Still photography and camcorder videography
Tuesday, July 25, 2006
Shutter speed issue
Unlike the stills camera, a camcorder records a continuous set of images. When they're played back at the right speed, they trick the brain into thinking that what you see is a moving picture, in the same kind of way as the zoetrope years ago. When the camcorder records an image, it's put together in two halves, known as fields. These fields make up alternate parts of the picture, making - in total - one frame. Camcorders usually record at the rate of 50 fields per second, though this depends on the television standard used by the camcorder. For instance, this is the case in the
Now if your camcorder is set to record at, say 1/250 second, the camcorder will still supply 50 charges per second to the CCD, but these will only last for 1/250th second each. This leaves 4/250th sec where the CCD doesn't register a charge. When the camcorder transfers the image to tape, the missing section causes a kind of 'hiccup' when the images are replayed, also making the picture look jerky.
Apparently, the higher the shutter speed, the more jerky the result sequence will look like.
So what is the usage of it? It is only useful when you need to render the fast motion sharper and can compensate in some sort of way the hiccups.
So the slow shutter speed is used for what, blurring effect? Yes, but also, for better exposure.
The problem is with the DOF. When you manually select a fast shutter, such as a sports mode, the camcorder will open up the iris to compensate for the reduction in light reaching the CCD. The result will be that the depth of field is reduced. In most cases, this is fine, but if you want the majority of your scene to be sharp, ie using a wide depth of field, you would need to shoot in much brighter light.
Zoom
The reason why today’s camcorder can afford such a high ratio zoom length is that their CCD size is relatively small. With a smaller area of pixels needed to convert light into an electrical signal, the lens itself can become smaller too. The first camcorders used 1/2in and 2/3in CCD chips, today 1/4in CCDs are the norm.
This relative size ratio needs to be applied to the nominative focal length in order to really understand their effect. For APS-c size CCDs, this conversion factor is 1.5, thus the 50mm used on APS-c can have actually the optical effect of 75mm. Reversely, in order to have the effect of 50mm in an APS-c model, the focal length of the lens should be around 36mm.
This reverse side is more critical since today’s camcorder is more than adaquete in their telephoto end. But due to their small CCD size, in order to have a wide angle shot, the focal length of the lens has to be really small. You should look for camcorders with a focal range starting at 3.6mm or less (if they have a 1/4in CCD). Recently, it has been Panasonic that has done best in this department, offering a range of cameras with minimum focal lengths of 2.9mm, equivalent to using a 28mm wideangle on an SLR.
Lets take a look at the Tech Specs of Canon XL H1, flagship HD machine compared with Nikon D50.
| | Canon XL H1 | Nikon D50 |
| Size of CCDs | 1/3 inch, 16:9 widescreen | 23.7 x 15.5 mm |
| Pixels on CCD | 1.67 megapixels per CCD | 5.1M |
| Video Effective Pixels | 1.56M HD | |
| Shutter Speed | 1/15,000-1/4 | 1/4000-30sec |
| Lens F-Stop | f1.6-3.5 | Depend on the lens |
| Focal Length | 5.4mm-108mm(20x) | Depend on the lens |
So how big is 1/3 inch?
Sensor Type Designation
Sensors are often referred to with a "type" designation using imperial fractions such as 1/1.8" or 2/3" which are larger than the actual sensor diameters. The type designation harks back to a set of standard sizes given to TV camera tubes in the 50's. These sizes were typically 1/2", 2/3" etc. The size designation does not define the diagonal of the sensor area but rather the outer diameter of the long glass envelope of the tube. Engineers soon discovered that for various reasons the usable area of this imaging plane was approximately two thirds of the designated size. This designation has clearly stuck (although it should have been thrown out long ago). There appears to be no specific mathematical relationship between the diameter of the imaging circle and the sensor size, although it is always roughly two thirds.
Common Image Sensor Sizes
In the table below "Type" refers to the commonly used type designation for sensors, "Aspect Ratio" refers to the ratio of width to height, "Dia." refers to the diameter of the tube size (this is simply the Type converted to millimeters), "Diagonal / Width / Height" are the dimensions of the sensors image producing area.
