Craft Photography and DoF

[casper]

I have been practicing my photography on my wife’s craft projects. Today I borrowed some of her “stick pins”, and found it difficult getting everything in sharp focus. DoF calculator suggested focal distance of 0.75m @ f/11 would give me focus from 0.71m to 0.80m – just enough depth at 0.09m, as subject was 0.075m front to back. I tried manual and auto focus, and eventually got something I was satisfied with – but it left me wondering: where is focal distance measured from (sensor, back of lens, or front)? I checked afterwards with Kuso, and it said my focal distance was 0.79m, which suggests I did not have enough near focus. I would appreciate any assistance in getting a better understanding, as well as any C&C. Images have been resized only, and all Exif data preserved.
Canon 7D, 50mm f/1.4, f/11, 1/40s.
1. Manual Focus


2. Auto Focus


3. set up (snapshot)

[Mark von Kanel]

Hi Noel,

I asked this question myself on one of my threads about togging bees, the focal distance is measured from the sensor, if you look at the top of your camera youll see a littke circle with a line through it this is the axis of your sensor and where you need to measure from.

These calculators arnt always as accurate as you might think and you may wish to experiment by taking pictures of a tape measure running away from you to calculate your own COC....

[Peeshan]

I'm not an expert on the subject, but maybe using a wider lens or a tilt&shift would help to get the whole glass in focus?

[george013]

Hi Noel,

I asked this question myself on one of my threads about togging bees, the focal distance is measured from the sensor, if you look at the top of your camera youll see a littke circle with a line through it this is the axis of your sensor and where you need to measure from.

These calculators arnt always as accurate as you might think and you may wish to experiment by taking pictures of a tape measure running away from you to calculate your own COC....

And mine answer was that all the formulas are calculated from the optical center point of the lens You can play with the different calculators to find out for your self.

@Peeshan,

There is only 1 plane that can be sharp, theoratical. The depth calculator is based on a magnification of the sensor so that you get an image of around 20 by 30 cm and viewed at a distance of around 1m. If you watch the image on your screen at 100% you probably watch an image of 150 or more cm depending on the quantity of pixels on the sensor and your screen.
George

And don't forget measurements are made from plane to plane parallel to the sensor.

George

[Mark von Kanel]

Noel,

theres a thread here about this:

DOF calculator

The consensus was that its measured from the focal plane which is the sensor.

George you did contribute to this thread but you didnt challenge the advice given to me at the time, have you read something new that i need to learn. and if your advice is correct about the optical center of the lens then how do you measure it? ive checked my lens's and can see no markings, and on a zoom would the optical center move with focal length?

[george013]

Noel,

theres a thread here about this:

DOF calculator

The consensus was that its measured from the focal plane which is the sensor.

George you did contribute to this thread but you didnt challenge the advice given to me at the time, have you read something new that i need to learn. and if your advice is correct about the optical center of the lens then how do you measure it? ive checked my lens's and can see no markings, and on a zoom would the optical center move with focal length?

@Mark,
This qoute is out of the message you mention.

Using the DOFmaster you can check it. Select a 100mm lens and put the subjectdistance at 10cm. Your DOF will be 0. Select for the distance 9cm and your DOF will be negative. So if you want the exact subjectdistance, and you measure from the film plane, you should correct this with the film-pane distance. I don't know how to do that, it's just theorie.

