Another thing that you must consider when looking at sensor size is something called “Diffraction Limiting”. For Standard Definition this is not as big a problem as it is for HD. With HD it is a big issue.
Basically the problem is that light doesn’t always travel in straight lines. When a beam of light passes over a sharp edge it gets bent, this is called diffraction. So when the light passes through the lens of a camera the light around the edge of the iris ring gets bent and this means that some of the light hitting the sensor is slightly de-focussed. The smaller you make the iris the greater the percentage of diffracted light with respect to non diffracted light. Eventually the amount of diffracted and thus de-focussed light will become large enough to start to soften the image.
With a very small sensor even a tiny amount of diffraction will bend the light enough to fall on the pixel adjacent to the one it’s supposed to be focussed on. With a bigger sensor and bigger pixels the amount of diffraction required to bend the light to the next pixel is greater. In addition the small lenses on cameras with small sensors means the iris will be smaller.
In practice, this means that an HD camera with 1/3? sensors will noticeably soften if it is more stopped down (closed) more than f5.6, 1/2? cameras more than f8 and 2/3? f11. This is one of the reasons why most pro level cameras have adjustable ND filters. The ND filter acts like a pair of sunglasses cutting down the amount of light entering the lens and as a result allowing you to use a wider iris setting. This softening happens with both HD and SD cameras, the difference is that with the low resolution of SD it was much less noticeable.
Over the next few posts I’m going to look at why sensor size is important. In most situations larger camera sensors will out perform small sensors. Now that is an over simplified statement as there are many things that effect sensor performance, including continuing improvements in the technologies used, but if you take two current day sensors of similar resolution and one is larger than the other, the larger one will usually outperform the smaller one. Not only will the sensors themselves perform differently but other factors come in to play such as lens design and resolution, diffraction limiting and depth of field, I’ll look at those in subsequent posts, for today I’m just going to look at the actual sensor itself.
Pixel size is everything. If you have two sensors with 1920×1080 pixels and one is a 1/3? sensor and the other is a 1/2? sensor then the pixels themselves on the larger 1/2? sensor will be bigger. Bigger pixels will almost always perform better than smaller pixels. Why? Think of a pixel as a bucket that captures photons of light. If you relate that to a bucket that captures water, consider what happens if you put two buckets out in the rain. A large bucket with a large opening will capture more rain than a small bucket.
Bigger pixels capture more light each.
It’s the same with the pixels on a CMOS or CCD sensor, the larger the pixel, the more light it will capture, so the more sensitive it will be. Taking that analogy a step further if the buckets are both of the same depth the large bucket will be able to hold more water before it overflows. It’s the same with pixels, a big pixel can store more charge of electrons before it overflows (photons of light get converted into electrical charge within the pixel). This increases the dynamic range of the sensor as a large pixel will be able to hold a bigger charge before overflowing than a small pixel.
All the electronics within a sensor generate electrical noise. In a sensor with big pixels which is capturing more photons of light per pixel than a smaller sensor, the ratio of light captured to electrical noise is better, so the noise is less visible in the final image, in addition the heat generated in a sensor will increase the amount of unwanted noise. A big sensor will dissipate any heat better than a small sensor, so once again the big sensor will normally have a further noise advantage.
So as you can see, in most cases a large sensor has several electronic advantages over a smaller one. In the next post I will look at some of the optical advantages.
Until a couple of years ago CMOS sensors were definitely the underdog, they tended to be very noisy due to electrical noise generated the on chip by the readout circuits and A/D converters. In addition they lacked sensitivity due to the electronics on the face of the chip leaving less room for the light sensitive parts. Today, on chip noise reduction has made it possible to produce CMOS sensors with very low noise and micro lenses and better design has mitigated most of the sensitivity problems. In terms of a static image there is very little difference between a CMOS sensor and a CCD sensor. Dynamic range is remarkably similar (both types of sensor use essentially the same light gathering methods), in some respects CMOS has the edge as they are less prone to overload issues. CCD’s are very expensive to manufacture as the way they are read out requires near lossless transfer of minute charges through a thousand or more (for HD) memory cells. The first pixel to be read passes down through over 1000 memory cells, if it was to loose 5% of it’s charge in each cell, the signal would be seriously reduced by the time it left the chip. The last pixel to be read out only passes through one memory cell, so it would be less degraded, this variation could ruin an image making it uneven. Although there is more electronics on a CMOS sensor, as each pixel is read directly a small amount of loss in the transfer is acceptable as each pixel would have a similar amount of loss. So the chips are easier to make as although the design is more complex, it is less demanding and most semiconductor plants can make CMOS sensors while CCD needs much more specialised production methods. Yes, CMOS sensors are more prone to motion artifacts as the sensor is scanned from top to bottom, one pixel at a time (A CCD is read in it’s entirety just about instantaneously). This means that as you pan, at the start of the pan the top of the sensor is being read and as the pan progresses the scan moves down the chip. This can make things appear to lean over and it’s known as skew. The severity of the skew is dependent on the readout speed of the chip. Stills cameras and mobile phone cameras suffer from terrible skew as they typically have very slow readout speeds, the sensors used in an EX have a much higher readout speed and in most real world situations skew is not an issue. However there may be some circumstances where skew can cause problems but my experience is that these are few and far between. The other issue is Flash Banding. Again this is caused by the CMOS scan system. As a flash gun or strobe light is of very short duration compared to the CMOS scan it can appear that only part of the frame is illuminated by the flash of light. You can reduce the impact of Flash Banding by shooting at the slowest possible shutter speed (for example shooting 25P or 24P with no shutter) but it is impossible to completely eliminate. When I shoot lightning and thunderstorms I often use a 2 frame shutter, shooting this way I get very few partial bolts of lightning, maybe 1 in 50. If you shoot interlace then you can use the Flash Band removal tool in Sony’s Clip Browser software to eliminate flash gun problems. CMOS sensors are becoming much more common in high end cameras. Arri’s new Alexa film replacement camera uses a CMOS sensor rated at 800asa with 13 stops of latitude. Red uses CMOS as does SI2K. Slumdog Millionaire (SI2K) was the first electronically shot film to get an Oscar for cinematography, so certainly CMOS has come a long way in recent years. CMOS is here to stay, it will almost certainly make bigger and bigger inroads at higher levels. Read speeds will increase and skew etc will become less of an issue. IMHO skew is not an issue to loose sleep over with the EX’s anyway. I shoot all sorts from hurricanes and tornadoes to fast jets and race cars. I have yet to come across a shot spoilt by skew, generally motion blur tends to mask any skew long before it gets noticeable. If you shoot press conferences or red carpet events where flash guns will be going off, then you may prefer a CCD camera as this is harder to deal with, but the EXs are such good value for the money and bring many other advantages such as lower power and less weight that you have to look at the bigger picture and ask what you expect from your budget.
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