Controlling exposure with polarizing filters

[Halsu]

Hi folks!

Here's a little trick to control the exposure on HV20, while keeping the camera's own exposure locked at f1.8, 1/50 shutter, 0dB gain.

First of all, here's how to lock the exposure to the above: Put on a lens cap, zoom all the way out and lock the exposure. Then set it manually to -7 (50i) or -11 (25p). This is for PAL models, similar steps should work for NTSC. This works only in Cine Mode, which prevents the gain setting from exceeding 9dB.

And now to the trick itself - here's how i do it:

I have not one, but two LINEAR polarization filters on my HV20. In addition to getting the expected effect of a polarizing filter, these can be used to control the exposure (which is the main reason i bought them). When the two filters are at 90 degree angle from each other, they will not let any light through, all other angles will let some, but not all of the light through.

Think of it as a variable ND filter.

Here's a test clip, locked exposure - all the changes in the image were made by rotating the filters, showing both exposure and polarizing effects.

http://eki.pp.fi/temp/Eki/HV20/Poltest.mpg

The whole set of filters i have on this clip is

43mm Skylight (to protect the camera lens)
Step up rings from 43 to 52 mm
Two 52 mm linear polarizing filters
52 mm UV filter (to protect the pol filters)

...this was my first test, it seems i need to get rid of the Skylight and/or the UV filter, the setup is too long and shows at the edges of the image. This wasn't visible on the LCD.

BTW: this will NOT work with spherical polarizing filters, they NEED to be linear.

BTW2: linear polarizing filters do NOT seem to cause any problems with the autofocus, which is nice.

[um3k]

I'm pretty sure only the front filter needs to be linear, the other can be circular. I believe it could also work with two circular polarizers if you flip over the one in front. This will eliminate any polarizing effect, which may be desirable in some cases. Just thought I'd mention...

[Halsu]

That's good to know, thanks!

[diggyk]

I'm pretty sure only the front filter needs to be linear, the other can be circular. I believe it could also work with two circular polarizers if you flip over the one in front. This will eliminate any polarizing effect, which may be desirable in some cases. Just thought I'd mention...

Have you tried it? I'd love to see what people's experience is before I go spend money.

[diggyk]

So, given that you can see some artifacts at the end of the screen, would you recommend a larger filter size? Maybe a 60 to 70 range instead?

[Halsu]

So, given that you can see some artifacts at the end of the screen, would you recommend a larger filter size? Maybe a 60 to 70 range instead?

I had two "unnecessary" filters on my stack (there just to physically protect the lens and the pol:s). Without them, there's no artifacts.

[Rikki]

I;d drop them if poss. You dont need a safety filter on the cam when using other filters and since PZ's are cheap you dont need one on the end either. Taking 2 extra slices of glass out of the equazion and the induced flare etc

[Halsu]

I;d drop them if poss.

I have. One reason for stacking thm all together was to test when they start showing at the edges - now i know ;-)

[lukemabat]

Have you tried it? I'd love to see what people's experience is before I go spend money.

I tried: only the front filter must be linear. In addition, if you put the circular one in the front, you can use it as a "night simulation" during the day, since the picture become blue (or yellow, it depends on the ring position). OK, not very useful if you can post-process by computer...

Ciao, Luca

[lukemabat]

So, given that you can see some artifacts at the end of the screen, would you recommend a larger filter size? Maybe a 60 to 70 range instead?

If you use larger filter, maybe you loose the instant auto-focus (the filter and step rings probably obscure the sensor).
I tried with 2 polarizer 43mm filter and there is no vignetting in 16:9 tape (or LW photo). You see a little bit vignetting on photo mode L (4:3 full sensor).
With 3 filter (UV+CPL+PL) you start to see vignetting also in 16:9. You see it only if you analyze the frame: you don't see it on the LCD screen.

Ciao, Luca

[Halsu]

Thanks for the info!

[dilly323]

This sounds like the best way by far to control exposure on the HV20.

You get an "exposure" ring
You get 1/48, 1.8, 0 gain every time
It's not very expensive
No need for the cell phone trick

What is the downside here? Why doesn't everyone do this? There's no vignetting with just the two polarizers apparently.

Dilly

[Michael Davis]

The downside is you have no idea how many stops of light you're cutting out. It's actually genius, I'm really impressed with the idea, but if you're trying to more accurately control the parameters of your shot (say, for repeatability), just spinning a filter until it looks right isn't very effective.

[Halsu]

repeatability

Actually, the filters i have came with angle markings, so i *can* get repeatable results if necessary.

The only big issue with these is loss of light - even when the polarizers are aligned, you loose maybe a stop or so. This technique works when there's plenty of light - it's best suited for daylight exterior use.

