Sunday, 27 February 2011

Using protective filters on lenses

A topic of much debate is whether or not to use clear glass filters on the front of lenses, for protection. Some would argue that putting on a clear filter protects the lens against objects touching it, and against water, dust, and so on. If something hits the front of the lens, they would say, it breaks the filter and not the lens itself.

Others argue that the front lens element typically is strong, and does not need extra protection. Some would even say that adding a filter may be negative, that it breaks more easily, and that the glass shards could add more to the damage of the front lens element.

Another argument is that an extra glass filter deteriorates the optical properties of the lens.

Now, I wouldn't want to drop my lens on the pavement to examine if using a filter helps protect it or not. Besides, as all lens accidents are different, one drop would not prove much anyway.

But to look at the resulting images when using a filter, as opposed to not using a filter, is possible. To examine the effect of using a clear glass protective filter, I took the same picture with and without a filter.

Example @ 200mm

Here is an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 200mm f/5.6.



without filter
with filter

This is an image with fairly low contrast, and on first glance, the two images are pretty similar. Let's look at an enlargement:



So is there any difference? Not really, I would say. Perhaps one could say that there is a tad bit more contrast in the left image, without the filter.

Example @ 14mm

Another example taken using the Panasonic GH2 with the Lumix G 14mm f/2.5 lens at f/5.6.



without filter
with filter

In this case, we barely need to look at an enlargement to see the differences. Clearly, there is more flare when using a filter. The right image also has some odd "phantom lights" in the frame, from light reflections between the filter and the lens. Here is one example in the centre of the frame:



Beyond the flare, it's hard to argue that the right image is worse in terms of sharpness, for example.

Example @ 45mm

Finally an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 45mm f/4.



without filter
with filter

In this case, it is very clear that the presence of the filter deteriorates the image. Not only is there much more flare, there is also a severe reduction in the contrast and sharpness across the whole frame.

Here is an enlargement which demonstrates this:



Conclusion

In low contrast situations, it is hard to find much evidence of negative impact from using a filter. However, when there is more contrast, e.g., a strong light source in front of the camera, the presence of a filter generates significantly more flare, and potentially also less contrast.

People who are proponents of using clear protective filters may still not agree with me. They could argue that if I had used higher quality multi coated filters, I would not have gotten these negative impacts. And perhaps they are right.

Since I don't believe in using filters for protection myself, I haven't invested in the most expensive filters. So it could be that higher quality filters would have reduced the deterioration.

Still, I believe it is fair to say that adding a filter will generate some deterioration of the image quality. But if using a protective filter gives you more peace of mind when using your expensive lens, perhaps that is an acceptable cost to you.

Personally, I prefer using hoods for basic protection of the front lens element.

Appendix

The filters used were: "Kenko Digital Filter UV" (52mm) for the Lumix G 45-200mm f/4-5.6 lens, and "Green Digital Filter UV" (46mm) for the Lumix G 14mm f/2.5 lens. Both filters are littered with terms like "High Quality" and "Premium Quality", but I would take that with a grain of salt.

Sunday, 20 February 2011

Bokeh comparison @ 14mm

The Lumix G 14mm f/2.5 pancake and Lumix G 14-42mm f/3.5-5.6 are both rather new lenses. What they also have in common, is that they have received a mixed reception online.

The pancake lens has some disappointed over lack of sharpness, and not being as fast as the Lumix G 20mm f/1.7. While the comment about not being as fast is correct, I have found the 14mm to be very good optically.

The new kit zoom lens is said to be worse than the lens it replaces, the Lumix G 14-45mm f/3.5-5.6. While it is probably true that the old kit lens was better, I have found the new one to be good, too.

In this article, I have taken a look at how the lenses render out of focus parts of the image, the bokeh. I did this by taking the same image with both lenses. The lenses were focused on the checkered pattern in the lower right part of the image, which is about 30cm from the camera. (click for larger images):



Lumix G 14mm @ f/2.5
Lumix G 14-42mm @ 14mm f/3.5

The pictures were taken with ISO160, base ISO for the Panasonic GH2.

