Dust bunnies, dirty lenses and stuck pixels

Modern cameras are absolute marvels at what they can do and how well they do it. Even with much abuse they produce pictures that are almost flawless. There are limits and little problems that will reveal themselves on close examination. This article explores some of those.

Dust Bunnies – the bane of interchangeable lens cameras

If you can remove the lens from your camera, it is almost certain that a little bit of dust will sneak in before you can cap it or install another lens. Such a speck of dust can settle on the sides, the mirror in a DSLR, or on the image sensor. Well, not exactly the sensor, that has usually some optical items stacked on it. If a speck settles on the filter in front of the sensor it will imped the light falling on a small number of the imaging pixels. You can see such specks in photos as “dust bunnies”. Before I go on, let me show you an example, a fairly drastic one I might add:

PNT1401-01-10693-07

PNT1401-01-10686-02There are many “dust bunnies” in the photo above, some of them are indicated by arrows. Here is a close-up view of the one indicated with a yellow arrow (top center) in the photo. The close-up shows the speck as photographed at f/36 while the photo above was taken at f/20.

The smaller the aperture, larger the f-number, the more noticeable will be the dust on the sensor. In fact at wide apertures and even mid-range (f/8 to f/11) the beam of light reaching any pixel is wide enough that a little bit of dust wont show. So it is a problem only when shooting at small apertures, for example in landscape photography when you want everything sharp from the foreground to the far mountains, when you want a really wide depth of field and thus choose a small aperture like f/22.

So, what can be done?

Obviously you will want to keep your camera as clean as possible. Always point the camera opening down when removing the lens – better to let dirt fall out than in.

Your camera most likely has a couple of tools for fighting dust bunnies. Most cameras have a dust-off feature. A bit of ultrasonic vibration is applied to the filter to shake any dust loose and have it, hopefully, fall to the bottom. There might be a surface there designed to be sticky to dust so it gets trapped. The camera can usually be set to perform this function regularly when you turn it on or off. This is like a dog shaking itself dry, but not nearly as vigorous, and hence not nearly as effective.

The second tool in your camera admits that some dirt may remain firmly stuck to the sensor. It lets you take a “dust reference photo”. Nikon calls it “dust off ref photo”. Check you camera manual to see how to record such a frame. The idea is to use this reference photo and subtract the spots in processing in your computer. Again Nikon has such a tool built into its Capture NX software (extra cost item), other manufacturers provide similar tools.

You can clean your camera sensor. That is a very delicate operation. First you need a clean room to do this in, and appropriate tools. Just like having a scalpel does not make you a brain surgeon, having a sensor cleaning kit does not impart the proper skills for operating on your camera. This task is best left to professionals. Your friendly camera store likely offers this service along with other camera maintenance services. A cleaned camera sensor will stay clean until you remove the lens – which like would be the very next day. So I have this service performed only when I want something else done to the camera.

Lastly there is the practical approach. Accept the fact that the world is not perfect. Inspect your photos – especially those that you will proudly show off to friends and strangers – and use the retouch tool, or healing brush, in your post-processing software to edit out any spots.

Dirt on the lens

The front element of your camera lens is exposed to the environment and dirt, even fingerprints, can easily get on it. With interchangeable lenses the back element can LJK10588-P2-Balso pick up dirt. Is that a problem? Most of the time not at all. The little bit of light blocked by even a fairly sizable spot will not affect the photo. There are conditions, however, under which dirt on the lens will show in photos and be very bothersome. Let me show you such an example.

The photo here shows a candle with a number of very much out-of-focus lights in the background. Some of these light patches are very sharply defined and you can see spots in them. Those little black spots come from dirt on the lens. You would definitely notice dirt of that size by looking at your lens. I am confident that you would take your lens cleaning brush and dust it off.

So what is this rare circumstance when dirt will show and why does it not show for all the out-of-focus light patches in this photo?

Dirt on the lens will show when the bundle of light coming from the lens is intercepted so it will be imaged as a large circle and when that light comes from a very small source, In the case of the photo here, some of the lights were pin-point small lights – LEDs. The patches not showing the dirt were made by larger lights. Essentially point sources are required to show up this problem.

