultra large maximum aperture, at f/0.95. And they are both completely mechanical constructions, with manual focus and aperture rings. They must be focused manually, and you must set the aperture manually.

Both are also very expensive.

The lenses can be useful when you want to photograph or video record in very low light situations, e.g., concerts, or when you want a very selective focus.

Specialty lenses

The Olympus 15mm f/8 lens cap and Lumix 12.5mm f/12 3D lens are rather unusual:



Olympus 15mm f/8 lens cap and Lumix 12.5mm f/12 3D lens

Olympus have released an inexpensive, very small lens. The Olympus 15mm f/8 is a wide angle lens, and it has a fixed aperture of f/8. It has no electronic contacts, meaning that no information is transferred between the lens and the camera. If you are using an Olympus camera, you need to manually set the focal length to 15mm for the In Body Image Stabilization (IBIS) to function.

There is no autofocus, however, with a small aperture of f/8, the depth of focus is quite large anyway. There is a possibility to change between "far" and "near" focus, which is enough for this aperture.

This is more of a novelty than a useful lens, in my opinion. It can only be used outdoors, or indoors with a flash at a short range.

The Lumix 12.5mm f/12 3D lens is also a bit of a novelty lens. With some of the more recent Panasonic cameras, it can be used to take 3D images. However, the lens has a very short 3D base, a short distance between the two sub lenses. Hence, the 3D effect is quite minimal.

I think it is rather useless as a 3D lens. If you are interested in 3D photography, it is better to get one of the dedicated 3D compact cameras, with two separate lenses and sensors at a more sensible distance.

At 12.5mm, the 3D lens goes into the wide angle section. However, it is not a wide angle lens, since there is an additional crop factor as the two lenses project smaller image circles inside the sensor. In reality, it is a short tele lens.

Not in the picture

Panasonic is planning to release a high end long tele prime lens, at 150mm f/2.8. This lens will probably have OIS, and could be available in 2014. This type of lens would typically be used by people who photograph wildlife, safari, birds, etc.

Compared with zoom lenses

Zoom lenses are of course fundamentally different from prime lenses, in that they do not have a constant focal length. Often, they also have a changing maximum aperture over the focal length range.

I have drawn up three zoom lenses in the diagram, the Lumix G 14-42mm f/3.5-5.6, Lumix X 14-42mm f/3.5-5.6, and the Olympus 12-50mm f/3.5-6.3:


As you can see, the zoom lenses often have a smaller maximum aperture than the prime lenses. This is one of the reason why some people like prime lenses.

Also, you can note that the Lumix G 14-42mm and the more compact Lumix X 14-42mm share the same end-points in terms of aperture range, but the more compact lens has a smaller available aperture inside the range. I have written about this phenomenon here.

Conclusion

Starting from only one single lens from both manufacturers Olympus and Panasonic, we now have an impressive array of native Micro Four Thirds prime lenses.

My favourites are the Lumix G 14mm f/2.5, for the good image quality, very small size, and fast, inaudible autofocus operation. And the Samyang 7.5mm f/3.5 fisheye lens, for the good image quality, and for providing a fisheye lens available at a very reasonable cost.

I also like the Olympus 45mm f/1.8, since it is a good portrait lens at a reasonable price.

See also

Lens buyers guide for Micro Four Thirds.

Micro Four Thirds lens lineup.

Sunday, 11 November 2012

IR filters

When taking pictures, we normally want the resulting image to accurately depict the reality. However, there are times when we rather want to depict a scene in a way that is not consistent with the reality as we see it.

This may sound strange. As the camera industry has spent an enormous effort to realistically depict the scene, why would we want to distort it?

However, keep in mind that one of the current crazes within popular photography is instagram, hipstamatic, and so on, which is all about distorting the images, and making them look unnatural.

Another way in which photos can be distorted is selective focus. The combination of a large sensor and a large aperture can give you an out of focus background when taking a portrait photo, for example. The rendering of out of focus regions is called bokeh, and the effect can be much more pronounced than what you would see with your own eye. Hence, one could argue that this is a distortion of the reality.

Another type of distortion is of course to use a lens with an unrealistic projection, for example a fisheye lens.

In this article, I am looking at another type of distortion: To use an IR filter to keep off the visible light, and hence, to photograph light outside of the visible spectrum.

A number of different types of IR filters are available. Usually, they are categorized by the cut-off frequency. Visible light roughly goes from 340nm to 740nm, with ultra violet (UV) to the left of the visible spectrum, and infrared (IR) to the right, see the illustration below.


The idea of the IR filter is to remove light which is to the left of the cut-off frequency, only leaving the infrared parts of the spectrum.

I tried two filters, both where bought online at a low price:


The filter to the left is labelled with "IR950", meaning that it cuts off all light to the left of 950nm. Hence, it blocks all visible light completely, and even some of the infrared light. The filter to the right is less extreme, labelled "IR680": It retains some visible red light, but cuts off the larger part of the visible spectrum. I start with the one rated the lowest:

IR680

This filter looks completely black, however, when holding it up towards the sun, you can see that it lets some red light through:


To see how it affects the images, I have taken the same image without and with the filter. Here they are:

Without filter, GH2 with Lumix 20mm f/1.7, ISO 160, f/5, 1/160s:


With IR680 filter, GH2 with Lumix 20mm f/1.7, ISO 160, f/2, 1/2.5s:


Based on the exposure settings, we see that the IR680 filter steals about eight stops of exposure, which is a lot. Normally, you need to use a tripod and a slow shutter speed when using an IR filter.

So how is this interesting? It is just a very red tinted photography. However, when opening the RAW image, one can adjust the white balance, which gives a result like this, leaving some false colours:


Another possibility is to desaturate the images, to create black/white photos. Here are the two images desaturated:



Without filter With IR680

The desaturated images are not that different.

Here is another example image, taken with the IR680 filter, and no desaturation in post processing:


IR950

This filter looks even blacker than the IR680 filter. When holding it towards the sun, it still looks completely black. I tried it on the GF3, GH1 and GH2 cameras, and they could all just see black when using the filter. So my conclusion is that this filter is too extreme to be used with most digital cameras.

Different sensors can have different types of filters in front of the photosites, and hence, react differently to infrared light. That is why I tested three cameras before giving up. It is well known that some cameras are better than others for infrared photography.

I also tested two different lenses, as I have heard that the coating on some lenses can block infrared light.

IR850

I also tried a third IR filter, with a cut-off frequency of 850nm, not pictured above. This filter looks completely black to the human eye, but the camera can see a little bit through it.

I tested it with the Lumix G 14mm f/2.5 lens. Here are pictures of the same scene without and with the filter:



Without filter, ISO 160, 1/4000s, f/2.5 With IR850, ISO 1250, 1/3.2s, f/2.5

The IR850 filter appears to remove too much light for the Panasonic GH2 camera sensor, and is not very useful.

Note that there is a lot of flare in the image taken with the filter. This is not due to the infrared light, but rather due to the strong light source in the centre of the image, and the poor quality of the filter glass. I have written about this effect of using filters here.

Conclusion

I think the IR680 filter lets through too much visible light, and hence, it doesn't create that special effects. On the other hand, the IR950 filter is too extreme. It removes all the light that the camera can see, and hence, is useless.

I would guess that something in between would give more interesting results. Perhaps a filter with a cut-off at around 720nm would be better. Such a filter removes virtually all visible light, and allows you to photograph a band of infrared light, given that the camera sensor is sensitive enough in this spectral range.