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Old January 16th, 2009, 08:26 PM   #61
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you are absolutely right.... about the size of the sensor ( or let's call it the format size ) has everything to do with it. The focal length of the lens does not..... a 50mm lens has a wider angle of view as the format size gets bigger... but, if you were to crop in on a larger sensors image to the size of a smaller format, the depth of field would be the the same as if you "zoomed in" to the same crop from a larger format with the smaller format.

Last edited by Christopher Witz; January 16th, 2009 at 11:42 PM.
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Old January 17th, 2009, 04:27 AM   #62
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Daniel Browning - thank you for an interesting lesson.

But:

DOF depends of the distance and the iris, like Simon says.

But f8 of a telephoto lens is larger than f8 of a wide angle lens.

So I wonder if it is the opening in mm that should be counted on. If so, longer focal lengths give shallower DOF. That is what I have learned ...

Last edited by Sverker Hahn; January 17th, 2009 at 04:45 AM. Reason: Moving words around ...
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Old January 17th, 2009, 11:49 AM   #63
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Quote:
Originally Posted by Sverker Hahn View Post
If so, longer focal lengths give shallower DOF.
If you backup enough to compensate for the longer focal length, the DOF remains the same.
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Old January 17th, 2009, 12:11 PM   #64
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Back in the early-mid 90's, I had a Sony Video 8 camera that had a feature on it called 'Portrait'. I remember first pushing that button. What did it do? It blurred out the background. Sort of a built in DOF adaptor. This was a 500 dollar or so camera. Definitely a cool feature, but as someone who shot mostly film (Super 8 and 16mm), it didn't ever make me think it looked like film.

If you watch the Zacuto comparison video, it's quite ironic that the 35mm portion has deeper depth of field than the DOF adaptors shown (which almost seem overkill). I do like how the adaptors give a more 'organic' look to video though.
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Old January 17th, 2009, 02:11 PM   #65
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Quote:
Originally Posted by Daniel Browning View Post
If you backup enough to compensate for the longer focal length, the DOF remains the same.
Long time ago I learned, contrary to what I said in my last post, that DOF depend on only two factors:

- iris
- scale, i. e. the subject/image ratio

According to this, focal length has nothing to do with DOF. An example was given in the Hasselblad Close-Up brochure:

- when at scale 1:1 and f 11, DOF is 2 mm, regardless of focal length and film size (those days I used 24x36 mm or 6x6 cm).

As Daniel has shown, also the background (and foreground, I suppose) blur has to be accounted for when composing an shoot.

Interesting thread.
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Old January 17th, 2009, 03:07 PM   #66
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Quote:
Originally Posted by Sverker Hahn View Post
DOF depend on only two factors:

- iris
- scale, i. e. the subject/image ratio
Correct. If you boil everything down into the simplest terms possible, DOF can be fully described by just f/number and magnification [setting aside bellows factor for this post]. If you change focal length and distance at the same time (back-up-zoom-in), magnification remains the same, so DOF remains the same. If you change *just* focal length, and nothing else, then DOF will change because magnification is changed. If you move closer to the subject, that too increases magnification, which changes DOF.

So f/number and magnification is one way. Another way to simplify it is physical aperture and focus distance. Physical aperture is the focal length divided by f/number; 50mm divided by f/2 is a physical aperture of 25mm. In this way, one number encodes the DOF effect of both the focal length and the f/number: both have an equal effect on DOF. So for a given composition (subject distance, angle of view), the lens with greater physical aperture will have thinner DOF. That is, 85mm f/1.4 on full-frame has thinner DOF than 50mm f/1.2 on APS-C, even though they both have the same field of view. This is because 85/1.4 = 60.7, and 50/1.2=41.6.

Quote:
According to this, focal length has nothing to do with DOF.
Correct, if magnification is kept constant.

Another way to say it is that focal length has nothing to do with DOF, as long as physical aperture is kept constant. 50mm f/2.0 has the same DOF as 200mm f/8, since both have the same physical aperture of 25mm. More about that in this thead:

http://www.dvinfo.net/conf/still-cra...pth-field.html
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Old January 17th, 2009, 07:02 PM   #67
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> "... focal length has nothing to do with DOF."

Sony don't seem to know this - on the EX1, without touching the aperture setting, the DOF indicator, when zoomed back to wide covers from zero to infinity, and as you zoom in in, this gradually gets smaller, finishing at a fraction of a metre.
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Old January 17th, 2009, 07:24 PM   #68
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Quote:
Originally Posted by Peter Wright View Post
> "... focal length has nothing to do with DOF."
The "..." that's missing from your quote is important. Sverker correctly pointed out that DOF depends only on iris (f/number) and scale (subject/image ratio, or magnification).

A better way to quote Sverker would have been:

Quote:
For a given f/number and scale, focal length has nothing to do with DOF.
Having a constant scale means any change in focal length must be compensated by another factor, such as moving the camera. If it is not compensated, then it becomes a change in scale, and therefore a change in DOF, just as Sverker said.

