Getting the right image ?
- Thread starter MrBeaker
- Start date
DVXGorilla
New member
Hello - I'm new to this forum, I hope I'm doing this the right way. My question is similar to Mr. Beakers, although a little more specific: I'm shooting green screen with my DVX, and aside from all the proper lighting and shooting with 24PA, I haven't a clue what other settings to properly utilize. I go into Scene File and I don't know where to start. I realize the DVX isn't the optimum camera to use for this, but it's what I've got and I'd like to use it properly. Any suggestions?
andy_starbuck
Well-known member
Chromakey
Getting good chromakey results depends on the entire workflow. You need to first light and stage a good keyable scene. Then you need to properly record it. And finally, you need to go through a post process to pull the key and create the composite using compositing software. Being able to pull the key at the end depends on staging and lighting at the beginning. So it's important to think of the entire workflow and not just one part of it.
There are a number of bits of advice that I have accumulated and will pass on to you here. A lot of the advice is about understanding what you are trying to accomplish in each part of the workflow and what has been found to be a useful technique by others.
The main thing you need to know is that your circumstances will be slightly different from anyone else's. The color of the screen may be slightly different. The color temperature, brightness, shape of the light cone, falloff, and distance of the lights may be different. The distance between the main subject and the backdrop may be different. Your software may be different. And all these differences combine to make your circumstance unique. So the best advice that I can give is to experiment with the entire workflow in an organized way so that you will come to understand what works best for your particular circumstances.
I will provide my best advice over the next several messages.
At the end I have provided a list of references to additional articles and resources you might find useufl.
Staging
Lighting the screen
Lighting the subject
Camera: Recording mode
Camera: White balance
Camera: Exposure
Camera: Gamma and Chroma settings
Post
There is a lot of help available in posts on DVXuser (search for it) and through online tutorials and on CDs/DVDs that can be ordered. Hopefully, the information I've provided will help you understand the overall workflow and what you are trying to accomplish in each part so that you will more easily be able to use these other resources to get the results you want.
a votre service,
Andy
Getting good chromakey results depends on the entire workflow. You need to first light and stage a good keyable scene. Then you need to properly record it. And finally, you need to go through a post process to pull the key and create the composite using compositing software. Being able to pull the key at the end depends on staging and lighting at the beginning. So it's important to think of the entire workflow and not just one part of it.
There are a number of bits of advice that I have accumulated and will pass on to you here. A lot of the advice is about understanding what you are trying to accomplish in each part of the workflow and what has been found to be a useful technique by others.
The main thing you need to know is that your circumstances will be slightly different from anyone else's. The color of the screen may be slightly different. The color temperature, brightness, shape of the light cone, falloff, and distance of the lights may be different. The distance between the main subject and the backdrop may be different. Your software may be different. And all these differences combine to make your circumstance unique. So the best advice that I can give is to experiment with the entire workflow in an organized way so that you will come to understand what works best for your particular circumstances.
I will provide my best advice over the next several messages.
At the end I have provided a list of references to additional articles and resources you might find useufl.
Staging
Lighting the screen
Lighting the subject
Camera: Recording mode
Camera: White balance
Camera: Exposure
Camera: Gamma and Chroma settings
Post
There is a lot of help available in posts on DVXuser (search for it) and through online tutorials and on CDs/DVDs that can be ordered. Hopefully, the information I've provided will help you understand the overall workflow and what you are trying to accomplish in each part so that you will more easily be able to use these other resources to get the results you want.
a votre service,
Andy
Last edited:
andy_starbuck
Well-known member
Chromakey: Staging and lighting
Chromakey: Staging and lighting
Staging
Good lighting means lighting the backdrop or screen properly and evenly. Placing the subject far enough in front of the screen that they can be independently lit with separate lights. And then ensuring that light from the screen doesn't reflect onto the subject (spill), and that shadows from the subject don't fall on the screen causing it to be unevenly lit.
The advice on staging varies, but it is often suggested that the subject be no closer than 10 feet in front of the screen, although 20 feet is better, and that the camera should be placed 20 feet in front of the subject. That's 30 to 40 feet. And rooms that size without windows (to provide better light control) are not so common. So very often we must make do with shorter distances and get the best lighting that we can under the circumstances, even though that means dealing with some spill and some shadows.
