Factors that influence depth of field

[Oldhouse]

So what is your point... you still only get about 3mm in focus on the bottom picture but about 10+mm in the top picture... (if it is legible or not is irrelevant).

 

[DaveC426913]

So what is your point...

I didn't beat around the bush.

The recognizable elements in both pics (22,23, 10,11) are exactly the same. The only concete metric we have - the tape measure's measurements - demonstrably show the DoF has not been altered.

you still only get about 3mm in focus on the bottom picture

How do you know that? The top image is too small to make out individual ticks or where they start and end. That information is literally not available in the sparse data. You are interpolating which ticks you conclude are in-focus and which are not.

That's the same circular argument. "All 10 ticks are in-focus because - despite being too small for my eyes to distinguish into discrete pixels - I am sure that must be in focus (way down there in fractions-of-a-pixel-land)."

How can you declare something that's less than one pixel wide to be "in focus"?

 

[Oldhouse]

I didn't beat around the bush.

The recognizable elements in both pics (22,23, 10,11) are exactly the same. The only concete metric we have - the tape measure's measurements - show the DoF has not been altered.


How do you know that? The top image is too small to make out individual ticks. That information is literally not available in the data. You are interpolating which ticks you conclude are in-focus and which are not.

That's the same circular argument. "All 10 ticks are in-focus because - despite being too small for my eyes to distinguish into discrete pixels - I am sure that must be in focus way down there in fractions-of-a-pixel-land."

Looks like you are slowly catching on to what DOF means:
It is all about being able to distinguish "discrete" points... If the image is small, you can't therefore it is considered in focus. The more you enlarge the image, the more discrete points you can distinguish.

Just think about it this way: If you had infinite resolution, you could keep enlarging the picture indefinitely. You would notice that the DOF shrinks more and more. The larger the picture, the less DOF you have. After all, as you know, only points from a specific distance are in focus (infinitely thin plane).

BTW, you still haven't given any answer to the posted formula...

 

[Drakkith]

The recognizable elements in both pics (22,23, 10,11) are exactly the same. The DoF has not been altered.

Yes it has. The blur size of every point in the image has been increased by several times its original size in the blown up image. The fact that you can still make out the tape measure is irrelevant.

 

[Oldhouse]

@Drakkith
Yes, agreed.

@DaveC426913
If you actually studied cinematography, you certainly are familiar with the “American Cinematographer Manual” (the Bible of Cinematography).

Here is an excerpt:
“Depth of field determines the range in front of and behind a designated focusing distance, where an object still appears to be acceptably in focus. A low resolving film stock or lens may appear to have greater depth of field, because the “in focus” image is already so soft, it is more difficult to determine when it goes further out of focus. Conversely, a very sharp, high contrast lens may appear to have shallow depth of field, because the “in focus” image has such clarity, it is much easier to notice when it slips out of a range of acceptable focus.

As you see, clearly speaks against your point and this comes from what is probably the most trusted source there is for cinematography related subjects.

 

[DaveC426913]

Looks like you are slowly catching on to what DOF means:
It is all about being able to distinguish "discrete" points... If the image is small, you can't therefore it is considered in focus. The more you enlarge the image, the more discrete points you can distinguish.

This is the same tautology. You've proven nothing.

Just think about it this way: If you had infinite resolution, you could keep enlarging the picture indefinitely. You would notice that the DOF shrinks more and more.

No it wouldn't.

Again, whether on a four inch screen or and four thousand foot screen, this scene will have an in-focus foreground and and out-of-focus background.

It has, and will always have, a narrow depth of field.

The larger the picture, the less DOF you have. After all, as you know, only points from a specific distance are in focus (infinitely thin plane).

Again: That infinitely thin plane is the focal plane in the camera. Not in the printer's ink cartridges.

BTW, you still haven't given any answer to the posted formula...

Nor do I need to.

 

[Drakkith]

Thread locked for moderation.

 

[Drakkith]

@DaveC426913 Here's the thing. I can find at least a few sources supporting what I've been saying. I can also find a few sources, including my Modern Optical Engineering book, that don't define Depth of Field to have anything to do with how the image is displayed, merely stating that depth of field is: "...the amount the object may be shifted before the (maximum) acceptable blur is produced."

Clearly there are at least two different methods of defining the depth of field of an image. One based on the blur size at the sensor/film, and the other based on the blur size in the displayed image. You can certainly argue that the latter definition is the more widely used one, which I would probably agree with, but unless you can provide something explaining how the former definition is incorrect then I don't see that there's much use to keeping this discussion open. If you can provide such a source, please feel free to message me and we can discuss it.

Until then, thread will remain locked.