Exposure & DOF control

XCheck

XCheck

Veteran
When reviewing some of my footage, I realized several things (yes they are obvious but I want to reiterate for those who still have doubts):

1) There is no way you can accurately judge sharpness using either of the two viewfinders

2) Shallow DOF is difficult to achieve without ND filters.

Now, if you use the built in ND filters, they give you 3 stops exposure value difference (8x and 64x). For practical purposes, I will assume the maximum aperture is F/2.0 and minimum aperture is F/16 (I know it's not, but it makes the math easier;)).

For any given ND setting, there is 'at least' 3 stops range of EV which can be achieved with on of the adjacent ND settings:

ND0: F/5.6 to F/16 is same as ND8: F/2.0 to F/5.6
ND8: F/5.6 to F/16 is same as ND64: F/2.0 to F/5.6

Tha's all nice and dandy, BUT: Say I want to use F/2.0 at all times for minimum DOF. That means I have up to 3 stops of exposure to bring down. Normally, this would be done by ND filters, but say I don't have those, and I want something quick (and I don't have a mattebox).

I'd like to know if anyone finds a problem with the following idea, which is to use the properties of polarizing filters.

If I combine two POLA filters, when their planes of polarization are aligned in parallel, it's like having only one filter (say, 1 stop light loss). When the filters are rotated so that the planes are perpendicular to each other, it's like having a cap on the lens (at least in theory). So, by rotating only one of the two filters, I can smoothly control the amount of light that goes through. I set the camera iris to F/2.0, and control exposure by rotating the front POLA filter. That way, I always have the shallowest DOF possible for given zoom setting, with added benefit of increased color saturation and glare reduction.

Is that feasible, or do I miss something?
 
Re: Exposure & DOF control

Polarisers an ND filters are different animals.

I see what you're saying, and to "some" degree it would work, but practical.....not really.

btw, set your viewfinder to B&W if you want to improve judging the sharpness.
 
Re: Exposure & DOF control

This is an interesting idea that comes up from time to time. An important point to remember is that the loss thru a polarizer is between 1.5 and 2 stops (varies with the brand and type) and that combining two polarizers doubles that loss, so you would start with a .9 to 1.2 ND (3 to 4 stops loss) which would increase as you rotated one polarizer to decrease the transmission.

Polarizers are not truly neutral and you would need to while balance at the setting you decided to use because the spectral transmission would vary some with the orientation.

All things considered a set of standard ND filters is a better solution and probably not more expensive than two polarizers.

Bill Turner
Century Division
Schneider Optics
 
Re: Exposure & DOF control

Thanks for the feedback, guys.

I figured that I'd have to white balance every time I'd change the polarizing filter orientation, but I thought it would be less of a hassle than screwing/unscrewing extra filters. I would LOVE to get a Chrosziel mattebox and follow focus and all those toys, but... $$$ :(. So I am looking at cheapo alternatives - does anyone know or has used Cokin filters with the DVX? The type where you screw a filter holder on your lens and then drop in filters like you would with a motion pictrue mattebox? They also have an extensible bellows - anyone has an experience with that?

John, when I use f-stop about 4.0 and higher, I have no problem due to the DOF, but setting a precise focus (without a sharpness chart) at medium distances is a bit of drag - too far to clearly see the difference in the viewfinder, and too close so if you miss it show clearly on the footage...

I guess I'll have to print the focus chart and have it with me - I am better at estimating distances in meters than mapping it to numbers. Getting there slowly, though...

Thanks again for all your replies.

J.
 
Re: Exposure & DOF control

Also, keep in mind that the ND ratings on the built-in filters are not actually accurate.  They're approximations.  The first filter (ND1/8 ) is actually about 3.5 stops, and the second filter (ND1/64) is actually about 4.5 stops or so, so using both in combination reduces light by 8 stops, not the six that you'd think by just reading their description.

So, definitely a 1-stop and 2-stop ND.

As far as the cokin filter holder, it's fine for what it is, but Cokin filters are not known for having superior optical quality.  Look into Lee resin filters, they're reportedly decently good quality and are quite affordable.
 
Re: Exposure & DOF control

Thanks guys - sounds like a trip to Vistek this weekend!

Oh yeah baby! Leave home without 'em! (the credit cards, of course...)

My wife, when she goes downtown, visits the Museum. So do I, so do I... ;D
 
Re: Exposure & DOF control

Can someone explain to me why adding nd filters/changing stops would affect the depth of field? i'm not following.
 
Re: Exposure & DOF control

The more you have the iris of the camera open the smaller the DOF gets. Just one of those things. It's related to the fact that the lens, when open, focuses much closer to the optics than when nearly closed.
 
Re: Exposure & DOF control

ok.. i thought the dof was strictly based on the zoom or lack of
 
Re: Exposure & DOF control

Read the DOF Tutorial for more info. DOF is controlled by zoom, aperture setting, and distance to the subject.
 
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