Lighting the screen
The screen needs to be lit and exposed so that it as provides as even a color as possible, of a medium to dull brightness. On those "making of" DVDs they always expose the shot for the subjects, and the green screens blow out to a day-glow green. That is definitely NOT what you want. Especially in closer distances, you want just enough color and brightness for your software to pull a decent key, but as dim as possible to keep spill light from bouncing onto your subject.
Getting even lighting isn't easy. Just point a light at a wall and take a look at it. The center is much brighter than the outside -- the light falls off brightness in a cone shape. And that shape, combined with intersecting of the cone-shape from other lights is going to create some kind of uneven light pattern on the screen. Soft lights can help. And also catching the edge of the light rather than the center bright spot on the screen can help light more evenly. So you will probably find that the instinct to point the light right at the middle of the screen will be the worst lighting you can accomplish. And if you have any way to block the light or control it, that will help.
The brightness of the lights is controlled mainly by moving them. When you move a light back from a target, when it seems like it is half-as bright, it's probably 1/4 as bright. The reason is that as the light is moved back it also illuminates a larger area -- so the light is spread out over a bigger square. Your eyes will fool you because they are great at adapting to different brightnesses of light. But it helps to know that moving a light can radically change the brightness. And a few inches can make a big difference.
As I said, your eyes will fool you. And that is also true of evenly lighting the screen. So what I would suggest is using your autoiris as a lightmeter. The way you do this is you zoom in on the screen and then watch the autoiris as you move the camera to different parts of the screen. The numbers will show you where the lighting is uneven and the screen is brighter or darker.
You will normally light the screen with two lights placed on either side of the screen and between 5 and 10 feet in front of it.
Lighting the subject
The ideal is that you should be able to light the subject separately from the screen. So turn off the screen lights and then light the subject. You will want a key light and a fill light to create modeling (the continuous change of light to shadow) that characterizes film and makes the subject look more realistic. You may need a rim light -- a low light that illuminates the back of the subject to make a rim or edge around one side. This will help cover spill and also provide a sharper edge for the pulling of the key. Finally, you need to light the subject so that they match the background scene they will be composited into. Sometimes strong lighting in the background scene that is copied into the lighting of the subject will "sell" the effect. Sometimes a very soft directionless lighting works well because the differences between the scene lighting and the subject lighting will be harder to detect.
See? You are trying to do three things at one time. First, you are trying to get the best lighting on the subject that you can to make that person look good. Second, you are trying to match the lighting from the background scene they are going to be composited into. And third, you are trying to light them for the key, to control spill from the screen onto the subject, and to avoid casting shadows from the subject onto the screen. That's what make lighting hard. It would be difficult enough to accomplish any one of these at a time, let alone all three at once.
Chromakey: Staging and lighting
Staging
Good lighting means lighting the backdrop or screen properly and evenly. Placing the subject far enough in front of the screen that they can be independently lit with separate lights. And then ensuring that light from the screen doesn't reflect onto the subject (spill), and that shadows from the subject don't fall on the screen causing it to be unevenly lit.
The advice on staging varies, but it is often suggested that the subject be no closer than 10 feet in front of the screen, although 20 feet is better, and that the camera should be placed 20 feet in front of the subject. That's 30 to 40 feet. And rooms that size without windows (to provide better light control) are not so common. So very often we must make do with shorter distances and get the best lighting that we can under the circumstances, even though that means dealing with some spill and some shadows.
Lighting the screen
The screen needs to be lit and exposed so that it as provides as even a color as possible, of a medium to dull brightness. On those "making of" DVDs they always expose the shot for the subjects, and the green screens blow out to a day-glow green. That is definitely NOT what you want. Especially in closer distances, you want just enough color and brightness for your software to pull a decent key, but as dim as possible to keep spill light from bouncing onto your subject.
Getting even lighting isn't easy. Just point a light at a wall and take a look at it. The center is much brighter than the outside -- the light falls off brightness in a cone shape. And that shape, combined with intersecting of the cone-shape from other lights is going to create some kind of uneven light pattern on the screen. Soft lights can help. And also catching the edge of the light rather than the center bright spot on the screen can help light more evenly. So you will probably find that the instinct to point the light right at the middle of the screen will be the worst lighting you can accomplish. And if you have any way to block the light or control it, that will help.
