How Phase Detection Autofocus Works

When it comes to DSLR technology, there seems to be quite a bit of confusion on how exactly phase detection autofocus works. While for most people this might not be a topic of great interest, if you are wondering how and why a camera could have an autofocus problem, this article will shed some light into what happens inside the camera in terms of autofocus when a picture is taken. There is an overwhelming amount of negative feedback on autofocus issues on such fine tools as the Canon 5D Mark III, Nikon D800, Pentax K-5 and other digital SLR cameras and it seems like most photographers do not seem to understand that the underlying problem is not necessarily with a specific model or type of a camera, but rather with the specific way these cameras acquire focus. If you search on the Internet, you will find thousands of autofocus reports on all kinds of DSLRs dating back 10+ years. Hence, the front focus and back focus issues we see in modern cameras are not anything new – they have been there ever since the first DSLR with a phase detect sensor was created.

How DSLR Cameras Work

To understand this issue in more detail, it is important to get to know how a DSLR camera works first. The typical DSLR illustrations only show a single reflex mirror positioned at a 45 degree angle. What they don’t show, is that there is a secondary mirror behind the reflex mirror that reflects a portion of light into a phase detect sensor. Take a look at the below simplified illustration that I made from a sample Nikon D800 image:

How Phase Detection Autofocus Works

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How to Quickly Test Your DSLR for Autofocus Issues

Below is the easiest and quickest way to test if your DSLR has an autofocus issue, along with a recommendation on what to do if there is a problem. This test can be used to detect front focus or back focus issues with a particular lens or a camera body. I will be using the Nikon D800E as a reference camera for this article, but any modern DSLR with Live View capability can be used for the same test (even entry-level DSLRs such as the Nikon D3200 have a Live View mode). Why would you want to test your camera for autofocus issues? Because if your camera or your lenses are defective or have a calibration problem, then you will not be able to obtain critically sharp images.

1) What You Will Need

For this test, you will need the following:

  1. Any DSLR with Live View mode capability such as the Nikon D7000.
  2. At least one lens, but preferably 2-3 lenses if you want to isolate the problem to the camera or your lenses.
  3. A good stable tripod.
  4. A flat vertical surface in a very brightly lit area. For example, your garage door or a wall inside your home that is adjacent to a very large window will do fine.
  5. Print out either this Siemens Star Focus Chart or this Focus Test Chart on regular letter size paper. You can print it on a laser printer or inkjet (doesn’t really matter). Make sure to print on regular paper, not anything glossy like photo paper.
  6. Scotch tape or some other adhesive material to keep the focus chart on the wall.

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Novoflex Nikon to Canon Lens Adapter Review

This is a review of the Novoflex Nikon to Canon Lens Adapter, also known as “Novoflex EOS/NIK-NT Lens Adapter”. This lens adapter is designed to be used specifically with Nikon G lenses that have no aperture rings. While most generic lens adapters can be easily used with older non-G Nikon lenses and you can easily control aperture by just rotating the aperture ring on the lens, there is no way to control aperture on all modern “G” type lenses with such an adapter. So if you used a generic lens adapter, you would be limited to shooting at minimum aperture of the lens (default) and there would be no physical way to adjust it while the lens is attached to the camera. To allow manual change of aperture on these types of lenses, Novoflex specifically designed an adapter with an aperture lever. In this review, I will talk about the pros and cons of using the Novoflex adapter and my overall experience with it when mounting Nikon lenses on Canon DSLRs.

Novoflex Nikon to Canon Lens Adapter

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Use Nikon Lenses on Canon DSLRs

While testing some Canon EF lenses on the Canon 5D Mark III, I decided to try to compare the lenses to the latest Nikon lenses and see how they perform side by side. First, my plan was to mount Nikon lenses on the D800 and Canon lenses on the 5D Mark III and look at images at 100% view, but then I realized that it would be tough to do a fair comparison, since the cameras are different. That’s when I thought about using Nikon lenses on a Canon DSLR with an adapter. I knew that it was possible, since some people love mounting the Nikon 14-24mm f/2.8G on Canon cameras. In this article, I will share my thoughts and experience on using Nikon lenses on Canon DSLRs.

Nikon 50mm f/1.4 with Novoflex Canon Adapter

1) Can Nikon lenses be mounted on Canon DSLRs?

As I have said above, yes, you can mount all Nikon F mount lenses (even the latest “G” type lenses without an aperture ring) on any Canon DSLRs – you will need a Nikon to Canon lens adapter to do that. There are plenty of options available from different brands. Generic adapters can be bought for less than $20, but those will only work with older Nikkor lenses with aperture rings. For “G” type lenses, you will need specialized adapters that could cost up to $300 USD.

2) Can Canon lenses be mounted on Nikon DSLRs?

