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Focal length, aperture and depth of field


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#1 Alexis Vanier

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Posted 31 May 2006 - 11:44 PM

Hi there,

I'm an amateur cinematographer, I've shot video and 35mm still, some super 8 on fixed lens cameras and now, as I just landed a job at the service departement of one of the biggest photo shops in Montréal, I am facing the thing I dread the most : digital cameras and the bloody conversion thing.

Here is my simple question : Are digital camera lenses partly responsible for the too deep depth of field associated to digital still cameras?

Here is my complicated sub-questions : Is, what is called, the actual focal length (the one written on the lens), really the actual focal length of the glass, or is the 35mm 'equivalent' the real focal length.

If the focal length is 'phony' then all the the F-markings on the lens are screwed-up and depth-of-field is, because DoF is relative to aperture diameter relative to focal length, the CoC and its progression over distance from focal point AND display circumstances RELATIVE to the capture area dimensions.

Summation : I'm completely lost. Help me.

Sub-summation quesdtion : Are 'digital' lenses real lenses of their own with 'focals' adapted to their capture area OR is the digital image 'a crop from a 35mm frame'.

==========

Again, please help me. I'm completely lost in all this.
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#2 Michael Nash

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Posted 01 June 2006 - 12:47 AM

Here is my simple question : Are digital camera lenses partly responsible for the too deep depth of field associated to digital still cameras?


Not the lenses per se, but the size of the sensor. The smaller the sensor, the shorter the focal length you need to maintain a consistent angle of view. The shorter the focal length, the deeper the depth of field (when aperture and subject distance remain unchanged).





Here is my complicated sub-questions : Is, what is called, the actual focal length (the one written on the lens), really the actual focal length of the glass, or is the 35mm 'equivalent' the real focal length.


The actual focal length.

If the focal length is 'phony' then all the the F-markings on the lens are screwed-up and depth-of-field is, because DoF is relative to aperture diameter relative to focal length, the CoC and its progression over distance from focal point AND display circumstances RELATIVE to the capture area dimensions.


The focal length marked is real, and makes no reference to 35mm. Rest assured, the laws of physics are intact. ;)


Sub-summation quesdtion : Are 'digital' lenses real lenses of their own with 'focals' adapted to their capture area OR is the digital image 'a crop from a 35mm frame'.


I'm not even sure what you're asking here, but there's nothing "phony" going on with digital lenses. The focal length is what it says it is, and the angle of view you get from that lens depends on the size of the sensor. Forget all this business about comparisons to or extractions from 35mm; it doesn't apply.
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#3 Laurent Andrieux

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Posted 01 June 2006 - 03:28 AM

Hello Alexis,

Don't know if it might help, but you ban have a look here :

http://www.cinematog...?showtopic=5207
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#4 Charles MacDonald

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Posted 02 June 2006 - 09:13 PM

The focal length marked is real, and makes no reference to 35mm. Rest assured, the laws of physics are intact. ;)
I'm not even sure what you're asking here, but there's nothing "phony" going on with digital lenses. The focal length is what it says it is, and the angle of view you get from that lens depends on the size of the sensor.

Of course the size of the sensor is buried in the fine print and can differ from Camera to camera. The smaler the sensor actually is, the more depth of field, and the shorter the lens focal lenth needs to be to aproximate a "normal" lens.

These days it is quite practical to have hi count sensors the size of a 16mm Fame.. I am not sure if you can "see" the sensor in most digital camera. (or HDTV camera for that matter. The technology is quite simalar.
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#5 Michael Collier

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Posted 02 June 2006 - 09:26 PM

The focal lenth is the actual focal length in the lens. 35mm equivalent is what the focal lenght would have to be if you are shooting on a 35 sensor (equivalent as far as angle of view) a 250mm (in 35mm) lens might look like a 100mm (in 2/3" sensor). As was mentioned before, the focal length is correct, and if you note, DoF IS equivalent to 35mm, if you look at it right. a 250mm 35mm lens should have the same DoF (approx of course) as a 250mm lens in 2/3" or even 1/3", but it would appear as though the lens is zoomed in much farther. (its aprox because the construction of most 35mm SLRs aren't the same as a digital lens, unless its a digi SLR)

One more quick thing....you can actuall tell the size of the sensor (if its not promenantly listed) the farther the actual focal length is from the '35mm' equivalent, the smaller the sensor (the ratios of focal lengths directly matches the ratio of sensor size.)

