Depth of Focus ... related to focal length or not?

[e._grim]

In View Camera Techniques, Leslie Stroebel writes "Depth of focus is not affected by focal length," and gives this formula: depth of focus = 2(f-number)(acceptable circle of confusion).

 

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In his review of the Walker XL in the July/August 2000 issue of View Camera magazine, Roger Hicks says that the camera makes it easier to get the front and rear standards parallel. This is critical with wide angle lenses because they have less depth of focus.

 

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This discrepancy is probably results from Hicks and Stroebel using the same terms to describe different things. Could some of you clarify this point for me? I am especially interested in the "problem," if problem there is, of alignment of front & rear standards with short lenses (75 to 120 mm).

 

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Thanks in advance.

[doug_paramore]

That's odd. Wide angle lenses are supposed to have more depth of

field than longer lenses. Maybe I put more emphysis on how it looks

than formulas. I don't know how they determine that, unless if it is

for the same object size on the neg. Anyway, part of the problem with

wide angle lenses is that many users of wide angle on LF do

not "focus in" as they should. Wide angle lenses should be focused

nearer the camera and let the depth of field take care of the distant

subjects due to more curvature of field of the wide angles.

 

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Regards,

[armin_seeholzer]

Hi mister Grim

 

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DOF is affected by the focal length. Thats the reason why many fashion

shooters take a f2.8 300mm lens wide open on the 35 mm camera and the

background is almost not visible because it is total out of focus.

Wider lenses has more DOF for example my 8mm fisheye has DOF from 20 cm

to infinity closed to f 5.6!

With short lenses the aligment is not so important as with longer

lenses but for good results is it always important.

Cheers!

[bob_salomon]

Depth of field is in front of the lens from the nearest point in

acceptably sharp focus to the furthest point from the lens

accepted as sharp.

 

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Depth of focus lies behind the lens and the film must lie within it

in order to be sharp.

 

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Depth of field is less with long lenses and greater with short

lenses.

 

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Depth of focus is shorter with wide lenses and deeper with long

lenses.

 

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The shorter the focal length the more s=critical the film plane

position becomes.

[bj_rn_nilsson]

Hi!

 

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Well, the real answer is that DoF is NOT related to focal length.

This is easiest to describe by using pictures, but here goes anyhow.

Let's start with a certain scene, say 20 feet wide. If you go back

the neccessary distance to fill that with a wideangle, you have to go

back e.g. 15 feet. To get the objects that in between 15 and 30 feet

from you within the DoF you have to set the aperture to e.g. f/32.

Now go back with the camera and mount a long focal length. See to

that you fill the same 20 feet. Now you will be at e.g. 60 feet from

the scene. In order to achieve the same DoF as before you have to use

the same aperture as with the wide angle. The thing that is the most

interesting about this is that the DoF using the longer focal length

sometimes appear (note: appear) to be larger.

The DoF is the same, but the wideangle lets you get closer to the

subject to get the shot, that's all.

Now, wideangle lenses does give a good DoF on subjects that has a

small degree of enlargement. I.e. you get a lot of area covered from

a short distance. But the very small distance in between the lens

plane and the film plane makes the setting of this distance critical.

Why is this? The circle of confusion (CoC) grows very rapidly in the

film plane when you move either plane back or forth. Compare this

with the relatively slow growth that you get when using a longer

focal length. (The cone is as long as the distance between the front

and rear standardts.) Given this, a misalignment of e.g. 1 mm

(~1/32") may be fully visible with an extreme wideangle lens, while

it may not be visible with a long focal length lens.

As I said in the beginning, some pictures/drawings would help to

describe these issues.

 

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Björn

[mike_mahoney]

Strobels� �View Camera Technique� offers good information on the

subject. The following two links offer interesting information:

 

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http://www.luminous-landscape.com/dof.htm

 

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http://www.luminous-landscape.com/dof2.htm

 

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See also the rather lively discussion thread in the general forum on

that same site related to digital DOF, which evolved into a

generalized discussion about DOF and circles of confusion.

