How to Avoid Moiré

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 Downsampling an Image Reduces Noise

Nikon D4 Sensor

One of our readers, Mike Baker, sent the below email to me today. I thought it was a great and interesting analysis of why downsampling an an image reduces noise, so I decided to share it with you (with his permission, of course). Trying to digest this stuff makes my head spin, but it is a great read. You might need to read it several times to understand what he means, especially with all the mathematical formulas (I had to):

You recently commented about downsizing a high-resolution image to a lower-resolution in order to reduce the apparent noise. While I knew that this is an effective way to reduce noise visible in the images, I had not thought in much detail about the technical reasons why this works.

After a long evening’s thought on the subject, and running a few questions past my friend and fellow engineer, I believe I have a (reasonable, though perhaps not perfect!) handle on the subject…

If the image signal and the image noise had similar properties, averaging neighboring pixels in order to reduce the resolution would not improve the signal-to-noise ratio. However, signal and noise have different properties.

There is (in general) no relationship between the noise in neighboring pixels. Technical junkies call this “no correlation”.

Correlation is the long-term average of the product of two signals N1 x N2. If two signals have no correlation, then the mean of their product is zero.

The signal in neighboring pixels has a high degree of correlation. If you add uncorrelated signals, then their “power” is added, meaning the combined signal is the square root of the combined power.

N_comb = sqrt(N1^2+N2^2) and for N1 = N2 = N we get N_comb = sqrt(2)*N, where N1, N2 are root-mean-square (RMS) values of the noise.

However, if signals are highly correlated, then their sum is effectively the sum of their magnitudes:

S_comb = S1+S2 and for S1=S2=S we get S_comb = 2*S

So, if we add the content of two neighboring pixels, we get:

SNR_comb = S_comb/N_comb = sqrt(2)*(S/N)

So, the signal-to-noise increases by square root of two, which is about 40%.

Now, you may say that the signal in neighboring pixels is not always 100% correlated. The correlation between the signals depends on the image content. If the image content is very smooth, the correlation is high. If the image content varies very fast, the correlation is low. Of course, noise will be more noticeable in smooth areas and the effect of resampling the image will be stronger.

Adaptive noise filters take into account the absolute signal-to-noise and the image content. They reduce the resolution more in areas that are smooth and have poor signal-to-noise and keep the original resolution in areas that have strongly varying image content and high signal-to-noise. You can think of it as a joint optimization of SNR and resolution.

Now, we also need to look into the different sources of noise:

  1. The first source of noise is dark current which is caused by electrons that accumulate in the individual pixel well, even if there are no photons entering (lens cover on). Dark current becomes dominant for very long exposures. For normal exposures the errors from trapped electrons are negligible.
  2. The second source of noise is the read-out noise. This is essentially generated by two sources: A) Noise added by the amplifier and B) Noise generated by the analog-to-digital converter. It is a fixed amount of noise that is added to each image during read-out. When you choose the ISO setting on your camera, you essentially set the read-out gain and therefore the read-out noise. The higher the ISO, the higher the read-out gain and the less read-out noise. Of course if you pick an ISO which is too high you will get signal saturation. So for low-light situations always pick an ISO that is no higher than needed to capture the image you want.
  3. The third source of noise is called “quantization noise” and is a bit harder to understand. It has to do with the fact that (in low-light conditions) we don’t sample a smooth, continuous flow of photons but rather discrete bunches of photons. The problem is, that a source of light does not produce a stream of photons that are spaced equally in time. So, if you image a low light source that sends out (on average) 100 photons per second, you may receive 90 photons for the first second, 105 for the second etc.. The average error will be on the order of the square-root of the number of photons (or electrons in the pixel sensor well). A typical sensor well contains between 20,000 and 60,000 electrons when fully charged. The maximum amount depends on the pixel size. A sensor well with 20,000 electrons has an error of approx +/-141 electrons when fully charged or +/-0.7%. A well with 60,000 electrons has an error of approx +/-245 electrons when fully charged or +/-0.4%. While we may be able to reduce dark current and read-out noise by cooling the sensor, there is essentially nothing we can do about it. If we keep on shrinking the pixels, we will have smaller and smaller electron wells and less and less electrons trapped.

    The above errors of 0.7% or 0.4% appear rather small and we would not be able to notice them. However, in low-light situations, sensor wells will be only partially filled. If we only manage to trap 1000 electrons, the error becomes 3%. If we only trap 100 electrons, the error becomes 10%.

    Notice that the term “quantization noise” has nothing to do with the signal quantization by the analog-to-digital converter. It has to do with the fact that your signal actually arrives in quantums of energy.

What do you guys think? Anyone wants to challenge Mike’s analysis? :)

What is Moiré?