The dof-calculator on CIC stops telling the DOF when it;s smaller then 1mm. Others go further http://www.dofmaster.com/dofjs.html
The theory behind this is, that if the subjectdistance is between the focalpoint and the lens, than there will be an imaginair image. Just using the formulas will give you a negative DOF.
So if you want to measure from the filmplane you have to add the focal lenght and some for the cameramovement to focus on the subject.
And I don't think there is a sign on your lens indicating the optical center point. With zoom-lenses it's moving anyway.
George

[ajohnw]

Looking at how the calculations work I feel that they are based round the distance from a fictitious lens that is an approximation of the one being used. Basically this lens is a single piece of glass of the same focal length as the lens being used. It's an approximation. Dof calcs aim in life is to produce a 12x8in print that can be viewed from 10in. Loosely speaking when a lens is focused at some distance the image there is dead sharp and deteriorates in both directions away from this point. Imagine the lens is focused on a zero sized point. The point will get larger and larger on the sensor further away from the focus. The image produced is called a circle of confusion. The limit for a full fame camera and the print size mentioned is usually when the circle of confusion reaches a diameter of about 0.030mm. Smaller sensors need a smaller circle of confusion as the sensor size has to be magnified more to produce the print. If the viewing distance is doubled the max size of the circle of confusion can also be doubled.

This makes measuring the distance from the lens for dof calculations difficult. The best approximation involves measuring it from the nodal point of the entire lens. This is the point where the lens could be mounted on a pivot and swung about without loosing focus on some point.. Another definition of this point is where it could be theoretically replaced with a single simple lens - this is what the dof calcs assume is being used. You will see comments about the front nodal plane of the lens but I believe that is a load of rubbish as the only place the lens can be replaced with a simple one is the one I just mentioned.

In practice none of this usually a matters - if shooting landscape etc then some 100mm error doesn't matter. It can matter when objects are closer especially in relationship to the focal length of a lens. This is why the distance scales on lenses crunch up more and more as the lens is focused further and further away. The distances are higher and higher multiplies of the focal length of the lens.

Ok so say some one measures the actual nodal point. Big problem, it's position will shift as the lens focus is changed just as a single simple lenses nodal point does. If any one wants to know more google and the term "thin lens approximations" should help. Dof calcs are based on thin lens approximations. A site that goes into this will probably also mention thick lens approximations. These use front and rear nodal points of thick single lenses.. Both can be used to approximate the behaviour of a series of thin or thick lenses.

Back to the OP's question. There will be some suck it and see. There are 2 aspects to this. AF is good at focusing on things rather than into them. It may be possible to force the camera to focus on something deeper into the subject - single point AF. A better option may be to use live view with a magnification level that makes up for the fewer number of pixels the view has. Say it's a 1mp view and a 16mp sensor - 16x would be the best bet but a lower mag may work out. I'd guess that 10x would be a minimum. It's possible to focus through something and centre the lens in the middle as focusing is usually done at a wider aperture than when the shot is taken.. Dof calculators can be used to get a feel for an aperture to use however there is usually no harm in shutting down another stop.

There is a decent pictorial view of what goes on here

http://berniesumption.com/photograph...eld-for-geeks/

And some more interesting approx calculations here

http://www.amateurphotographer.co.uk...ield-explained

A typical use here

http://www.stephenwilliamsphotograph...-formulas.html

That goes back to where it's measured from. If the measurement is taken from the front of the lens as in this case which seems to be fairly common practice and the camera focused to this point the actual focal distance the lens is set to is greater because it measures from the sensor plane so the dof will be marginally greater. So in real terms all of the above complications don't matter. The only one that does is that it is all an approximation.

There are other ways of dealing with depth of field. A couple are mentioned in this thread. Both can be very useful.

Depth of field 2/3 behind and 1/3 in front???

John
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[Mark von Kanel]

this from wiki.

The distance scales on most medium- and small-format lenses indicate distance from the camera’s image plane. Most DOF formulas, including those in this article, use the object distance from the lens’s front nodal plane, which often is not easy to locate. Moreover, for many zoom lenses and internal-focusing non-zoom lenses, the location of the front nodal plane, as well as focal length, changes with subject distance. When the subject distance is large in comparison with the lens focal length, the exact location of the front nodal plane is not critical; the distance is essentially the same whether measured from the front of the lens, the image plane, or the actual nodal plane. The same is not true for close-up photography; at unity magnification, a slight error in the location of the front nodal plane can result in a DOF error greater than the errors from any approximations in the DOF equations.

so there you go, if you want accurate DOF, photograph a tape measure and look at what it actually is rather than what the calculator says it is! this is actually what i did to work out what i needed to do to get the dof i wanted to photograph the bees the calculator wasnt accurate enough when getting close. this is probably due to the fact that its difficult to find the elusive nodal point.