Edited by Michael Collier, 02 June 2006 - 09:27 PM.

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#6 Alexis Vanier

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Posted 02 June 2006 - 09:36 PM

I think you all cleared this thing up.

But that

and if you note, DoF IS equivalent to 35mm


is most interesting, although a more thourough and mathematically sustained explanation might be even more revealing to my still confused comprehension.

I like the laws of physics. B)
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#7 Chris Keth

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Posted 02 June 2006 - 11:55 PM

I like the laws of physics. B)



There's one tricky bit that's not a law of physics, but that also affects the depth of field characteristics of a certain format. On smaller sensor/film sizes, we tend to use a wider lens than in 35mm to do particular things. Say you like the look of 100mm close-ups when shooting 35mm film. Well, if you went to 16mm film, you would want something more like a 50mm lens, which would, at the same subject-to-camera distance, have greater depth of field. This lens choice issue is, I think, the biggest annoyance.

Just the same, by the way, on 8x10 format film (yes, 8 by ten INCHES) a standard portrait lens is somewhere in the realm of 19" (nineteen INCH) focal length and your depth of field at f22 sometimes won't even cover from the nose to the ears of a subject of a portrait.
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#8 Michael Nash

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Posted 03 June 2006 - 12:09 AM

There's one tricky bit that's not a law of physics,


No, I'd say what you just described IS physics, or at least mathematics. It very easy to calculate or even look up the angle of view and depth of field.

What isn't so much a law of math though is the manufacture of lenses, where you tend to get more barrel distortion with really short, wide lenses. For example, a 3mm lens on a 1/3" chip camera will usually have a similar angle of view, but much more barrel distortion, than a 7mm lens on a 16mm film camera.

is most interesting, although a more thourough and mathematically sustained explanation might be even more revealing to my still confused comprehension.


What don't you understand? We'll try to be patient...
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#9 David Mullen ASC

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Posted 03 June 2006 - 02:59 AM

Isn't the point of filmmaking to get away from thinking technically too much of the time? There is a level of technical knowledge about filmmaking technology that is not really practical, it just satisfies the curiosity of tech heads.
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#10 Olex Kalynychenko

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Posted 03 June 2006 - 08:25 AM

Hi.

Very important and necessary question.
If you shoot, you need know sharpness depth of field distance of lens.
I use DOF table for calculatioon.
But, i have DOF table of 35 mm prime lenses from Arri site.
( very good technical information ).
A some small table of 35 mm Cooke S4,
Table of DOF for Super 16 Cooke zoom lens 9-50 mm.
But, i have not DOF tables for Super 16 prime lenses.
If you know, Where i can load DOF table for Super 16 prime lenses, please, help me.

From technical side, The use of DOF tables from 35 mm prime lenses with Super 16 lenses not possible, because, 35 mm DOF tables calculate of point of unsharpness ( Cooke 0.001 in= 0.025 mm, russian engineering standard 0.03 mm,).
The DOF tables of Super 16 lenses must use point of unsharpness 0.0005in = 0.0127 mm ( russian engineering standard 0.015mm ).
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#11 Stephen Williams

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Posted 03 June 2006 - 01:57 PM

From technical side, The use of DOF tables from 35 mm prime lenses with Super 16 lenses not possible, because, 35 mm DOF tables calculate of point of unsharpness ( Cooke 0.001 in= 0.025 mm, russian engineering standard 0.03 mm,).
The DOF tables of Super 16 lenses must use point of unsharpness 0.0005in = 0.0127 mm ( russian engineering standard 0.015mm ).