To better evaluate the images, here are 100% crops from the left, centre and top parts of the images, respectively. The images were taken with apertures ranging from f/2.5 to f/8:




(Click for larger images.)

Due to diffraction, one should avoid using apertures smaller than f/8, i.e., avoid larger aperture numbers. This is especially true if you are going to be studying enlargements from the images. If you plan on publishing the images in web size only, go ahead and use apertures as small as f/22. The diffraction will not be a problem if you are going to downscale the images that much.

Conclusion

I think this test does not reveal any bokeh problems with any of the lenses. Perhaps the test could have been made more challenging by having some strong highlights in the background.

The patterned textile in the first compilation image is in focus. We can see that it very sharp already from the smallest aperture. This appears to verify my initial claim that both lenses are in fact quite good, despite their rather bland reception.

The Lumix G 14mm f/2.5 pancake lens appears to be sharpest. This was to be expected, I think, since it is not a zoom, and hence has fewer optical compromises. Further, it also lacks an OIS lens group, and has a simpler construction with only six lens elements. The 14-42mm zoom lens has twice as many lens elements. Generally, more lens elements can lead to worse optical performance, for example reduced contrast. However, modern quality zoom lenses can retain superior optical performance with 15-20 lens elements.

Friday, 18 February 2011

TTL flash metering and flash delay

Flash metering has come a long way the recent decades. TTL flash metering for SLR cameras was first introduced by Olympus in the mid 1970's. TTL refers to Through The Lens. The camera measures the amount of light coming onto the film through the lens during the exposure, and cuts off the flash as the exposure is sufficient.

Film based SLR cameras

For film based SLR cameras, this is usually implemented by having a flash light meter in front of the film plane. The amount of light reflected off the film from the flash is metered, and the flash is turned off when there has been a sufficient amount of light for the desired exposure. See the illustration below.


Film based SLR camera with lens

In this illustration, the mirror is raised for exposing the film.

This generally worked well, at least as long as the subject was not too dark or too light, in which case you needed to manually adjust the flash exposure.

Digital SLR cameras (DSLR)

With digital cameras, this does not work well, since the imaging sensor, replacing the film, is not reflective enough. To overcome this problem, most DSLR cameras fire a pre-flash before raising the mirror, and then fire the main flash after exposing the sensor.

The pre-flash is used to determine the amount of flash needed for the exposure. With this method, the TTL flash meter is no longer needed, the camera's ordinary light meter is used. See the illustration.


DLR camera with lens

There are some DSLRs that still measure the amount of light reflected off the sensor chip, and avoid the pre-flash. The Fujifilm S1 and S3 does this.

Mirrorless cameras

As you know, Micro Four Thirds is a mirrorless camera system. The camera has no light sensor anymore. The imaging sensor is the light sensor. So to find the correct flash exposure, a pre-flash is triggered while the sensor is exposed. Then the camera must make the sensor ready for a second exposure, and fire off the flash with the correct amount of light. This typically takes a bit more time than with a DSLR. The DSLR used the separate light meter for the pre-flash, and could expose the main imaging sensor only once.

Here's a basic illustration of a mirrorless camera with lens. It is much simpler, since there is no mirror, pentaprism, or light meter.


Mirrorless camera with lens

Flash and pre-flash timings

To examine the pre-flash and main flash timings, I have video recorded operating the Panasonic Lumix GH1 and GH2 cameras. I also measured the Pentax K10D, which is an older DSLR from 2006. I turned off autofocus, to measure the flash delay only, and not also the autofocus delay.

I used 50fps when recording, which gives an accuracy of approximately 0.02s.

GH1GH2K10D
Time to pre-flash0.16s0.12s0.08s
Time to main-flash0.12s0.14s0.12s
Total flash delay0.28s0.26s0.20s

The first timing is the delay from pressing the shutter until the pre-flash is fired. The second is the delay from the pre-flash until the main-flash. The third figure is the sum of the two first: The total delay from pressing the shutter until the main flash is fired.

You can see the recordings here. I uploaded them as 25p videos, so they are not as good for verifying the actual timings.



