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Old October 3rd, 2002, 02:58 PM   #16
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If you mean the most (largest DoF) the lens with the smallest aperature (largest F number) may have the most DoF. On the other hand if you mean least (smallest DoF) the lens with largest aperature (smallest F number). The F number express' a ratio which is why the inverse relationship to large opening and small F number. This example assumes your are trying to achieve a constant target size as outlined above. You might want to try reading sections A to E above and see if that helps. If you're not findding your answer, post back and I will try to help.

Jeff
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Old October 3rd, 2002, 03:01 PM   #17
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well when i saw a 35mm lens demo for the t+s technik they said that 35mm has a very shallow depth of field, I'm not quite sure what that meant, but if I can get the best as possible results that are reaching for the 35mm depth of field, then I'd like to know what lens is best for the XL1 which has similar qualities.
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Old October 3rd, 2002, 03:07 PM   #18
 
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They were referring to the format size, NOT the lens focal length.
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Old October 4th, 2002, 08:56 PM   #19
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Ok Jeff, just one more clarification due to my sad maths ability to work it out myself, but if you had 2 cameras with different CCD sizes and framed a subject identically and for some reason had the fStop at the same number, then would you always get more DoF with the one with the smaller CCD's no matter what you tried to do?


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Old October 4th, 2002, 09:25 PM   #20
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The smaller CCD would have more DoF if all other factors (A thru E above) and target size stay the same. If you haven't seen this thread yet, http://www.dvinfo.net/conf/showthread.php?s=&threadid=4103
you might be interested in the answers at the end.

Most of the situations presented talk about a constant target size (subject). The target size isn't always fixed. The Director or DP may decide that the target size can change to allow for more or less DoF. This is a creative decision and it's what film making is partly about. That's why it is important to understand DoF and how it works when the target size is not fixed.

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Old October 4th, 2002, 11:38 PM   #21
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Thanks for that Jeff, it's what I thought. The reason I was asking is that I was interested in whether focal length was an "absolute" for any lens, ie does a 50mm focal length on one lens ALWAYS produce the same size image (Assuming everything is else is same, distance, aspect ratio etc) as any other 50mm lens. It seems from what I've tried to find out that they are.

Thanks again
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Old October 5th, 2002, 05:57 AM   #22
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This may sound flippant or rude and I don't mean for it to be, but . . . a millimeter is always a millimeter. Josh asked about what the focal length really is on this thread http://www.dvinfo.net/conf/showthread.php?s=&postid=25113 You might be interested in some of the definitions.

Jeff
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Old October 5th, 2002, 08:49 AM   #23
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Don't worry no offense taken. Thanks for that link it explained everything cause it was the definitions I wanted to know, so I could actually understand what XXmm meant in real terms.

Thanks
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Old November 22nd, 2002, 10:12 AM   #24
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I recently received an email from Jean King concerning DOF and with her permission I am reprinting it here with my reply

Hi Jeff,
I too, am wanting to achieve a shallow depth of field effect where a focal
point is razor sharp but everything around the subject is blurred. I read
your posts with much interest, as far as the controlling and independent
variables that are in place. Without insulting you, I however, need the
"dumbed" down version, relative to using a Canon GL2. In other words, to
achieve shallow DOF, with a GL2 are there some basic steps that I can
perform, given the presets??
(i.e., adjusting f-stop and aperture, the general distance I need to be from
subject). Thanks for your help.

Jean King


The three basic things (A, B, C above) that affect DOF are focal length of the lens, taking aperture and distance to your subject. In some cases focal length of the lens and distance to subject cancel each other out. How do they do that? As you move further from you subject, by picking up your camera and moving it away from the subject, you increase depth of field. However, depending on your subject, the subject may get too small. That's only common sense and easy to demonstrate with your camera. But how do we get the subject back to the same size? We typically zoom the lens until the subject is larger. By zooming and making the subject large we have decreased the DOF (longer focal length, less DOF). The Law of Reciprocity cancels the two changes out (longer focal length=less DOF and moving away from subject=more DOF). Your DOF says exactly the same if the subject size is the same.

How do I change DOF? In most case if your subject size has to remain the same size (say a Models head size) decreasing the aperture (small numerical F Number) will decrease the DOF. Use aperture around F1.6, F2, F2.8 to achieve a shallow DOF. If your scene is too bright, using a Neutral Density filter (ND) will decrease the light entering the lens and forcing the aperture to go numerically smaller.

The subject size does not always stay the same size. If I just move the camera further from the subject (and not zoom back in) DOF will increase. If I zoom in (and not move the camera back) DOF will decrease. Combine those changes with changes to your aperture and large changes in DOF can be made.

Why do you dwell so much on subject size? In the real world, the subject size (persons head size) is dictated by the TV set or the script for the movie. Who would watch the news if the persons face was the size of a quarter? Or who would be afraid of Dirty Harry is his face were small on the screen? So, in conclusion, size does matter.

Jeff
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Old November 22nd, 2002, 11:03 AM   #25
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A couple of months ago, this is what I posted about DOF (sept 25)
"Getting a given depth of field with a wide angle lens is just as easy/difficult as getting it with a zoomed in lens and back of to keep the same object dimensions. Only the F-number and the target(CCD)dimensions determine the DOF. Given the fact that a zoomed-in lens generally has a higher F number than when zoomed out, the DOF can even be shallower in wide mode (and getting close to the object), if other constraints allow close setting"
All established members seemed to disagree... now everybody seems to know and agree that, if target size must be kept constant, focal length (wide or tele) doesn't play any role in the DOF equation.
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Old November 22nd, 2002, 11:06 AM   #26
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Re: Depth of field

Jeff,
Thanks a lot --that helps a great deal. I will experiement with those three variables when I rent a camera. You might have guessd by now that this effect is important to me. So, which camera in your opinion, would offer the greatest flexibility to "play" with depth of field?? (XLS1, PD150, GL2, etc.)
Thanks
Jean
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Old November 22nd, 2002, 03:01 PM   #27
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The CCD size is important for DoF, the bigger the CCD size the shallower the DoF. So the PD150 and Xl1s would be better than the Gl2. Also, from what jeff said above, then the Xl1s might be better if you can live with the subject being bigger than what you would have envisaged, because then you can utilise the bigger zoom on the xl1s (Am I right here Jeff?). Also if you have the budget and consider it important, rent a mini35 Adapter and get 35mm style DoF.
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Old November 22nd, 2002, 08:08 PM   #28
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Re: Thanks!

This information will help me narrow my camera choices. Wish me luck in shopping!
Jean King
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Old November 22nd, 2002, 08:16 PM   #29
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Yes, as Aaron points out, the larger chips in the PD150 and XL1S will provide a shallower DOF (everything else being equal). More things go into a film look than just DOF. Some are camera techniques involving settings and filters. Part is lighting, part is motion and in my mind (and eye) part is DOF. There are many threads on it here http://www.dvinfo.net/conf/forumdisp...?s=&forumid=34. As far as playing with DOF I think the XL1S with a manual lens will give you the most control and flexibility with DOF.

Jeff
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Old April 17th, 2003, 06:09 PM   #30
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Having people in Focus & background Not

I have been trying to get the camera to focus on people a few feet away and then blur the background.

What is the trick to this with the GL2?
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