What can you do? Clean your lens. Be very careful doing that, your lens has thin layers of anti-reflection coatings. You do not want to scratch those.  Follow the instructions in your manual.

If you are bothered by the fact that some of the bokeh patches, that’s what they are called, are not round but rather boat-shaped, hold your question for a discussion of optics. We won’t get into that in this article.

Stuck pixels

Your camera sensor has millions of individual light sensors, “pixels”. As with all electronic devices some of those tiny sensors may go bad. Having a few non-functioning pixels is not a serious problem and you may not even notice them. A non-functioning pixel might be stuck off and not provide any signal. That would  result in a black spot. A black spot would go unnoticed in almost all photos.  It could also be stuck on and produce a full output signal. That would show as a tiny white spot. We are sensitive to spots of light and a stuck on pixel is much more noticeable.

 

PNT1401-01-8698-01Now just one pixel would seem that it should not be of any concern at all, but there is more to the story. In order to produce color photographs, the sensors in the camera are equipped with color filters. Really tiny filters, some pixels have green, others blue, and others red filters. The pixels with their various filters are arranged in patterns. Rather than provide images made up of red, green and blue little patches, cameras provide three color signals for every pixel. That increases the apparent resolution and goes back to the early days of digital cameras when sensors had only thousands of pixels rather than millions as they do today. To produce the color information for each pixel the computer in the camera looks at the signals from adjoining pixels and calculates what the missing two colors should be. Why this lecture on minutiae? Because a stuck pixel will not show up as a single pixel but rather a patch since it affects what is calculated for adjoining pixels.

The photo here shows an evening scene with the moon and a star. Well, that star, so nice and sharp in this picture, is a stuck pixel rearing its little head.

Let’s take a closer look at that “star”. PNT1401-01-8698-02

You can see that at least an area of nine pixels is affected. In the actual file data an even larger number can be identified as different from the surrounding area.

Stuck pixels are permanent defects that may be affected by various environmental conditions including sensor temperature. They will tend to be more noticeable in long exposure photos because for those the sensor is operating for a longer period of time and getting warmer is it does its job.

Here again a reference exposure can be used to eliminate the artifacts in post-processing. If you are into astronomical photography you will definitely want to do that.

For the rest of us a stuck pixel will go unnoticed in most photos. If it shows up in a contrasting area, it can be easily touched up in post-processing. There are not likely to be many of those in your camera.

Well, I hope you have gained some useful information from my story on dust bunnies, lens dirt, and stuck pixels. Now I have to go and see if I can get my dirtied-up camera clean again. Ah, the things we do for our readers!

[This article was first published in Photography Notes & Tips.]

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© 2014 Ludwig Keck

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Photography Notes and Tips

Another member of the clan

Please welcome Photography Notes and Tips to the Café Ludwig clan of sites.

This new blog offers, just as the name proclaims, notes and tips about photography, much of it oriented to black and white photography. It is in a wide layout so photos can be shown large without needing to click and go to another site to view them. Comments and other features are oriented toward Google+.

You can find the listing of the latest posts at PN&T in the sidebar on the right – just scroll down a ways.

Here is an image linking to the blog.

Photography Notes and Tips

Take a look at the blog. You might find it interesting and helpful.

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© 2013 Ludwig Keck

Dramatic B&W Photos

Create dramatic dark skies with simple tools

In black and white photography it is all about the light, shapes and lines. Such images offer a totally different appeal from their color photo counterparts. Dramatic dark skies are a hallmark tool for stunning images. Here are some tips for getting the most from your color originals using simple tools.

Let me start with a demonstration.

Men of Steel

This delightful sculpture showing workers taking a break on a steel beam gleams in this photo. The dark sky and deep shadows bring life to this image.

It was derived from a rather ordinary color photo. The normal process is to use a color-filter conversion when making it into a B&W image. Pretty much all the photo editors provide such tools.

In PaintShop Pro you can pick the exact filter color from a color wheel as illustrated below. PaintShop Pro - Black & White Film tool

Similarly, Picasa has a “Filtered B&W” effect and the filter color is picked from a palette. Picasa - Filtered B&W

 

Windows Photo Gallery offers a selection of B&W effects with several filter colors. Windows Photo Gallery - Effects

With all of these tools the reddest filter does not provide that dramatically dark, almost black, sky for a normal blue sky in a photo. There is an added trick to achieve the deep dark sky: Adjust the color saturation to a very high level. The image below shows the results. The outside (left and right) portions show the normal photo with the normal result through a red filter.