The brightness of the lights is controlled mainly by moving them. When you move a light back from a target, when it seems like it is half-as bright, it's probably 1/4 as bright. The reason is that as the light is moved back it also illuminates a larger area -- so the light is spread out over a bigger square. Your eyes will fool you because they are great at adapting to different brightnesses of light. But it helps to know that moving a light can radically change the brightness. And a few inches can make a big difference.
As I said, your eyes will fool you. And that is also true of evenly lighting the screen. So what I would suggest is using your autoiris as a lightmeter. The way you do this is you zoom in on the screen and then watch the autoiris as you move the camera to different parts of the screen. The numbers will show you where the lighting is uneven and the screen is brighter or darker.
You will normally light the screen with two lights placed on either side of the screen and between 5 and 10 feet in front of it.
Lighting the subject
The ideal is that you should be able to light the subject separately from the screen. So turn off the screen lights and then light the subject. You will want a key light and a fill light to create modeling (the continuous change of light to shadow) that characterizes film and makes the subject look more realistic. You may need a rim light -- a low light that illuminates the back of the subject to make a rim or edge around one side. This will help cover spill and also provide a sharper edge for the pulling of the key. Finally, you need to light the subject so that they match the background scene they will be composited into. Sometimes strong lighting in the background scene that is copied into the lighting of the subject will "sell" the effect. Sometimes a very soft directionless lighting works well because the differences between the scene lighting and the subject lighting will be harder to detect.
See? You are trying to do three things at one time. First, you are trying to get the best lighting on the subject that you can to make that person look good. Second, you are trying to match the lighting from the background scene they are going to be composited into. And third, you are trying to light them for the key, to control spill from the screen onto the subject, and to avoid casting shadows from the subject onto the screen. That's what make lighting hard. It would be difficult enough to accomplish any one of these at a time, let alone all three at once.
andy_starbuck
Well-known member
Chromakey: Camera settings
Chromakey: Camera settings
Camera: Recording mode
24PA -- 24 frame per second, Progressive, Advanced. Okay, but why?
Progressive frames don't have interlacing artifacts. And interlacing creates horrible artifacts in chromakey that are really difficult to work with. Basically, if an object (or the camera) moves too quickly, the blurred area will have "comb teeth" one pixel wide. And this creates problems pulling a good key.
24 frame per second -- to match your film project -- but also because the final composite has to be rendered. And 24 frame per second is 6 frames per second less than 30 -- so there is 20% less rendering work.
And.. Advanced mode... because the frames are stored in a way that makes it easier for your capture software to remove the "fill-in" frames to get from the 60 fields per second back to the original 24 frames per second. And you need to get rid of the fill in frames because they are made of the top field of one frame and the bottom field of another frame -- creating more interlacing artifacts that won't key cleanly.
Camera: White balance
Remember to set the white balance with the screen NOT lit and the subject lit. In an ideal lighting circumstance, there should be no spill on the subject. In most situations, where the room is not large, there will be some ambient soft spill light over the entire scene which will screw up the coloring of the subject. So typically, with a green screen, the spill light will cause the subject to be slightly reddish-orange tinted. What you are looking for is a scene in which the subject can be color-corrected in post to compensate for the ambient spill. If you white balance with the screen lit, then you may get a better original color on your subject, but it will be at the expense of the coloring of the screen itself. We want the screen colored in the extreme so that it's easier to pull a key.
Camera: Exposure
Various suggestions on relative brightness between the subject and the background exist. One rule of thumb is that if you look at the autoiris on the screen alone, and then you look at the autoiris on the subject alone, the subject should be about 1 full stop brighter than the background. That difference in brightness will help your software pull a cleaner key. That will help you to make sure the background isn't too brightly lit.
Remember to expose for your subject and then set the iris to manual before turning on the background lighting.
Camera: Gamma and Chroma settings
The gamma and chroma settings can make a big difference, but it has to be matched up with the color of your screen. What you might want to do is run through the various gamma and chroma combinations, taking sample shots of a still figure or model, and then bring the footage into your compositing program and SEE which combination produces the best key.
In my circumstance, the best key came from Cine Gamma D and Cine Color, which was fortunate since that was what I was shooting the rest of the movie with, and it probably helped the subject fit the background. But experiment -- your circumstances may be different.
Chromakey: Camera settings
Camera: Recording mode
24PA -- 24 frame per second, Progressive, Advanced. Okay, but why?
