How do auto-iris/auto aperture and auto gain/auto ISO work together?

[Chris7]

Hello. I'm a HF100 owner looking to move up to the prosumer world (Canon XH A1 is on my wish list). Got a few questions on some videography fundamentals:

Can someone get me to the right links or otherwise explain how auto-iris and auto gain work in consumer/prosumer cameras like a Canon HV20 or XH A1? I'm assuming that, when my HF100 is set to full auto, the camera adjusting both the aperture and gain automatically?

1. Is this correct? Is this option also possible on a prosumer camera like the XH A1?

In a DSLR, you just set the ISO (gain) and choose shutter speed prio or aperture prio, allowing one or the other to be controlled automatically (unless you set everything manually). Of course on a video camera, the shutter speed must be fixed. I would imagine I could from there also set either the aperture and let the gain be automatic, or set the gain and let the aperture be automatic. Except, if the aperture is way too high or way too low I would be over/under-loading the sensor to the point that it could not compensate with gain.

2. Do experienced videographers generally manually set the aperture and let the gain be automatic, or vice versa?

3. Last question: Many pros choose to disable both auto gain and auto aperture. Do broadcast cameras ($20K+) generally have the option of auto gain and auto aperture?

Thanks in advance,
Chris

 

[ppc_michael]

I believe first the iris is opened as much as it can, and once the iris is wide open, if it still needs more it will raise the gain. I don't think shutter strays automatically unless you have some other funky settings going on.

Professional videographers in a controlled setting (such as, say, a sitcom taping) will set everything to full manual and try hard to adjust only the iris. If it's still too dark you add more light. In less controlled environments such as documentary or ENG where the point is to capture the event no matter what, then sometimes you'll see gain kicked up.

Broadcast cameras do tend to come with some form of auto exposure, but I've never been in a situation where I've used it.

 

[MrLatte23]

Canon's Auto-shutter...

Some Prosumer Canons have a setting that compensates for too much light by turning the shutter on automatically. This keeps the exposure within an acceptable range but also strobes any fast moving action. They also have as you stated TV and AV settings which allow you to choose your aperture or speed and let the camera determine the rest.

 

[LethalWolfe]

Experienced shooters rarely use auto anything because they don't want the camera to do something stupid and screw up the shot.


Lethal

 

[Chris7]

Here’s a link to a video of a pro cinematographer shooting on a RED One with a $57K lens. Manual focus, gain, and aperture. What’s interesting to me is the footage he shows about half way through the video. The first clip where the female subject runs to the window looks a lot like what one might come across in a documentary, and it’s constantly a stop over or underexposed, as the shooter describes.

http://www.studiodaily.com/main/video/9187.html

Broadcast cameras do tend to come with some form of auto exposure, but I've never been in a situation where I've used it.

Experienced shooters rarely use auto anything because they don't want the camera to do something stupid and screw up the shot.
Lethal

Wow. So are they adjusting gain/aperture in the middle of a shot? Seems like when shooting sports, for example, one end of the football field will have more sun than the other, and what about when the ball’s in the air?

I believe first the iris is opened as much as it can, and once the iris is wide open, if it still needs more it will raise the gain...

OK this makes sense intuitively. The cameral would be programmed to open the iris as much as possible, then start boosting the gain.

Thanks,
Chris

 

[LethalWolfe]

Wow. So are they adjusting gain/aperture in the middle of a shot? Seems like when shooting sports, for example, one end of the football field will have more sun than the other, and what about when the ball’s in the air?

You expose for the subject and let every thing else fall where it may. A camera on auto doesn't know what the subject is and will try and expose for the whole frame which could ruin the shot. In your example about following the ball you'd just have to ride the iris and do it smooth enough so it doesn't look distracting. It's been a while since I've shot, but there were times where I adjusted focus, zoom, and iris all at the same time. For big events the camera ops typically only control zoom and focus and a guy back in the production truck controls the exposure and color for each camera to try and make all the cameras have a consistent look.