The inside pair shows a deeply saturated sky and the the resulting dark black and white image.

Lunch break

One note of caution: The resulting black and white image will show a good deal of mottling in the formerly blue areas. A uniform sky will make this even more noticeable. So make the conversion in the largest image that you have. Make any size reductions afterwards to maintain a clean sky.

This post was also published at Photography Notes and Tips.

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© 2013 Ludwig Keck

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Color Temperature and White Balance

As I looked at the photos from a short walk in the park I realized that I had inadvertently illustrated a lesson on color temperature and white balance. Four of my snapshots are shown here.

CT WB-01We all know about color temperature. From the orange-y glow from a candle, the warm light of incandescent bulbs, the cool of fluorescents, the brilliant white of sunshine, to the steel-blue light in open shade under a blue sky outside. Us humans have no trouble at all recognizing a white sheet of paper as such under any of these lighting conditions. Modern cameras are clever enough  to automatically adjust to do the same – and they do a good job of it.

There are occasions when a camera might not get the automatic adjustment right on. I was photographing food items last week under warm incandescent lights. LK_000746-C-400I wanted to make sure that the “white balance” was good – that the white plates were indeed white in the photos, so I used the manual setting. D3100Mode-C2-400Most cameras have a group of manual settings in addition to a number of automatic modes. The manual settings are typically labeled M, A, S, P, for completely Manual, Shutter speed selected, Aperture selected, and a manually definable Program mode (shown in yellow in my illustration).

In the manual modes on most cameras a variety of settings can be specified, like white balance, and the camera will remember those settings. I had done this, as I mentioned, for my “food shoot”. So now to my afternoon walk.

With my camera either on AUTO, Camera-mode-AUTO, or “close up”, Camera-mode-CU, I can take photos without much fuss. The camera takes care of the work – including setting for color temperature. On this walk there were several scenes where I wanted more depth of field so I switched to “A”, aperture preferred, and set the lens to f/11. I did not notice that on the manual setting the camera remembered the white balance setting for “incandescent”. For that setting, since there is a lack of blue light coming from lamp bulbs, the camera multiplies the amount of the blue signal many times (and also the green to a lesser extent) in order to render white objects as white in the photo. Under daylight now, with plenty of blue light, resulting photos, were, let’s say, worthy of being called “cyanotypes”. Since the camera was still compensating the exposure there was precious little information in the reds.

imageThis much color temperature error was way more than most editors can handle. Lesser error are easily compensated for in imagePhoto Gallery.

Picasa has a neat eyedropper control that lets you pick an item in the photo that is supposed to be white, gray, or black, and and corrects the color temperature with a single click.

As for my beyond-correction photos – why, I just turned them into “art”!

LJK_7067-P4-1024

LJK_7084-B3-1024

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© 2012 Ludwig Keck

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Setting up your SkyDrive

SkyDrive is a service of Microsoft for storing and sharing photos and documents online – “in the cloud”. If you have a Windows Live ID, or a “Microsoft account”, you have a SkyDrive with 25 GB of free cloud storage. With a Hotmail email account you already have a SkyDrive.image

If you need to set up a new Microsoft account, go to this site:

http://explore.live.com/skydrive then click on Get SkyDrive.

You will be presented with the usual sign-up form with fields for your name and other information. You can get a new email account, @hotmail.com or @live.com, or you can register with your current email address from another provider.

If you register an existing email account, you will receive a confirming email and need to click a link to verify that you indeed have that account. You sign in to your account at skydrive.com.

A brand new SkyDrive looks about like the illustration here.

image

You will see three folders, Documents, Public, and Pictures. You can add additional folders. The sharing of the Documents and Pictures folders are set to “Just me”, that is, they are not shared. The Public folder, as you might guess, is set to “Everyone (public)”.

If you have an older, existing account, the array of folders might be a little different.

Now you are ready to store documents and photos in your SkyDrive. How you do that is covered in another article.

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© 2011 Ludwig Keck