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Keeping Focus with Wide Open Aperture and Distance

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Steve Crow
Keeping Focus with Wide Open Aperture and Distance
on May 5, 2011 at 4:36:57 am

First some technical details of my setup:

* Canon T2i
* Canon 50mm 1.4 prime lens
* Z-Finder (3x model I believe)
* ND Fader with 8 stops of light reduction

Okay, I know from the various depth of field calculators (or at least think I know!) that with any aperture setting you are going to have a certain distance in front of your subject that is out of focus, a mid range distance that is in focus and then a certain distance behind the subject that is again out of focus.

Since I like to film interviews with open apertures, I am now getting nervous if I am risking out of focus shots because I am not taking the distance to the subject into consideration during filming.

What I am asking is should I be doing something different to maintain focus with a wide open aperture when I am 8 feet away from the subject versus 20 feet, as an example?

Essentially all I am doing is using the digital magnification function on the Canon T2i (5x and 10x) to set focus and then using a combination of ISO settings and maybe a Fader ND filter to get my exposure compensation meter to sit somewhere in the middle of the range or slightly below.

So far I haven't noticed any out of focus shots but I am getting increasingly nervous because, frankly, I am not consciously taking into account distance to subject when it comes to my focus.

I'm just figuring "well, if it looks sharply in focus in the LCD monitor with my 3x Z-Finder plus the 5x/10x magnification trick then I'm probably okay" - but am I really?





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Jason Jenkins
Re: Keeping Focus with Wide Open Aperture and Distance
on May 5, 2011 at 5:48:04 am

You might give yourself a little breathing room by shooting at f2 instead of f1.4. I don't know about your Canon lens, but my Nikon 50mm f1.4 is definitely softer at f1.4 than at f2. Personally, I wouldn't be comfortable shooting a shallow depth-of-field interview without an external monitor. They call them "confidence monitors" for a reason. I use a SmallHD DP6. Its 6" 1280x720 resolution screen gives me a great image that is easy to keep an eye on throughout the interview.

Jason Jenkins
Flowmotion Media
Video production... with style!


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Dave Tally
Re: Keeping Focus with Wide Open Aperture and Distance
on May 5, 2011 at 1:22:37 pm

First off, you need to understnd DOF, the 1/3, 2/3 rule and that DOF is different for every lens. To answer your question, Yes.

Selective focus is for placing the audience where you want them to be, what you want them to "focus" on.

Use lighting to create visual depth in you shots, selective focus is a tool not a style.


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Steve Crow
Re: Keeping Focus with Wide Open Aperture and Distance
on May 5, 2011 at 1:36:13 pm

Thank you Dave for your response - to clarify - if my subject is in focus as seen through my camera lens/LCD can I be assured they will also be in focus in the video itself?

My concern is that my subjects will be out of focus because I am not taking into account the issue of distance,

On the other hand they are in focus when viewed on my DSLRs LCD (using the Z-Finder with 5x/10x display magnification) so can I trust that or not?


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Dave Tally
Re: Keeping Focus with Wide Open Aperture and Distance
on May 5, 2011 at 2:14:31 pm

Yes, if what you are seeing on your monitor is sharp then your video will be as well. Rely on only a quality monitor, but at 10X on the camera, if its sharp then it's sharp.

If I had one wish for DSLR video... throught the lens focus.

If you are moving the camera (dolly) a focus puller is a must at shallow depth of field. Set marks for confidence, if the focus puler feels he missed his mark, check the video.

Dave Tally
Director of Photography / Cinematographer


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