Not to state the obvious, but w/still cameras you get to reset your shot between pictures but w/moving picture cameras you have to do everything on while you are taking pictures. This is why lenses for still cameras don't make very good lenses for video or film cameras. Video and film lenses are geared to tighter tolerances so you can smoothly adjust focus, zoom and/or iris while shooting where as still lenses aren't because they don't need to be. Well, that might change down the road as still and video cameras continue to converge.


Lethal

 

[Gymnut]

Hello. I'm a HF100 owner looking to move up to the prosumer world (Canon XH A1 is on my wish list). Got a few questions on some videography fundamentals:

Can someone get me to the right links or otherwise explain how auto-iris and auto gain work in consumer/prosumer cameras like a Canon HV20 or XH A1? I'm assuming that, when my HF100 is set to full auto, the camera adjusting both the aperture and gain automatically?

1. Is this correct? Is this option also possible on a prosumer camera like the XH A1?

In a DSLR, you just set the ISO (gain) and choose shutter speed prio or aperture prio, allowing one or the other to be controlled automatically (unless you set everything manually). Of course on a video camera, the shutter speed must be fixed. I would imagine I could from there also set either the aperture and let the gain be automatic, or set the gain and let the aperture be automatic. Except, if the aperture is way too high or way too low I would be over/under-loading the sensor to the point that it could not compensate with gain.

2. Do experienced videographers generally manually set the aperture and let the gain be automatic, or vice versa?

3. Last question: Many pros choose to disable both auto gain and auto aperture. Do broadcast cameras ($20K+) generally have the option of auto gain and auto aperture?

Thanks in advance,
Chris

I've owned the HV20, HV30, XHA1, sold them and now using a Panasonic HMC150. The XHA1 has Auto Gain Control which I recommend leaving off because who knows how much gain the camera is kicking in. The XHA1 doesn't exactly hold up well in low light.

Prosumer cameras, like the XHA1, enable you to preprogram differing levels of gain(Low/Med/Hi), where varying levels of gain, measured in db(decibels) can be assigned, i.e. +3, +6, +12, -5, etc.

Like the HV20/30/40, you have various options of recording on the XHA1: Av, Tv, M. Recording in Av(Aperture Value) mode will allow you to manipulate the aperture while the camera will compensate by adjusting the shutter speed(This can be hazardous depending on your subject). Tv(Time Value), allows you to manipulate the shutter speed while the camera automatically adjusts the aperture.

Like Lethal pointed out, recording full manual obviously gives you greater control without the camera doing something stupid like dropping the shutter speed while you're recording something with fast motion.

If you can find an HMC150, which it seems a lot of retailers are backordered, I'd recommend you consider it over the XHA1 if you're able to go tapeless.

Edit: Forgot to add, yes you can shoot full auto on the XHA1 as well, but at your own risk.

 

[Chris7]

I’m still wondering, does the prosumer crowd have any use for auto-iris? People shooting documentaries without an assistant, or wedding videographers, perhaps?

...It's been a while since I've shot, but there were times where I adjusted focus, zoom, and iris all at the same time...

I actually didn't know it was possible to predict what the light is as you pan to lighter/darker areas. I’m inferring that this any pro videographer would have this level of skill – I had no idea what the standard was.

...Like the HV20/30/40, you have various options of recording on the XHA1: Av, Tv, M. Recording in Av(Aperture Value) mode will allow you to manipulate the aperture while the camera will compensate by adjusting the shutter speed(This can be hazardous depending on your subject). Tv(Time Value), allows you to manipulate the shutter speed while the camera automatically adjusts the aperture...

Having the camera automatically set shutter speed does indeed seems like trouble– I thought you almost always set the shutter to twice the frame rate (e.g. 24P would be 1/48). But what about setting auto-iris on a XH A1 or HMC150? Can you tell the camera to do auto-iris and then kick in the gain once necessary (after the iris is totally open)? ppc_michael mentioned above that this how my HF100 probably works, when set to a constant shutter speed.

If you can find an HMCX150, which it seems a lot of retailers are backordered, I'd recommend you consider it over the XHA1 if you're able to go tapeless.