[george013]

The front nodal-point is nearly the optical center point. The distance scales on the lens, that's something you can't work with. http://en.wikipedia.org/wiki/Nodal_p...nts_and_planes
The dof-calculators are all based on some formules, take a look http://en.wikipedia.org/wiki/Depth_of_field somewher in the middle.
Did you understand what I meant with the negative DOF?

@Casper,

DoF calculator suggested focal distance of 0.75m @ f/11 would give me focus from 0.71m to 0.80m – just enough depth at 0.09m, as subject was 0.075m front to back.

I'm not sure what you measured, but from this point of view it should be something top-front to bottom-back.

George

[ajohnw]

All from the wiki so it must be correct.. Trouble is that when photographers are involved it often isn't as simple as that.

John
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[William W]

I also replied to that other thread. And I stand by what I wrote on that other thread.

That is to say, that DoF Calculators assume that the Photographer will use the distance as being measured from the FILM PLANE to the SUBJECT.

The FILM PLANE is where the sensor is located.

The FILM PLANE is indicated by the Greek letter phi ɸ which is located on the camera body.

In that other thread I have provided there is an image illustrating.

*

This conversation is confusing the use of DoF calculators and the use of Distance Markings on the Lens's Turret barrel and the formulae for exact DoF calculations.

These are not necessarily the same.

One uses a DoF Calculator to make a very reasonable estimate of the DoF and one pumps in the numbers and uses a CoC (Circle of Confusion) that is reasonable – in so doing the Distance pumped into the DoF calculator is as described above: there is no fiddling around with what lens it is or the lens design or the dissection of the particular lens’s geometry – it is a simple measurement from the symbol ɸ to the Subject.

That’s one main reason why the ɸ mark is on the camera in the first place. Noting that there were (before personal computers and apps) ‘DIAL-type’ DoF Calculators. (And If I have time later today I will post an image of one)

Another main use of the FILM PLANE mark ɸ on the cameras is for Magnification Calculation, when using Bellows or Tubes.

When wishing to make DoF calculations for MACRO WORK, it is usual to use the MAGNIFICATION in that calculation, and I note, that is how the CiC tutorial describes.

WW

[William W]

I have been practicing my photography on my wife’s craft projects. Today I borrowed some of her “stick pins”, and found it difficult getting everything in sharp focus.

Ignoring for a moment the: questions; answers; and the few incorrect uses of technical terms about DoF and DoF Calculators and Calculations . . .

If you want to attain a GREATER DoF across the OBJECT for the general shooting scenario that you have described and illustrated, then here are four practical methods you might consider:

1. Keep the same CAMERA VIEWPOINT but use a WIDER LENS and then crop in post production to attain the FRAMING that you require.
This will keep the PERSPECTIVE the same as the image that you have in your illustration.
Rough guess, based upon experience – try about 18 ~20mm Lens as a starting point. (Kit Zoom lens would be good if you have one)


2. Use the same 50mm lens, but move the CAMERA VIEWPOINT farther away from the OBJECT and then crop in post production to attain the FRAMING that you require.
This will CHANGE the PERSPECTIVE from the image that you have in your illustration.


3. Use a combination of suggestions 1 & 2 above.
This is to preserve the PERSPECTIVE as close to the illustarion as much as possible by NOT moving the CAMERA VIEWPOINT back too much, from the OBJECT.
Rough guess, based upon experience, move back a bit and use about a 30mm lens, a 35 prime if you have one or the Kit Zoom Lens at about 30mm.


