Calculating Depth of Field

I am shooting a high school class photo tomorrow of around 400 students. They will be arranged on the gym bleachers in 7-8 rows. If I follow this chart correctly:

http://www.dofmaster.c​om/dofjs.html

At any given distance and f-stop, as focal length increases, depth of field decreases. So for my camera (Canon 70D) but I used a 60D on the chart - I get these results.

Distance: 30 ft.
f/8
Focal Length 35
DOF = 69.2 ft.

Distance: 30 ft.
f/8
Focal Length 125
DOF = 2.33 ft.

Does this look right to you? With 7 or 8 rows, I need to make sure I have a depth of field of at least 15 feet I'd say.

Would appreciate any thoughts!

The standard DOF table/calculator uses the erroneous assumption about human visual acuity...not even as good as optometrists' goal for human vision....20/20 (in US).
IOW, a person with 20/20 vision will be able to DETECT 'out of focus' blur!

Cambridge Color has a DOF calculator with 'advanced mode' that allows the user to select 20/20 vision (rather than 'manufacturer standard' DOF) and allows the user to also specify the print size and the viewing distance. https://www.cambridgei​ncolour.com/tutorials/​DOF-calculator.htm
That calculator says 35mm f/8 has DOF zone of 28', while 125mm has DOF zone of 1.85'

Thanks Wilt.

Wilt wrote in post #19445182
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The standard DOF table/calculator uses the erroneous assumption .....
.

.
Great point, Wilt.

Depth of field is not a mathematical, quantitative thing, even though some left-brained people insist on making it that way. . Depth of field is a subjective thing in which the greatest factor is not the size printed, "circle of confusion", aperture, or distance between subjects. . The greatest factor is each viewer's personal preference, and what they like most, or like least, or what they can look past or tolerate, when it comes to sharpness vs. varying degrees of blurriness.

Almost everything about photography is subjective, and based on what each person likes most or least, and yet certain people with certain brain types keep trying to make it objective and mathematical. . Photography is 100% art and 0% science. . One can disagree with this, but then one would be wrong

.

Dof is absolutely a quantitative thing regardless of how those not caring about math might otherwise say.

The subjective part is the quality or desirability of the bokeh depending on lens and how the subject material look to you individually on the edges of that field. There is no equation to that.

For example, lighting is all mathematically computable based on the gear and settings but how the lighting is arranged, the level of contrast vs softness, etc is more subjective.

Much of life is the conglomerate of objectivity, logic and computation along with subjectivity, personal goals and desired outcomes, you just have to know where that line is when factors shift from one to the other.

tspencer1 wrote in post #19445132
At any given distance and f-stop, as focal length increases, depth of field decreases.

You need to redo your cases so the framing at the subject is the same. So in the second case, your camera has to move way back, to a distance of 107 feet, to give the same framing. Then redo the calculations.

I couldn't replicate your results, but give it a try and tell us what you get.

Generally ONLY THE APERTURE affects DOF for the same picture (same subject framing). Just stop down to get the DOF you need.

tspencer1 wrote in post #19445132
I am shooting a high school class photo tomorrow of around 400 students. They will be arranged on the gym bleachers in 7-8 rows. If I follow this chart correctly:

http://www.dofmaster.c​om/dofjs.html

At any given distance and f-stop, as focal length increases, depth of field decreases. So for my camera (Canon 70D) but I used a 60D on the chart - I get these results.

Distance: 30 ft.
f/8
Focal Length 35
DOF = 69.2 ft.

Distance: 30 ft.
f/8
Focal Length 125
DOF = 2.33 ft.

Does this look right to you? With 7 or 8 rows, I need to make sure I have a depth of field of at least 15 feet I'd say.

Would appreciate any thoughts!

What was the final product? A printed picture? Images on a screen? As the discussion in the thread suggests, it isn't as simple as plugging numbers into an algorithm. Note that the 125 mm focal length isn't going to give the same image as the 35 mm lens, as @Archibald mentioned.

As this is was posted a month ago, I'd have suggested taking several images with different f-stops to cover your bases.