Nikon D800 vs D800E Moire

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

Case Study: Skin Color Problems

Skin Color Problem

Another case study was submitted on Nikon D7000′s handling of colors. Here is what our reader writes:

Hello Nasim, 2 months ago I bought my first Nikon camera – D7000. I’ve read much about it and decided that this is best camera for me, but recently I am noticing that in certain lighting conditions colors are inadequate. There is an awfull yellow-green color, especially noticeable on people’s faces. Skin on pictures is also has strange color. Changing wb temperature is hardly helping. As an owner of the D7000 could you tell me if this is the problem of all D7000 cameras or is it malfunction of mine? What can i do to fix this?

And here is a sample image that was attached to the case study:
Skin Color Problem

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Case Study: Image Spots and Streaks

Spots and Streaks on Image

One of our readers sent me an image with the following question as a Case Study:

I have no idea what this streak is on my pictures could you give me an idea? I bought a new lens, because there was a small scratch on my old one. However, the same streak appears in the exact same place. It is a line about 1 inch on the top right of my pics. Usually seen when shooting skylines, clouds. etc.

Here is the attached image:
Spots and Streaks on Image

So, what are those spots and streaks that are clearly visible in the above image? First, the good news – the above spots and streaks have nothing to do with the lens. In fact, lens problems and even major scratches on the front lens element rarely ever show up in images. Unless the rear lens element is damaged/scratched, you should not see any lens defects show up in your images. Those of you who have seen my articles on cleaning DSLR sensors probably already know what these are. They are dust spots, along with a piece of hair that is sitting right in the middle of the camera sensor (the long dark line streak). Now the bad news – whenever you see something like this consistently show up in your images when shooting at small apertures, you will have to either clean the camera sensor yourself or send your camera for cleaning in order to get rid of all this dirt on the sensor. The latter is a safer method, but will cost you a lot of money to continue sending your camera every time you need it cleaned; plus, you won’t be able to take pictures while it is in service. The cheapest method is to clean your camera sensor yourself. As I have shown in the my cleaning DSLR sensor article, you can clean a sensor very quickly without any hassles, as long as you have the proper tools. Is it risky? Unless you do something stupid, the procedure is very safe (obviously, I take no responsibility for any potential damage to your camera). Just watch the video and then watch the more detailed videos on how to clean DSLR sensor and keep your camera gear clean for more info.

Let me know if you have any questions!

Case Study: Bird Photography

Bird with distracting BG

I have finally been able to more or less clean up my mailbox and sort through most of the emails that keep pouring in from our readers. The case studies that our readers are sending have been piling up in my mailbox and my to-do list, so I will try to do a better job in posting these on the blog from now on. Let’s start with a case study from our reader Gaurav Rajaram, a bird lover and photographer from Bangalore, India. Here is what he sent me:

I use a Nikon 300mm f/4 paired with a Nikon D200 for my bird photography. While shooting, I notice that I do not get a clean background, which I would expect from a prime lens. I have got such a background in one image of mine, however, the subject is a little too soft for my liking (the picture is attached). Is there any way to get a clean background so as to help the viewers’ focus remain on the subject (the bird in this case)? Could you share a tutorial with us? I’m attaching sample images for this case study in JPEG format with full EXIF info.

And here are the two images Gaurav attached:

Bird with clean BG but too soft

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How to Store Memory Cards

Nikon D7000 Dual Slots

After losing a memory card with the best pictures from a trip I took across the western USA, I decided to write a quick article on how to store memory cards and how not to lose photographs during long trips. It was a lesson learned the hard and painful way, so a couple of days after the loss, I came up with a plan to protect my data going forward and try not to lose it any more in the field. Below you will find my plan and my recommendations.

Losing images from a long-planned and expensive trip can be very painful. After it happens, you realize that it is not the financial aspect of it, but the effort you put into creating those images instead that hurts the most. We as photographers have to work with the best light during the day, which happens at sunrise and sunset times, no matter where you are located. In Glacier National Park, the sunset times in summer can be as late as 10 PM and as early as 5 AM in the morning. Northern Canada and Alaska are even worse, with sunset times close to midnight in July and sunrise in less than 5 hours. Add +1 hour after sunset and -1 hour for sunrise to get back and to the location, and we are talking about less than 3 hours of sleep at night. In addition, those late hours are also the peak and active time for wildlife, making it dangerous to hike to get to a good spot. And I am not even talking about the weather, which can go against you in those twilight hours. In addition, you carry the heavy weight with you and spent a lot of time tweaking your equipment and composing your shots using different spots and angles. So with so much effort put into making those images, the last thing you want is to lose them. What’s worse is, if you have been shooting for a while, you know if you got a great photo right at the time you take it. You take a look at the camera LCD and you know it is a keeper, a potential for your showcase portfolio. Once you lose photographs, you start to remember those keepers and deep regret hurts even more. So, why even take the chance? Take all the steps you can to protect your photographs when traveling and working on the field.

