shutter speed and aperture corelation

[Ali Jafri]

Okay I've been meaning to ask this question for a while now. Just hadn't built up the nerve to yet:

How do we make the shutter speed work alongside the aperture setting? What are their functions and applications in different scenarios? I record in PAL always and predominantly use the DV setting on my FX1. I still haven't found the perfect balance between these two. Could someone please explain and demystify this?

[Greg Laves]

Shutter speed and aperature both control the amount of light that gets to the image devices, but they do it in completely different ways. But they are not the only controls that effect the amount of light and all the controls are interdependant to achieve the proper exposure. Shutter speed obviously controls how long the shutter is open. The longer it is open, the more light comes in. The aperature (also known as the iris) is a variable opening in the lens. The wider the iris, the more light comes in. If all other camera controls are not altered, if you let in more light with the shutter, you have to compensate by reducing the light that the shutter allows in. But there is no such thing as a universal solution to the relationship of shutter speeds and aperature.

[Don Bloom]

generally speaking, the slower the shutter speed the smaller the iris and vice versa. Faster shutter means bigger iris. Keep in mind though that the iris is number from f/{low number i.e. 1.6} (big and wide) to f/{high number i.e. 22}small and narrow)
For example using a shutter speed of say 1/60th and an iris of say f/4.8 and you decide you need more DoF by changing the iris to f/11. The problem is that by doing that you will be quite under-exposed. So to compensate you have to change the shutter speed by an equal amount of steps. In this example since you changed the iris from 4.8 to 11 (closed) thats 3 stops(5.6, 8,11) soyou need to slow the shutter down by 3 stops. From 1/60th to 1/8th.Since tape need light just like film to PROPERLY expose it once you've set the CORRECT exposure for the lighting anytime you make a change to 1 of the elements, (iris, shutter, ND filters, gain) you need to change the others to make up the difference).
Small iris means deep DoF, wide iris means shallower DoF.
Many years agowhen I was a still photog (way before auto anything on cameras) my standard shutter speed was 1/60th using Triax 400ASA film pushed to 800 or 1200 depending and what Iwas shooting. For sports I went to 1/250th at f/2.8 or f/4 depending on the lighting and pushed the film to 1600. So by changing 1 thing I had to change them all.
Tape be it shooting in DV or HDV/HD can't be 'pushed' like film so you're a bit more limited but that's the jist of it. Change 1 thing change 'em all. Slow shutter/small iris (high number) Fast shutter/big iris (low number).
HTHs
Don

[Khoi Pham]

With Z1 and FX1, if you press Shutter and not Apperture button, you can set you shutter speed and the apperture will adjust according to your shutter speed to give you corrected exposure and vice versa, if you press both and also gain button, then you are in full manual, now you must rely on your zebra for correct exposure.

[Ali Jafri]

Thanks for the explanations guys :) with all that you've said i understand that keeping the iris open all the way yields shallower depths of field which is what most videographers strive for to help achieve a more film-like feel. So wouldn't it make sense to always keep the iris wide open while using the shutter speed and neutral density filters to control exposure. Would that make sense though? What i end up with is different shutter speeds for different shots and angles with the same fully wide open iris.

[Adam Gold]

Your best bet is to set up a Picture Profile with an AT IRIS LMT of F4, then shoot in auto using this PP. The cam will always want to stay at a shutter speed of 1/60 (1/50 in PAL land) so it will tell you to adjust your ND when appropriate. It'll only change shutter speeds as a last resort if you cannot compensate enough with your NDs.

You could also buy additional screw-on NDs if necessary.

[Ali Jafri]

Adam, that's an excellent idea :) Now someone please tell me why I would want to shoot in a fixed frame rate all the time. I really don't have any idea about this so any help would be appreciated. As explained above, I do want to get a shallower depth of field and I do that by keeping the iris open to its widest and controlling light by ND filters and shutter speed. Is there any disadvantage of doing that. I noticed that there is a change in colour saturation when going to slower shutter speeds (below 50 in PAL) why is that? Is that any good? Why would anyone want to shoot in higher shutter speeds? Why not just put extra ND filters on, or even close the iris a little bit.

This is all so confusing to me, someone please explain.

[Boyd Ostroff]

Originally Posted by Ali Jafri

Adam, that's an excellent idea :) Now someone please tell me why I would want to shoot in a fixed frame rate all the time.

For starters, don't confuse "frame rate" with "shutter speed", they are two completely different things. The frame rate is the number of images which the camera captures each second. The shutter speed is the length of time during which each frame is exposed.

Since interlaced NTSC video consists of 60 fields per second (there are two fields to each frame), it makes sense to match that with a shutter speed of 1/60 (1/50 PAL). If you shoot at 1/1000 sec, for example - then there will be a gap of about 0.98 seconds before the next field is captured. This can cause fast motion to look strange. It can be used as an effect, but you should experiment around with different shutter speeds, and probably stick with 1/60 (NTSC, 1/50 PAL) unless you have a specific reason not to.

If you choose a shutter speed which is less than 1/60 (NTSC), then the shutter remains open longer than the time it takes to capture one frame. This results in something called "field doubling" where the same data is written to both interlaced fields. If you are shooting in standard definition, or downconverting, this won't be much of a problem. But in HDV mode, you will be losing half of the information (capturing 540 lines instead of 1080).

The cineframe modes on the Z1 and FX1 use some trickery to simulate frame rates of 24p, 25p and 30p. See Adam Wilt's article on this topic here: AJW's HDV Info: Cineframe modes explained

[Don Bloom]

also keep in mind that the distance you are from the subject will affect the DoF. Close to subject wide iris generally means showller Dof. Shooting from 50 feet back the DoF is going to be pretty deep no matter what.

Don

[Adam Gold]

Originally Posted by Ali Jafri

Why would anyone want to shoot in higher shutter speeds?

Some people like it. At a much higher shutter speed your action will look "stroby," but some people like it that way, especially if you're going to slow the footage down in post. Slower shutter speeds will introduce more motion blur, all things being equal.