The second part of the tripartite exposure settings (other than sensitivity and aperture) is shutter speed. The shutter opens or shuts to let in light. The longer it remains open, the more light is let in. Shutter speed is generally expressed in seconds or, more often, in fractions of a second. A shutter speed is said to be "faster" than another shutter speed when it lets in less light.
The shutter can be mechanical, electronic, or a combination. Mechanical shutters are often designed as focal-plane or leaf types. Other types are also possible, but these two are the most common types. Most modern DSLRs use focal plane shutters. A focal plane shutter is a shutter that is attached near the focal plane (e.g., film or sensor) of the camera. Focal plane shutters normally have one or more curtains. In one common design, one shutter opens and a second shutter closes just behind it. The gap between the two moves at the selected shutter speed. This design enables camera manufacturers to provide very fast shutter speeds. For example, the fastest shutter speed on one of my first cameras was 1/250 second. Modern day DSLRs can provide shutter speeds of 1/4000 second or faster. Thus, although each shutter curtain may open and close slower than the fastest shutter speed, the gap is only visible by any position in the frame at the selected (e.g., fastest) shutter speed.
Shutter speed can affect how much blur you see in an image. When your subject is moving a faster shutter speed will cause the subject to appear still whereas a slower shutter speed will capture some motion blur. For example, in fast-moving sports, you may want a shutter speed of 1/250 second or faster. A human walker may only require 1/125 second. A stationary (e.g., still life or architctural) subject will let you any shutter speed you want because it isn't going anywhere. A second concern in blur is whether you are using a tripod. With good camera holding technique, most people require a shutter speed of 1/f seconds where f is the effective focal length of your lens. For example, if you are using a cropped frame sensor with a 70-200 mm lens with the lens set at 100 mm, then your effective focal length is approximately 100 mm and so you would want a shutter speed of no slower than 1/100 seconds. If your technique is not very good, you may want a faster shutter speed, such as 1/200 seconds. If your lens or camera has vibration reduction (also known as image stabilization; VR for Nikon and IS for Canon), then you may be able to get away with a slower shutter speed (e.g., 1/100 seconds with poor technique or even 1/50 seconds with good technique). I try to go with 1/f even with vibration reduction. If your camera is on a tripod, you don't need to worry about the 1/f formula at all.
Most cameras have a maximum flash sync speed (also known as x-sync speed). This is the fastest shutter speed the camera can use with a flash. If a faster shutter speed is used, the frame will be partially blocked by the moving shutter curtains. In automatic exposure modes, the camera probably will not let you select a faster shutter speed when the flash is turned on. In manual exposure modes, you need to make sure that you don't select a faster shutter speed unless you want a portion of the frame to be unaffected by the flash.
Sensitivity, aperture, and shutter speed are all equally important. Once we learn about aperture, we can discuss how we can select appropriate combinations based on what we are trying to accomplish.
Thursday, March 5, 2009
Sunday, February 22, 2009
Sensitivity
Speed kills. That's a common saying you hear in both defensive driving courses and in drug use prevention. In the world of photography, however, we've been exploring ways to get faster for a very long time. We get more speed with faster shutter speeds, faster flash synchronization speeds, wider apertures, and faster sensitivity. It's the sensitivity variable in exposure that we will explore in this post.
Sensitivity is expressed in ASA or ISO and is a measure of how sensitive film or a DSLR's sensor are. The lower the expressed ISO, the less sensitive the film or sensor. The higher the expressed ISO, the more sensitive the film or sensor. ASA is the original scale that many of us grew up with. We commonly used ASA speeds 64, 100, 200, 400, 800, 1600, etc. Each doubling meant that the film was twice as fast. For example, if the "correct" exposure is 1/125 second at f/8 using ASA 100 film, we could use 1/500 second at f/8 with ASA 400 film.
ISO stands for the International Standards Organization. The ISO set at least a couple of standards for expressing sensitivity. The standard we use today approximates the old ASA we used to use. I guess old habits die hard. So, when you set your DSLR's ISO to 100, 200, 400, 800, etc., it would be the same as if you inserted that speed film.