Quote:
Sony don't seem to know this - on the EX1, without touching the aperture setting, the DOF indicator, when zoomed back to wide covers from zero to infinity, and as you zoom in in, this gradually gets smaller, finishing at a fraction of a metre.
Right. That does not conflict with what Sverker wrote. Changing the scale by using a longer focal length causes a change in DOF.

If you had kept magnification constant (by using the back-up-zoom-in technique), you would find that the EX1 DOF indicator would remain the same, despite the much longer focal length. That is what is meant by "focal length has nothing to do with DOF".
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Old January 17th, 2009, 08:17 PM   #69
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Sorry - I'm not trying to be obstinate, I'm trying to understand ...

If I shoot a given frame with my zoom back at full wide, my EX1 tells me depth of field is from zero to infinity.

If I go back a few metres and zoom in to the same given frame, the EX1 tells me the DOF is now less than half a metre.

Should I not believe the camera?
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Old January 17th, 2009, 09:04 PM   #70
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Originally Posted by Peter Wright View Post
If I go back a few metres and zoom in to the same given frame, the EX1 tells me the DOF is now less than half a metre.
Did you remember to change the focus distance? (If you backed up 10 feet, the focus should also be changed by 10 feet.)
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Old January 17th, 2009, 09:50 PM   #71
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I didn't necessarily have to - if I make my subject sharp after moving back and zooming in, it is still in focus after moving closer and zooming back to wide.

But whether or not I did change what was in focus does not affect the fact that the camera reports a much shallower DOF after moving back and zooming in, and this is backed up by what I see in the viewfinder.
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Old January 17th, 2009, 11:26 PM   #72
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Originally Posted by Peter Wright View Post
I didn't necessarily have to - if I make my subject sharp after moving back and zooming in, it is still in focus after moving closer and zooming back to wide.
That is only possible if the depth of field is deeper than the distance you move the camera. For example, say you start out wide and focus on a subject at 1 meter. Then you back up 6 meters and zoom in so the subject takes up the same portion of the composition again. When you focus on the subject the focus distance is now 7 meters. If you return to the original position without correcting the focus, your focus will now be off by 6 meters. If the subject is still in focus, then the DOF must be deeper than 6 meters.

In most circumstances, even with the tiny 1/2" sensor of the EX series, the depth of field will not be deep enough to make the subject sharp when focus is off by such a great amount. Such chance arrangement of DOF and subject depth does not allow us to draw conclusions about the DOF indicator in the camera: the focus must be centered on the subject for that purpose.

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But whether or not I did change what was in focus does not affect the fact that the camera reports a much shallower DOF after moving back and zooming in, and this is backed up by what I see in the viewfinder.
You're right: moving back and zooming in, without changing focus distance, causes the DOF indicator to show thinner DOF. But once you also change the focus distance, I think you will find that the DOF indicator shows the same DOF.

Let me illustrate with a few DOF numbers from my Canon 5D2 using a CoC appropriate for 1080p (12 microns).

Code:
50mm lens, focus distance 1 meter, f/2.0 DOF = 17.3mm
100mm lens @ 2 meters, f/2.0 DOF = 17.3mm
200mm lens @ 4 meters, f/2.0 DOF = 17.3mm
400mm lens @ 8 meters, f/2.0 DOF = 17.3mm
500mm lens @ 10 meters, f/2.0 DOF = 17.3mm
800mm lens @ 16 meters, f/2.0 DOF = 17.3mm
The increase in focal length would normally make the subject take up a larger part of the frame. Moving backwards compensates for that so the subject takes up the same portion of the frame.

The longer focal lengths also normally cause the DOF to be thinner. The increased focus distance compensates for that so the DOF remains the same.
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Old January 18th, 2009, 06:01 AM   #73
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One thing I didn't read mentioned here is CoC or more to the point that it isn't constant for a given sensor size. I've shot with both a 1/2" SD camera and the EX1 and DOF is dramatically different because of the higher resolution of the EX1 when viewed in HD close the the monitor.
The other thing I notice with HD compared to film is there's a different aesthetic to the DOF. With projected film you see the subject pretty much all in focus and the background very blurred.With HD including the EX1 you can have the subject's nose clearly in focus, the ears slightly out of focus and the background not as blurred as film.
I believe this difference is at least in part due to the way film looses resolution through the printing process, there's certainly a very different look to film scanned and digitally projected compared to a traditional print. Perhaps the 35mm 'DOF' adaptors achieve much the same by reducing resolution and at the same time reducing DOF.
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Old January 18th, 2009, 07:20 AM   #74
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Bob:

Having used nearly all of the high-end 35mm sensor cameras (Genesis, F35, Red, D21), I can report that contrary to your theory (if I understand it properly), these deliver a very sharp image with LESS depth of field than the 35mm film equivalent, if anything. My focus pullers certainly have a tougher time of it than they would with film, they constantly report that they find it shallower and more critical. I've had a number of theoretical discussions with various people as to whether this is due to the lack of depth to a sensor vs film emulsion etc.
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Old January 18th, 2009, 12:45 PM   #75
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Seems simple enough what Simon is saying. DOF is defined as the "Area in focus". Zooming doesn't change the area in focus, it just further softens what's already out of focus. DOF measure AREA, not DEGREE of softness. It's a state of range, not intensity.
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