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Sony NXCAM NEX-FS100 CineAlta
An interchangeable lens AVCHD camcorder using E-Mount lenses.


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Old July 4th, 2012, 11:00 PM   #1
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Fs 100 dof calculator and occasional focus problems

Is there a dof app for the iphone yet?
I have dof master and it doesn't mention either the f3 or fs100. IOt has a selection for the Canon 5d, and I wonder how close that would be.
I also downloaded indiefilmgear's depth of fielf calculator. It has a selection for the f3, but not the fs100.

The reasomn I'm asking is that I've had focus problems, on beach shots especially, and wanted to check what my dof really was. I checked the 5d on both apps and the F3 on the indiegear app. The settings I entered were what I think was using for one scene at the beach on a shoot yesterday. I had the Canon 70-200 zoomed in to 200mm, I was about 12- 15 feet away ( talent was in water, I was on sticks on sand). I was usind a circular nd filter and had the iris at around 12 or 13. The dof that both apps gave me for the 5d, and one for the f3 was just under a foot When I entered f21, the calculators only gave me a foot and a half.

I have the zacuto z finder and also usually check peaking on the sony loupe, but I jsu wasn't on the money yesterday on the zoomed in shot.
Also, is there any possibility that a certain preset would make focus harder? I tried out for the first time Abel Cines's no. 2 prest, which they call ( I think) highlights. It supposed to be foe bright out door shots and high contrast..
Any suggestions?
Bruce Yarock
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Old July 5th, 2012, 07:17 PM   #2
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Re: Fs 100 dof calculator and occasional focus problems

First let me say I've never used a DOF calculator so I may be way off base. Are any other Nex cameras listed on DOF master, since they all have a 1.5x crop, wouldn't the DOF be the same across the line? This is just a guess...
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Old July 6th, 2012, 09:58 AM   #3
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Re: Fs 100 dof calculator and occasional focus problems

Chris,
The only one listed was the f3, and I forget whether or not that's an nex. I think that the dof on the fs100 is probably similar to the 5d , 7d, R3.
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Old July 6th, 2012, 11:37 AM   #4
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Re: Fs 100 dof calculator and occasional focus problems

5d is full frame so the DOF is quite a bit shallower at any given focal length, the 7d is a 1.6x while the Nex/FS is a 1.5x so that will be pretty close. There's a noticeable difference (in a good way) since I moved from the 5d2 to the FS, its easier to keep moving subjects in focus since the DOF is not as razor thin as the FF sensor.
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Old July 9th, 2012, 02:48 AM   #5
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Re: Fs 100 dof calculator and occasional focus problems

Bruce,

A calculator is not going to help you, there are too many variables (size of circles of confusion, pixel site size etc). I

The cold reality is that lens at 200 mm (equivalent to 300mm on the FS) is going to give you a few inches of DoF at that short distance. Holding crisp focus for any sort of movement is going to be extremely hard, so don't beat yourself up. One of the major limiting factors is the viewfinder - there is simply not enough resolution on any of the electronic finders for the ultra-precise focus you need.
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Old July 9th, 2012, 12:18 PM   #6
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Re: Fs 100 dof calculator and occasional focus problems

Nick,
Thanks for the info. It is really hard to get crisp focus in that situation. For other reasons, we had to re do parts of the shoot a couple of days ago. This ime ( since low light wasn't an issue) I used the 18-200 kit lens with a variable nd filter. I put the rig on my manfrotto monopod with the three support feet and a fluid head, went into the water closer to the talent, and kept the zoom much wider. Between the fs focus assist and the Zacuto ( I wish they'd add red peaking), and the wider shot, plus an occasional push on the af button, everything looks great.
Beautifull image, but very challenginng camera.
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Old July 11th, 2012, 01:50 PM   #7
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Re: Fs 100 dof calculator and occasional focus problems

I might be totally off here but, isn't the fs100 coc right around 0.021mm?