4. Use a Tilt Shift Lens. For the size OBJECT the TS-E 45 or the TS-E 24 would be suitable, the TS-E 45 would be my first choice.
However to attain the full benefits of using a TS-E lens you should also use a 135 format camera (aka ‘full frame’).
Note this option is expensive.

*

Here is a simple example of how moving the CAMERA VIEWPOINT farther away, and then cropping in Post Production back benefits a greater DoF for photographing a small OBJECT and you can see how little area the OBJECT occupies of the original framing in the camera:



WW

[george013]

I also replied to that other thread. And I stand by what I wrote on that other thread.

That is to say, that DoF Calculators assume that the Photographer will use the distance as being measured from the FILM PLANE to the SUBJECT.
.......
WW

It surprise me that nobody is reading my arguments. A main formula is the lens formula.
1/focal distance= 1/object distance + 1/image distance
The CoC gives you a minimal and a maximal image distance. By calculating the according object distances and subtracting them, you get the DOF.
Now look at the lens formula. As soon the object distance get smaller as the focal distance the image distance gets negative, an imaginair image.And resulting in a negative DOF.

Would it be as you say that the calculator measures from the film plane, than this would happen at a distance of 2 times the focal distance.

Pure theory. No camera can focus at those distances.

By using a wider angel lens and then crop the image, you should also use another CoC. I think the result will be the same.

Casper didn't react anymore, so I am guessing.

as subject was 0.075m front to back

I think that should be more.

George

[Mark von Kanel]

All from the wiki so it must be correct.. Trouble is that when photographers are involved it often isn't as simple as that.

John
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Yup wiki is God!! you cant argue with doing your own tests though

[Mark von Kanel]

William I BELEEEEEVE YOU!! and your answer of moving back and cropping to get the DOF needed to photograph my bees is exactly what i did.

Goerge im certain that what you saying is true, but how do you apply it in the real world? finding a nodal point on a lens that will move with internal focusing or zooming is just totally impractical.

Its ok having mathematical theories but we need to make it work in the real world, so Williams suggestions work better, at least we know where the sensor is. For critical DOF requirements, if DOF calculators use the nodal point then they are just plain useless and you are better off experimenting to ascertain ACTUAL DOF/COC with your equipment rather than a theoretical one.

[george013]

That are two different things: how it is and how you want it to be. I only try to tell you how it is with some arguments how I got to that conclusion.
This year I was on vacation in the northern part of Spain, Picos de Europa, and wanted to try make pano photos. Than you need the nodal point, especially when you have a close foreground in it. My search to that information brought me to the following site http://wiki.panotools.org/Entrance_Pupil_Database. I'm sorry, it's a wiki again. At this moment they have some trouble, but in it there is a database of camaras and lenses so you can calculate your nodal point. Again, I don't know if this is useful, but it gives you an idea.
But if you have a tripod you can easily find the nodal point for that camera and that lens.

But also important is what Casper measured as the desired DOF. I think there is his main problem.

George

[William W]

It surprise me that nobody is reading my arguments. A main formula is the lens formula.
1/focal distance= 1/object distance + 1/image distance
The CoC gives you a minimal and a maximal image distance. By calculating the according object distances and subtracting them, you get the DOF.
Now look at the lens formula. As soon the object distance get smaller as the focal distance the image distance gets negative, an imaginair image. And resulting in a negative DOF.

George,

I read your arguments. The response is contained here (my bold & underlined for emphasis):

This conversation is confusing the use of DoF calculators and the use of Distance Markings on the Lens's Turret barrel and the formulae for exact DoF calculations.

These are not necessarily the same.

etc . . .

In other words for the PRACTICAL purposes of USING a DoF CALCULATOR for General Photography – the EXACT formulae are not important.

Using the DoF Calculator:

1. The measurement is simply taken from the marking of "phi" on the camera to the Subject.
2. Then the camera format is selected and (maybe) a suitable CoC for the camera format is put into to the calculator (usually the CoC agreed by convention or is 'suitable' to the individual Photographer as s/he has determined 'suitable').
3. Then the Aperture is put in to the calculator.
4. Then the Focal Length of the lens is put into the calculator

Then BINGO a DoF 'estimate'.