If you’ve to time for a quick comparison, I’ve been incredibly curious about how the HMC150 looks/handles compared to the XH A1.
PS: In a review of the XH A1, camcorderinfo noted, "When shooting 30F and 24F, you’ll definitely notice a stutter in moving objects." Is this a problem with with the HMC150 in 24p?

Thanks again,
Chris

 

[-DH]

2. Do experienced videographers generally manually set the aperture and let the gain be automatic, or vice versa?

3. Last question: Many pros choose to disable both auto gain and auto aperture. Do broadcast cameras ($20K+) generally have the option of auto gain and auto aperture?

2. Most professional videographers never use AUTO anything. You manually compose, pan, tilt, ride iris and focus ... all at the same time. In studio setups, the iris, pedistal, gain, chroma level, subcarrier pahse and horizontal phase is often done remotely through a Camera Control Unit (CCU) by a dedicated operator separate from the camera operator. Even though it is done remotely, they generally use manual adjustments and don't rely on any AUTO settings.

3. Broadcast cameras don't come with lenses**; the lens choice is optional. Most lenses do have auto iris capabilities but is seldom used. Most cameras also have an ALG (auto gain) setting (the camera can be set to a predetermined gain maximum), but that is also seldom used. An exception for both of those would be for 'run-n-gun' type of shooting like you'd see on Cops.

-DH

** = unless you're purchasing or renting a package deal.

 

[LethalWolfe]

I’m still wondering, does the prosumer crowd have any use for auto-iris? People shooting documentaries without an assistant, or wedding videographers, perhaps?

IMO, no because Mr. Auto will always react to stimuli even if it's to the detriment of what is being shot. For example, lets say you are shooting an interview outside on a sunny day near an intersection and a white semi truck pulls into frame. If you are on manual the semi will be blown out but the subject will still be properly exposed. If Mr. Auto is driving he'll see that a big over-exposed area is suddenly in the frame, freak out, quickly drop the exposure to compensate and the semi will look good but your subject will suddenly be in the mud.

IIRC some newer cameras have a "soft adjust" type feature w/the auto iris so the camera will more smoothly adjust exposure as opposed to abrupt adjustments but I haven't used one of those so I don't know how well it works.

I actually didn't know it was possible to predict what the light is as you pan to lighter/darker areas. I’m inferring that this any pro videographer would have this level of skill – I had no idea what the standard was.

An experienced person will be able to guesstimate how many stops difference there is and then use the zebras to fine tune the exposure. Just like an experienced camera op can track an person/object and keep them in focus the whole time.

If you’ve to time for a quick comparison, I’ve been incredibly curious about how the HMC150 looks/handles compared to the XH A1.
PS: In a review of the XH A1, camcorderinfo noted, "When shooting 30F and 24F, you’ll definitely notice a stutter in moving objects." Is this a problem with with the HMC150 in 24p?

The lower the frame rate the less smooth moving objects, or camera moves, will appear.


Lethal

 

[Gymnut]

indeed seems like trouble– I thought you almost always set the shutter to twice the frame rate (e.g. 24P would be 1/48). But what about setting auto-iris on a XH A1 or HMC150? Can you tell the camera to do auto-iris and then kick in the gain once necessary (after the iris is totally open)? ppc_michael mentioned above that this how my HF100 probably works, when set to a constant shutter speed.

If you’ve to time for a quick comparison, I’ve been incredibly curious about how the HMC150 looks/handles compared to the XH A1.
PS: In a review of the XH A1, camcorderinfo noted, "When shooting 30F and 24F, you’ll definitely notice a stutter in moving objects." Is this a problem with with the HMC150 in 24p?

Thanks again,
Chris

Yes you can have the camera automatically adjust the aperture and apply gain when needed, on the XHA1. Record in Tv(Shutter Priority) enabling you to select a shutter speed of your choice, and leave AGC(Auto Gain Control) to let the camera apply gain when the lighting situation drops. I forget whether you can set a threshold for the maximum amount of gain for Auto Gain Control, but you can see the danger in leaving this on because it can really make your image grainy, and with HD content, that's the last thing you want. I'm quite certain you can set a maximum amount of gain for AGC in the sub menus. Yes, on the HMC150 you set AGC limits and have the camera control the aperture. I don't recommend having AGC on, because the XHA1 and HMC150 both have gain selector knobs so it's relatively easy to turn on a gain setting of MY choice. The 150 is leaps and bounds better in low light than the XHA1 and the IQ doesn't suffer much when adding gain.

