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#1 Jay Taylor

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Posted 08 May 2006 - 11:10 AM

Hey everyone. I'm new so bare with me!

My grandad just gave me his old Bell & Howell 16mm Filmo! Nice camera! Non-reflex, but still nice. What I'm curious about is setting up dof effects with a movie camera. I understand setting the f-stop accordingly, but you can't really change the shutter speed to compensate for any loss of light.

What I'm interested in is shallow dof. I don't see a way to do this without a way to control shutter speed. I've seen that certain zeiss lenses have an added control, t-stop, that apparently controls the light seperately from the f-stop. Would I need a lens like this in order to achieve shallow dof? If so, is there any affordable lenses that offer this!? The zeiss lenses are $3k and up! The filmo uses c mount lenses, so I guess I might need an adapter as well.

Anyways, thanks for your help!

Jay
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#2 David Mullen ASC

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Posted 08 May 2006 - 11:22 AM

Shutter angle does not affect depth of field, except that it may help you reduce outdoor bright light further so you can open up the iris, but ND filters are better for that.

Basically you need to use very fast lenses, ideally in the range of T/1.4 to T/2.0, wide-open, to get a shallow focus look in 16mm. Or shoot only telephoto shots. But most 16mm fast lenses that are affordable are rather soft optically when used wide-open, so just be aware of that. Plus of course, focus pulling becomes a lot more critical at wide-open apertures, but that's the nature of shallow-focus photography.

T-stops and f-stops are the same thing -- the marks on the ring controlling the lens aperture/iris -- just the markings are shifted over with T-stops on modern lenses from the original f-stops marks in order to compensate for any internal light loss so that exposures are more accurate (in other words, you read f-stops on your meter and use the T-stop on your lens. So a T-stop is just a more accurate f-stop mark on the barrel.) But honestly, there is only a minor difference between the two marks so don't sweat over it.

In other words, a lens has an f-stop mark based on the size of the aperture in relation to the focal length, a physical dimension. A lens maker reads the amount of light passing through the lens and notes that even if the lens is set to f/2.8, for example, he is getting, let's say, 1/6 of a stop less than that out of the rear of the lens due to some light loss from all the glass elements, so he moves the f-stop mark over on the ring so that he gets f/2.8 exactly in terms of the light passing through. That adjusted mark is called a T-stop ("T" for "transmission").
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#3 John Pytlak RIP

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Posted 08 May 2006 - 11:23 AM

Hey everyone. I'm new so bare with me!

My grandad just gave me his old Bell & Howell 16mm Filmo! Nice camera! Non-reflex, but still nice. What I'm curious about is setting up dof effects with a movie camera. I understand setting the f-stop accordingly, but you can't really change the shutter speed to compensate for any loss of light.

What I'm interested in is shallow dof. I don't see a way to do this without a way to control shutter speed. I've seen that certain zeiss lenses have an added control, t-stop, that apparently controls the light seperately from the f-stop. Would I need a lens like this in order to achieve shallow dof? If so, is there any affordable lenses that offer this!? The zeiss lenses are $3k and up! The filmo uses c mount lenses, so I guess I might need an adapter as well.

Anyways, thanks for your help!

Jay


Depth of field is more shallow as you open up the lens iris --- and you get more exposure as you open up the lens. If necessary, you can either reduce the light level, or use neutral density (ND) filters to adjust exposure to compensate for using a larger aperture setting:

http://en.wikipedia.org/wiki/F-number

http://www.faqs.org/...oto/lenses/faq/

http://www.dofmaster...hyperfocal.html
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#4 Laurent Andrieux

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Posted 08 May 2006 - 11:25 AM

I've seen that certain zeiss lenses have an added control, t-stop, that apparently controls the light seperately from the f-stop.


The difference is only that f-stop are "mathematically" calculated, while T-stops take account of the light loss due to the lens. They are very close to each other. Usually, a lens is calibrated with T-stops, anyway. You are supposed to set the iris ring according to the amount of light you have, that's all.

If ever you have a lens calibrated in f-stops, and is a prime lens, it might mean that the loss is negligeable so that you can just set it as if it was a T-stop. If you have a zoom lens or a lens that you think is not loseless, open up 1/3 or even 2/3 of a stop and that should be fine
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#5 Jay Taylor

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Posted 08 May 2006 - 12:03 PM

Wow, that was fast! So let me see if I'm understanding all this?

DOF becomes shallower as you open up the lens iris. Which in turn means more light would be hitting the lens, so I'd have to control that. So does that mean if I wanted a shot with a large DOF I would shoot with a larger f-stop? Wouldn't that mean I'd need MORE light for those types of shots?