The huge advantage, of course, of DSLRs is that you don't need to rewind and insert new film as light conditions change. You simply change the ISO. During the course of a day, you can use ISO 200 outside and ISO 400 or more inside. That's fantastic flexibility.
In most presently sold cameras, the higher the ISO, the more noise that is introduced. For example, you will see much more noise at ISO 6400 than at ISO 200. However, manufacturers are building increasingly better sensors. For example, the Nikon D3 produces fantastic images at ISO 6400 -- much better than any older Nikon.
As a starting point, I use ISO 200 outdoors and ISO 400 indoors. When I use my Nikon D3, I don't hesitate in bumping up the ISO, when needed indoors or for sports, to ISO 800 or higher. By increasing the ISO in lower light, my shutter speed and aperture remain within normal working conditions. If I didn't increase the ISO, I may need a shutter speed that is too slow to handhold or an aperture that is either unreasonably wide (e.g., providing too little depth of field -- a topic we will explore later -- or much larger than the lens is capable of). I usually don't let the camera select the ISO because I want to know what's happening with the light.
Play around with different ISO settings in your camera when using it with different lighting conditions so that you understand how ISO affects your shutter speed and aperture.
Sensitivity is expressed in ASA or ISO and is a measure of how sensitive film or a DSLR's sensor are. The lower the expressed ISO, the less sensitive the film or sensor. The higher the expressed ISO, the more sensitive the film or sensor. ASA is the original scale that many of us grew up with. We commonly used ASA speeds 64, 100, 200, 400, 800, 1600, etc. Each doubling meant that the film was twice as fast. For example, if the "correct" exposure is 1/125 second at f/8 using ASA 100 film, we could use 1/500 second at f/8 with ASA 400 film.
ISO stands for the International Standards Organization. The ISO set at least a couple of standards for expressing sensitivity. The standard we use today approximates the old ASA we used to use. I guess old habits die hard. So, when you set your DSLR's ISO to 100, 200, 400, 800, etc., it would be the same as if you inserted that speed film.
The huge advantage, of course, of DSLRs is that you don't need to rewind and insert new film as light conditions change. You simply change the ISO. During the course of a day, you can use ISO 200 outside and ISO 400 or more inside. That's fantastic flexibility.
In most presently sold cameras, the higher the ISO, the more noise that is introduced. For example, you will see much more noise at ISO 6400 than at ISO 200. However, manufacturers are building increasingly better sensors. For example, the Nikon D3 produces fantastic images at ISO 6400 -- much better than any older Nikon.
As a starting point, I use ISO 200 outdoors and ISO 400 indoors. When I use my Nikon D3, I don't hesitate in bumping up the ISO, when needed indoors or for sports, to ISO 800 or higher. By increasing the ISO in lower light, my shutter speed and aperture remain within normal working conditions. If I didn't increase the ISO, I may need a shutter speed that is too slow to handhold or an aperture that is either unreasonably wide (e.g., providing too little depth of field -- a topic we will explore later -- or much larger than the lens is capable of). I usually don't let the camera select the ISO because I want to know what's happening with the light.
Play around with different ISO settings in your camera when using it with different lighting conditions so that you understand how ISO affects your shutter speed and aperture.
Thursday, February 19, 2009
Exposure
We were supposed to start discussing exposure this week in an effort to get everyone on the same page. However, at the last minute, I surprised my wife with a trip to Barrow, Alaska. Within the span of a few days, it was our 15th wedding anniversary, my wife's 39th (plus 1) birthday, my daughter turned 13 (man, I'm feeling old), and St. Valentine's day. Why Barrow, you ask? Well mostly because not many people can say that they have been 350 miles north of the Arctic circle in the winter. Can you? Well, now my wife, kids, and I can. We were also fortunate enough to see polar bears and about 10,000 eskimos who had gathered in Barrow for their annual Kigviq festival. The location changes annually. Unfortunately, we saw very minimal northern lights (aurora borealis) -- it has apparently been a bad year all over for the lights. I apologize that our discussion was delayed.
