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Why is Sensor Size Important: Part 2, Diffraction Limiting

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.

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Why Is Sensor Size Important: Part 1.


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:

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.

pixels Why Is Sensor Size Important: Part 1.
Small pixels capture less light each

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.

Noise:

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.

Sony PVM-740 OLED Monitor. Like looking through a window!

pvm-740 Sony PVM-740 OLED Monitor. Like looking through a window!
Sony PVM-740

Sony PVM-740 OLED monitor

One of the things that really caught my eye at NAB was Sony’s new PVM-740 field monitor. This is one of the first professional monitors to use OLED technology (Organic Light Emitting Diode). Traditional LCD screens work by using a backlight that has a liquid crystal panel in front of it. When a charge is applied to the liquid crystals they change the polarisation of the light passing through them, this light then passes through a second polariser and between them they vary the amount of light passing through the panel to the viewer (If you have ever seen a VariND filter or tried twisting one polarising filter relative to another you can see how this works). While on the whole this works reasonably well there are some issues with this technology. The first is that the liquid crystals never fully block the passage of all the light, so black is never truly black, some light always seeps through. In addition brightness is limited to that of the backlight and the light is attenuated as it has to pass through the crystals and polariser. In addition if the backlight is too bright then the blacks get brighter too which limits the overall contrast range. Another issue is that LCD’s take time to change state from on to off and off to on. This leads to lag and smear with fast motion or high refresh rates. While a lot of money has been spent over the years developing LCD technology and there are some excellent LCD monitors available, these issues still exist and LCD performance still lags behind that of CRT’s.

Enter OLED. Organic Light Emitting Diode displays use a grid of light emitting devices, each pixel is a separate emitter, so when it’s off, it’s truly off. This means that blacks are completely black. When the emitter is on the light it emits is not passing through a polariser or crystal so it’s brightness is not diminished, this means that whites are really bright. In addition you can switch an LED on and off pretty much instantly so there is no lag or smearing. When you see the new Sony PVM-740 OLED monitor side by side with a similar LCD monitor the difference is striking! It’s like looking through a window, the image is clear and crisp, blacks are… well.. black and whites are bright and sparkle. The pictures from the PVM-740 are much more like the images you would expect to get from a top spec CRT monitor, yet the 740 is light weight, compact and uses less power. It should also be more robust and will not be affected by magnetic fields like a CRT monitor.

You really need to see this monitor in the flesh to appreciate the images it produces.

Sony To Launch 35mm compact camcorder


Well one of the big NAB announcements was the intention by Sony to release a compact 35mm equipped camera. A prototype unit was briefly shown at the press event but little details were given. I’ve been trying to find out more but Sony are being very tight lipped. It was announced that it will be available prior to next NAB, so that means that there is likely to be some other launch event in, at my guess 6 to 8 months time, of course there may be more news before then. Last year Sony announced the PMW-350 at Satis which this year is October 19th/20th, so I would expect more news by then. What I would say is that the prototype appears to be more than just a simple mock up as it shows some new switch and control designs that I’ve not seen on a Sony camera before. It has also been stated that this new unit is just part of Sony’s 35mm road map so perhaps there will be more than one new camera. As for pricing, well all that Sony will say is “affordable”. My guess is it will be in the XDCAM EX1 price area depending on lens options. If it uses DSLR lenses and can be purchased without a lens, my guess is that it would be cheaper than an EX but more expensive than a Canon 5DMk2, my guess would be £4k.

So what features can we expect to get? The sensor should have large pixels so it should be good in low light and offer high dynamic range. As it has a 35mm sensor I expect it will shoot 24P, 25P, 30P plus I would hope over-cranking at up to 60fps. It would be really nice if it did 1080P60. It should have decent audio controls and it’s going to need a really, really good viewfinder. I would imagine that you will have a number of different lens options via some form of adapter, possibly being supplied with a Sony Alpha lens mount. The prototype was shown with a PL mount lens and very few users will be able to afford to use them, so there will have to be a lower cost option.

These are exciting times. In the next 12 months we will see a huge change in the tools available for video production. The new 35mm and 4/3? cameras from Sony, Panasonic and Canon will change the way TV is made forever. They won’t be ideal for some applications such as news or run and gun, where you don’t want the focus problems that a big sensor will bring, but for drama, documentary and low budget movies I think we will see a dramayic change in the way things are done.

The Great CMOS Debate. More on CMOS vs CCD.

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.

PMW-350 Scene Files for Download

PMW-350 Scene Files for Download

Alister-350-Scene-flies1

Click on the link above to download a set of my latest scene files. Un-zip and copy to the root of an SxS card, the in the file menu load the files.