Hi,

From my understanding, if you use the 35mm values 2 stops lower you will get what you want.

Stephen
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#12 Olex Kalynychenko

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Posted 04 June 2006 - 07:13 AM

Hi,

From my understanding, if you use the 35mm values 2 stops lower you will get what you want.

Stephen


I don't know, what version of comparison can be correct.
35 mm and Super 16 lenses with one focal distance
or 35 mm and Super 16 lenses with one angle of view.

If i take Cooke Super 16 zoom lens on 25 mm position of zoom and 1 meter of focus, and T5.6
the DOF distance 0,933....1.079 m
( Russian DOF 0.88...1.15 m)

Distagon 24mm Arri35, focus - 1 m, T5.6
DOF 0.872...1.322 m
( russian DOF 0.79...1.37 m )

If i take formula 16 mm DOF = 35mm DOF - 2 stops,
24 mm Distagon Arri 35 - 1 m T2.8
DOF 0.939....1.148 m

The DOF 0.938...1.075 have T2.0 ( - 3 stop)

But, i not check this formula with other lenses and other distance of focus.


If we take lenses with similar angle of view
25 mm ( 16 mm )angle 23.8'
50 mm ( 35 mm ) angle 23.4'

Planar 50 mm Arri35, focus- 1 m, T5.6 ,
DOF 0.968...1.057 m

You can see, if you take 16 mm lens and 35 mm lens with similar angle of view, you can have near resemblance pf DOF, or use formula -2 ( -3 ) stops.
( I write this formula on my note book. Thank )

But, the better, if you have DOF data chart of Super 16 lens.
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#13 Stephen Williams

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Posted 04 June 2006 - 07:36 AM

If i take Cooke Super 16 zoom lens on 25 mm position of zoom and 1 meter of focus, and T5.6
the DOF distance 0,933....1.079 m
( Russian DOF 0.88...1.15 m)


Olex,

You can not compare a zoom lens with a prime lens. The front nodal point of the zoom will be in a different place. DoF is calculated from the front nodel point of a lens, not the film plane. The image size on the film plane is also different. DoF is calculated from F stops and many motion picture lenses are only marked in T stops.

Size of the exit pupil can also have an effect on DOF. Simple lens theory ignores this and assumes a 'thin' lens. That is why a Zeiss or Cooke lens chart are not identical.

DoF charts are a starting point to test from. Many people will say that DoF is the same regardless of format, think of zooming in with a Telecine to a S16mm frame from a 35mm negative.

Looking through the eyepiece can tell you an awful lot!

Stephen
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#14 Olex Kalynychenko

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Posted 05 June 2006 - 06:01 AM

Hi Stephen.

Thank you for your answer.

I wish underline, i had not idea to show of full theory of DoF at one short letter.
I know not West theory of DoF and can told about russian theory of DoF only.
The russian book have many pages with diagrams of theory DoF.
And DOF have nonlinear dependence with focal distance of lens, with objective aperture (T),
from distance of focusing, very small distance of focusing, normal of macro lens.
The DoF calculation must take account of additional lens extension at focusing and many other
factors.
But, this is theory and you have not free time make complex calculation on shooting area.
That's why, The use of data chart of lens manufacturer will better.

You can not compare a zoom lens with a prime lens. The front nodal point of the zoom will be in a different place. DoF is calculated from the front nodel point of a lens, not the film plane.

Stephen


I can agree, but partly.
The base formula of DOF from theory ( i can not show formula correct on text)
of DoF for Zoom lens and prime lens full similar.
You correct.
The formula use distance of focusing - distance between optical centre ( base point )of lens and subject.

