Question on Depth of field

John Adolfi · July 8, 2006

After reading the threads about 65mm film they said that the depth of field(focus?) was more critical on 65mm. Does that mean the further down the film format guage you go the lees critical depth of field is. Therefore with super 8 you can just about point and shoot? Help me understand whay 65mm is so different than say 35mm on this issue.

Chris Keth · July 9, 2006

After reading the threads about 65mm film they said that the depth of field(focus?) was more critical on 65mm. Does that mean the further down the film format guage you go the lees critical depth of field is. Therefore with super 8 you can just about point and shoot? Help me understand whay 65mm is so different than say 35mm on this issue.

The difference bwteen formats is the lens you tend to use, more than anything else. On 8mm, you will use far wider lenses than on 65mm. Remember that focal length is one thing that affects depth of field. So for any given job, you will use a much longer lens for 65mm than you would for 8mm. This effectively makes 65mm have much less depth of field, given the same object distance and framing, than other formats.

Alan Duckworth · July 9, 2006

First off, depth of field and depth of focus are two different things. Depth of focus occurs behind the lens and is an issue with things like lens mounts - depth of focus problems usually need a technician to diagnose and fix.

Depth of field occurs in front of the lens and is a way of expressing just how much of the image will appear to be in focus on the screen. A shallow depth of field will have only a small portion of the image, say a face, actually in focus and everything else will become progressively "fuzzier". A deep depth of field (also called "deep focus", somewhat confusingly) will have virtually everything in the image frame appear to be sharp.

As the the hole in the iris becomes smaller (that is, moves towards f22) the depth of field becomes deeper. As the hole in the iris becomes larger (that is, moves towards f5.6 or f2.8) the depth of field becomes shallower. This statement applies for ANY format.

Now, to your basic question - as the format size increases (eg 65mm) the depth of field becomes shallower at any given f-stop, and the opposite is true. This is due to something called "circles of confusion" [i did not make that up!], which is essentially about how the lens projects its image onto the film plane - as the film plane gets bigger, the circles of confusion get bigger because the distance from the lens to the film plane also gets bigger. It's also why wide-angle lenses appear to have more depth of field than telephoto lenses - telephoto lenses are optically further away from the film plane [but not necessarily physically further away]. That's a bit of a simplistic summary of the science, and in practical terms, this means that in the larger formats the focus must be placed very critically, as at even something like f8 the focus fall-off to "fuzzy" happens very quickly. But, most cinematographers like to work with a shallow depth of field as it isolates the subject from the background. Hence the need for focus pullers, because as your subject moves it needs to be kept in focus.

The small formats (including most video cameras - the chips are small in all except the new Genesis style) have enormous depth of field at f8 and up, and still have lots of depth at even f2.8 or so. This makes them great for consumer use where critical focussing is rarely done, but hard to work with if you need the isolation. So, yes Super 8 can often be "point and shoot" [within reason], as it is in most consumer single chip video cams. The "autofocus" in those consumer cams doesn't have a whole lot of work to do!

There was a thread on this (in I believe "First Time Filmakers") a few weeks back, well worth looking up. Sorry if I have been a bit long-winded, but I wanted to touch all the bases for clarification.

(I was typing this as Christopher's post came in, so a little repetition)

John Adolfi · July 9, 2006

Fantastic guys thanks!

David Mullen ASC · July 9, 2006

The short imprecise answer is that the smaller the target area, the shorter the focal lengths get on average, and shorter focal lengths generally create more depth of field than longer ones. You'll note that the typical zoom range for Super-16 and 2/3" CCD cameas is something like 10mm-100mm, etc. whereas for 35mm it would be 25mm-to-250mm, etc. With Super-8 and 1/3" CCD cameras, it's more like 4mm-to-40mm, etc.

With 35mm anamorphic and 5-perf 65mm, the range would be more like 50mm-to-500mm rather than 25mm-to-250mm to achieve the same fields of view.

Stephen Williams · July 9, 2006

The short imprecise answer is

Hi,

If you a looking for a longer answer try:-

http://www.vanwalree.com/optics/dof.html

http://www.dofmaster.com/dof_defined.html

There is currently a thread on CML as to why a Genesis has less DOF than S35mm.

Stephen

John Adolfi · July 16, 2006

The short imprecise answer is that the smaller the target area, the shorter the focal lengths get on average, and shorter focal lengths generally create more depth of field than longer ones. You'll note that the typical zoom range for Super-16 and 2/3" CCD cameas is something like 10mm-100mm, etc. whereas for 35mm it would be 25mm-to-250mm, etc. With Super-8 and 1/3" CCD cameras, it's more like 4mm-to-40mm, etc.

With 35mm anamorphic and 5-perf 65mm, the range would be more like 50mm-to-500mm rather than 25mm-to-250mm to achieve the same fields of view.

So its not that you have a larger area to photograph on but the focal length that determins it. So if in super 8 I'm, using a 20mm fixed lense and on a 35mm camera I use a 80mm fixed lense then the out come would be the sameas far as depth of field is concerned. All things being equal outside of lens and negative size.

David Mullen ASC · July 17, 2006

So if in super 8 I'm, using a 20mm fixed lense and on a 35mm camera I use a 80mm fixed lense then the out come would be the sameas far as depth of field is concerned.

NO. Forget the format for a moment, what matters is the focal length you have to use, assuming you use the same f-stop and are focused at the same distance.

So if you have to use a 20mm on one type of camera to get the same field of view as an 80mm on the other type of camera, then you'd have more depth of field on the camera using the 20mm lens -- because a 20mm has more depth of field than an 80mm if both are focused at the same distance and are at the same f-stop. Just look at a depth of field chart and compare a 20mm lens and an 80mm both focused at 5', let's say, and both set to an f/2.8.

What would be the same is the field of view in your example, not the depth of field. A 20mm lens on a Super-8 camera sees roughly the same view as an 80mm on a 35mm camera, but you'd have less depth of field on the 35mm camera because you're using an 80mm lens instead of a 20mm lens.

John Adolfi · July 17, 2006

So what you are saying is David its harder to keep the sharpe focus when shooting 35mm than super8. David I understand now, thanks.

Edited July 17, 2006 by John Adolfi

Ilmari Reitmaa · July 17, 2006

Hello everyone. Long time no see.

I may be just adding to the whole depth of field -confusion here, but there is an alternate, however equivalent, approach to depth of field and related concepts, as described by Harold Merklinger (not affiliated) in the book Ins and Outs of Focus, particularly in chapter five. I've always found it to have certain appeal.

The basic idea is this: You draw an imaginary line from the perimeter of the lens diaphragm to the point of focus, and beyond. Now, at any given distance, the distance from the optical axis to the drawn line equals the radius of a disk of which a point-like detail looks like, at that distance (think about a long shot in the night, with an actor in the foreground, in focus, and blurry disks of city lights in the background). At the point of focus, a point-like detail remains point-like. Closer to the camera, a point-like detail is no longer point-like but never exceeds the size of the diaphragm. Approaching infinity, a point-like detail grows infinitely.

What gives? The radius of the disk is proportional to size of the smallest resolvable detail at that distance (roughly, half of the size). Focal lengths and format sizes (and actually, even depth of field) need not be considered at all, until one begins to consider how to frame the image (that is, what is the angle of view or the ratio of the format size to the focal length). And even then, the radiuses of the disks at all distances remain the same; it's just a matter of their relative sizes in the image, which is what depends on the angle of view.

It's a kind of a backward method, and I may not have been able to put it down too clearly here, but there's certain elegance to it, I think. For a better explanation, refer to Merklinger's book.