[chad_jarvis1]

Bob got it right, because it would appear he is the only person who

read the question thoroughly enough. Depth of field and depth of

focus are two different concepts.

[bill_glickman]

Just making it perfectly clear since people have commented

incorrectly after Bob's correct response. There is two issues here,

Depth of Field and Depth of Focus. Depth of Field is what everyone

except Bob is talking about above. It is defined as the amount of

acceptable "out of focus" which can be tolerated in reference to the

point of exact focus, or the plane of sharp focus. This "out of

focus tolerance is defined by the Circle Of Confusion. (coc) This is

what 99% of people talk about and quite often confuse with Depth of

Focus.

 

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However, Depth of Focus, is the opposite of the above, but

eqaully defines focus tolerances. Depth of Focus defines the amount

the film can be out of alignment vs. the acutal, or true film plane.

The film must be within this tolerance as defined below to maintain

the desired cc:

 

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f stop * the allowable circle of confusion (used in the Depth of

Field calculation)

 

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As you can see from the formula, Depth of Focus has no bearing

on the fl of a lens. The answer to this formula defines how much the

film can buckle or be out of alignment for any reason, in either

direction of the true film plane and still maintain the desire circle

of confusion on film. With improper film alignment or film buckle

Depth of Field is useless as Depth of Focus becomes the bottleneck to

the resolution acheived on the film.

 

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So both Depth of's, are equally important to acheive the

resolution you desire on film. Howver, most people tend to only deal

with depth of field since the depth of focus is out of most peoples

control. Depth of Focus is also the achillies heel of most fast

lenses, they are often limited by film flatness, not the quality of

the lens.

 

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Also effecting resolution on film is slew of other factors

such as moving subject, moving camera from wind, shutter vibration,

shutter speeds which spend a majority of their time opening and

closing in the light path which adds to diffraction, diffraction

limited f stops, lens and film resolving powers defined by the 1/R

formula in the Fuji Film handbook, miror slap...etc. It's almost

amazing anyone can ever get a sharp image!

 

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to answer your other question.... In his review of the Walker

XL in the July/August 2000 issue of View Camera magazine, Roger Hicks

says that the camera makes it easier to get the front and rear

standards parallel. This is critical with wide angle lenses because

they have less depth of focus.

 

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OK, what I think happened here is this... although the depth

of focus formula does not account for the fl of the lens, it seems

this is a bit of a shortcoming of the formula itself. Or possibly

the formula is only really designed for normal lenses and through the

years this disclaimer has beend dropped. Because the further the

lens is from the film plane, the more shallow the light angles

hitting the film. The closer the film plane is to the lens, as in

wide angle lenses, the greater the angles of light are approaching

the film. So, I do agree with what he is saying, but you are right

in that the formula does not account for it. The same is true using

the same fl lens and focussing at infinity vs. focussing very close

causing the 3x the bellows draw...you have friendly angles to work

with at 3x bellows draw, but no allowance in the formula for such.

Not all photographic formulas are all encompassing. Even Depth of

Field formula does not account for diffraction...it's something you

just need to know and apply it accordingly. Hope this helps...

[hector_e._pena]

Bob is right.

 

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Also, depth of field is not related to focal lenght. Depth of field

is related to magnification. So if you use a 300 mm lens and the

subject in the ground glass measure 39mm, and then you change to a 90

mm lens and get closer to your subject in a way it will measure 39 mm

in the ground glass, you will have the same depth of field than with

the 300 mm lens at the same f stop.

[matt_sampson]

It has always been my understanding that short lenses mean very

shallow depth of focus at the film plane. This leads to the

rationale behind the Silvestri and Ebony non folding

cameras....everything stays absolutely parallel so that theoretically

the only variable left is the flatness of the film. Very short

lenses such as the Scheneider 47xl produce such shallow depth of

focus that it is extremely important that great care is taken when

using them. Worn backs and slightly missaligned ground glass will

lead to unsharp pictures no matter how good the lens.