And need to take note of geometrical size of optical system, because, real lens have front and rear optical centre.
But, I use DoF table of lens from real distance of focusing of lens, the distance between film plane and subject.
I think, you mean additional lens extension with focusing on disfference distances at calculatiion of DoF by formula.
The zoom lens and prime lenses have difference length of optical scheme, difference distance between front and rear base optical point and lens extension will difference too.

But, from other side.
The distance of focusing of lens for any prime and zoom lens - distance between film plane and subject of shooting and printed on lens.
This is distance included distance between subject and front optical point + distance between front and rear optical point of lens + distance between rear optical point and film plane.
The data chart of DoF of lens show me distance of focusing lens from film plane, not from optical centre of lens.

That's why, if we compare data chart of lenses with distance of focsuing of lens, this digital included all geometrical size of this lens.

DoF is calculated from F stops and many motion picture lenses are only marked in T stops.


Yes, the many lenses are marked in T stops.
The mechanical aperture ( F ) - design value, depend from size of lens.
The effective value of lens aperture ( T ) = F- ( negative profit of volume of light inside lens ).
The negative profit of light inside lens have small volume, that's why, you can use F for calculate DoF.

You told all Correct, yes, the " Simple lens theory ignores this and assumes a 'thin' lens. "
That's why need use a data chart of DoF of manufacturer of lens, but, if i have not this data, i use
similar or other simply calculation.
If you have other simply way, i will happy to use this too.
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#15 Stephen Williams

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Posted 05 June 2006 - 06:14 AM

Hi Stephen.

Thank you for your answer.


Olex,

Remember that if the object size stays the same on the negative, the DoF will remain constant regardless of lens focal length. (at the same F stop). Sometimes its hard to believe but easy to see in the macro world.

Stephen
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#16 Olex Kalynychenko

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Posted 05 June 2006 - 08:47 AM

Olex,

Remember that if the object size stays the same on the negative, the DoF will remain constant regardless of lens focal length. (at the same F stop). Sometimes its hard to believe but easy to see in the macro world.

Stephen


Full agree .
If you have the similar size of object on the negative ( ground glass ) with difference lenses ( difference focal lenth ) and difference of distance of focusing, the DoF will have constant volume.
I know this and use this at my work.
But, this work good for close-up shots and a some for medium shots
I have a few data chart for difference ratio of size of subject and size on the negative.

This is russian data chart with point of unsharpness 0.032mm

35 mm lenses
If we have ratio 1:25 ( close-up )
lenses 18...150 mm, DoF
F2.0 DoF 0.078m...0.0785 m, F4.0 0.1560..0.1562m F8.0 0.3132...0.3510m

the ratio 1:100 ( medium shot )
lenses 18..100 mm DoF
F2.0 1.22..1.37m
F4.0 18 mm 4.36m, 25 mm 3.14m 35...100 mm 2.75...2.46m
F8.0 18 mm - infinity, 25 mm 61.84m 35 mm 6.3m, 50...100 mm 5.37..5.14m

the ratio 1:200 ( general plan)DoF
F2.0 18 mm 8.68m, 25 mm-6.27m, 35 mm-5.46m 50..100mm 5.12..4.90m
F4.0 18 mm- infinity, 25mm-123m, 35mm-18.21m, 50 mm-12.83m 75...100 mm, 10.24m
F8.0 18 mm- infinity, 25mm-infinity 35mm-infinity, 50 mm-246m, 75 mm-32.68m, 100 m- 25.07mm

( i hope, this information can be interesting)
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#17 Alexis Vanier

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Posted 05 June 2006 - 05:50 PM

Remember that if the object size stays the same on the negative, the DoF will remain constant regardless of lens focal length. (at the same F stop). Sometimes its hard to believe but easy to see in the macro world.


That's true unless you change format, isn't it?

So, is there a way to do a quick and dirty calculation of depth of field by including format (well that would be the standard CoC) in the formula?
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