Exposure is really just about one main thing: recording the "correct" amount of light. Correctness is really subjective. It depends on what you consider to be the main subject in the frame and how you want to portray that subject. Dark or light? Sharp or blurry? Those are decisions you as the photographer make.
Exposure is controlled by three variables: aperture, shutter speed, and sensitivity (ASA or ISO). By varying one or more of these, you control how much light is recorded. If you change any one, you can change how much light is recorded. If you don't want to change how much light is recorded but want to change one of the factors for effect (e.g., sharpness, blurriness, grain, etc.), you can vary two or more of the variables. We'll discuss all of those coming up. First, let's discuss what each one is.
Aperture controls the lens opening. It is indicated in "f-stops." On your lens or camera, it may be indicated as f/2.8, f2.8, or 2.8 -- all mean the same thing. By convention, it is expressed fractionally. Thus, larger numbers mean smaller openings. For example, f/4 is a smaller opening than f/2.8 but larger than f/5.6.
Shutter speed controls the duration that light is allowed to enter the camera body and is usually measured in seconds or fraction of a second. On a sunny day using ISO 200 and an aperture of f/16, you will probably use a shutter speed of around 1/250 second.
ISO (ASA in the old days) is a measure of the sensitivity of film or sensor. Larger numbers mean higher sensitivity. In most current DSLRs, the higher the ISO, the more noise you introduce.
We'll talk about each of these in the next few postings.
Exposure is really just about one main thing: recording the "correct" amount of light. Correctness is really subjective. It depends on what you consider to be the main subject in the frame and how you want to portray that subject. Dark or light? Sharp or blurry? Those are decisions you as the photographer make.
Exposure is controlled by three variables: aperture, shutter speed, and sensitivity (ASA or ISO). By varying one or more of these, you control how much light is recorded. If you change any one, you can change how much light is recorded. If you don't want to change how much light is recorded but want to change one of the factors for effect (e.g., sharpness, blurriness, grain, etc.), you can vary two or more of the variables. We'll discuss all of those coming up. First, let's discuss what each one is.
Aperture controls the lens opening. It is indicated in "f-stops." On your lens or camera, it may be indicated as f/2.8, f2.8, or 2.8 -- all mean the same thing. By convention, it is expressed fractionally. Thus, larger numbers mean smaller openings. For example, f/4 is a smaller opening than f/2.8 but larger than f/5.6.
Shutter speed controls the duration that light is allowed to enter the camera body and is usually measured in seconds or fraction of a second. On a sunny day using ISO 200 and an aperture of f/16, you will probably use a shutter speed of around 1/250 second.
ISO (ASA in the old days) is a measure of the sensitivity of film or sensor. Larger numbers mean higher sensitivity. In most current DSLRs, the higher the ISO, the more noise you introduce.
We'll talk about each of these in the next few postings.
Sunday, February 8, 2009
Sensor Summary
Here are are some summary points to consider when selecting your first or next DSLR before we progress to photographing basics next week.
1) Sensors are an important part of selecting a DSLR - some are better than others. Whereas in the past the conventional wisdom was to invest in good glass (lenses) even at the expense of camera bodies, we also used to be selective about film. Today, the glass and the camera are both important because different cameras use different sensors -- we no longer use film. We spent some time discussing sensors. If you are interested in a more in-depth treatment, several books are available.
2) Don't focus on resolution (pardon the pun). Unless you are going to be printing huge images that people will look at up close (i.e., very large prints spanning across pages in a magazine -- double truck, large posters, etc.), 10 megapixels should be sufficient.
3) Pixel density will improve over time. As it does, resolution will also improve. Don't stress over 50% more pixels in the newest model or the other brand next year. Under Moore's law, that seems to apply to all silicon-based technology, it will get faster and better next year at a reduced cost. Get the right equipment now and then upgrade later only if you need to.
Next week, we'll talk about the rudimentary aspects of photography to get everyone on the same page. Then, we can together explore hwo to get more out of our very advanced cameras.