It is really - that simple.

***

By using a wider angel lens and then crop the image, you should also use another CoC. I think the result will be the same.

Again (I assume) that you are referencing using a DIFFERENT CoC because of the resultant enlargement when we crop the frame tighter in Post Production.

Again my response is similar: The CoC is NOT that critical.

This is NOT a theoretical question in an exam where formulae are necessary to make the perfect equation but rather this is the practical application of theory to arrive at a shooting distance and aperture which will make good a shot that exhibits a suitable DoF for the uses of the image.

For PRACTICAL PURPOSES for the job as described in the Opening Post: using a wider lens at the same CAMERA VIEWPOINT - the PERSPECTIVE will be maintained and the DoF will be increased, provided both the Aperture and the Camera Format remain the same.

WW

[William W]

William I BELEEEEEVE YOU!! and your answer of moving back and cropping to get the DOF needed to photograph my bees is exactly what i did.

Ta.

For clarity - I did NOT thank you for "helpful post" because you believed me.

I marked you post as "helpful post" because your post articulated in a different way that this is a discussion primarily about the PRACTICAL applications of theory and NOT about theory, per se.

I thanked you for "helpful post" on behalf of George in the sincere hope that he comes to understand the difference.

WW

[Stagecoach]

Hi Noel,

I will not get into the discussion as to where the distance as given in DOF calculators is taken from on the camera/lens rig other than to say with close up work like this how easily and accurately could you measure it anyway?

From the information you have given you state the subject distance was 75mm 'front to back'. The question arises as to how you determined this, whether on the true horizontal plane OR the subject on the same plane as your camera which in the photos clearly shows to be angled down.

If you measured the 75mm on the horizontal plane the subject distance 'front to back' is going to be greater with your camera set as it is.

Grahame

[ajohnw]

It surprise me that nobody is reading my arguments. A main formula is the lens formula.
1/focal distance= 1/object distance + 1/image distance
The CoC gives you a minimal and a maximal image distance. By calculating the according object distances and subtracting them, you get the DOF.
Now look at the lens formula. As soon the object distance get smaller as the focal distance the image distance gets negative, an imaginair image.And resulting in a negative DOF.

Would it be as you say that the calculator measures from the film plane, than this would happen at a distance of 2 times the focal distance.

Pure theory. No camera can focus at those distances.

By using a wider angel lens and then crop the image, you should also use another CoC. I think the result will be the same.

Casper didn't react anymore, so I am guessing.

I think that should be more.

George

I have to disagree with Bill here. For instance

http://www.trenholm.org/hmmerk/DOFR.html

Actually for most uses the method described on those pages is easier to use than hyperfocal distance. which does have the shortcomings he outlines.

If some one feels need of the derivations complete with nice diagrams they are here

http://www.imajtrek.com/new_page_10.htm

These are effectively using the nodal point of the thin lens being used which wont relate to an actual camera lens. The front nodal point comments that are about probably comes from another method that use entrance and exit pupil sizes of the lens being used. It's still an approximation.

The best option is to measure from the front of the lens. The fact that this is likely to give a marginally greater APPROXIMATE depth of field is irrelevant as the sums fall flat on their face as object distances get shorter. Hence macro uses an entirely different sum but even that is still an approximation based on magnification factors. In principle it should be possible to use these at any distance and they are likely to give a better guide at shorter distances.

Should add
Where the sensor plane marking come in is if a distance scale on a lens is used. Not a good idea if precision is needed as the tolerance on the flange to sensor distance is unbelievably tight. It also has it's usage for macro work wtth bellows but once upon a time manufacturers provided rather large tables covering this use and also scales on the bellows designed to fit in with the camera and lens.

John
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