No, Canon lenses cannot be mounted on Nikon DSLRs. Technically it is possible to design an adapter to do it, but you will not be able to focus to infinity. This is due to the fact that Nikon DSLRs have a longer distance between the lens flange and the sensor (focal plane), which would make Canon lenses behave almost like extension tubes. Nikon has a flange focal distance of 46.5mm, while Canon’s EF mount is 44mm as can be seen in this chart. So while a 2.5mm thick adapter could be used on Canon DSLRs, it would be impossible to go in reverse direction on Nikon DSLRs.

3) Why Do It?

So, why mount a Nikon lens on a Canon DSLR? Normally, you would not want to. Nikon lenses are designed to be used with Nikon DSLRs and Canon lenses are also specifically designed to be used with Canon DSLRs. At times, however, there might be a need to do it. Here are some reasons I could think of:

  1. You shoot with both Nikon and Canon DSLRs and you have some good Nikon lenses that you want to be able to use on your Canon DSLR. You do not feel like buying a similar lens from Canon, so buying an adapter would be a cheaper alternative.
  2. You shoot videos on a Canon DSLR and you want to be able to change lens aperture silently using an adapter, rather than rotating the lens aperture ring or the dial on the camera.
  3. You really love the Nikon 14-24mm f/2.8G and want to use it on your 5D Mark III.
  4. You converted from Nikon to Canon and you still have some classic Nikkor lenses that you do not want to part with. Using them with an adapter on a Canon DSLR sounds like a good option.
  5. You just want to do it for fun!

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Introduction to Infrared Photography

Infrared, or “IR” photography, offers photographers of all abilities and budgets the opportunity to explore a new world – the world of the unseen. Why “unseen”? Because our eyes literally cannot see IR light, as it lies just beyond what is classified as the “visible” spectrum – that which human eyesight can detect. When we take photographs using infrared-equipped film or cameras, we are exposed to the world that can often look very different from that we are accustomed to seeing. Colors, textures, leaves and plants, human skin, and all other manner of objects can reflect IR light in unique and interesting ways, ones that cannot be mimicked with tools such as Photoshop (yes – there are limits to what Photoshop can do!). Like any form of photography or art however, it is a matter of taste. I would strongly urge people to explore the world of IR. As the number of cameras-equipped devices proliferates and the associated technologies improve, IR photography may offer the opportunity for photographers to expand into new arenas and differentiate their offerings from those of others.

Barboursville Vineyards

1) Terminology

For purposes of this article, I will refer to the infrared light spectrum as “near infrared”, or simply, “IR”. Near infrared refers to the spectrum of light just beyond the range humans can detect with their eyesight. This light range is between 700 – 1200 nm (nanometers). Another aspect of the IR spectrum, above near IR, is associated with thermal imaging. Thermal technology was popularized by movies such as, “Patriot Games” and other thrillers, whereby intelligence agencies or military personnel were able to detect villains by measuring their body heat under nighttime conditions. Today’s common digital camera sensors are not able to detect thermal images. Under the right circumstances however, digital cameras can do an excellent job of recording IR.

2) History Of Infrared Photography

The first forays into IR photography, using special film plates, began in the early part of the 20th century. During WWI, IR photography proved extremely valuable, as images using the IR spectrum were not affected as much by atmospheric haze as normal photos. IR images were also able to show stark distinctions between vegetation and buildings, better identifying potential enemy targets such as camouflaged munitions factories and other key sites. Rivers, streams, lakes, and other waterways were depicted in a very dark hue, making them much more obvious.

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Taekwondo Photography Tips

I had an opportunity to photograph a local Taekwondo sparring event last weekend and I decided to share some of the photographs from the event, along with some photography tips and lessons learned. I have been involved in Taekwondo since I was 12 and while I spent many years taking part in this beautiful and highly energetic (and sometimes even brutal) sport, I never had a chance to photograph it. While I have been suffering from pneumonia during the last 2 weeks, I could not skip a Taekwondo sparring with some of the best athletes in Colorado. I got my daily doze of antibiotics, then quickly made a plan and took off.

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How to Avoid Moiré

In this quick article, I will talk about how to avoid moiré if your camera is not equipped with a low-pass / anti-aliasing filter, or if it has a special low-pass filter like the Nikon D800E that is also prone to moire. Moiré can be quite painful to deal with in post-processing, so it is best to avoid it in first place. Below you will find a list of steps you can take to avoid moiré while shooting patterns.