1) Sensors are an important part of selecting a DSLR - some are better than others. Whereas in the past the conventional wisdom was to invest in good glass (lenses) even at the expense of camera bodies, we also used to be selective about film. Today, the glass and the camera are both important because different cameras use different sensors -- we no longer use film. We spent some time discussing sensors. If you are interested in a more in-depth treatment, several books are available.
2) Don't focus on resolution (pardon the pun). Unless you are going to be printing huge images that people will look at up close (i.e., very large prints spanning across pages in a magazine -- double truck, large posters, etc.), 10 megapixels should be sufficient.
3) Pixel density will improve over time. As it does, resolution will also improve. Don't stress over 50% more pixels in the newest model or the other brand next year. Under Moore's law, that seems to apply to all silicon-based technology, it will get faster and better next year at a reduced cost. Get the right equipment now and then upgrade later only if you need to.
Next week, we'll talk about the rudimentary aspects of photography to get everyone on the same page. Then, we can together explore hwo to get more out of our very advanced cameras.
Thursday, February 5, 2009
Sensor sizes
Sensor sizes can vary across camera types and even across DSLRs. Point-and-shoot cameras typically have quite small sensors. Most DSLRs have larger sensors. They are called APS-C or "cropped" frame sensors because their surface area is smaller than a 35 mm film frame. Some DSLRs have "full frame" sensors that are approximately the same size as a 35 mm film frame. There are even much larger medium format sensors.
There are advantages and disadvantages to having full frame sensors to cropped frame sensors. Full frame sensors can provide more resolution or larger photosites. More resolution generally means better capability for very large prints. Larger photosites generally means lower noise. On the other hand, cropped sensors can be advantageous when additional reach is needed with lenses, such as for sports or wildlife photography.
Currently, prosumers have several choices between full frame and cropped frame cameras. Professionals have been steadily moving to full frame, though some remain with cropped frame cameras because of the advantages stated above.
The article linked below provides a laundry list of other considerations.
Links:
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
There are advantages and disadvantages to having full frame sensors to cropped frame sensors. Full frame sensors can provide more resolution or larger photosites. More resolution generally means better capability for very large prints. Larger photosites generally means lower noise. On the other hand, cropped sensors can be advantageous when additional reach is needed with lenses, such as for sports or wildlife photography.
Currently, prosumers have several choices between full frame and cropped frame cameras. Professionals have been steadily moving to full frame, though some remain with cropped frame cameras because of the advantages stated above.
The article linked below provides a laundry list of other considerations.
Links:
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
Wednesday, February 4, 2009
Photosites and pixels
Each sensor has arrays of hundreds of thousands of "photosites" that detect light. More photosites generally translate to more pixels of resolution. Because each pixel can contain several photosites (e.g., to detect red/green/blue wavelengths), and there is different technology for sensors and photosites, the number of photosites does not necessarily correspond to the number of pixels.
Nevertheless, camera manufacturers usually specify the resolution of the cameras based on the number of pixels that the sensor generates (usually in a native "raw" format). Most DSLRs can store images in different resolutions. For example, Nikon DSLRs enable users to specify small and large JPEGs. When saving native raw files, however, most DSLRs save the full resolution emitted by the sensor. An exception is if the camera is a "full-frame" DSLR that has a "cropped frame" mode. For example, the Nikon D3 has a native FX mode and a DX mode to emulate older Nikon DSLRs. (We'll discuss full frame and cropped frame next time.)
There are numerous advantages to having more pixels. Generally, the more pixels available, the larger a printed image can be without degradation in quality (e.g,. sharpness). This is only really a problem if you are planning to print larger than poster size. Very few prosumers will do that, but it could be nice to know that you can do that.
There are also drawbacks to having too many pixels. All things being equal, with higher pixel density (i.e., pixels within some defined area) generally comes more noise. The files stored by the camera are much larger. When postprocessing files, larger files take much longer to postprocess.