These are mainly matrix tweeks. neut2 is one I like that gives rich primary colours while still reasonably true to life. Cine1 is a sudo filmic look Film1 is meant to emulate well saturated film stock DSC-1 is based on Chroma-Du-Monde chart for accurate daylight color Neut is my first matrix tweak for a less green look and warmer skin tones.

Vapour: The Sky in motion.

The last couple of days provided some really spectacular skies with amazing vapor trails and fluffy clouds. So I grabbed a selection of cameras and shot a bunch of timelapse which you can see cobbled together in this clip. I used a Sony PMW-350 and a Canon 350D and 550D. Amazing how well they match after a quick grade. I also included an previously un-used Aurora shot from Iceland at the end. The name of the clip comes from the fact that clouds are made of water vapor and ice crystals.

You can also watch it in HD on Exposure Room.

Canon T2i, first impressions, initial tests (frame grabs and video supplied).

t2i-EX Canon T2i, first impressions, initial tests (frame grabs and video supplied).
EX1 on left, T2i on right

As you may have seen from my earlier post I became the owner of the new Canon T2i (or 550D as it’s known in the UK) at the weekend. Clearly before using any camera in anger it’s important to see what it can and can’t do. I will say that I am not a Canon DSLR expert. I have been following the fuss and much admire some of the work done with these cameras by Phil Bloom, but frankly after playing with the Canon over the weekend I have to say I’m disappointed. Yes you can achieve shallow depth of field very easily and you do get a filmic look to the pictures, but look at the footage on a big monitor and it just looks soft. At first I wondered if this was the lens I was using, so I tried a couple of others including a nice Tamron 28mm prime. I tried different apertures, shutter speeds etc, but every clip I’ve taken looks soft. In isolation, on scenes with low detail this isn’t immediately apparent, but anything with lots of fine detail looks soft. Some of this is aliasing, look at the roof of the house in the T2i image, it appears to have diagonal roof tiles, this is a pretty typical aliasing artifact. I shot some closer shots of the buildings and the brickwork aliased like crazy.

t2i-flowers Canon T2i, first impressions, initial tests (frame grabs and video supplied).
EX1 on right, T2i on left

Looking at the flowers picture you can see that the EX1 has picked up more of the subtle texture, or at least it has recorded more of the texture. I’m sure some of the Canon’s softness is due to compression artifacts. The other thing that I found is that it is tending to crush blacks a bit. I have played around with the picture styles and you can reduce this a bit, but there is very little detail in deep blacks, which would IMHO make grading tricky. The one good thing I did find was that it is very noise free at 200 and 400 asa, it’s also useable up to 800 asa or at a push 1600asa, so it would make a good camera for very low key scenes, provided you use a good fast lens. Looking at the Canon pictures there was something pleasing about the deep, almost crushed blacks. I think this helps contribute to the Canon DSLR “look” so I quickly threw together a new picture profile for the EX1/3 and PMW-350, but I’m afraid that the details of that will be the subject of another post, as I have work that I must do first! The EX images in the frame grabs were shot with this picture profile. As we all know the ergonomics of the video DSLR’s is pretty poor for video. It’s tricky to hold and you have to use an add on Loupe to make the LCD useable as a viewfinder. You can’t zoom mid shot and without peaking or zebras adjusting exposure and focus accurately is difficult. I was hoping to be able to use the 550D as a B camera for those situations where I need a small, discreet camera, but having seen the pictures, so far, for me it will be reserved for holidays and shooting where you not supposed to video and for shoots where supper shallow DoF is essential. I have to say I’m really disappointed, I wanted this camera to be so much better, I knew it would suffer from aliasing, but I wasn’t expecting the soft pictures, I guess some will say that the softness adds to the filmic look, but I’d much rather do that with some nice pro-mists or filtration in post production rather than starting out with soft pictures. Perhaps I’ve done something wrong? If I have please add a comment!

