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Changing light when in front of the lens with video with a projector behind me

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Tom Nash
Changing light when in front of the lens with video with a projector behind me
on Apr 30, 2015 at 2:56:02 pm

OK so I have this fun project I'm working on and hit a bit of a snag. I have a Canon 6D, a 702p projector, a 10" wide screen (16:9) Screen is 2' off the ground.

I'm attempt to film an audience and project back live as if they are sitting in front of a mirror. I'll have a hole in the center of the screen towards the bottom with lens angled slightly up.

Projector is on a 6' high stand projecting down.

My issue is that if I'm sitting in direct line of the camera and projector is behind me over my head, if I move around, I see the camera (I think its the camera) adjust for the light change and I see a flare. I can see the projector flare on the top of the screen as I would expect since the projector would have to be a lot higher to get off screen, but I don't know what's happening when I'm in direct line of the camera and I see sudden bright flares. If I'm to the right or left, it doesn't happen.

Any thoughts?


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on Apr 30, 2015 at 6:42:07 pm

I think you are not filming in full manual mode so that the camera is making some auto corrections to the exposure as the overall light conditions change. Set your exposure manually and then leave it alone and I think the problem will go away - let us know if that works for you.

Steve Crow


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on Apr 30, 2015 at 7:55:41 pm

Thanks Steve. I was trying that last night but couldn't get the exposure right since my test setup has the audience a bit under lit and was worried if I opened it up, people in the third row would be out of focus. Gave up when it got late last night but I'll try again.

BTW, any chance you know why when shooting video, if you're in Av, you can't control aperture where it sets it at f/4 (widest for this lens). Seems like Av and Tv are the same in video.


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on Apr 30, 2015 at 9:42:47 pm

Not sure I 100 percent got your question but let me take a shot at it, and forgive me if I blow it.

For your filming situation I would set my aperture way more towards the "closed" end because that way you will have a deep depth of field and most everyone in your audience is going to be in focus. F8 or even higher.

I think at first I misunderstood your question and I wanted to point out to you that your exposure and your focus are two separate things. I thought perhaps what you meant was that you were afraid that the some of the audience members would be overexposed or blown out but now I think you were actually talking about Depth of Field issues.

Anywho....unless you set up a professional lighting grid to ensure even lighting throughout the audience area, yes there are going to be different levels of exposure.

What I'm saying is go into manual mode and then pick an exposure that works out the best OVERALL even if some areas are SLIGHTLY over or under exposed. For an F8 aperture, I would expect to have to use a fairly high ISO because 95% of the time I keep my shutter speed at 1/50th or twice my 24fps frame rate.

AV and TV modes are not DOING the same thing in video. In AV mode you are selecting the aperture and the camera is selecting the shutter speed and ISO while in TV (t for "time") you are selecting the shutter speed and the camera is selecting the aperture and ISO for you. Now, in both cases, the camera will try and work out a combination of settings that gets you an acceptable exposure, somewhere right in the middle between not too dark or too bright, and the resulting shot may very well look exactly the same in terms of overall exposure in both AV and TV modes. The only difference is HOW the camera is achieving that exposure.

But of course, yes, different apertures, shutter speeds and ISO can make a HUGE difference in the overall look of the video - the depth of field, the "smoothness" of the movement and exposure. That's why I only really shoot in manual mode for my projects, because I want to control all 3. If I were doing fast moving action type stuff yes then I might choose AV or TV mode (most probably AV) but in your case, I'd go manual all the way since it doesn't sound like the lighting conditions are going to change.

Steve Crow


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 1:11:27 am

Hey Steve

Thanks for that. I understand Av vs Tv and needing to pump up ISO if I close it a bit to like f8 but for some reason when in video mode, I can't get it to do anything but f/4 which is odd. Though maybe it's something that I couldn't do in Av when in video mode but then what's the sense of Av if I can't set the aperture. Weird. Will play around with that again.

I also didn't take into account FPS. Wasn't sure if that mattered in terms of the exposure. I'm set at 1920 with 30fps the highest in the camera. Since I was projecting back live I thought the highest FPS might be best but maybe not.

Thanks


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 1:18:12 am

So you're saying while in aperture priority mode the only aperture you can set is a 4.0 and it won't let you set it to anything else? What happens when you try and select another aperture like 8.0?

Is this a fixed aperture lens by any chance? What lens do you have?

When taking still photographs can you use other apertures besides 4.0?

Steve Crow


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 1:52:13 am

It's the 24-105. Normally I can set it like you would expect but won't budge in movie mode in Av. I'll keep playing with it and let you know if I figure it out.


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 1:53:59 am

Dude, that lens is fixed aperture at 4.0 meaning that's the only aperture it has!! :-)

Steve Crow


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 5:00:16 pm

Ahh, right Variable vs Fixed. So how does this work with a fixed aperture but with an aperture range of f4-f22 (according to BH). In Av, while taking pictures, I can scroll through F4-F22. Not sure I understand how that works then.

