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Your Preference for Filming Daylight Exteriors - Tiny Aperture & Little ND Filtration, or Larger Aperture & Heavy ND Filtration?


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#1 Karl Lee

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Posted 26 October 2014 - 09:25 PM

Let's say that you're filming 16mm daylight exteriors with 200T or 250D, and your metering indicates proper exposure at f/22 (for the moment, let's forget about the 85 filter and 2/3 stop compensation when shooting 200T).  Using my particular setup as an example, an SR3 with a Canon 11.5 - 138 zoom, and based on your personal experience and preference, would you prefer:

 

A.  Shooting at T22 with no ND

B.  Shooting at T8 with an ND.9

C.  Shooting at T4 with an ND.9 and ND.6 (effectively ND1.5)

 

Given these three choices, which would be your preference, and the reasoning behind it?  Does the decision ultimately come down to desired depth of field, or is there more to the decision than just that?  I know lenses are sometimes considered their sharpest at certain apertures, so if DOF isn't that big of a factor, I'm curious if it's best to shoot closed way down without ND filtration, relatively wide open with heavy ND filtration, or somewhere in the middle.

 

Realistically, a meter reading around f/32 isn't uncommon when shooting 200T or 250D in direct sunlight, in which case shooting without any filters isn't really an option.  For the sake of discussion, though, let's stick with the hypothetical f/22 reading.

 

Thanks!


Edited by Karl Lee, 26 October 2014 - 09:26 PM.

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#2 Bill DiPietra

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Posted 26 October 2014 - 09:54 PM

Given those limitations, I would choose "C" simply because I like a slightly softer image.  Even some older lenses can give a crisp look at higher apertures, which is sometimes exactly what you want.

 

But if I know I'm going to be shooting exteriors in direct sunlight, I automatically choose a slower stock to avoid the issues you are talking about.  Usually, 50D will top out at around f/16 in areas where I shoot.  With an ND .6 I can stop it down to f/8 which is about as high an aperture as I like to shoot at.


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#3 Karl Lee

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Posted 26 October 2014 - 10:05 PM

But if I know I'm going to be shooting exteriors in direct sunlight, I automatically choose a slower stock to avoid the issues you are talking about. 

 

Good point, Bill.  I've already shot a few rolls of 50D, so now I'd like to try experimenting with 200T and 250D and throwing some of my filters into the mix :)


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#4 David Mullen ASC

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Posted 26 October 2014 - 10:18 PM

If you aren't aiming for a specific depth of field, you'd use enough ND to get into the middle f-stop range. You can lose sharpness due to diffraction if stopped down all the way, though many zooms do OK at f/16.
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#5 cole t parzenn

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Posted 30 October 2014 - 12:15 PM

Why would a zoom suffer less from diffraction? At what aperture do you find diffraction start to become noticeable, in motion picture?


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#6 David Mullen ASC

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Posted 30 October 2014 - 02:18 PM

I don't know why zooms can be stopped down a bit more (to my eye) but it probably is related to the fact that the widest aperture is limited by the telephoto end of the zoom, so think of using a zoom like you would a telephoto lens that goes to the same focal length as the long end of the zoom. I could be wrong, it just may be that the zooms are less crappy stopped down than opened up and only seem to feel more like a prime lens' sharpness when stopped down.

As far as seeing the softening from diffraction, that's also a bit subjective unless you are shooting charts. If you shoot a close-up of a face stopped down on the long end of a zoom, your eye isn't going to be craving the same degree of detail as it would for a distant subject that is small in the frame.

On "Citizen Kane", Gregg Toland replaced the iris blades in his 25mm lens with Waterhouse stops when he wanted to shoot at f/16.
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#7 Dom Jaeger

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Posted 31 October 2014 - 12:11 AM

Why would a zoom suffer less from diffraction? 

 

It's an interesting observation that zooms can be stopped down a little more than primes. Mathematically, given the same f stop, the size of the blur pattern of a point source caused by diffraction should be the same, so the degree of 'softness' should also be the same. There might be some variation caused by different aperture shapes, in that a polygon will create spikier blur patterns than a circle, but I'm not sure that zooms are any more or less prone to having polygonal apertures.

 

One small factor could be that cine lenses are commonly marked in T stops, and since zooms will usually lose more light than a prime, the same T stop in a zoom will be a smaller f number in a prime. In other words, a zoom needs to be stopped down a little more in terms of T stops to reach the same f stop as a prime. But with modern lenses we're probably only talking half a stop difference at the most.

 

I think David's observation that sharpness is often subjectively appraised is the main explanation. Apart from the sorts of shots a zoom might be used used for (like the close up of a face as mentioned), the improvements gained by stopping down a zoom probably shift the sweet spot balance further towards a smaller aperture, outweighing the softening effects of diffraction.

 

 


At what aperture do you find diffraction start to become noticeable, in motion picture?

 

Like pin-pointing an exact depth of field, nominating an aperture where diffraction starts to be noticed is somewhat arbitrary. When is sharp no longer sharp? How good is your eyesight? We can determine mathematically the size of an Airy disc (the interference pattern of a point source caused by diffraction) directly from an f number:

diameter of airy disc = 0.000045 x (f number) in inches (from Arthur Cox, "Photographic Optics")
At least this is for green light - a mid spectrum wavelength. Blue light will create a smaller disc, red light a larger one.
 