UPDATE: I was so convinced that I must be doing something wrong that I shot some more clips, this time with less harsh lighting. No, change however, the T2i is still soft and the new clips show just how big a problem aliasing is. You have to consider that the coloured moire patterns are recorded like that, no amount of grading will get rid of it. A small amount of diffusion on the camera should help, but then your going to have to work out how much to soften and diffuse each shot to make sure your not making the pictures even softer than they already are.

t2i-aliasing Canon T2i, first impressions, initial tests (frame grabs and video supplied).
EX top, Canon T2i bottom

The frame grabs are all 1:1 pixel for pixel, no trickery has been used! You can download some further examples by clicking here. Even if you were shooting stuff for the web this level of aliasing could cause big problems as it’s really obvious. For this shot I had the Canons sharpness setting turned all the way down. I have also turned down the contrast setting as this gives better dynamic range with less crushed blacks. My workflow is to import the H264 files from the camera and then convert them to ProResHQ. This helps a little with sharpness over working with the native H264, but for me this last test was the nail in the coffin for DSLR’s as footage like this would simply be unusable. If you watch the YouTube clip please make sure you watch it full screen or at least at the 480P setting. The small embedded size doesn’t show the aliasing as much as the bigger versions.
FURTHER UPDATE:
OK, so it’s defiantly not just me doing something wrong. When in focus the T2i/550D aliases (as do all the current Canon DSLR’s). This is a grab from Philip Blooms latest Canon short. For once this is a daylight piece and as I expected it exhibits a lot of aliasing. The grab is actually taken from the thumbnail on his exposure room page. I’m really pleased to see this as it shows that aliasing is a problem for the experts too. You start to appreciate why so many of the Canon shorts are shot at night, with millimeter deep DoF… it’s to stay clear of having stuff in focus that will alias. there are filters from Caprock that are supposed to help, but you need a different filter for each focal length and aperture that you use, they also soften the picture somewhat.

PBaliasing Canon T2i, first impressions, initial tests (frame grabs and video supplied).
Aliasing in Phil Blooms latest edit

If you want my opinion, then it has to be that the Canon’s are close, but still a mile away. The aliasing issue is a biggie. Sort it out and the skew, jello and overheating can be worked around, but if you have to worry about simply having a piece of wood in focus and whether it’s going to exhibit rainbows of colour or whether cobble stones will twitter and change colour (At 00.35 and this is from Canon) then it will limit what you can do. There is quite a lot of aliasing in Phil’s new daytime clip, basically anytime anything is steady, has texture and is in focus, it aliases. I’ve been shot down in flames on other forums for saying that this is a problem, but if even the experts can’t deal with it then what hope does everyone else have? I would love to have the option of shooting with the shallow DoF that the Canon’s offer, but not at the expense of having to avoid any kind of texture. Perhaps Red and Scarlet will be better, perhaps Canon will sort it out, or perhaps not, as the cameras are clearly selling like hot cakes, even with the issues. If they do fix it then the camera will almost certainly be for video only.

XDCAM EX Firmware for new cards.. Available now!

Sony have just released the firmware required to use the new 32Gb SxS-1 cards, the Memory Stick  and SD Card adapters. This firmware also adds the ability to have much longer clip titles (46 Characters!) on the PMW-350 and EX1R. You can do the update yourself, you don’t have to send the camera back to Sony (hooray) to do the update. CLICK HERE to go to the firmware on the Sony UK site. Initial reports indicate that the update improves the performance of 3rd party SD card adapters and is straight forward to install. There are also indications that original EX3?s boot up a bit faster. At the moment the EX1 update appears to be missing from the download page, but I have been assured this will be rectified in the next 24 hours.

How I shoot the Northern Lights

Well I have just returned from Iceland where I held a couple of 3D stereoscopic master classes and a workshop on video for the internet. They went well and we all had fun despite almost a foot of snow fall the morning of the classes. On the last day of my trip I decided to try and get some more Northern Lights footage. As I am often asked how I do this I put together the clip below which explains what settings I use for the Aurora and also gives a brief description of S&Q on an XDCAM EX. Basically what I do is use the EX Slow Shutter at 32 or 64 frames to increase the sensitivity of the camera. For a dim Northern Lights display I use 64 frames but for a bright display I drop down to 32 frames. The slow shutter acts like a long exposure on a stills camera. I then combine this with interval record shooting at 1 frame every second. I did also have a Canon DSLR with me and tried to shoot the Aurora with that. I found I needed a 10 second exposure at 800 asa to get a similar result to that achieved with the EX. The 10 second exposure means that it would take longer to get a decent length video sequence and most of the motion of the Aurora would be lost. Some of the exposure difference was I admit to the slower F4 lens on the Canon compared to the Sony EX’s F1.8, so perhaps with a faster lens you could bring the exposure down to around 5 seconds and this is something I hope to try when I go Aurora chasing next winter.

If you watch the video make sure you stay to the end to check out my attempt to record a piece to camera in 60 mph blowing snow! Don’t know why I even thought it would work. What I will say is that my new Vinten 5AS did a great job of keeping the camera steady in some pretty extreme conditions.

Camera setup, reviews, tutorials and information for pro camcorder users from Alister Chapman.