Progressive frames don't have interlacing artifacts. And interlacing creates horrible artifacts in chromakey that are really difficult to work with. Basically, if an object (or the camera) moves too quickly, the blurred area will have "comb teeth" one pixel wide. And this creates problems pulling a good key.
24 frame per second -- to match your film project -- but also because the final composite has to be rendered. And 24 frame per second is 6 frames per second less than 30 -- so there is 20% less rendering work.
And.. Advanced mode... because the frames are stored in a way that makes it easier for your capture software to remove the "fill-in" frames to get from the 60 fields per second back to the original 24 frames per second. And you need to get rid of the fill in frames because they are made of the top field of one frame and the bottom field of another frame -- creating more interlacing artifacts that won't key cleanly.
Camera: White balance
Remember to set the white balance with the screen NOT lit and the subject lit. In an ideal lighting circumstance, there should be no spill on the subject. In most situations, where the room is not large, there will be some ambient soft spill light over the entire scene which will screw up the coloring of the subject. So typically, with a green screen, the spill light will cause the subject to be slightly reddish-orange tinted. What you are looking for is a scene in which the subject can be color-corrected in post to compensate for the ambient spill. If you white balance with the screen lit, then you may get a better original color on your subject, but it will be at the expense of the coloring of the screen itself. We want the screen colored in the extreme so that it's easier to pull a key.
Camera: Exposure
Various suggestions on relative brightness between the subject and the background exist. One rule of thumb is that if you look at the autoiris on the screen alone, and then you look at the autoiris on the subject alone, the subject should be about 1 full stop brighter than the background. That difference in brightness will help your software pull a cleaner key. That will help you to make sure the background isn't too brightly lit.
Remember to expose for your subject and then set the iris to manual before turning on the background lighting.
Camera: Gamma and Chroma settings
The gamma and chroma settings can make a big difference, but it has to be matched up with the color of your screen. What you might want to do is run through the various gamma and chroma combinations, taking sample shots of a still figure or model, and then bring the footage into your compositing program and SEE which combination produces the best key.
In my circumstance, the best key came from Cine Gamma D and Cine Color, which was fortunate since that was what I was shooting the rest of the movie with, and it probably helped the subject fit the background. But experiment -- your circumstances may be different.
Last edited:
andy_starbuck
Well-known member
Chromakey: Post and More Information and Articles
Chromakey: Post and More Information and Articles
Post
For standard definition DV video, the trick to getting a decent composite is to apply a small blur -- maybe a couple of pixels -- to the chroma (color). DV is, more ore less, a full resolution monochrome image with a 1/4 resolution color image overlaid on top of it. (I'm generalizing here, but you get the point). The eyes of the viewer have cones and rods -- we sense gray scale separately from color. The two images are put together in the visual processing systems of the brain, where the larger regions of color are interpreted by the brain and matched up with the higher resolution monochrome image -- creating the illusion of a full color, full resolution image. When we change the color -- through compositing using chromakey -- the illusion is disturbed -- creating all kinds of digital artifacts. Most commonly, the edge shifts from one big color pixel to another as the subject moves, giving a weird cut-out effect where the edge is always in motion and little green dots of background color are randomly appearing and disappearing. You can compensate for this by blurring the chroma edge before the key is pulled.
There are a number of tutorials available depending on the compositing software you are using and also the keying utility. There are some GREAT professional keying packages available today. Just prepare to spend some time learning how to use them.
My Favorite Chormakey References
Microfilmmaker.com http://microfilmmaker.com/ is an online magazine dedicated to low-budget to no-budget video. And they provide a lot of great information for free.
