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DOF between film/digital formats


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#1 Mihai Nicolau

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Posted 01 November 2009 - 03:20 PM

Hello.
The big question which has been bugging me all day long is: if I have a 50mm lens at F1.4 on a 35mm film/digital camera and shoot a close up of someone then I put that same lens with the same F number on a 16mm film/digital camera which stays in the same position as the 35mm one will I get a different DOF ?
From what i've read and understood up until now I would say no but today I started reading about this thing called circle of confusion stuff and it all came crashing down on my head.
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#2 Stephen Williams

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Posted 01 November 2009 - 03:42 PM

Hello.
The big question which has been bugging me all day long is: if I have a 50mm lens at F1.4 on a 35mm film/digital camera and shoot a close up of someone then I put that same lens with the same F number on a 16mm film/digital camera which stays in the same position as the 35mm one will I get a different DOF ?
From what i've read and understood up until now I would say no but today I started reading about this thing called circle of confusion stuff and it all came crashing down on my head.



The DOF will be identical but the picture will be much more close up. 16mm is just a crop of the 35mm frame.
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#3 Mihai Nicolau

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Posted 02 November 2009 - 02:21 AM

Thank you for the answer. That is what i thought but based on what i've read is appears this it not exactly the way things are.

So this is what Wikipedia is saying :
"Another approach is to use the same focal length with both formats at the same subject distance, so the magnification is the same, and with the same f-number,


so the DOF ratio is in direct proportion to the format size. The perspective is the same for both formats, but because of the different angles of view, the pictures are not the same.

Cropping an image and enlarging to the same size final image as an uncropped image taken under the same conditions is equivalent to using a smaller format; the cropped image requires greater enlargement and consequently has a smaller circle of confusion. The cropped image has less DOF than the uncropped image."

http://en.wikipedia....and_diffraction --> lower on the page at DOF vs. format size
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#4 Dominic Case

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Posted 02 November 2009 - 05:29 PM

The DOF will be identical but the picture will be much more close up. 16mm is just a crop of the 35mm frame.

This sums it up about as simply as can be. Clear thinking!

But we can make it more complicated quite easily! Yes, Circle of Confusion raises its ugly head.

Because 16mm is a smaller frame, we conventionally allow a smaller circle of confusion (in proportion). Therefore, with the same lens, you would have less depth of field, even though theimage goes out of focus in exactly the same way.

But to get a comparable image with the same field of view, you would use a shorter lens in 16mm, and this increases the depth of field back to more than you'd get for the same fov in 35mm.

From my elderly AC manual, taking each lens at f/2 and field of view (width) at 25 ft.

gauge coc lens fov dof
35mm 0.01" 35mm 14'11" 19'0" - 37'0"
16mm 0.006" 35mm 8'4" 21'0" - 30'10" (fov for super 16)
16mm 0.006" 20mm 14'6" 15' - 120' (approx - I interpolated between two tables)


apologies for the layout - html insists on reducing the spaces even with a proportional font
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#5 Stephen Williams

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Posted 03 November 2009 - 02:45 AM

Cropping an image and enlarging to the same size final image as an uncropped image taken under the same conditions is equivalent to using a smaller format; the cropped image requires greater enlargement and consequently has a smaller circle of confusion. The cropped image has less DOF than the uncropped image."


Hi,

I think your making things too complicated, Where you sit in the cinema makes a difference, should the CoC be calculated for people in the front row, middle or back?

Stephen
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#6 Mihai Nicolau

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Posted 03 November 2009 - 12:19 PM

Hi,

I think your making things too complicated, Where you sit in the cinema makes a difference, should the CoC be calculated for people in the front row, middle or back?

Stephen



I know there is that rule that you should sit at a distance of 1.5 times the length of the screen for optimal viewing conditions if I remember correctly. So people in the front rows will see the image more blurred.

@Dominic:

Thanks for the answer. However I don't understand your table very well.
In this first row of data 35mm 0.01" 35mm 14'11" 19'0" - 37'0" --> do 14'11" 19'0" refer to the vertical and horizontal FOV ? And is it such a big difference in DOF between super 16 and 35 with the same lens...almost 7 feet ?

You also said that they go out of focus exactly the same way ? What does this mean ?

So for practical purposes...is there any difference between formats when using the same lens regarding DOF when taking into accountthe CoC or is this just formulas and theory ?

And last question...what actually defines this CoC in any format. I mean how can 16mm have a smaller CoC than 35mm when it's resolution is also smaller. Isn't Coc actually a point created by the inability of a lens to actually focus the rays of light into a spot (dot) but our eyes still perceive it as a dot because of our resolution capatity. So shouldn't CoC in a film or digital format be defined by it's resolution ?
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#7 Stephen Williams

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Posted 03 November 2009 - 01:40 PM

I know there is that rule that you should sit at a distance of 1.5 times the length of the screen for optimal viewing conditions if I remember correctly. So people in the front rows will see the image more blurred.