Conclusion

One could say that the GH2 improves slightly on the GH1 in terms of flash delay. However, the difference, 0.02s is not significant with my way of measuring. So we can only say that they are comparable.

When comparing with the older Pentax K10D, we see that the GH1 and GH2 perform almost as good. The difference between 0.20s and 0.26s is not very large. Probably, the autofocus speed is more important to the average user, and we have seen that both the GH1 and GH2 perform very well in terms of autofocus.

One way to avoid the TTL pre-flash, is to use a flash in Auto mode.

Now, we should not conclude that the extra flash delay with Micro Four Thirds cameras is exclusively a bad thing.  As opposed to DSLRs, which have a limited number of light metering sensors, the Micro Four Thirds cameras essentially take one full picture to determine the correct exposure.  This means that the camera has at least 12 megapixels of information available.  It probably doesn't use all of this information.  But what it can potentially do, is to use the information about where in the frame faces are, to enhance the exposure.  Also, the camera knows which areas are in focus, and can make sure that these areas are properly exposed.  The extra information the camera has can be put to good use to give you a better exposure.

Sunday, 13 February 2011

Self portrait with a fisheye lens

Most of what I write in this blog is non-personal. While the technical analysis is generally based on fairly objective observations, it is supplemented with subjective comments. But it's still far from personal.

With this post, I take a turn towards the more personal side with a self portrait:


It was taken with the Lumix G 8mm f/3.5 fisheye lens. It has an amazingly short minimum focus distance of 0.1 meter. This is measured from the sensor, which means that the distance from the front lens element is around 2cm, or approximately one inch. The field of view is still very wide, so it cannot be called a macro lens. Here is one example where I used the lens to photograph a LEGO figure up close.

At this distance from my face, the perspective gets very distorted. You can see this in the self portrait, since my nose and eye looks enormous compared to the rest of the face. Of course, the fisheye lens is far from what you would call a portrait lens.

To get as much as possible in focus, I set the aperture to f/9. There was still enough light to get a shutter speed of 1/20 second at ISO320.

Composing the image was fairly easy. I could just flip out the LCD screen on the Panasonic Lumix GH2 and see how the resulting image would be, while holding the lens towards my eye. I used autofocus, which is very fast with this lens, even at this close distance.

Tuesday, 8 February 2011

Example video capture, GH2+Lumix-Leica 45mm f/2.8 macro

Here is an example video capture using the Panasonic Lumix GH2 camera with the Panasonic Leica Lumix DG Macro-Elmarit 45mm f/2.8 1:1 macro lens.

The camera was handheld, and I left the aperture at f/2.8, shutter speed around 1/100 second. I used the 720p mode, which, since I have the European camera model, gives me 50fps. It was downmixed to 25fps using Openshot video editing software.



I left the autofocus on, using the face detection focus mode. Initially, the camera chose to focus on the toy men on the train station platform, which suited me fine.

As the train enters the frame, the focus starts wandering off, which doesn't really help in this case. However, the focus returns to the toy figures afterwards, which is desirable. In this case, it would probably have been better to turn off autofocus during the video capture, since it doesn't help much at all.

Other lenses, like the Lumix G 14-42mm f/3.5-5.6 would probably have focused faster. However, with a maximum aperture of f/5.6 at 42mm, it would not have given the nice blurred effect of items out of focus. On the other hand, a large part of the frame is in fact out of focus in this video. So perhaps f/5.6, giving more of the subject in focus, would have been a better choice here anyway.

Saturday, 5 February 2011

Chromatic Aberration and lens correction

Chromatic Aberration (CA) is a type of lens distortion. It is caused by light of different colours being refracted differently by the glass lens elements.

This is one reason why lenses often consist of pairs of lenses grouped together: The two lens elements in the pair are made of different glass types, and have different optical properties. The aim of the construction is to neutralize the effect of chromatic aberrations, so that all visible colours focus in the same place.


Here is an example image illustrating CA. The image was taken with a Panasonic GH1, using the Olympus Zuiko 9-18mm f/4-5.6 wide angle zoom lens at 9mm f/4:


An enlargement of the lower left corner reveals the CA artifacts:


You'll see that there is red and green fringing off the high contrast areas, where the white and black paint meet. These artifacts are typically seen in the corner of the frame, while the centre of the image is generally free from them.