| | | | Sensor (mm) | ||
| Type | Aspect Ratio | Dia. (mm) | Diagonal | Width | Height |
| 1/3.6" | 4:3 | 7.056 | 5.000 | 4.000 | 3.000 |
| 1/3.2" | 4:3 | 7.938 | 5.680 | 4.536 | 3.416 |
| 1/3" | 4:3 | 8.467 | 6.000 | 4.800 | 3.600 |
| 1/2.7" | 4:3 | 9.407 | 6.721 | 5.371 | 4.035 |
| 1/2.5" | 4:3 | 10.160 | 7.182 | 5.760 | 4.290 |
| 1/2" | 4:3 | 12.700 | 8.000 | 6.400 | 4.800 |
| 1/1.8" | 4:3 | 14.111 | 8.933 | 7.176 | 5.319 |
| 1/1.7" | 4:3 | 14.941 | 9.500 | 7.600 | 5.700 |
| 2/3" | 4:3 | 16.933 | 11.000 | 8.800 | 6.600 |
| 1" | 4:3 | 25.400 | 16.000 | 12.800 | 9.600 |
| 4/3" | 4:3 | 33.867 | 22.500 | 18.000 | 13.500 |
| 1.8" (*) | 3:2 | 45.720 | 28.400 | 23.700 | 15.700 |
| 35 mm film | 3:2 | n/a | 43.300 | 36.000 | 24.000 |
(*) Also called "APS-C". Many variants exist. APS-C film measures 25.1 x 16.7 mm, Sony's APS-C measures 21.5 x 14.4 mm, Nikon "DX" sensors measure 23.7 x 15.7 mm, while Canon has several (smaller and larger) variants, e.g. 22.2 x 14.8 mm and 28.7 x 19.1 mm.
It is fairly easy to see that the CCD size of Nikon D50 is 1.8 inch, which is roughly 21.5 times the size of a 1/3 inch CCD. This small size is a key reason for the image quality difference, especially in terms of noise and dynamic range. And here is why even the best camcorder today cannot be compared to real 35mm film. On the other hand, DSLR is quickly approaching the 35mm line of size. Some of high-end cameras is already equipped with a full frame CCD (Kodak DSC-14n, Canon EOS-1Ds Mark II). It is not improbable that in a few years major compact cameras will be equipped with 35mm sized CCD and professional cameras will be equipped with CCDs whose size mount to traditional 120 (645) or even 5x7, 8x10.
Here is a list of the size of other canon less expensive camcorders. 2006-7
| Specification Sheets | ||||||
| | CCD | # of | Optical | Still | LCD | Lowest |
| 1/4.5 | 1 | 18 X | 1280 x 960 | 2.5" | ||
| 1/3.4 | 1 | 11 X | 1632 x 1224 | 3.5" | ||
| 1/3.4 | 1 | 12 X | 1632 x 1224 | 2.5" | ||
| 1/6 | 1 | 18 X | 640 x 480 | 2.5" | ||
| 1/6 | 1 | 20 X | 1024 x 768 | 2.5" | ||
| 1/6 | 1 | 22 X | 1024 x 768 | 2.5" | ||
| 1/6 | 1 | 22 X | 1024 x768 | 2.5" | ||
| 1/6 | 1 | 20 X | 1024 x 768 | 2.5" | ||
| 1/4 | 1 | 16 X | 1280 x 960 | 2.5" | ||
| 1/4.5 | 1 | 16 X | 1280 x 960 | 2.5" | ||
| 1/4.5 | 1 | 14 X | 1280 x 960 | 2.5" | ||
| 1/3.4 | 1 | 14 X | 1632 x 1224 | 3.5" | ||
| 1/3.4 | 1 | 10 X | 1632 x 1224 | 2.5" | ||
| 1/3.4 | 1 | 10 X | 1632 x 1224 | 2.5" | ||
| 1/6 | 1 | 20 X | 1024 x 768 | 2.5" | ||
| 1/6 | 1 | 20 X | 1024x768 | 2.4" | ||
| 1/4.5 inch | 1 | 18 X | 1280 x 960 | 0.0" | | |
| 1/4.5 | 1 | 18 X | 12180x960 | 2.5" | | |
| 1/4.5 | 1 | 20 X | 1280x960 | 2.5" | ||
| 1/4.5 | 1 | 18 X | 1280 x 960 | 2.5" | ||
| 1/3.4 | 1 | 14 X | 1632x1224 | 2.5" | ||
| 1/4.5 | 1 | 14 X | 1152x864 | 2.4" | ||
| 1/3.4 | 1 | 10 X | 1632x1224 | 2.5" | ||
| 1/2.8 | 1 | 10 X | 2340x1736 | 2.5" | | |
| 1/6 | 1 | 25 X | | 2.7" | | |
| 1/6 | 1 | 25 X | 1024 x 768 | 2.7" | | |
| 1/5 | 1 | 20 X | 1152 x 864 | 2.7" | | |
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