 

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On the subject of depth of field, (which as someone eventually

pointed out is VERY different to depth of focus),I ,again, have

always been lead to believe that regardless of the focal length of

the lens, depth of field is always the same only perspective changes

and I think that Hector's insightful contribution bears that out. At

the end of the day it doesn't matter things are as they are and you

have to work with the laws of physics not against them.

[bob_salomon]

"only perspective changes"

Not really. Perspective changes with the angle of the camera to

the subject not with the focal length.

 

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What is in or out of focus changes with focal length as does the

amout of area captured on the film and the relative sizes of

objects. But the perspective remains constant if the camera

position remains constant.

[jeff_schraeder1]

Think about the definition of f stop. it's a ratio of the diamenter

of the aperture to the focal length of the lens. f32 on a 400 mm lens

is a bigger hole than f 32 on a 35 mm lens. The size of the hole

(aperture) determines DOF. So...are they related or not? Any lights

going on?

[matthieu_levi_strauss]

I totally agree with Mr. Grim: there is a clear discrepancy

between the writings of Hicks and Stroebel. Although it may seem

counterintuitive, I agree with the fact that depth of focus depends

only of the f-stop (and of course of the chosen value for the circle

of confusion). This is indeed easy to demonstrate using a little

drawing showing the light cone (for instance emitted by a luminous

point) coming from the lens onto the focus plane. The depth of focus

depends only of the angle of this cone which depends itself of the

ratio "focal length/physical aperture of the lens" which is the

definition of the f-stop.

Therefore, I cannot understand why the alignment of the front

and rear standards should be more critical with short focal length

lenses. The only explanation that I can find for this putative small

depth of focus of the short lenses is that, due to their large depth

of field, short lenses are often used more open (smaller f-stop)

than long lenses.

[bob_salomon]

"I cannot understand why the alignment of the front and rear

standards should be more critical with short focal length lenses"

 

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Because when no movements are applied the entire area at

infinity should be equally sharp. This means that the film plane

must be totally within the area of depth of focus. If not some

areas will be unsharp. This requires that a camera be in proper

alignment.

 

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We had a case with the NY Times where they were trying to use

a 47mm on a 23 Technika that was 25 years old. While the

camera appeared to have no visible wear it was impossible to

focus the lens at infinity and have an overall sharp image.

 

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To cure the problem the camera had to go back to the factory

and be re-built to the original factory specs.

 

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Depth of focus is very critical with wide angle lenses.

 

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Perhaps an easy way to remember the difference is that Depth

of field is on the subject side of the lens. Depth of focus is on the

image side of the lens.

[noshir_patel2]

Bob, how do you reconcile your statement about depth of focus varying

by focal length with Stroebel's formula (which doesn't contain focal

length)? This would seem to be critical to the optimum sharpness f

stop selection technique that I have seen on the LF homepage and in a

magazine (maybe View Camera, I don't remember).

[bob_salomon]

"Bob, how do you reconcile your statement about depth of focus

varying by focal length"

He obviously never made a camera that uses extreme wide

angle lenses that needs to be sharp all over at infinity.

 

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I am more concerned what the makers of cameras and lenses

say then theorists. They don't have to make the product.

[bill_glickman]

Bob, Noshir asked a very good question above. You did not answer his

question, but have been around long enough to maybe know the answer.

Can you elaborate on this?

 

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Almost everyone can see from diagrams and drawings that wide

angle lenses require much more precision in film flatness and the

paralellness of the standards. But the question still remains, why

doen't any of the Depth of Focus formulas utilize the fl into the

formula? I have never seen any Depth of Focus formulas utilize fl as

a variable. Does anyone have a good answer for this?

 

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I am sticking by my theory. The formula was originaly designed

for normal fl lenses. Longer lenses are are easier to work with than

normal lenses in this regard, so no reason to re write the formula.

But wide angle lenses require even greater precision than normal and

long lenses....and no one every bothered to figure out what the exact

relationship between fl and Depth of Foucus is, hence the formula

really only applies to normal and longer lenses...