  1. Analyze patterns in your images on your camera’s LCD at 100% view – while looking at your images on the camera LCD at 100% view can be very inefficient and time-consuming (especially on the Nikon D800E with its massive 36.3 megapixel images), if you are shooting anything with repeating patterns, you need to know whether there is moiré in your images or not. Looking at the thumbnail on the LCD might not reveal moiré, so you will have to zoom in to see it. Very strong / nasty moiré might be visible even at 50% view or less, while you will only spot mild moiré at 100% pixel level view. If you see moiré and want to avoid it, proceed to step #2 below.
    Quick useful tip: if you own an advanced Nikon DSLR like Nikon D300s or higher, you can set the multi-selector center button on the back of the camera to instantly show 100% view without having to press the zoom button several times. You have to enable this feature here: Custom Settings Menu->Controls->Multi selector center button->Playback mode->Zoom on/off->High magnification. Once set, you will be able to zoom in to your images at 100% by simply pressing this button in playback mode.
  2. Change camera to subject distance or adjust focal length – if moiré is visible in your image, the best thing you can do is change the distance to your subject. You can either physically move closer or away from your subject, or you can zoom in/out with your lens. Remember, moiré only happens when the pattern you are photographing exceeds sensor resolution, so all you have to do is move to a safer distance. Sometimes this means moving just inches away from your subject.
  3. Adjust focus to a different area – while this is not always practical, adjusting the focus a little away from the patterns will remove moiré.
  4. Change the angle of the camera – simply changing the angle of the camera a little can completely eliminate even very strong moiré patterns.
  5. Stop down the lens to f/11-f/16 – when lenses are stopped down beyond a certain aperture (depending on the lens and the sensor size), an optical phenomenon known as “diffraction” kicks in. Diffraction effectively reduces resolution, which also eliminates moiré. While I would personally avoid doing this, if you cannot change your subject to camera distance or adjust your focal length for whatever reason, this technique surely works.

If you did not do any of the above and ended up with an image that has visible moiré, then your only option is to try to fix it in post-processing.

Why sensor dust is more visible at small apertures

Another reader of ours, Frank Di Luzio, sent the below image that explains exactly why sensor dust is more visible at small apertures. While I have explained this phenomenon to some of our readers before (see the comment section), I have not had a chance to write a separate article with a proper illustration, demonstrating how aperture size affects the shape and size of dust particles. Thanks to our generous readers like Frank, I now do not have to do it, because the below illustration is perfect.

Dust on Sensor

In summary, when the size of aperture is large (a small F-number like f/2.8), light rays reach dust particles that are sitting on the sensor filter from different angles. Remember, although I refer to this as “sensor dust”, dust actually never touches the sensor, because there is a thick filter (actually, more like a number of filters packed together to form a single filter) that sits in front of the camera sensor. Therefore, by the time light reaches the physical sensor, it is spread out on a very large area, making dust appear as a large blob with a soft ring. When using very large apertures like f/1.4 on fast prime lenses, these blobs might be so washed out that they might be practically invisible to your eye. That’s why portrait photographers notice dust less often than landscape photographers!

Now when the lens is stopped down and aperture is significantly smaller, say at f/16, light rays coming from the lens diaphragm are perpendicular to the sensor filter. Because the angle is more or less straight, dust specks also cast direct and defined shadows on the sensor. That’s why dust shows up in images much smaller, darker and with more defined edges at small apertures.

Big thanks to Frank for sending the illustration!

Benefits of a High Resolution Sensor

As camera manufacturers are continuing the megapixel race, with Sony releasing a bunch of 24 MP APS-C (1.5 crop-factor) cameras like Sony A77, A65 and NEX-7, and Nikon releasing a high resolution 36 MP Nikon D800, many of us photographers question the need for such a high resolution sensor. Some of us are happy while others are angry about these latest trends. Just when we thought companies like Nikon abandoned the megapixel race, instead of seeing other companies do the same, we now see Nikon back in the game with a new breed of product with a boatload of pixels. Why did Nikon all of a sudden decide to flip the game? Why does everyone seem to be going for more pixels rather than better low-light / high ISO performance? Does a high resolution sensor make sense? What are the true benefits of a high resolution sensor? In this article, I will provide my thoughts on what I think has happened with Nikon’s camera strategy, along with a few points on benefits of a high resolution sensor.

Nikon D4 Sensor

Pixel Size, Pixel Density, Sensor Size and Image Processing Pipeline

OK, this topic is rather complex if you do not know anything about pixels and sensors. Before you read any further, I highly recommend to read my “FX vs DX” article, where I specifically talk about pixel and sensor sizes and their impact on image quality.

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What is Moiré?

Moiré pattern occurs when a scene or an object that is being photographed contains repetitive details (such as lines, dots, etc) that exceed the sensor resolution. As a result, the camera produces a strange-looking wavy pattern as seen below:

Moiré

(Image courtesy of photo.net)

See how noticeable the moiré pattern is on the jacket? That’s moiré for you, at its worst. Moiré is almost never seen in nature, but is very common in everyday objects and items around us – you might see it in all kinds of fabric, straight hair, architecture, etc. You might have even seen it on your television. In photography, moiré happens mostly because of the way light reaches the sensor and how the sensor interprets the light through the bayer interpolation filter.

While there are methods to effectively reduce moiré, there is no easy way to completely remove it in post-processing software. Lightroom 4 will ship with a moiré reduction tool and Nikon will also ship its next version of Capture NX with built-in moiré reduction functionality, but neither one will be able to fully get rid of the worst moiré pattern occurrences.

Here is a comparison between the Nikon D800 and D800E (the latter is prone to moire), which clearly shows Moiré on the Nikon D800E (Image courtesy of Nikon):
Nikon D800 vs D800E Moire

See “How to Avoid Moiré