And last question...what actually defines this CoC in any format. I mean how can 16mm have a smaller CoC than 35mm when it's resolution is also smaller. Isn't Coc actually a point created by the inability of a lens to actually focus the rays of light into a spot (dot) but our eyes still perceive it as a dot because of our resolution capatity. So shouldn't CoC in a film or digital format be defined by it's resolution ?


Hi,

Sitting close to the screen, will have the most resolution IMO.

The CoC is something you choose yourself after testing. You must decide what is acceptably sharp. If your S16 lenses resolved 3 times mare than your 35mm lenses then the CoC would be about the same.

If final viewing is on an iPhone your results will be different to viewing in a cinema.

Stephen
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#8 Dominic Case

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Posted 03 November 2009 - 05:53 PM

Thanks for the answer. However I don't understand your table very well.
In this first row of data 35mm 0.01" 35mm 14'11" 19'0" - 37'0" --> do 14'11" 19'0" refer to the vertical and horizontal FOV ?

For each row of data I showed gauge coc lens fov dof (see the top row of the table)
So for the first row as you ask:
gauge = 35mm
coc = 0.01" = circle of confusion - this is a standardised, agreed figure for each gauge, see below
lens = 35mm = focal length of lens (I chose 35mm and 16mm focal lengths to match fov (roughly))
fov = 14'11" = horizontal field of view (as noted: for 16mm I chose super 16) - quoted to show the relevant choices of lens
dof = 19'0" - 37'0" the near and far distances: everything between these distances is "in focus"

And is it such a big difference in DOF between super 16 and 35 with the same lens...almost 7 feet ?

For 35mm the range is 19' to 37'. For 16mm using the same lens, it is 21' to 30'. The DOF is about half as much for 16mm with the same lens purely because we have a smaller circle of confusion.

You also said that they go out of focus exactly the same way ? What does this mean ?

My point was, that a hypothetical point of light that is perfectly in focus at 25 feet, would be out of focus, appearing as a disk of diameter 0.006" at 21' or 30', regardless of the gauge of the film, all other things being equal. But if it's 35mm we'd say that was still in focus as it's smaller than the COC. If it's 16mm, we'd say that is right on the dge of being out of focus. The image is the same, it's how we describe it that is different.

Just to reiterate this point. If we applied the same COC to 35mm and 16mm, then with the same lens the DOF would be identical. (But FOV would of course be different.) And it's not really appropriate to have the same COC for both gauges - see below.

So for practical purposes...is there any difference between formats when using the same lens regarding DOF when taking into accountthe CoC or is this just formulas and theory ?

There is no such thing as "just theory". Theory should describe what actually happens. If it doesn't, then the theory is wrong.

Your question isn't really very clear. The best way to think of it is NOT to compare the same lens with different formats (though that is useful to explain how it all works). In practice, when you compare 35mm and 16mm, you would use different focal length lenses, in order to get the same FOV. And then, as the rest of my table shows (and as we know), you actually get more depth of field in 16mm with the same field of view. (The focal length makes more difference than the COC).

And last question...what actually defines this CoC in any format. I mean how can 16mm have a smaller CoC than 35mm when it's resolution is also smaller. Isn't Coc actually a point created by the inability of a lens to actually focus the rays of light into a spot (dot) but our eyes still perceive it as a dot because of our resolution capatity. So shouldn't CoC in a film or digital format be defined by it's resolution ?


As Stephen has pointed out, things like viewing distance from the screen (and screen size) also come into play when we consider what is acceptably sharp. There are so many variables here, but if you are comparing the same shot, viewed by the same audience in the same row of the same cinema, taken in 35mm and 16mm, then the 16mm will have to be blown up roughly twice as much to fill the screen. So the smallest dot that we can distinguish on the screen will have been half as big on the original 16mm negative.

The clearest way to eliminate all the other variables like viewing distance, is to assume that COC is a proportion of the size of the negative. Of course it's only rough, as no-one distinguishes between standard and super 16 (or standard and super 35 for that matter) even though the negative image is wider in the super formats.

So for 35mm, a COC of 0.01" is 0.12% of the frame width.
For 16mm, a COC of 0.006" is 0.13% of the frame width.

That's less than 1K resolution in digital terms. So as you can see, it's nothing to do with the limiting resolution of the lens or the stock or the digital sensor. It's to do with what we can actually see in normal viewing.
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