ND filter?
 

Anyone experimented with a ND filter for a GoPro? Maybe a small piece of window tint or something? It could be useful since we all use them outside...

you can get them at eyeofmine.com

,,,as well as a killer underwater flat port adapter to sharpen up the submerged video.

yeah I tried some underwater stuff with the normal housing and it looked awful..so after I was thru freaking out I read about the flat port housing and got one, makes all the diff..

Re: ND filter?
 

The GoPro really cries out for ND when used in sunlight. To avoid diffraction the iris stops closing at about f/5.6 and from that point on the shutter speed increases. Increases to such an extent that frames taken from a cross-country trip on my bike are all pin sharp, and the wheel spokes show only the tiniest amount of image blur. The jello effect also means the spokes are all shown bent, but that's another story.

Of course this very high shutter speed (I estimate it at 1/750th sec, something like that) gives the footage an incredibly staccato look - which, when added to the non stabilised image - does look a bit crazy. Mercalli helps on the stabilisation timeline though.

tom.

Re: ND filter?
 

There is no iris, the exposure is controlled by shutter and gain.

Re: ND filter?
 

Take some stills with the GoPro Ian, pull them into Photoshop or Picasa and look at the detailed info. You'll see the camera uses all sorts of different apertures. Or are you saying these are extrapolated from internal (and undocumented) ND filters?

tom.

Re: ND filter?
 

If you pull the GoPro apart Tom, you'll find there is no physical aperture in the lens. Video exposure is controlled by shutter and sensor gain. The Gopro aperture is fixed at f/2.8, that why it has an almost infinite depth of field. Everything is in focus in any light.

The EXIF data is usually reported incorrectly, most programs interpret it as f3.61 regardless of lighting conditions.

See this sample photo, the aperture is being reported as f/492.2 ......... yep f/492.2

Exif | GoPro Flight by GoPro Photos | Flickr - Photo Sharing!

If you add an ND filter, all it will do is slow down the shutter speed and then if the GoPro senses it requires more light. it wall crank up gain. It reaches a point eventually where the shutter is too slow and then the noise makes the video unusable.

If you know otherwise please educate me, but even GoPro resellers like the above quoted "Eye of Mine" reseller GoPro HD Hero for Professionals - EyeOfMine Multimedia on this page under EXPOSURE, quotes ""It is fully-automatic. The lens aperture is fixed - you get one big T-stop:) Because the lens is fixed, the only other variable is shutter speed which obviously can cause issues in bright daylight with stuttering because of fast shutter speeds. ""

This is why we have been nagging GoPro for a firmware upgrade to enable Exposure or Shutter Lock and it is coming in the next model.

Cheers Ian
BTW run this GoPro user forum, heaps of info there about everything to do with GoPro and projects.You're welcome to join us.
GoProUser.freeforums.org

Re: ND filter?
 

'fixed at f/2.8, that why it has an almost infinite depth of field' you say, but considering the small chip size and the very short focal length used, then once the hyperfocal has been set up the aperture makes very little difference to the dof in reality, just allowing (should you have the option to stop down some) for slightly closer focusing.

tom.

Re: ND filter?
 

Focus from about 12inches to infinity regardless of light, pretty much fool proof focus wise.

Re: ND filter?
 

1/2" sensor is not really small considering the size of the GoPro.

Re: ND filter?
 

I haven't played with the ND much but I did add a polarizer and it makes a lot of difference.

From another forum:
'The idea was to mount a threaded upsize ring to a case permanently, without vignetting.

I used a method that I found on the board and took a 40.5 to 58mm Fotodiox ring and tried it with a 58mm circular polarizer and just held it in place. Worked fine with no vignetting but the shutter surround was in the way. I milled a tad off of the surround so the ring cleared and epoxied the upsize ring to the bezel.

Perfect fit except the bubble was higher than the top of the ring so I ordered a spacer ring from Fotodiox today as well as a 58 to 77mm ring. It looks like the spacer will just clear the top of the shutter button and that will let me add the 77 mm ring.

All my better filters are 77 mm so it would be nice if I cane use them with the go pro too. The big question will be if the added ring height will vignette.

I'll have the spacer and ring in a couple of days and will post the results."

http://i198.photobucket.com/albums/a...g?t=1301879828

http://i198.photobucket.com/albums/a...g?t=1301880361

http://i198.photobucket.com/albums/a...g?t=1301880600

Re: ND filter?
 

I did this test about 12 months ago, there is a slight increase in contrast with a polarizer and it does reduce highlights a bit, but a variable ND just introduces too much noise once you approach the point where the shutter speed is low enough to allow a nice motion blur.

I used a lineal polarizer in the example below, so i had to rotate it to get the effect, a circular polarizer won't need as much rotation.

Re: ND filter?
 

Hi Ian,
the CMOS chip (complete with masses of jello-wobble) is listed as a 1/2.5'', so exactly mid way between the 1/3'' (say Sony Z5) and ½'' (say EX1-R) chips. Small for any camera, but huge in a body the size of a GoPro.

Interesting your thoughts on the vari-ND introducing noise as the gain-up point is reached, a point we'll never be told about presumably. But the very high shutter speeds in sunlight sure spoil motion footage in my view, and motion blur (as shown in your MB3 video) definitely helps the impression of speed.

Next point - a polarisor shouldn't reduce highlights. It is after all simply an ND for wavelengths in one direction only.

tom.

Re: ND filter?
 

Yes you're right 1/2.5" that was a typo, still big for a GoPro as we agree.

In keep telling the guy's on my forum the Polarizer won't reduce the blown highlights in Snow and theoretically it shouldn't, but if you watch my test above carefully, the Polarizer when rotated does reduce the blown white clouds just above the horizon and the waveform monitor in Sony Vegas confirms it. It has me stumped, but hey if it works, it works. But it is hard to dial in for sports use where conditions are changing constantly as they do when you use a POV Cam.