Videocopilot.net http://www.videocopilot.net/ has a LOT of free tutorials for After Effects and some of the greatest effects and training packages available. SeriousFx is just about the best training for After Effects I've seen and it covers how to do a greenscreen composite using DV footage. http://www.videocopilot.net/product...filmmaker.com/tipstrick/Issue22/baschrom.html
"Tips for Shooting Greenscreen with the HVX and DVX cameras" http://microfilmmaker.com/tipstrick/Issue22/DVXHVX1.html
"Learning to Light: Distance vs. Power" http://microfilmmaker.com/tipstrick/Issue12/light1.html
Greenscreens
"The Great Greenscreen Roundup" http://microfilmmaker.com/tipstrick/Issue22/grnround1.html
"Build a $60 Portable Greenscreen" http://microfilmmaker.com/tipstrick/Issue22/buildgrn.html
Software Information
"Primatte Keyer 4" http://microfilmmaker.com/reviews/Issue31/PKeyer4_1.html
"Ultra CS3" http://microfilmmaker.com/tipstrick/Issue22/Ultra_1.html
"zMatte" http://microfilmmaker.com/tipstrick/Issue22/zMatte_1.html
"Fixing Poor Greenscreens with CompositeLab" http://microfilmmaker.com/tipstrick/Issue13/poor_grn.html
Chromakey: Post and More Information and Articles
Post
For standard definition DV video, the trick to getting a decent composite is to apply a small blur -- maybe a couple of pixels -- to the chroma (color). DV is, more ore less, a full resolution monochrome image with a 1/4 resolution color image overlaid on top of it. (I'm generalizing here, but you get the point). The eyes of the viewer have cones and rods -- we sense gray scale separately from color. The two images are put together in the visual processing systems of the brain, where the larger regions of color are interpreted by the brain and matched up with the higher resolution monochrome image -- creating the illusion of a full color, full resolution image. When we change the color -- through compositing using chromakey -- the illusion is disturbed -- creating all kinds of digital artifacts. Most commonly, the edge shifts from one big color pixel to another as the subject moves, giving a weird cut-out effect where the edge is always in motion and little green dots of background color are randomly appearing and disappearing. You can compensate for this by blurring the chroma edge before the key is pulled.
There are a number of tutorials available depending on the compositing software you are using and also the keying utility. There are some GREAT professional keying packages available today. Just prepare to spend some time learning how to use them.
My Favorite Chormakey References
Microfilmmaker.com http://microfilmmaker.com/ is an online magazine dedicated to low-budget to no-budget video. And they provide a lot of great information for free.
Videocopilot.net http://www.videocopilot.net/ has a LOT of free tutorials for After Effects and some of the greatest effects and training packages available. SeriousFx is just about the best training for After Effects I've seen and it covers how to do a greenscreen composite using DV footage. http://www.videocopilot.net/product...filmmaker.com/tipstrick/Issue22/baschrom.html
"Tips for Shooting Greenscreen with the HVX and DVX cameras" http://microfilmmaker.com/tipstrick/Issue22/DVXHVX1.html
"Learning to Light: Distance vs. Power" http://microfilmmaker.com/tipstrick/Issue12/light1.html
Greenscreens
"The Great Greenscreen Roundup" http://microfilmmaker.com/tipstrick/Issue22/grnround1.html
"Build a $60 Portable Greenscreen" http://microfilmmaker.com/tipstrick/Issue22/buildgrn.html
Software Information
"Primatte Keyer 4" http://microfilmmaker.com/reviews/Issue31/PKeyer4_1.html
"Ultra CS3" http://microfilmmaker.com/tipstrick/Issue22/Ultra_1.html
"zMatte" http://microfilmmaker.com/tipstrick/Issue22/zMatte_1.html
"Fixing Poor Greenscreens with CompositeLab" http://microfilmmaker.com/tipstrick/Issue13/poor_grn.html
I, too, have become a lot wiser thanks to Andy's and Barry's posts in this thread. For everyone's benefit (including mine) I copied all the good stuff out of this thread and made a Word document out of it for fast referencing. Especially if you're out shooting.
:dankk2:
View attachment Ultimate DVX Listing.zip
:dankk2:
View attachment Ultimate DVX Listing.zip
CameronMcC
Well-known member
this is a great read, but i wish the original posts by andy_starbuck could be cleaned up to not have all the QUOTE&# stuff that probably came with a html switch somewhere.
ward70morris
New member
Getting the right image ?
A fluid header needs to be sliced into 2 or 3 pieces. If it's a 2 pc header, you'll have the logo and the header background that repeats across the width of the forum. If it's a 3 pc, you'll have a logo on the left, piece of the header on the right, and again the header background that repeats to cover the width of the forum.
________________________
building chicken coops
A fluid header needs to be sliced into 2 or 3 pieces. If it's a 2 pc header, you'll have the logo and the header background that repeats across the width of the forum. If it's a 3 pc, you'll have a logo on the left, piece of the header on the right, and again the header background that repeats to cover the width of the forum.