Tom


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Blaise Douros
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 6:03:09 pm

The answer is to not use any of the program modes when shooting. You need to expose this scene manually if you want it to work. Av and Tv modes are not as smart as you are when it comes to what in the scene needs to be exposed correctly. Additionally, they will make automatic adjustments to the brightness of the scene, which is undesirable.

If you can't change your aperture in Av mode, there may be a problem with your camera. I have the same camera and lens, and have no problem doing exactly what you describe. Make sure you're not making a bonehead mistake like trying to use the wrong dial for the aperture. Or, make sure you're not trying to adjust it using the exposure compensation setting.

In the case of the 24-105 f4L, the fixed aperture means that your lens can shoot at a maximum of f4 at any focal length; other lenses will automatically change their maximum f-stop (almost always to a higher number) as you zoom in. It does NOT mean that you can't change the lens from f4. You can dial the lens to f8 if you want, or f22, whatever, at any focal length. But it doesn't mean that f4 is its only aperture.

In this case, here's what you need to do: go into manual mode. Set your shutter speed to double your framerate (1/60 for 30 fps, 1/50 for 24 fps). Set your aperture to a midrange number like f8, to get you some depth of field. I would suggest zooming out, and placing your camera slightly closer to your audience; a wider angle on the lens means a deeper DOF, so you won't have to stop up quite so much to get more DOF. Now, set your ISO. Find a balance of ISO and f-stop that works to get you enough DOF, without having to set the ISO too high. In my experience, the 6D's ISO can go pretty damn high without too much noise; 2000 or 2500, even 3200 can be perfectly acceptable.

Consider adding additional light on the audience. This will help you better control your exposure.


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 6:38:27 pm

Wow Blaise, I've just been schooled! :-)

I always thought that a fixed aperture lens meant it had ONLY that one aperture to select from throughout its entire focal range (24-105mm in this case) but what I guess you are saying that F4 is the most open aperture available on that lens that works throughout the entire focal range but that at the same time you can still shoot at 5.6, 8,11,16 and 22. Is that right?

Would those other apertures beyond 4.0 also be available then throughout the entire range or only part of it or....?

Steve Crow


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Blaise Douros
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 6:55:14 pm

Yes, all of the apertures are available at all focal lengths--the key on a fixed-aperture lens is that it doesn't change when you zoom in. It's a nice-to-have feature, but not all of the L lenses have it--the 70-300 L, for example, goes from a max aperture of f4 to f5.6 when you zoom it in.

Note, of course, that this does not mean that you'll have an equal T-stop :) f4 at 24mm lets in more light than f4 at 105mm, simply because there is a wider angle of view at 24mm than 105 mm for the light to enter the lens. Thus, as you zoom in, less light gets in, and the image will darken noticeably as you zoom in, even at a constant aperture.

If I remember right Steve, you tend to favor Sony and Panasonic, and as far as I know, not a lot of their lenses are constant aperture (but "as far as I know" isn't very far when it comes to those lenses!), so you can be forgiven for not knowing Canon's terminology.

You are correct in stating that there are lenses out there that have a totally fixed aperture, but these lenses are usually hyper-telephoto, usually fixed around f8, and are intended more for use in telescopes--often they've been adapted for photographic use.

Tom, I just noticed that on my 5DIII (I don't have the 6D in front of me), when I switch from manual to Av mode, the aperture is controlled by the other dial. So double check this on your setup if you're really sold on using Av. I still recommend using manual.


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 7:05:45 pm

Thanks. Just did. The dial on the back won't budget the aperture. But will when not in video mode. Odd. Go to Manual and it does. Hmmm


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 7:06:45 pm

"If I remember right Steve, you tend to favor Sony and Panasonic, and as far as I know, not a lot of their lenses are constant aperture (but "as far as I know" isn't very far when it comes to those lenses!), so you can be forgiven for not knowing Canon's terminology. "

Thanks for the thought but actually I shoot exclusively (for now) with Canon so I don't have this excuse as to why I didn't know this...guess it slipped by me. Somehow I was thinking of constant or fixed aperture in the same way as a prime lens with a single focal length.

The T-stop issue clouds the whole thing for me again however because what I understand you saying is that the exposure may still darken as you zoom in despite the F-stop staying the same. The reason why that happens you explained quite well but then I have to ask, aren't you then losing one of the benefits of paying for a more expensive fixed aperture lens - at least when shooting in manual mode? I can see that in an aperture priority mode the camera would compensate for this "darkening" by increasing the ISO and/or assigning a slower shutter speed so you may not even notice - but in manual mode you'd have to do that adjustment yourself.

Steve Crow


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 6:57:16 pm

Great stuff. I get the idea of fixed aperture across the focal length now. With an older camera and lousy lens it used to adjust the aperture as I zoomed which must have been variable. Got it.

Odd that on video mode I can't adjust aperture but can in image mode using the same dial. Will try that again but manual I think is the way to go as suggested by you guys to help reduce the auto exposure issue I was having with flares while in auto and in Av.