[huw_evans2]

'<i> Almost everyone can see from diagrams and drawings that wide

angle lenses require much more precision in film flatness and the

paralellness of the standards. </i>'

 

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In that case 'almost everyone' needs to do their diagrams and

drawings more accurately. The depth of focus is determined by the

angle subtended by the aperture at the film plane. This is the same,

for any specified f-number, regardless of focal length <i> because

the f-number is equal to the focal length divided by the aperture

diameter</i>. Stroebel's formula is correct, and could be verified

by any reasonably bright child with a decent grasp of elementary

geometry.

 

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What <i>might</i> be confusing the issue is that with a short lens an

error in focussing of, say, 1mm will make a much larger difference in

the position of the object plane of focus than it would with a longer

lens. However, the effect on the circle of confusion of a point

source at the intended object plane will be the same - which is just

another way of saying that for a given aperture and a given focus

distance the shorter lens has more depth of field.

[thilo_schmid]

Depth of Focus is related to the focal length and Stroebel stated the

right formula for it. This is best known in Astrophotography, because

there is all about Depth of Focus and they don't care about Depth Of

Field. Take a look at

<a

href="http://www.astropix.com/HTML/I_ASTROP/FOCUS/FOCUS025.HTM#Depth">

http://www.astropix.com/HTML/I_ASTROP/FOCUS/FOCUS025.HTM#Depth</a>

for further explanations.</p>

<p>Another important issue with short lenses is "Field

Curvature". The

focal plane is not a plane, but merely a part of a sphere. The

shorter the depth

of focus the more critical is field curvature and this is the reason

why short

lenses do require higher precision.</p>

Regards,

[thilo_schmid]

Sorry, for the addional confusion, I forgot the essential word "not".

Depth of Focus is *not* related to the focal length. But the focal

sphere has a smaller radius with a shorter lens and is thus more

aparent.

 

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Regards,

[huw_evans2]

I think we're now getting into the realms of equivocation - if we

weren't there already.

 

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You could indeed say that depth of focus is related to focal length

if you are speaking in the context of fixed aperture <i>size</i>. If,

OTOH, you speak in terms of a fixed <i>f-number</i> then it is

independent of focal length.

 

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I must say, though, that I find the entire discussion somewhat

bizarre, since the whole point of LF cameras in general is the

ability to change the relationship of the front and rear standards in

order to exercise control over the image, including the tilting

and/or swinging of the plane of focus - all of which is done by eye

and not by theoretical considerations. To do this reliably means

that you need the alignment of the ground glass to be as accurate as

possible in relation to the positioning of the film holder. However

this is vital regardless of whether short or long lenses are in use.

[huw_evans2]

Oops! My last reply was composed before I had seen Thilo's

clarification, and so looks somewhat strange now. There's no

equivocation any more.

[bill_glickman]

Huw, thank you for the insult. Sorry your answer did not register

with me before I made the post. I understand what you are saying and

it makes perfect sense, I was mistaken, but don't feel I deserved

such ridicule. Have you ever made a mistake in your life?

[huw_evans2]

I'm sorry Bill - I didn't intend it to come across like that. I'll try to be more careful how I express myself in future.

[huw_evans2]

Hmmm. Reading it again, my apology doesn't sound entirely adequate - perhaps I had better explain further.

 

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I've read a lot of posts on forums such as this that have rubbished theoretical answers offered to explain some of the more arcane aspects of photography, optical theory, or whatever. The posters usually insist that their 'experience' is more to be trusted than anyone else's 'theory', no matter how well established it may be.

 

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I had one just the other day on the Medium Format Digest, where I was effectively dismissed as a 'gearhead' and 'beancounter'. One of Bob Saloman's posts put me somewhat in mind of this, and so when eventually I decided to post on this thread I was more in a mood to 'wield the big stick' than to 'speak softly'.

 

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The first paragraph of my first post, while making specific reference to Bill Glickman's post, was actually typed with the more general intent of rebutting those who dismiss the optical theory because they think their own observations contradict it, and further, it was typed in a state of some annoyance. With hindsight I conceed that it appears to be ridiculing Bill, and for that, again, I apologise.