Wilt wrote in post #19445182
The standard DOF table/calculator uses the erroneous assumption about human visual acuity...not even as good as optometrists' goal for human vision....20/20 (in US).
IOW, a person with 20/20 vision will be able to DETECT 'out of focus' blur!

Cambridge Color has a DOF calculator with 'advanced mode' that allows the user to select 20/20 vision (rather than 'manufacturer standard' DOF) and allows the user to also specify the print size and the viewing distance. https://www.cambridgei​ncolour.com/tutorials/​DOF-calculator.htm
That calculator says 35mm f/8 has DOF zone of 28', while 125mm has DOF zone of 1.85'

Meanwhile, the 125 mm image isn't framed the same way, and so is a comparison between apples and oranges. The standard DOF calculation. Most people don't have 20/20 vision without corrective lenses. The Cleveland Clinic suggests only 35% people have this level of vision. Even with corrective lenses, the vision holds only at a certain distance.
https://my.clevelandcl​inic.org …n%20means,away%​20(20%2F10).

Tom Reichner wrote in post #19455308
.
Great point, Wilt.

Depth of field is not a mathematical, quantitative thing, even though some left-brained people insist on making it that way. . Depth of field is a subjective thing in which the greatest factor is not the size printed, "circle of confusion", aperture, or distance between subjects. . The greatest factor is each viewer's personal preference, and what they like most, or like least, or what they can look past or tolerate, when it comes to sharpness vs. varying degrees of blurriness.

Almost everything about photography is subjective, and based on what each person likes most or least, and yet certain people with certain brain types keep trying to make it objective and mathematical. . Photography is 100% art and 0% science. . One can disagree with this, but then one would be wrong

.

I wonder how you can have a working camera of any type beyond a daguerreotype without science.
For a digital camera, you need chemistry, quantum physics, optics, and computer science all working together properly. All those scientists and engineers provide science so you can do art.

TeamSpeed wrote in post #19455310
Dof is absolutely a quantitative thing regardless of how those not caring about math might otherwise say.

The subjective part is the quality or desirability of the bokeh depending on lens and how the subject material look to you individually on the edges of that field. There is no equation to that.

For example, lighting is all mathematically computable based on the gear and settings but how the lighting is arranged, the level of contrast vs softness, etc is more subjective.

Much of life is the conglomerate of objectivity, logic and computation along with subjectivity, personal goals and desired outcomes, you just have to know where that line is when factors shift from one to the other.

I'd argue that DoF (depth of field) is more qualitative than quantitative in photography. The equation gives a reasonable approximation. The DoF calculation is derived from other equations, notably diffraction, some of which are valid only at one wavelength- green (520 nm) is usually chosen. As we usually use white light for photography, we reach our first approximation. Wilt's link lists some other assumptions made to give a usable approximation for a particular situation.

It's just geometry.

The light arriving at the sensor is a cone. The more-or-less circular base is at the diaphragm inside the lens and the apex is at the sensor. This is for a single point at the subject.

When in focus, the cone makes a point at the sensor. When it is out of focus, the sensor intersects the cone making a circle. The circle gets bigger the more we are out of focus.

As you close the aperture down, the base becomes smaller and cone becomes narrower. That means the circle doesn't get as big when the sensor is moved back and forth. We have more DOF. All this can be calculated exactly from geometry.

For real systems with lens aberrations and diffraction, the picture becomes more complicated, but the simple geometry model teaches us a lot.

Archibald wrote in post #19455381
It's just geometry.

The light arriving at the sensor is a cone. The more-or-less circular base is at the diaphragm inside the lens and the apex is at the sensor. This is for a single point at the subject.

When in focus, the cone makes a point at the sensor. When it is out of focus, the sensor intersects the cone making a circle. The circle gets bigger the more we are out of focus.

As you close the aperture down, the base becomes smaller and cone becomes narrower. That means the circle doesn't get as big when the sensor is moved back and forth. We have more DOF. All this can be calculated exactly from geometry.

For real systems with lens aberrations and diffraction, the picture becomes more complicated, but the simple geometry model teaches us a lot.