________________________
building chicken coops
Prather Productions
Member
Quote:
2. SELECTING THE SCENE FILE SETTING
There is a dial on the back left of the camera with numbers on it going from "f1" through "f6". These are scene files, and they control a number of internal settings of the camera. I will assume for the moment that you haven't modified any of these scene files using the on screen menu, and that they are still at their factory defaults. The most important positions for you to know are "f1" and "f5". You can read about the rest of the default scene files in the manual. "f1" gives you 60 frame per second interlaced video, which looks sharp like a news video. "f5" gives you 24 frame per second video with color correction that looks a lot like the images you see in a movie -- from film. The main things you need to know about these two settings is that the AutoFocus works very slowly in 24p (f5) mode. It takes several seconds to refocus -- not like a consumer video camera. The AutoFocus in 60i (f1) mode operates as you would expect. The other thing that you need to know is that you need to choose either 60i or 24p for an entire project or video. You can't easily mix the two different types of material on a single time line in a non-linear editing program. Select (f1) or (f5).
Thanks Andy for that. Could you possibly take this a step further? I'm thinking of trying the F1 scene file at that 60i for my kids football game this Saturday. I used a different file for last Saturday's game (I think F3?) and was on 24P, I believe. Came out good, but would like to try something new. I had just got a Hoya circular polarizer (HD CIR-PL) and had that on the lens last week too (used 1/8 ND filter). My question is, since all the presets are at 0 right now on my F1 scene file, are there any corrections you would suggest as far as coring, detail, etc. for a sports shot on a bright sunny day AND would edit on my PC Avid software work ok with 60i? THANKS!
Ps. I thought you had to turn the circular polarizer just right, to get it to work, but did not notice any change as I was turning.
Dave
2. SELECTING THE SCENE FILE SETTING
There is a dial on the back left of the camera with numbers on it going from "f1" through "f6". These are scene files, and they control a number of internal settings of the camera. I will assume for the moment that you haven't modified any of these scene files using the on screen menu, and that they are still at their factory defaults. The most important positions for you to know are "f1" and "f5". You can read about the rest of the default scene files in the manual. "f1" gives you 60 frame per second interlaced video, which looks sharp like a news video. "f5" gives you 24 frame per second video with color correction that looks a lot like the images you see in a movie -- from film. The main things you need to know about these two settings is that the AutoFocus works very slowly in 24p (f5) mode. It takes several seconds to refocus -- not like a consumer video camera. The AutoFocus in 60i (f1) mode operates as you would expect. The other thing that you need to know is that you need to choose either 60i or 24p for an entire project or video. You can't easily mix the two different types of material on a single time line in a non-linear editing program. Select (f1) or (f5).
Thanks Andy for that. Could you possibly take this a step further? I'm thinking of trying the F1 scene file at that 60i for my kids football game this Saturday. I used a different file for last Saturday's game (I think F3?) and was on 24P, I believe. Came out good, but would like to try something new. I had just got a Hoya circular polarizer (HD CIR-PL) and had that on the lens last week too (used 1/8 ND filter). My question is, since all the presets are at 0 right now on my F1 scene file, are there any corrections you would suggest as far as coring, detail, etc. for a sports shot on a bright sunny day AND would edit on my PC Avid software work ok with 60i? THANKS!
Ps. I thought you had to turn the circular polarizer just right, to get it to work, but did not notice any change as I was turning.
Dave
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No Label Productions
New member
WoW!!! great post.. it helped a lot...thanks
flash roberts
Member
Very good & quite helpfull advise thanks Andy Starbuck,
but I wish you didn't keep using the ( "A", "B", and "Preset") stuff, as it makes the sentences & info hard to read through properly !
but I wish you didn't keep using the ( "A", "B", and "Preset") stuff, as it makes the sentences & info hard to read through properly !
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flash roberts
Member
Hahaha someone said " Andy should write a book," I agree he is very knowledgeable & informative, but that would be a very difficult book to read with Andy's, "A" "B" and "Preset" stuff, just saying ? 
flash roberts
Member
Most on here are talking about DVX100 & 100B
I have a DVX102B anyone tell me the difference between them, they all seem to have the same or similar form factor ?
I have a DVX102B anyone tell me the difference between them, they all seem to have the same or similar form factor ?