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Depth of Field...again


Charles DeRosa

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I've recently (thanks to this site) come under the impression that going to a tighter lens, but keeping the subject of a frame at the same relative size (ie moving the camera back) will not dramatically change the depth of field - all other parameters (t-stop) staying the same. That in fact to change depth of field, without changing the relative size of the subject, is a t-stop issue.

 

Yet, reading in Blain Brown's book "Cinematography: Theory and Practice" he provides an example which returns me to a "you want less depth of field, just throw a tigheter lens on the camera and move back" state of thinking.

 

Brown has two pictures (pg 53) of the same woman. In each picture her relative size in the frame is more or less the same. In the first picture you can see clearly into the background. In the second picture the background it completly out of focus.

The caption reads:

"A very long lens throws the background out of focus and the viewer's entire attention is drawn to the character."

 

Can somebody clarify this for me one more time?

thanks

 

Chuck

Los Angeles

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I've recently (thanks to this site) come under the impression that going to a tighter lens, but keeping the subject of a frame at the same relative size (ie moving the camera back) will not dramatically change the depth of field - all other parameters (t-stop) staying the same. That in fact to change depth of field, without changing the relative size of the subject, is a t-stop issue.

 

How did you come to that impression?

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You're correct that depth of field stays the same when you change lenses and distance but keep the same relative frame size (focusing on the same subject). A depth of field chart will confirm this.

 

But what a depth of field chart DOESN'T show you is how out-of-focus the background looks when it's outside the depth field. A longer lens farther away will make the background softer because the focus falls of more quickly, compared to the shorter lens at the closer distance.

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Another point is that actually, the depth of field is nearly the same if the size of the foreground object is the same. But it's obvious that concerning the background, its size is not the same when you use a longer focal length while putting the camera further from the foreground as to keep the same sizing.

 

As Michael says, when you are out of the dof, things don't happen the same with different focal lenth. When you do focus calibration tests, you see this clearly.

 

The thing is that the formulas that are used are based upon relative premisses, that could be questionned. They give you precise numbers, though it doesn't mean much to give precise numbers when talking about dof. Dof depends on so many factors, don't forget that the CC number for instance, is a matter of statistics, and is no way valuable in "any" situation. So actually, dof doesn't "suddenly cuts" at the figure you find by the formulas, but slowly or quickly falls down, according to different factors, the focal length being a major one.

 

Another point is that when you focus close to an hyperfocal distance, the rules are changed a bit, so that some situations cannot be compared, while others can be. In general I would say that a given situation at a focal length of 35 mm wouldn't be much different than comparing to a 40 or 50 mm, but comparing a situation at 18 mm to a 105 mm situation (though you'd have the same "sizing" on the forground) can be pretty different.

 

The situation I was in myself, as a focus puller, was to have someone (director, dp) asking if it would be worth using the 35 instaead of 50 for a precise shot, for instance, where dof was a point. And the answer would be no in 99 % of the cases. But the difference is so big beetween a 105 and a 18 in terms of shot composition, that when you have a question about dof, it's never changing that much the shot parameters that gives you a satisfying answer on a shooting...

 

A friend of mine, who is a teacher at Louis Lumière has written an whole thesis about DOF, if you can read french, let me know, I could send you some extracts from his work, since I don't know if anybody would translate this into english. May be I'll suggest him ...

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The nice thing about all this is that numbers all relate geometrically, which makes it easy to keep straight in your head.

 

To cut the field of view exactly in half, you exactly double the focal length. And to keep the same shot size when you double the focal length, you exactly double the distance. I love it when I don't have to do math in my head!

 

This also makes it easy to choose lenses when setting up shots. I don't have to try out different lenses to see what framing they'll give me; I already know a 75mm will give me half the field of view of the wide shot that was framed on the 35, if the distance is the same.

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The nice thing about all this is that numbers all relate geometrically, which makes it easy to keep straight in your head.

 

To cut the field of view exactly in half, you exactly double the focal length. And to keep the same shot size when you double the focal length, you exactly double the distance. I love it when I don't have to do math in my head!

 

This also makes it easy to choose lenses when setting up shots. I don't have to try out different lenses to see what framing they'll give me; I already know a 75mm will give me half the field of view of the wide shot that was framed on the 35, if the distance is the same.

 

But we are a bit away from the DOF calculations, here.

 

About the figures in the DOF formulas, I posted something somewhere else in this forums somedays, that didn't get much reactions about... :

 

Do you know that the hyperfocal distance, written 1/H = eN/f² is false, for instance ? It should be f"normal"X f"of the shot" instead of f²... In other words, f ² is correct only if you apply the formula to the "normal " focal length in the considered format (40mm or so in 35 mm...) and is wrong if using another one...

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The whole notion that wide-angle lenses give you more depth of field and telephoto lenses give you less is partially a visual trick, as Michael points out. Depth of field is a range of acceptably sharp focus forward and back from the point of focus.

 

However, a telephoto lens enlarges a smaller area of the background so that it's true focus (or the amount it is out of focus) can be seen more clearly. If you took the shot with a wide-angle lens, where the background details were smaller and looked farther away, and cropped and enlarged the frame in post to match the telephoto shot, you'd see that the same background details were just as soft in the wide-angle photo.

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I posted something somewhere else in this forums somedays, that didn't get much reactions about... :

 

Do you know that the hyperfocal distance, written 1/H = eN/f² is false, for instance ? It should be f"normal"X f"of the shot" instead of f²... In other words, f ² is correct only if you apply the formula to the "normal " focal length in the considered format (40mm or so in 35 mm...) and is wrong if using another one...

 

 

What did I say about doing math in my head? ;)

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