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Nikon AE-L / AF-L Button

Nikon D3100 AE-L AF-L Button

Whether you are using an entry-level DSLR like Nikon D3100 or a top of the line DSLR like Nikon D3x, there is a special button on the back of your camera labeled “AE-L / AF-L” that can be quite useful in many situations. After I wrote the Autofocus Modes article, I received several requests from our readers, asking me to explain what the AE-L / AF-L button does, when it should be used and how it can be combined with different autofocus modes. In this article, I will try to go through this button in depth and explain how I personally use it on my cameras.

Nikon D3100 AE-L AF-L Button

1) AutoExposure-Lock / AutoFocus-Lock

The AE-L / AF-L button stands for “AutoExposure-Lock and AutoFocus-Lock” and its primary function is to lock camera exposure and/or focus. What does this exactly mean? If you are using any of the camera modes like Program, Aperture Priority, Shutter Priority or other scene modes, the button could be used to force the camera to use a certain value for shutter speed, aperture, ISO and white balance. Since in any of the automatic modes the camera uses its metering sensor to determine the optimal exposure, instead of having your camera re-evaluate the light every time you recompose, you could lock the exposure to a value you are comfortable with – hence the term “AutoExposure-Lock”. There are many cases where using this feature is very helpful. One example is when you photograph panoramas. It is extremely important to use exactly the same exposure from frame to frame in panoramic photography. If one exposure differs from another, it is practically impossible for panoramic software to stitch images together in a consistent, continuous form. Another good example is if you are photographing a subject with a constantly changing background and you want to expose the subject exactly the same way from shot to shot. Basically, any time consistency of exposure is required and you do not want to switch to a full manual mode, the AE-L button can be very useful.

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How Much is Too Much?

Shoes 1

When it comes to our wedding business, Lola put me in charge of “QA” (Quality Assurance) before images are delivered to our clients. After she is done with all post-processing work, we sit down together and review all images. She is naturally good at working on images and her creative and artistic side really comes into play when she photographs and then edits images. I have a very different approach to photography and I often pay lots of attention to such things as image sharpness, detail, framing and angles. It is surprising to see how well our different perspectives merge into a productive environment – while we sometimes do disagree, we both understand that our ultimate goal is to provide the best results to our clients.

On average, Lola and I come back with approximately 1,000-1,500 images per wedding, all shot in 14-bit NEF (RAW) format. Once we sort everything out and pick our favorites, we leave approximately 500-600 images that will be delivered to our clients, with only about 1/10th that are retouched in Photoshop. I was once talking to a photographer based out of Florida, who told me that he only takes between 100-250 images per wedding. When I told him how many images Lola and I take, he laughed, arguing that most images are probably duplicates of each other and that we should learn how to take fewer, but higher quality images. Being in business for over 30 years as a successful full-time pro, he definitely knew what he was talking about. But I also realized that his habit of taking fewer pictures definitely has to do with film days, where more photos meant more photo lab work. With digital SLRs that have a shutter lifespan of at least 150 thousand clicks, we no longer need to worry about the cost or working with chemicals in a lab. So, what is better – take fewer, but higher quality images right on the set, or take as many pictures as possible at different angles, perspectives and settings? As far as I’m concerned, I am somewhere in between. I think that taking very few pictures is risky, as you might think you got everything right on camera, but you might have missed some details – eyes closed, focus not 100% accurate, etc. The last thing you want is a frustrated customer that asks for another picture you do not have. On the other hand, walking with a DSLR and shooting non-stop is also counter-productive, since then you have to deal with too many images and the process of choosing the best ones and working on them later might take hours of precious post-processing time. Wedding photography is a tough business to be in mainly because of a photographer’s time, especially if complex Photoshop retouching is involved. Details can take away too much time and the question that I have been asking myself lately is “how much is too much”, when we work on images.

Take a look at the below image, as it came out of the camera:

Shoes 1

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Case Study: Image Quality

Sharpness

One of our readers sent me some sample images from his camera, asking why his photos are not sharp and often too bright and flat-looking. He is using a pro-level body (Nikon D700) and very good lenses like the Nikon 24-70mm f/2.8 and the Nikon 16-35mm f/4 that he bought after reading my reviews and he is disappointed with his setup. Here is what he wrote me:

I really need your help.

I own the Nikon D700 + Nikkor 24-70 f/2.8 + recently bought the Nikkor 16-35 f/4 after reading you review. I wanted the 14-24mm f/2.8, but without filter it is a big problem for me. Anyway, I have owned the camera for about 8 months and I am not satisfied with the results…

I mostly shoot in RAW with Active D Lighting set to “Auto”. My photos never seems as sharp as the samples you put on your site and they always looks too bright and flat. It’s like they are “dead” without contrast and color and I don’t know what’s wrong with my setup. Maybe it’s a problem with the camera sensor or I don’t know what… I am not a pro photographer and not even close, but I expert much better results from what I have. I mean I can always fix in post-processing software like Aperture 3 which I have, but i want great photos out of the camera without playing with it too much in post.

Please let me know if you see what the problem is and if there’s something wrong with what I am doing? I totally feel hopeless…

Thank you for your time.

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