If I was shooting outdoors, lots of sunlight, and I wanted just about everything in focus, would I need more light? Or would the sun cover it? I'm sure these questions are idiotic! lol.

So lets see?

Smaller f-stop = shallow dof = lots of light / Larger f-stop = large dof = less light

Is that right? Or is it the other way around?

Jay
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#6 Laurent Andrieux

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Posted 08 May 2006 - 12:14 PM

DOF becomes shallower as you open up the lens iris. Which in turn means more light would be hitting the lens, so I'd have to control that. So does that mean if I wanted a shot with a large DOF I would shoot with a larger f-stop? Wouldn't that mean I'd need MORE light for those types of shots?


Yes to all questions

Smaller f-stop = shallow dof = lots of light / Larger f-stop = large dof = less light

Is that right?


That's correct if you don't miss that small aperture is given by a big number (11, 22 for instance) and provide a large DOF ; and a big aperture, given by small numers (2, 2.8 for instance) provide a shallow DOF.

Mind that the focal length (wide angle as opposite to telephoto) influence DOFa lot as well : a wide angle gives a lot of DOF, while a telephoto gives a shallow DOF
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#7 Jay Taylor

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Posted 08 May 2006 - 12:20 PM

Awesome! I think I understand dof a lot better now. You guys are fast and thorough! I appreciate your help!

Jay
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#8 David Mullen ASC

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Posted 08 May 2006 - 12:28 PM

Luckily if you want a deep focus look, it's easier in 16mm than with 35mm because the focal lengths of the lenses used tend to be half as long. But conversely it's harder to create a shallow focus look in 16mm.
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#9 John Pytlak RIP

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Posted 08 May 2006 - 12:28 PM

So lets see?

Smaller f-stop = shallow dof = lots of light / Larger f-stop = large dof = less light

Is that right?
Jay



Thats right. Let's say you are using a film like Kodak VISION2 50D Color Negative 7201 outdoors on a bright sunny day. A 170-degree shutter will give you an exposure time of 1/50 of a second, and your meter will likely tell you to expose an EI-50D film at f/16. At f/16, you will have lots of depth of field.

Now since you want less depth of field, you decide to open up the lens to f/4.0, which would give four stops additional exposure. So you need to use a 1.20 density neutral density filter (4 stops x 0.30 density per stop = 1.2) to compensate.

Another reason not to use f/16 is "diffration limiting" --- most lenses have optimum sharpness in the middle range of their f/stops. As you close the iris down to a "pinhole", diffraction of the light though the small opening limits the sharpness you can achieve.
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#10 Andy_Alderslade

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Posted 20 May 2006 - 01:04 PM

Luckily if you want a deep focus look, it's easier in 16mm than with 35mm because the focal lengths of the lenses used tend to be half as long. But conversely it's harder to create a shallow focus look in 16mm.


David can you please clarify what you mean by 'deep focus'?

Is that when the perspective is crushed and the background is magnified, but does not refer to the depth of field?

Thanks, Andy
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#11 David Mullen ASC

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Posted 20 May 2006 - 06:09 PM

David can you please clarify what you mean by 'deep focus'?

Is that when the perspective is crushed and the background is magnified, but does not refer to the depth of field?

Thanks, Andy


No, that's referring to the compressed perspective of a telephoto lens.

"Deep focus" refers to objects from very near to very far all looking reasonably in focus.
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#12 Alan Duckworth

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Posted 30 May 2006 - 02:32 AM

It's not the movies, but check out almost any of Ansel Adams' black & white landscape photographs - they are the definition of "deep focus". Mr Adams used mostly 4x5 and 8x10 cameras, with apertures set down as far as f64. He also was using view cameras, which allow for camera adjustments to realign the subject, lens and film plane to maximize the depth of field - a neat trick not available on movie cameras (or is it? - anybody know?).

A classic example of deep focus in the movies is to be found in Antonioni's "Blow-Up" (MGM 1966). About 10 minutes in there is a shot of a model laying on the floor filling the full width of the foreground, and the "hero" (a photographer played by David Hemmings) is relaxing on a couch in the far background - all in clear focus. And it is an interior shot at a real location.
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#13 Stephen Williams

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Posted 30 May 2006 - 02:45 AM

Yes to all questions
That's correct if you don't miss that small aperture is given by a big number (11, 22 for instance) and provide a large DOF ; and a big aperture, given by small numers (2, 2.8 for instance) provide a shallow DOF.


Hi,

The reason the no gets bigger while the apeture gets smaller is that they are fractions.

1/2, 1/2.8, 1/4, 1/5.6 ,1/8, 1/11 ,1/16 etc

Stephen
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