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

[Blaise Douros]

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.

[Tom Nash]

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

[Tom Nash]

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

[Steve Crow]

“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

[Tom Nash]

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

[Steve Crow]

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: https://cinemashock.org/2012/07/30/45-degree-shutter-in-saving-private-ryan/

* More “video like” (as opposed to cinematic)

Steve Crow

[Blaise Douros]

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.

[Blaise Douros]

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.

[Tom Nash]

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

Tom

[Tom Nash]

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