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Depth of Field On 16mm Worse Than Video

Published : 6th February 2008

(I would like to Thank Doug Hart for his co-operation in allowing CML to reproduce parts of his book for this discussion)

Hello everybody!

I have made my first stand as a DoP (I actually am a photographer). I used a 16mm Bolex with Nikon 35mm lenses from 24mm to 200mm.

All my life as a photographer I was fighting against low depth of field (and I mean really low - focus on eyes and ears are out, thus when person moves you have focus on the nose while eyes are out... a.s.o.)

and now it seems all the contrary: I have a terrible depth of field in the shots - like from "NY to LA". I have three examples of shots on my website. If you would please look at them and tell what you would recommend I should do. It's almost worse than video...

The URL for the pictures is : www.soheili.eu/cine/index.html

Is it in general that "the smaller the image the more DoF" ?
But what is it with super 8? I never saw that effect there...

I am:
Phil Soheili
Corporate Portrait, People & Fashion Photography
ANKH Studios Munich & Milan
http://www.soheili.eu


Phil Soheili writes :

class="style9">>>and now it seems all the contrary: I have a terrible depth of field in the >>shots - like from "NY to LA". I have three examples of shots on my >>website. If you would please look at them and tell what you would >>recommend I should do.

It is a lot easier to reduce depth-of-field than it is to increase it. Neutral Density filters will open your aperture and reduce d-o-f.

Using slower film stocks will also open that aperture. Probably 95% of all film shoots work in the "less than T4" range, and probably 90% in the "less than T2.8" range.

Doug Hart
1AC, NYC


Hi Doug

…and thanks for your help.

Just one more question:

Could you explain the difference between F and T..
I used to name aperture with f. Here I hear almost never f but T. What is the point?

And thank you very much again.

Phil Soheili
Photographer
People, corporate portrait & fashion
ANKH Studios
Munich & Milan
+49 172 74.898.74
+39 335 835.8991
http://www.soheili.eu


The "f" and "t" you are referring to is f-stop and t-stop.
An f stop is the mathematical relationship of the focal length of the lens to the diameter of the lens opening. f=D/F
A t stop (more exact) is a mechanical exact measurement of light transmission as measured at the factory where the lens is made.

Rich Cascio
SF Cameraman


class="style9">>>Just one more question:
>>Could you explain the difference between F and T…I used to name >>aperture with f. Here I hear almost never f but T.

Phil -

Here is an excerpt from Chapter 9 in my book (The Camera Assistant: A Complete Professional Handbook, Focal Press 1996) to answer your question:

>>>>
f/stops and T/stops

The relative size of the lens aperture compared with the focal length of the lens is measured in Af/stops.@ F/stops are mathematical computations of how much light should get through the lens, in an ideal world with an ideal lens, with perfect glass. The Af/stop@ is also sometimes described as the Ageometric aperture.@

Take the focal length of the lens being used, say 100mm, and divide by the actual diameter of the iris, say 25mm, and that=s an f/4 on the lens. A 50mm lens with a 25mm aperture would be at f/2.

It=s a simple formula: the f/stop equals the Focal Length of the Lens divided by the Iris Diameter.

                                                           Focal Length

                      f/stop       =        -------------------------------------------

                                                           Iris Diameter

However, we live in a most imperfect world, no glass is optically perfect, no lens design is mathematically perfect, and no iris is perfectly round. The f/stop deals simply with the physical dimension of a hole in the diaphragm of the lens. It does not deal at all with the amount of light actually reaching the film. Some of the light passing through the lens is lost to diffraction, refraction, and reflection, before reaching the film plane.

Lenses, and zoom lenses in particular, have many elements of imperfect glass inside. The f/numbers printed on the lenses are good for theoreticians but don=t have much to do with how much light actually gets to the film. Therefore, we also have AT/stops,@ measured with a device called a T/stop machine. This is really only a light meter with a lens mount on it, and actually measures how much light gets through all those layers of imperfect glass and through the scattering that=s being done by all the air/glass surfaces inside the lens. This is much more accurate than mathematically obtained f/4, which may only be T/4.2 in reality.