All above is true. The PERCEPTION of 'in focus' lies in the eye+brain ability to SEE a circle of light as a point or as a circle of light. An eagle is better at that than a human...so an eagle eye would perceive 'out of focus' even when the 20/20 human eye perceives a 'point of light' but which is really a 'smallish circle of light' and not a true 'point' (perfect focus).

If standard DOF tables are based on the human eye being fooled with a circle of light which is 0.025mm and the eye thinks 'this is a point', then someone with better visual acuity will detect 'circle of light' even though the person with 20/30 vision is fooled otherwise. As explained within the Cambridge DOF calculator program

"People with 20/20 vision can perceive details which are roughly 1/3 the size of those used by lens manufacturers (~0.01 in features for a 8x10 in print viewed at 1 ft) to set the standard for lens markings.
...even if you can detect the circle of confusion with your eyes, the image may still be perceived as "acceptably sharp."

So DOF calculations are quantitative, but the qualitative part is based upon the individual eyesight...they are fooled or not fooled by 0.025mm circle of light, and the qualitative aspect of what is 'acceptably sharp' to one person vs. to another!
And then some DOF programs might use 0.030 as the Circle of Confusion size, and some others use 0.033mm, or 0.025mm...the 'confusion' behind the CofC size!

What is relevant for the OP and others needing to know DOF for a shooting situation, is that if you are going to do calculations, ALL the relevant parameters need to be taken into consideration.

So OP might have heard that focal length affects DOF, goes to the online calculator, and confirms that it is so - but ignores subject distance, which also affects DOF, and in the opposite direction. Therefore the exercise gave the wrong answer for the OP's shooting situation.

And yes, we can calculate DOF to 6 decimal places, but what does it avail us? We can use the math to gain an understanding of how it works, if we want. Then the best is to use the knowledge gained or experience to make our decisions. The best DOF rule is that only aperture affects DOF (assuming same framing, sensor size, viewing distance, and maybe some other things I forgot).

Archibald wrote in post #19455397
What is relevant for the OP and others needing to know DOF for a shooting situation, is that if you are going to do calculations, ALL the relevant parameters need to be taken into consideration.

So OP might have heard that focal length affects DOF, goes to the online calculator, and confirms that it is so - but ignores subject distance, which also affects DOF, and in the opposite direction. Therefore the exercise gave the wrong answer for the OP's shooting situation.

And yes, we can calculate DOF to 6 decimal places, but what does it avail us? We can use the math to gain an understanding of how it works, if we want. Then the best is to use the knowledge gained or experience to make our decisions. The best DOF rule is that only aperture affects DOF (assuming same framing, sensor size, viewing distance, and maybe some other things I forgot).

Indeed. We might know DOF is from 13-42', but what objects in the scene fall within that zone? How do we measure distances out in the field, to determine what is going to be 'in focus' vs. 'out of focus'? Camera scales on the lens are seldom precise enough. Laser rangefinders cannot be seen past a certain distance (to properly aim them at a target) in bright light, so are not useful in the sun. Golf rangefinders assume the fixed height of the hole flag.

And if we choose later to print to 40" x 32" rather than merely to 10" x 8", and to view at 10' rather than at 10", what does that all do to our DOF calculations that we might have at time of shooting?! I have to say that in amost 60 years in photography, I have NEVER calculated DOF nor Hyperfocal distance for a given shot. For me, DOF calculation is purely a theoretical discussion of the concept, one not applied in practice. The CONCEPTS are applied, not the calculation. And since so many lenses are zoom FL, one does not have DOF zone marks to help with zone focusing, either.

Archibald wrote in post #19455381
It's just geometry.

The light arriving at the sensor is a cone. The more-or-less circular base is at the diaphragm inside the lens and the apex is at the sensor. This is for a single point at the subject.

When in focus, the cone makes a point at the sensor. When it is out of focus, the sensor intersects the cone making a circle. The circle gets bigger the more we are out of focus.

As you close the aperture down, the base becomes smaller and cone becomes narrower. That means the circle doesn't get as big when the sensor is moved back and forth. We have more DOF. All this can be calculated exactly from geometry.

For real systems with lens aberrations and diffraction, the picture becomes more complicated, but the simple geometry model teaches us a lot.

A cone is a reasonable approximation, but one can't just use any cone. That's where the calculations come in.