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Old October 11th, 2006, 01:52 PM   #1
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DOF w/ 13x lens

Since the focal length of the 13X lens is 3.5mm-46mm and the focal length of the stock 17x lens is 5mm-85mm, does that mean the 13x lens wiould have a tougher time achieving a shallow dof, particularly in interview situations?

Thanks.
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Old October 11th, 2006, 02:50 PM   #2
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No, it's the same, lens for lens.

DOF is a product of focal length and aperture, not a product of the focal length capability of a lens.

If you're asking if you'll have a hard time throwing the background out of focus at 3.5mil, well, then the answer is yes. It'd be impossible, actually. But if both lenses are set at, say, 30mm, then they'd both create the same DOF.
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Old October 11th, 2006, 02:52 PM   #3
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Depends on whether you tend to zoom tighter than 46mm when shooting interviews.

Bear in mind also that depth of field is identical when the subject is framed the same, regardless of what focal length you're shooting at. If your interview shot is head and shoulders, no matter how far you had to zoom in to get that framing, the background will be identically blurry. The only thing that changes is perspective on the subject and background, zooming in too tight can actually reduce the apparent separation between subject and background, like how a pitcher and batter appear on TV to be practically right next to each other.
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Old October 11th, 2006, 03:03 PM   #4
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Quote:
Originally Posted by Stephan Ahonen
Bear in mind also that depth of field is identical when the subject is framed the same, regardless of what focal length you're shooting at.
Sorry Stephan, that's not correct. Two shots framing somebody the same, one at 4.5mm and one at 80mm, both with aperture wide open, will NOT have the same DOF.

Maybe you meant something else and I misread, but as you state it, that's incorrect.
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Old October 11th, 2006, 03:05 PM   #5
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When I shoot my interviews, I create as much distance behind my subject as possible and put the camera about 15 feet in front of my subject. I zoom all the way in, pretty much filling the frame with the face, and focus. I then pull back to head & shoulders and move in periodically. I have noticed that the background appears closer than I'd like, but when it's out of focus, it seems to work. I just want to make sure I won't lose that ability with the 13x lens, but it sounds as if it won't be a problem. Can you think of any other way that the image in this circumstance would be affected by the 13x lens? I'm assuming the image would be better.
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Old October 11th, 2006, 03:21 PM   #6
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Quote:
Originally Posted by Nate Weaver
Sorry Stephan, that's not correct. Two shots framing somebody the same, one at 4.5mm and one at 80mm, both with aperture wide open, will NOT have the same DOF.

Maybe you meant something else and I misread, but as you state it, that's incorrect.
It sounds hard to believe, but here's a practical experiment that proves it:

http://www.luminous-landscape.com/tutorials/dof2.shtml
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Old October 11th, 2006, 03:23 PM   #7
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Quote:
Originally Posted by Nate Weaver
No, it's the same, lens for lens.

DOF is a product of focal length and aperture, not a product of the focal length capability of a lens.
.
Everyone has talked about the focal length, however the aperture (f stop) has much to do with DOF.

Using ND filters and or shutter speeds (depending on your mode of shooting) will give a lower f stop f2 to f2.8 and a much shallower depth of field than lots of light and f11 or f16.

That's on of the reasons why subjects shot in deep shade often look so good.
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Old October 11th, 2006, 03:36 PM   #8
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The experiment in the linked text is just blowing up pictures and asking the user to judge apparent sharpness. Real DOF calculations are based on COC numbers. I think the text is flawed.

It's common knowledge that if you're having trouble with a busy background and want to throw it out, you back up, put on a longer lens and open up.

It seems like you're arguing with that methodology even though we all know it's the real world way of controlling DOF.

Even though I think the article is flawed, understand I'm not going to argue about it...I have no interest in trying to prove what I know to be my daily experience when shooting.
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Old October 11th, 2006, 03:44 PM   #9
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You're showing a profound ignorance of the definition of "depth of field."

Depth of field is the range of distances from the lens that will be rendered acceptably in focus. Backing up and zooming in retains an identical, or at least very similar depth of field, but changes the perspective so that less detail appears in the background and foreground. This doesn't change the fact that depth of field has not been affected, only the perspective on the scene.