While pixel count should be a consideration when selecting your first or next DSLR, it need not be your primary consideration. Most modern DSLRs have more than 10 or more megapixels (10 million pixels), which is plenty for printing a poster.
Links:
1) Wikipedia article on Foveon X3 Sensor used in some Sigma digital cameras
Nevertheless, camera manufacturers usually specify the resolution of the cameras based on the number of pixels that the sensor generates (usually in a native "raw" format). Most DSLRs can store images in different resolutions. For example, Nikon DSLRs enable users to specify small and large JPEGs. When saving native raw files, however, most DSLRs save the full resolution emitted by the sensor. An exception is if the camera is a "full-frame" DSLR that has a "cropped frame" mode. For example, the Nikon D3 has a native FX mode and a DX mode to emulate older Nikon DSLRs. (We'll discuss full frame and cropped frame next time.)
There are numerous advantages to having more pixels. Generally, the more pixels available, the larger a printed image can be without degradation in quality (e.g,. sharpness). This is only really a problem if you are planning to print larger than poster size. Very few prosumers will do that, but it could be nice to know that you can do that.
There are also drawbacks to having too many pixels. All things being equal, with higher pixel density (i.e., pixels within some defined area) generally comes more noise. The files stored by the camera are much larger. When postprocessing files, larger files take much longer to postprocess.
While pixel count should be a consideration when selecting your first or next DSLR, it need not be your primary consideration. Most modern DSLRs have more than 10 or more megapixels (10 million pixels), which is plenty for printing a poster.
Links:
1) Wikipedia article on Foveon X3 Sensor used in some Sigma digital cameras
Tuesday, February 3, 2009
What is a digital sensor?
As we discussed last time, a digital sensor and memory are what separate a DSLR from prior film-era SLRs. A digital sensor (or "image sensor") is an electronic device that converts light it senses into a digital signal.
Initially, sensors were charge-coupled devices ("CCDs"), but many if not most sensors are now complementary metal-oxide-semiconductors ("CMOS"). It used to be the case that CCDs produced superior images. However, manufacturers have poured much more money into CMOS R&D over the last decade or longer because CMOS-based devices are cheaper to make and require much less circuitry. As a result, CMOS sensors rival the performance of CCDs and today may even be better.
Just as with lenses, you need to keep your sensor clean. While that's not so hard with point-and-shoot cameras because they are sealed units, your sensor will likely get dirt on it over time if you change lenses.
The sensor (or, to be more precise, the low pass filter on top of the sensor) must be kept clean so that you don't have distracting blotches from accumulated dirt. Some cameras can clean their sensor automatically, but most require occasional manual cleaning. You can either clean the sensor yourself using available kits or you can have it professionally done. There's even a whole web site dedicated to sensor cleaning by a camera cleaning outfit. Note, however, that you should make sure that you are using the right equipment for the job. DO NOT use the compressed air cans -- you can damage your camera severely if you do, and are likely to introduce moisture.
Links:
1) Wikipedia article
Initially, sensors were charge-coupled devices ("CCDs"), but many if not most sensors are now complementary metal-oxide-semiconductors ("CMOS"). It used to be the case that CCDs produced superior images. However, manufacturers have poured much more money into CMOS R&D over the last decade or longer because CMOS-based devices are cheaper to make and require much less circuitry. As a result, CMOS sensors rival the performance of CCDs and today may even be better.
Just as with lenses, you need to keep your sensor clean. While that's not so hard with point-and-shoot cameras because they are sealed units, your sensor will likely get dirt on it over time if you change lenses.
The sensor (or, to be more precise, the low pass filter on top of the sensor) must be kept clean so that you don't have distracting blotches from accumulated dirt. Some cameras can clean their sensor automatically, but most require occasional manual cleaning. You can either clean the sensor yourself using available kits or you can have it professionally done. There's even a whole web site dedicated to sensor cleaning by a camera cleaning outfit. Note, however, that you should make sure that you are using the right equipment for the job. DO NOT use the compressed air cans -- you can damage your camera severely if you do, and are likely to introduce moisture.
Links:
1) Wikipedia article
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