The image quality is comparable between the XHA1 and the HMC150, with the XHA1 being slightly sharper because of its native 1440x1080 pixels, whereas the HMC150 utilizes pixel shifting. Both cameras have their strengths and weaknesses, although I favor the HMC150, partly because I no longer have to mess with tapes. The XHA1 has been around since 2006, I believe, and it's still a very robust and capable camera; The 20x zoom lens is nice, and I like that there are three adjusting rings on the lens, focus, zoom, and iris, whereas the HMC150 only has two, and a thumbwheel for iris, although the focus ring can be traded out to be the iris ring.

However, the HMC150 has much better low light sensitivity, a much more efficient codec, records to dirt cheap SDHC cards(16GB for $25.99), it's much lighter than the XHA1 because there's no tape transport, the lens, at its widest is 28mm, which means you don't need a Wide Angle lens attachment, focus assist is much better than the A1, Waveform & Vectorscope monitor, much larger LCD over the A1, and I like the control layout over the XHA1, which has you going through a lot of menus.

I suggest you head over to DVXuser.com or DVinfo.net for any side by side comparisons of the two cameras. The 150 is not for everyone, but it's certainly a spectacular camera for the price.

Edit: If you're shooting fast moving objects, like Lethal pointed out, you'd be best served recording at 60i and with a fast shutter speed. I never encountered any problems with recording moving objects(not sports/fast moving obj) in 24F or 30F. Yes, if you're recording in 24P, a shutter speed of 1/48 is ideal to mimicking a filmic look. Of course, depending on your lighting situation, your shutter speed may have to go up or down if you need more or less light and are already shooting wide open or fully stopped down.

http://www.dvxuser.com/V6/showthread.php?t=160383

http://www.eventdv.net/Articles/ReadArticle.aspx?ArticleID=52607

 

[Chris7]

IMO, no because Mr. Auto will always react to stimuli even if it's to the detriment of what is being shot...
An experienced person will be able to guesstimate how many stops difference there is and then use the zebras to fine tune the exposure...

Yes you can have the camera automatically adjust the aperture and apply gain when needed, on the XHA1...

Many thanks to both of you for the info.

I never encountered any problems with recording moving objects(not sports/fast moving obj) in 24F or 30F.

On my HF100, the stutter is so bad in 24p and 30p that it is not usable for panning. I looked at a HV30 in a store, and it seemed to have the same problem, although I didn’t record anything – just looked at the LCD. I never see a stutter in film. Even on action scenes, film seems to just properly blur the picture so one frame slides properly into the next. I don’t understand what’s causing the stutter on these consumer cams, it would seem that recording in 24p at 1/48 should do exactly the same thing that a film camera does (1/48 second of picture, one 1/48 second black; on DVD 24p playback, no black between, just hold the 1/48 second shot for 1/24 second, and instantly on to the next). But I’m glad that you're not experiencing what camcorderinfo noted, maybe the XH A1 was updated since that 2006 review (camcorderinfo noted the same stutter problem on my HF100 – I read about it after I bought and used the camera).

 

[Gymnut]

Hmm..I think the stuttering when panning may be associated with the rolling shutter that's associated with the CMOS sensor, which Canon's consumer lineup utilizes. The XHA1 uses CCD's(even though the sensor is interlaced), which have their own disadvantages.

Edit: http://dvxuser.com/jason/CMOS-CCD/

 

[Chris7]

Hmm..I think the stuttering when panning may be associated with the rolling shutter that's associated with the CMOS sensor, which Canon's consumer lineup utilizes. The XHA1 uses CCD's(even though the sensor is interlaced), which have their own disadvantages.
Edit: http://dvxuser.com/jason/CMOS-CCD/

Thanks for the link. You just saved me quite a bit of time and frustration. This article or something simular should come with all consumer comcorders. It would seem you are correct, and I would bet that rolling shutter would always stutter in 24p, as the slower shutter used with 24p requires motion blur to make it work, and blur is exactly what rolling shutter cannot do properly (I'm sure the other CMOS artifacts don't help). Also good to know now not to try any tracking effects with my camera -- I was just about to start experimenting with this in Motion and I would have wondered what I was doing wrong.