In the extreme corner, the artifacts take up about 2-3 pixels, which is a moderate effect.

CA artifacts are generally found in the corner, especially when using wide angle lenses.  The lack of CA artifacts indicate a high quality lens design.

Automatic lens correction

Panasonic Lumix G lenses are automatically corrected for CA artifacts by the camera. In the JPEG output files, the camera has adjusted the images with the intention to remove these distortions.

When using Panasonic Micro Four Thirds lenses on Olympus cameras, the CA corrections are not done with current camera models. Future cameras from Olympus may employ the same technology as Panasonic uses, and adjust for these effects.

We can still see the original exposures by opening the RAW image files in a converter which allows for not implementing the CA adjustments. One such RAW converter is UFRAW. By using this converter, we can compare the original exposure with the out of camera JPEG output, to see what kind of adjustments were done.

I am fully aware that there are RAW converters which will do the CA corrections as well.  So this is not a test of the RAW converter, but rather a look at the image before the CA correction, to see what kind of CA artifacts the lenses generate.

I have done these comparisons for three lenses. The images were taken with the Panasonic GH1 at ISO 100:

Lumix G 8mm f/3.5 fisheye

Here is the full image, take with a wide open aperture at f/3.5:


And the upper right corner at f/3.5 and f/5.6:


In this example, we can see that the fisheye lens is very sharp in the corner, even wide open at f/3.5.

In the extreme corner, there is some red and green fringing where white meets black.  Perhaps around 2-3 pixels of colour artifacts.  They are corrected well in the JPEG output, though.

Lumix G 14mm f/2.5 pancake

Here is the full image, take with a wide open aperture at f/2.5:


And the lower right corner at f/2.5 and f/5.6:


We see here that when using the lens wide open, there is some vignetting in the corner, and also, the sharpness is not optimal. This is quite common for any lens, really, and I wouldn't say it is a problem.

Looking at the CAs, it looks like there is about 1-2 pixels of red and green fringing in high contrast areas in the original RAW image. In the adjusted JPEG image, there is still some purple fringing.

Lumix G 20mm f/1.7 pancake

Here is the full image, wide open at f/1.7:


And the lower right corner at f/1.7 and f/5.6:


My comments here are mostly the same as for the Lumix G 14mm lens: There is vignetting, and there is dullness in the corner at f/1.7. Again, this is not uncommon, and especially so for a low light lens with a large aperture.

Before conversion to JPEG, there is some small amount of green and red fringing. In the converted JPEG image, the green fringing is gone, but there is still some purple fringing.

Olympus Zuiko Digital 50mm f/2 1:2 macro (Four Thirds lens)

This lens is rather well known for it's CA artifacts. Here is a video showing the lens being used on a Panasonic GH1 camera (with an adapter). When focusing manually close to the minimum focus distance, you can see clearly black text on white background is black only when in perfect focus. Focusing a bit longer gives a green outline. And focusing closer gives a red outline. Doubleclick on the video to get a larger view, up to 720p is possible.



Here is a 100% view of a photo taken using the Olympus 50mm f/2 macro at 1:2 magnification, and f/2 aperture. As you see, text which is beyond the focus point has green fringing, while text nearer has red fringing.




The Panasonic-Leica 45mm f/2.8 macro lens for Micro Four Thirds does not exhibit these kinds of artifacts.

Conclusions

When using Panasonic lenses on Panasonic cameras, some CA artifacts are corrected automatically. However, there are still some purple fringing left in the corners.  It is generally restricted to around one pixel width, which is not much.

Even before the CA correction, the CA artifacts are very moderate on the Lumix G 20mm f/1.7 and the Lumix G 14mm f/2.5 pancake lenses. The Lumix G 8mm f/3.5 fisheye lens has somewhat more CA effects in the extreme corner, but it is well corrected by software.

All in all, I think that CA artifacts are nothing to worry about with these lenses. Even when using the lenses on Olympus cameras, without in camera CA correction, this should not bother you much.