So in simple terms at f/11 any lens will turn a point source into a blur circle of around 0.0005" in diameter. 
 
We now need to decide if a 0.0005" diameter blur is an acceptably sharp rendering of a point source. Traditionally, to determine DOF tables, lens manufacturers and cinematographers have relied on a defined maximum allowable blur circle diameter that could still be considered sharp - a Circle of Confusion figure. These figures may vary wildly depending on factors like the format size, the final viewing size and distance, and these days also the size of the camera sensor pixels. But typically, 35mm movie lenses have used a CoC of 0.001" (0.025mm) and 16mm lenses a CoC of 0.0006" (0.015mm). 
 
So looking at those figures, the blur circle at f/11 is nearly as large as the historically accepted Circle of Confusion for 16mm format. By contrast, the historical 35mm CoC figure is not reached until about f/22. Many people might argue that the modern CoC value for 35mm format should be tightened to 0.00075" (0.019mm), which places the acceptable diffraction boundary at about f/16. Some even argue for 0.0005", which means f/11.
 
With digital cameras, the pixel size gives us a physical limit to the effects of diffraction. Alexas, for example, have photosites that are 0.00825mm wide, which after de-bayering and downscaling effectively become 0.0124mm wide. So diffraction has no discernable impact on Alexa's resolution until around f/11, where the blur circle begins to fill a pixel. But just how noticeable the softening would be at f/16 depends on lots of variables.

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#8 Dom Jaeger

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Posted 31 October 2014 - 12:26 AM

On "Citizen Kane", Gregg Toland replaced the iris blades in his 25mm lens with Waterhouse stops when he wanted to shoot at f/16.

 

I wonder if that was to help with diffraction or because the iris scales on old lenses used to bunch up as you closed down, making it tricky to set an accurate f/stop. With Waterhouse stops you know exactly what stop you're getting.

 

Can I ask where you read that snippet David? I'm always curious to read about technical details like this from cinematographic history. 


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#9 David Mullen ASC

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Posted 31 October 2014 - 12:41 AM

I found a mention of Waterhouse stops in Robert Carringer's book on the making of "Citizen Kane" on my bookshelf.  He says that the Waterhouse stops were used to reduce halation for the scene where Toland pointed the camera into the lights of the opera house. But I would think that at very small apertures, having a perfect circular hole would also improve sharpness but perhaps only on the edges of the frame.  But looking around at some photography forums, the only advantage of Waterhouse stops seems to be (other then accuracy) that the edges of the aperture can be less reflective.

 

There is also a mention of Toland's use of Waterhouse stops in the footnotes of David Bordwell's book on Hollywood film style -- he quotes Joseph Walker, who said accuracy in f-stop markings was not great in those days between sets of lenses and Toland's solution was to use Waterhouse stops for critical exposures.

 

George Turner here:

https://www.theasc.c...r99/best/m1.htm

says that the Waterhouse stops were used to be able to shoot into bright lights without ghosting problems.

 

This entry on Stanley Cortez:

http://www.cinematog...DoPh/cortez.htm

has a quote by Cortez saying that he and Toland used Waterhouse stops but doesn't say why.

 

Turns out that the AC article by Toland does not mention Waterhouse stops.


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#10 Dom Jaeger

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Posted 31 October 2014 - 07:18 AM

Thanks David!

A veritable feast of links to follow! I get the feeling you're rather well versed in this subject. :)
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#11 Carl Looper

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Posted 31 October 2014 - 05:44 PM

By definition the extent to which some area in the image is in/out of focus, is determined entirely by the circle of confusion at the film plane so it wouldn't matter what lens you used as the definition doesn't care - it only concerns itself with the end result - the point spread at the film plane. Which is perfectly quantifyable.

 

But of course, what we're looking at in an image is not just single areas in the image or even the sum of such, but the differences between various parts of the image, in time (between shots) as much as space. It's this difference that is far more obvious than any fixed value.

 

But leaving that aside there is one aspect of the difference between a wide lens and a telephoto lens that might be pertinent. But I imagine, if it holds, it would be very subtle.

 

Assuming the same aperture, the angle at which light converges to a point at the film plane, will be a wider angle on the wide lens, than it is on the telephoto lens. On a wide lens it means the circles of confusion would increase more rapidly as you move away from the focal plane. In other words there should be (as far as I can tell mathematically) a small "counter-force" to the rather larger magnification the telephoto lens otherwise has on the circle of confusion.

 

Whether or not this 'counter-force' is cancelled out in any way I haven't explored. It just came up the other day when doing some calculations for a setup. As a rule the greater the focal length the more shallow will be the depth of field, all else being equal.

 

But the very interesting thing here is precisely this "all else being equal". In the studio we tend to make everything be equal, in order to isolate some aspect of a system that isn't. But out in the field, in a creative environment, it's almost always the opposite practice. Unless you are otherwise doing a zoom shot from a single location, changes in focal length will often be accompanied by changes in camera position, ie. the "all else being equal" wouldn't get much of a say. And this can easily cancel out any significance or usefulness the rule might possess. The vertigo effect is perhaps the most exaggerated form of this.

 

C


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