Only one question: The article says, "(For point of clarification, CCDs use a “global shutter”; CMOS can use either “rolling” or “global” but as a practical matter all the camcorders on the market use the “rolling shutter” technology)." I'm assuming the author meant to write, "... all the consumer camcorders on the market use "rolling shutter" technology." Yes?

Thanks again,
Chris

 

[Gymnut]

Thanks for the link. You just saved me quite a bit of time and frustration. This article or something simular should come with all consumer comcorders. It would seem you are correct, and I would bet that rolling shutter would always stutter in 24p, as the slower shutter used with 24p requires motion blur to make it work, and blur is exactly what rolling shutter cannot do properly (I'm sure the other CMOS artifacts don't help). Also good to know now not to try any tracking effects with my camera -- I was just about to start experimenting with this in Motion and I would have wondered what I was doing wrong.

Only one question: The article says, "(For point of clarification, CCDs use a “global shutter”; CMOS can use either “rolling” or “global” but as a practical matter all the camcorders on the market use the “rolling shutter” technology)." I'm assuming the author meant to write, "... all the consumer camcorders on the market use "rolling shutter" technology." Yes?

Thanks again,
Chris

Yes, from what I understand, a global shutter would raise the price of a CMOS camera significantly. Even the Sony EX1, which has CMOS sensors, has a rolling shutter and is not immune to this problem. However, for a $6k camera, it produces a stunning image, and I'm sure has enough flexibility in its controls to circumvent the issues of a rolling shutter in the hands of an experienced operator.

 

[LethalWolfe]

Even on action scenes, film seems to just properly blur the picture so one frame slides properly into the next.

The guys shooting movies know how to shoot around the limitations of 24p. 24p is accepted as the standard for motion pictures, but it's not a frame rate for all occasions. For example, professional sports are shot and broadcast at either 720p60 or 1080i60 to get the smoothest possible motion. If FOX, for example, shot and aired football at 24p it would look horrible.

Only one question: The article says, "(For point of clarification, CCDs use a “global shutter”; CMOS can use either “rolling” or “global” but as a practical matter all the camcorders on the market use the “rolling shutter” technology)." I'm assuming the author meant to write, "... all the consumer camcorders on the market use "rolling shutter" technology." Yes?

AFAIK no video/digital cinema cameras on the market today using CMOS have a global shutter. The Red One doesn't have a global shutter and, to the best of my knowledge, the other Red cameras that won't be released for at least 12-18 months aren't currently slated to have global shutters either. There may be a specific problem w/the particular camera you are using, or possibly a problem w/the user (), but I don't believe the problem is inherent to CMOS technology. Small cameras are also more susceptible to image stabilization problems because their extreme light weight and compact size makes it harder to keep them steady and thus harder to shoot at lower frame rates. W/that being said the Canon HV20/30/40 have become the darlings of the 35mm adapter crowd because of the amount of image quality you get for such a low cost.


Lethal

 

[Chris7]

The Red One doesn't have a global shutter...
There may be a specific problem w/the particular camera you are using, or possibly a problem w/the user (), but I don't believe the problem is inherent to CMOS technology...
W/that being said the Canon HV20/30/40 have become the darlings of the 35mm adapter crowd because of the amount of image quality you get for such a low cost.

Interesting. I did not know RED used rolling shutter -- no problems with their progressive images.

The link Gymnut posted said there were tracking problems associated with any kind of CMOS. Anyone know about this?

Do many/most of the prosumer cameras use CCD's? All CCD's are global shutter, yes?