Can't you just use that?
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Old July 11th, 2012, 01:59 PM   #8
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Re: Fs 100 dof calculator and occasional focus problems

Chris,
Excuse my ignorance, but what exactly is the cirle of confusion? never really understood it.
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Old July 12th, 2012, 09:10 AM   #9
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Re: Fs 100 dof calculator and occasional focus problems

Bruce, I'd suggest you google it for a better understanding because I would most likely confuse you more with my understanding of it. (or confuse myself more...)
but here goes...
COC is basically a portion of the equation that lets you see something "In Focus".
When the light relected off something (the models right eye) comes in the lens, it's bent a little and if you focus the glass so her right eye is perfectly focused, it's perfectly focused on the sensor/film.
Her right ear (which is farther away from the cam) MAY still be in focus (depending on you glass/F stop/distance to subject etc) but it will not be in AS PERFECT a focus as the right eye.
The chair back (which is even farther away from the cam) MAY still be in focus, or MAY be getting a little soft.
This (obviously) is depth of field. Any camera, any lens, any fstop, There is only one point/distance from the camera that is perfectly "in focus". Any other points farther (or closer) may "appear" to be in focus, but are slightly off. When it gets so far off that your eye (or the sensor/film stock) sees it getting soft then you're getting out of the DOF.
That area of the "focus" (from the point closer to the cam to the point farther from the cam) is DOF IF it's in front of the camera.
If it's on the inside of the camera (on the other side of the lens where it's hitting the sensor/film) THAT is the COC. The size of the sensor/film stock will change the effective COC so different cameras will/may have a different coc. Since the semsor/film stock size won't vary on the same cam the coc for a particular cam will always be the same. I.E. the fs 100 will have a coc of 0.020 to 0.021 (I believe it's actually something like 0.0204 if I remember correctly) So I misspoke earlier when I said it's 0.021, It's actually closer to 0.020.
A large format cam (say a 70 mm Imax) will have a larger coc. A small format cam, a smaller COC for it's sensor.

All in all, if the region where the light hits the sensor and that area is "in focus" then the coc is less than that which the sensor can handle. (0.020 in the case of the FS100)

I think the confusing part is that we refer to the coc as 2 seperate things.
1) the area of the light coming out of a lens that IS (or more accurately, what is percieved to be) "in focus" (even if the lens isn't even on a camera)
2) the area of a sensor/film stock where that light would be "in focus" (how big that area is--0.020 in the case of the fs100)

Now my head hurts......


.All, if I got that wrong, please correct me.
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Old September 16th, 2012, 08:10 PM   #10
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Re: Fs 100 dof calculator and occasional focus problems

Bruce,

The F3 and the FS100 share the same sensor. They are both Super35 sized chips. This is a cine format. So when using any DOF calc you can always choose S35.

There are a bunch of really good apps. One of the best is Toland by Chemical Wedding.

Several factors could be contributing to lack of sharpness:

Lens mount adapter for your Canon lens.
Low quality adapters may not give the proper flange depth. That is the proper distance of the lens to the sensor. If this is off, your focus is off.

Filters.
Low quality glass is a bummer.

Camera, or subject movement after setting focus.
You only have about thirteen inches of focus to play with at the settings you described.

Inaccurate Focus.
If you zoom in, focus, and then zoom back out with a still photo lens, your focus will change as you change focal lengths. There are few, if any stills zoom lenses that perform like a cine/video lens. Still photo lenses are very sharp and have a lot of great characteristics, but being parfocal is not one of them. Parfocal refers to the ability to hold focus through the zoom range of the lens. Stills lenses also breathe a lot when you zoom. This means that as you change focal length, the image will appear to zoom, or "breathe" a bit. Hiding the zoom with a camera move helps.

Also there may be some diffraction introduced as you stop the lens down. This will also make the image less sharp, as you stop down to smaller apertures, F/16, 22, etc.

If you are using a follow focus, check and make sure that the gear belt around the lens is tight and that the all the parts of the FF are locked down: swing arm, gear, the FF itself.

I use the built in peaking function on the camera's LCD, in addition to my Cineroid EVF.

Hope this helps.

You might want to do a focus check. Try it with and without the ND filter(s) you use and at different apertures, focal lengths and distances. Focus using the EVF and then double check accuracy with a larger monitor. Maybe check with a tape measure and see if your barrel distances are close to the actual tape. If they are off, that might indicate a problem.

Best,

-Mark
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