In darker conditions, i just take the filters off and shoot the regular way. This is not really as bad as it sounds because when it's too dark to use the polarizers, the best exposure for HV20 will be pretty close to 1/50, f1,8 etc. anyway.

This technique has made HV20 much more usable for real production work for me, and i'd strongly suggest everyone interested in more control of the cam to give it a go.

[Yojimbo]

Hi Halsu

I know this thread is now a few months old, but may I ask Halsu and the other posters what brands of filter they would recommend (or not recommend)? I particularly like the sound of the ones Halsu mentions with the angle markings (I've not been able to find any that have these).

Thanks

[CycleWriter]

Wasn't the whole purpose of the original trick to lock exposure and reduce gain for low light situations? How does this help that?

[Yojimbo]

Hi Cyclewriter,

I'm interested in this trick cos I want to achieve the "poor man's selective focus" effect. I know the Shallow DOF Police are going to jump on me for this, but I'm happy with the kind of selective focus you can get with a little bit of zoom (say, a third. BTW is there anyway to measure how much zoom you have on the HV, in the interest of repeatability?), and the aperture as wide as it'll go. This is perfectly possible indoors with controlled lighting, but not outside (I want to keep the shutter speed around 1/50). So I could use a neutral density filter, but this double polariser trick would seem to give you total control over how much light enters the camera.

Haven't tried it myself yet! But will place an order soon...

[Rumpelgeist]

This is cool usage of linear pols, this is the same idea that was around since 1960-ies to cut out light from oncoming cars: they have pol layer on the headlights at 45 degree, and your car has it on the windshield at 45 degree, so they cancel each other. But your car's headlights are at 0 degree with your windshield, so you can see your own light. Did not work because they could not make a whole windshield polarized.

But linear polarizers can affect metering and/or autofocus. The HV has iAF sensor, but it may be obscured by a large filter.

[askomiko]

I tried this some years ago with a still camera, and it really messed up the colors. Everything was green and purple. That was funny effect actually, and it only appeared when the polarizers were set so that almost no light goes through, which is obviously useless for video use.

[Halsu]

I tried this some years ago with a still camera, and it really messed up the colors. Everything was green and purple. That was funny effect actually, and it only appeared when the polarizers were set so that almost no light goes through, which is obviously useless for video use.

The filters i have give a blue shade when set so that no light goes through - could be useful for day for night i guess, hehe... Only situation where i could imagine someone needing that much ND is shooting a closeup of the sun... and trying to underexpose it ;-)

In normal operation, i haven't noticed any problems with color reproduction, focus or exposure metering.

[Halsu]

I know this thread is now a few months old, but may I ask Halsu and the other posters what brands of filter they would recommend (or not recommend)? I particularly like the sound of the ones Halsu mentions with the angle markings (I've not been able to find any that have these).

Mine are manufactured by HOYA - they're 52mm diameter, so one needs adapter rings to get them to work with HV20.

I haven't tried other brands - these have worked well enough for me.

[doruondun]

I prefer 4x4 filters for my mattebox. So you buy it once and you can use it with every lens or converter...

[HueyNRolf]

Very cool!
What is the difference between linear and circular?
For some reason I thought linear filters were fixed and circular ones rotate. Guess I was wrong.
I have a circular so, to give this a try, should I get a linear or second circular?

Cheers,
HnR

[treyvollmer]

I'm pretty sure only the front filter needs to be linear, the other can be circular. I believe it could also work with two circular polarizers if you flip over the one in front. This will eliminate any polarizing effect, which may be desirable in some cases. Just thought I'd mention...

I keep reading "circular" filter. What does this mean?

Thank you,

Trey

[Halsu]

I keep reading "circular" filter. What does this mean?

There's two kinds of polarizing filters, which generally work the same way, but differ in special occasions like this, and i.e. when focusing on a SLR camera.

From wikipedia:
http://en.wikipedia.org/wiki/Polariz...lter#Polarizer

There are two types of polarizing filters generally available, linear polarizers and circular polarizers (or CPL filters). With the exception of how they interact with some autofocus and metering mechanisms, they have exactly the same effect. Both transmit one of two states of linearly polarized light. The difference is that a circular polarizer alters the light leaving the filter and entering the camera by using a quarter-wave plate to circularly polarize that light. This has the same effect photographically as a linear polarizer, reducing glare in the scene. The metering and auto-focus sensors in certain cameras, including virtually all auto-focus SLRs, will not work properly with linear polarizers because the beam-splitters used to split off the light for focusing and metering are polarization-dependent. Circular polarizers work with all types of cameras, because mirrors and beam-splitters reflect both circular polarizations equally.