You may be right about the problem just being my particular camera. The HV30's progressive image looked choppy/stuttered on the LCD at the store but, again, I never looked at recorded progressive images of this cam. I looked up the HV30 and found this on a camcorderinfo review of the HV30: "When we looked at Canon’s AVCHD camcorders, the HG10 and HR10, the 24P mode produced a strange, choppy effect that was not apparent in their HDV models. Canon could not explain why this was, but we bore witness nonetheless. And once again, the 24P on their HDV models looks fine. The blur is quite exaggerated when your subject is moving quickly, but there’s no choppiness."

Also, there's plenty more problems with my shooting than just the stutter/choppiness in progressive scan.

 

[LethalWolfe]

Interesting. I did not know RED used rolling shutter -- no problems with their progressive images.

The link Gymnut posted said there were tracking problems associated with any kind of CMOS. Anyone know about this?

Skew, wobble, and partially exposed frames all inherent of rolling shutter CMOS systems, but can vary in degree between cameras depending on how aggressively the camera makers attempt to combat the problems.

Do many/most of the prosumer cameras use CCD's? All CCD's are global shutter, yes?

I believe all CCDs, or at least all CCDs commonly used in cameras, use global shutters. Most cameras out there are CCD, but there seems to be a general move towards CMOS cameras that started a year or two ago mainly because they are easier and cheaper to produce, consumer less power, and give off less heat.


Lethal

 

[Chris7]

Most cameras out there are CCD, but there seems to be a general move towards CMOS cameras that started a year or two ago mainly because they are easier and cheaper to produce, consumer less power, and give off less heat.

The previous posts on this thread as well as reading about Panasonic's choice to use less but larger photosensors prompted me to do some reading this weekend and a few questions are nagging at me. These questions are really moot in a practical sense -- my eventual choice will be between a Scaret 2/3" fixed lens and a Panasonic HMC150, with perhaps a HV30 in the interim. Still, my curiosity's getting to me:

1. I believe in a RED One the lens projects light to a space of 36mm wide, so that some of the light is just lost outside of the 22mm sensor. I think 22mm is the useable with of 35mm film, due to perforations and the optical soundtrack. Why not just use a 36mm sensor like a fill sized DSLR?

2. The RED Scarlet 2/3" obviously uses a 2/3" sensor. Why in the world did they not just make it a 22mm, or for that matter, a full 36mm sensor, with huge photosensors (same 3K horizontal resolution)?

3. Apparently, CMOS sensors can be set to rolling or global shutter, although all the ones on the market use rolling shutter. Why not just make them with global shutter and leave out the associated artifacts?

4. How much do these sensors cost to make? Are the smaller sensors much cheaper? Seems like the cost would be in making many tiny photosensors, not in making larger sensors, meaning RED and other manufactures could easily make 36mm 3K sensors with huge photosensors?

Anyway, thanks for the help,
Chris

Here's a link to a blog by a cinematographer who prefers dynamic range over pixels, if anyone's interested.
http://prolost.blogspot.com/2008/11/too-much-is-not-enough.html

 

[LethalWolfe]

1. Probably a better Q to ask at the RedUser boards.
2. Probably a better Q to ask at the RedUser boards.
3. Going global has some problems of its own given current technology, link.
4. Probably a better Q to ask at the RedUser boards.
5. Stu Maschwitz has great blog (as well as a great book) but he isn't a cinematographer.


Lethal

 

[KeithPratt]

1. It's meant to be a movie camera. Why not make it equivalent to motion picture film?

2. Because some people want 2/3". They say they're going to make 35mm versions too. And have you ever tried focus-pulling with 35mm/a full-sized sensor? "Take 36..."

3. If it was as easy as that I'm sure they would.

4. Don't know, but I'd imagine the answer is not a simple one.

I'm with Stu on the dynamic range issue. People salivate over resolution numbers like they do over car engine size or BHP. When you get to using one, however, you realise a lot of the performance comes from the weight, chassis and brakes.

 

[Chris7]

1. Probably a better Q to ask at the RedUser boards...

Thanks for your help and thanks for the link. I'll post these on reduser.net. Curiosity is getting the best of me -- getting a little sidetracked into the technical.
-Chris

Edit: Thanks for the info, Keith.