As for the link, you say it's flawed because it doesn't use hard numbers, then you say that you don't need to prove what you see in everyday shooting. The article is illustrating the principle using everyday shooting techniques.
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Old October 11th, 2006, 03:51 PM   #10
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All I know is that when I shoot 4 feet in front of my subject, the background is in focus and when I shoot 10 or 15 feet in front of my subject, the background goes out of focus. I've never paid much attention to the perspective, but it certainly makes sense. I'll look for it.
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Old October 11th, 2006, 04:14 PM   #11
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Another interesting Depth of Field article which explores the relation of the aperture to DOF.

http://www.cs.mtu.edu/~shene/DigiCam...-of-field.html


Let's not sully the discussion with declarations of "profound ignorance", everybody learns more without the putdowns.

The Luminous Landscape article was very interesting by the way.
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Old October 11th, 2006, 04:17 PM   #12
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Quote:
Originally Posted by Stephan Ahonen
You're showing a profound ignorance of the definition of "depth of field."
I am? How so?

So what you're saying, based on that article, is that all the DOF calculators that have been created over the years, calculators that are based on 3 criteria:

1-Focal Length

2-Aperture

3-Circle of Confusion

I'll, for the moment, toss everything I've learned as an assistant cameraman and DP over the last 16 years out the window and honestly listen to you. For real. No pissing contest.

In all sincerity, where does this leave this leave my Samuelson DOF calculator, which I assume is based on hard math and optical theory? And every other DOF calc?
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Old October 11th, 2006, 04:40 PM   #13
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Quote:
Originally Posted by Stephan Ahonen
Depth of field is the range of distances from the lens that will be rendered acceptably in focus. Backing up and zooming in retains an identical, or at least very similar depth of field, but changes the perspective so that less detail appears in the background and foreground. This doesn't change the fact that depth of field has not been affected, only the perspective on the scene.
I have been working as a professional still photographer/videographer for the past 15 years and it is the field that I studied in college. I think that I would have a little bit of experience.

I think that what you are describing is the same field of view not depth of field. If you frame the shot the same using a 5mm lens and then do the same thing with a 45mm lens, the only way possible to do this would be to move either the subject or the camera. By doing this and changing the focal length of the lens (or change the lens if using fixed lenses) you would change the depth of field.

In school we were always that the best way to affect your depth of field there were several ways to do this. They were: change focal length, open or close your apeture (depending on the effect you were trying to achieve), change shutter speed (thus effecting your apeture), change your film speed (thus changing your apeture).

In fact I would have long discussions with classmates on the best way to adjust exposure when shooting stills. Some would argue change the apeture and leave the shutter speed alone. My way is to change the shutter speed because changing the apeture can change the composition of your shot if the depth of field changes.

Hope I didn't go off on a rant here. There are many ways to control DOF, but if you change focal lengths and still have the shot framed the same, i.e. you had to move the camera, the DOF should be different.

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Old October 11th, 2006, 04:43 PM   #14
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Quote:
Originally Posted by Stephan Ahonen
no matter how far you had to zoom in to get that framing, the background will be identically blurry.
This is simply not true. At the widest end of a lens on a 1/3" ccd camera almost everything is in acceptable focus. At a longer focal length the dof narrows. No disrespect, but it's hard to understand how this point is in question.
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Old October 11th, 2006, 05:57 PM   #15
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Here's an example that illustrates it. Exact same subject size, exact same aperture, exact same framing. The shot on the left was accomplished by backing up and zooming in. The shot on the right was accomplished by zooming all the way out and moving in closer.

http://www.icexpo.com/dvx100/DOF-Combined.jpg

Now, it is true that they have the same DOF. But it sure doesn't look like it! The shot on the left, anyone looking at it would say "the background is out of focus." And on the shot on the right, anyone would say "that background is in focus."

Point of fact is that you could magnify the shot on the right's background until it was the same size as the shot on the left, and under such magnification it would look comparably blurry.

But does it matter? The point is -- if you want the background to look like it's out of focus, you can get that by backing up and zooming in. Whether it's "technically" shallower DOF or not, it gives the look the shooter was going for.
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