I'll try the approach suggested starting with FPS work from there with ISO being last setting. I'm only 15 ' from the audience or so which has me at 35mm which is nice and wide. The audience should be lit well so I think I'm good there.

Going to run a test tonight once it's darker (for projector). I'll post back.

Tom

Tom


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 7:22:54 pm

Great stuff. I get the idea of fixed aperture across the focal length now. With an older camera and lousy lens it used to adjust the aperture as I zoomed which must have been variable. Got it.

Odd that on video mode I can't adjust aperture but can in image mode using the same dial. Will try that again but manual I think is the way to go as suggested by you guys to help reduce the auto exposure issue I was having with flares while in auto and in Av.

I'll try the approach suggested starting with FPS work from there with ISO being last setting. I'm only 15 ' from the audience or so which has me at 35mm which is nice and wide. The audience should be lit well so I think I'm good there.

Going to run a test tonight once it's darker (for projector). I'll post back.

One add'l question if you don't mind. I saw a post of your on 30vs60 FPS. Any recommendations if my objective isn't what I'm going to do with the footage after but the immediately live display of the recording. I'd like to record so we can watch the audience later but it really is all about an audience singing along to a song when they don't know they are being recorded and don't know that they are about to be looking at themselves on screen. There's a slight lag but I'm thinking that is the lag from camera to HDMI switch to projector. Since there's going to be a fair amount of movement (not dancing but definitely some having fun and watching ourselves) does 30/60 make a difference in blur? I suspect what's recorded is more or less what is displayed on the screen of the camera which is what we project and maybe as 'live' as I can get it.

Tom


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Steve Crow
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 7:36:13 pm

Hey Tom,

"I saw a post of your on 30vs60 FPS. Any recommendations if my objective isn't what I'm going to do with the footage after but the immediately live display of the recording"

I don't know if you were referring to something I posted but here are my thoughts anyways. FOR ME I think of 60fps as something to use when I know I want to create a slow motion effect with that video later on during editing. I shoot and deliver my video the vast majority of times at 24fps so I don't even go as high as 30 because I associate that with DV video, rightly or wrongly.

For a live screening situation like your are describing, you may notice the video on the screen is:

* More jerky or jittery at 60fps than 30fps...I have trouble sometimes noticing the difference myself but people with better eyes than mine say there is a more "staccato" like feeling to the video and often refer to the battle scenes on the beach in the opening of the movie "Saving Private Ryan" as an example - although I am not sure what frame rate or shutterspeeds they actually used. You can research that here if you want: http://cinemashock.org/2012/07/30/45-degree-shutter-in-saving-private-ryan/

* More "video like" (as opposed to cinematic)

Steve Crow


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Blaise Douros
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 9:53:25 pm

The T-stop issue clouds the whole thing for me again however because what I understand you saying is that the exposure may still darken as you zoom in despite the F-stop staying the same. The reason why that happens you explained quite well but then I have to ask, aren't you then losing one of the benefits of paying for a more expensive fixed aperture lens - at least when shooting in manual mode?

Not quite. There's a great discussion of it here.

Re: 30 vs 60 fps: You don't want to use 60 fps unless you intend to post-process for slow motion. You will automatically cut in half the amount of light entering the lens, which you can't afford anyway, and it won't do you any good motion-wise. For a static shot, even one with lots of subtle movement in it, even 24 would be fine.

As for shutter speed vs framerate: The Saving Private Ryan example is referring to shutter speed only, not to framerate. That footage was shot at 24fps with a 45 degree shutter, which ends up being something like 1/192. This means that the action is not slowed down or sped up, but each frame is super crisp, with no motion blur. It ends up looking really choppy and harsh.

A lot of guys get hung up on shutter speed vs framerate; just remember that your framerate is how many images your camera is taking per second, and your shutter speed is how fast each of those images is being exposed. The faster the shutter speed, just like in photography, the less motion blur there is in the image.

For Tom's purposes, he doesn't need anything other than a standard shutter speed and framerate. So 24 fps @ 1/50 or 30 fps @ 1/60 should be your starting point. If you're really in a bind, 24 fps @ 1/40th might get you a little extra light, at the expense of a bit more motion blur in your subjects.


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 2, 2015 at 1:57:46 pm

Hey Steve thanks for the help. I really appreciate the posts and getting back so quick. Thanks agin.
Tom


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Blaise Douros
Re: Changing light when in front of the lens with video with a projector behind me
on May 1, 2015 at 9:57:44 pm

Odd that on video mode I can't adjust aperture but can in image mode using the same dial.

Again, try the other dial. On some Canon cameras, the aperture control shifts from one dial to the other in different modes.


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Tom Nash
Re: Changing light when in front of the lens with video with a projector behind me
on May 2, 2015 at 1:55:30 pm

Thanks for all of your help. This was a very helpful thread. Learning a lot and having fun with this project.

Tom


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