>>>>

Hope this helps.

Doug Hart
1AC, NYC


Part 2

 

T/ stands for Atransmission@ or Atrue@ stop. The AT/stop@ is also sometimes described as the Aphotometric aperture.@ It=s the light that is actually transmitted through the lens and reaching the film plane, and will be different on every lens, depending on the design of the lens, how many elements are included, what type of glass, and what type of coatings. This is a correction, to allow more accurate and controllable exposure of the photographic image.

Before zoom lenses and super-fast lenses came into common use, it was less complicated, the f/stop was an accurate enough measure of the amount of light that got through the lens. Now we’ve got lenses that contain many elements of glass inside, and coating on all surfaces, and it=s become a lot more complicated.

We are also now more exacting in our shutter speed computations and requirements. A hundred years ago, a two or three minute exposure would be necessary to photograph a portrait or a landscape. A few seconds more or less exposure didn’t make much difference. Now we’ve got to get 24, 30, 100, or even 500 pictures done each second with a camera. A much more accurate measurement is needed.

In prime lenses, the difference between the f/ and T/numbers is very small, because there are a limited number of elements of glass inside. Some lenses will have both f/ and T/scales on the lens. There might be two indices on your aperture scale, one for f/stops and one for T/stops. On prime lenses, the difference between the f/ and T/stops is usually less than 1/4 of a stop.

Sometimes the lens has different scales on opposite sides of the lens. On the Angenieux 12B120mm Zoom, for example, for years the standard lens for 16mm filming, the top scale has f/stops in white, and on the bottom of the lens is a T/stop scale in red.

In general, the more elements there are in the lens, the farther apart the f/ and T/numbers are going to be. The relationship between a lens= T/numbers and its f/numbers is shown in the following formula:

[Sorry - I couldn't make the formula appear correctly in an email - I've never tried to get the "square root" symbol into an email before. You'll just have to buy the book!]

You can see from this formula that if the lens were perfect (a transmittance of 100 percent), the T/ number and the f/ number would be the same. For a lens with a transmittance of 90 percent, f/2 would translate to T/2.1, and for a lens with transmittance of 80 percent, f/2 translates to T/2.3, etc.

>>>>

Hope this helps.

Sorry - I had to break up the passage into smaller pieces for CML.

Doug Hart
1AC, NYC


Part 3

Panavision, Zeiss, and some other lens manufacturers have decided to just totally ignore f/stops on lenses. These lenses only have a T/stop scaleCthere is no f/stop scale anywhere. The T/stop will always be a higher number than the equivalent f/ because light is lost inside the lens. Every time light comes to an air/glass surface, a small percentage of that light is Ascattered,@ or refracted and reflected back. Even with a lens with only a few elements, less light gets through the lens than what entered the lens.

Every piece of glass, because there is no such thing as a perfect piece of glass, is also going to absorb a little bit of light. No glass is absolutely, perfectly, transparent. There is always some colour, some impurity, in the glass. There will always be less light coming out of a lens than was put in. So T/s are always higher numbers (less light) than f/s.

There is no such thing as a light meter that will give you readings in T/stops. A meter tells you mathematically what the exposure should be and how much light needs to reach the film for proper exposure. This reading from the meter will always be in f/stops. You then set it on the lens using the T/stops. The amount of light actually reaching the film through the lens will be the correct amount, according to the exposure meter.

Any calculation of exposure must be made in f/stops. Once you have the exposure reading that you want, in order to deliver that exposure to the film you’ve got to set the lens to the T/stop.

>>>>

Doug Hart
1AC, NYC


Thanks Doug!

That's great!
So you're a writer, too..
Great company here...

Be good, guys

Phil Soheili
Photographer
People, corporate portrait & fashion

ANKH Studios
Munich & Milan

+49 172 74.898.74
+39 335 835.8991


Over the years I've tried to explain this to many assistants and trainees but never so concisely.

Good job

Julian Bucknall
1AC LONDON



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