ND filter versus faster shutter speed. Please educate me. - Page 2 at DVinfo.net

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Sony HVR-Z1 / HDR-FX1
Pro and consumer versions of this Sony 3-CCD HDV camcorder.


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Old October 8th, 2007, 08:22 AM   #16
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Thanks for the link - interesting thread Nick.

There's nothing special about 'video lenses' as Sony seems to infer - all lenses suffer from diffraction losses the smaller the aperture that's used. In theory all lenses are at their very best wide open, but in practice flare from the interior of the cylinder, vignetting of the frame and decentering of the elements ensures this is never so.

Our Z1s have a 'sweet spot' in the aperture scale at f/4. Any wider than that and the lens vignettes the corners of the frame, and the zebras can clearly show that wide open the edges of the frame are a good 1.5 stops under-exposed.

Of course for general movie making this doesn't matter a toss and can even be used to good effect - 'highlighting' the object in the centre of the frame quite nicely. If you use your camera for making panoramas you'll quickly notice that frame vignetting makes matching up sucessive shots very difficult.

Cameras that have automatic (and generally undocumented) internal ND generally stop the lens closing below f/4 these days, the ND taking over as the owner thinks he's stopping down. (Sony PDX10, A1 etc). This is to stop diffraction blurring the image, a problem that is exacerbated by short focal lengths.

Pinhole cameras (remember them?) needed a tiny hole to keep the image sharp(ish), but not so small as to have diffraction take precidence.

So with our tiny chips and short focal lengths (and even shorter focal lengths if you use a wide-angle converter) you must stay well clear of small apertures. In fact our Z1's menu offers you the option of not allowing you to stop down past f/5.6 or so. If you're shooting HDV, take this option. I'm much less afraid of using maximum aperture than I am of using minimum.

But never be afraid to experiment I say. Video is the greatest of teachers.

Opening slo-mo shot on Splash was a Canon 1014E at 60 fps.

tom.
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Old October 8th, 2007, 08:36 AM   #17
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I think you mean in the picture profile? You can limit the iris to no smaller than F11, F6.8 or F4 in there.

I have often shot with a picture profile limiting the iris to F6.8 and will do this religiously from now on.

I have my maximum gain in PP1 set to 12dB and I'm going to keep sharpness at 9 as well from now on having read comments from DSE and others about the over-sharpness of HDV cameras and noticing this myself in my recent footage shot with no picture profile set.

I'm also wondering about a bit more colour. I notice a lot of people choosing around +2. What do you do?
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Old October 20th, 2007, 09:47 AM   #18
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Originally Posted by Tom Hardwick View Post
OK, a close-up lens (in reality simply a positive dioptre single element - though the Century one may be a cemented doublet to justify its cost and to reduce aberrations), is designed to work with your camera's lens focused on infinity...
Tom, just coming back to this, does this mean I really shouldn't use the diopter on anything else except infinity? Should I even go as far as locking the focus on infinity and then just adjusting the other parameters (zoom, distance to subject etc.)?

Just wondering about this because I'm cutting my macro footage now and a lot of it is not quite sharp. Some of such footage may well have not been on infinity focus.

Last edited by Nick Hope; October 20th, 2007 at 11:07 AM.
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Old October 20th, 2007, 09:58 AM   #19
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If you fit a close-up lens it simply means that infinity as marked on your lens is a lot closer. You can still use the camera's focusing quite happily, but of course adding the extra element(s) brings with it pretty nasty abberations. Generally the centre of the image should be ok, but it's the edges that suffer. Stopping down helps, but then again the more you stop down the more you head towards diffraction. You can't win.

So many manufacturers are making their lenses 'tele-macro'. The HC7 is a case in point - you have to use max telephoto (because of vignetting) but it does allow great closeups from a distance.

tom.
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Old October 20th, 2007, 11:46 AM   #20
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Thanks Tom. Well the achromatic Century diopter is definitely sharper around the edges the single-element diopter I was using before. But I'm only getting razor-sharp clarity and detail on the occasional shot. Many of the other macro clips shot with the Century are ever so slightly blurry, but still usable. The blurriness can't be seen on the camera's LCD. Only on my 24" monitor. A winning combination of lighting, iris, focus and distance from subject is pretty difficult to achieve (particularly underwater!).

I wish the exposure metadata could somehow be captured along with the footage, not just displayed on the camera during playback. Would make it much easier to learn from mistakes.

Attached is a still from some of the better underwater footage with the Century diopter. I guess that slug's about 2 inches long.
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ND filter versus faster shutter speed. Please educate me.-nudi.jpg  
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Old October 20th, 2007, 01:06 PM   #21
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Originally Posted by Nick Hope View Post
the achromatic Century diopter is definitely sharper around the edges the single-element diopter I was using before. But I'm only getting razor-sharp clarity and detail on the occasional shot. Many of the other macro clips shot with the Century are ever so slightly blurry
Nick, that's because using the diopter makes your DOF a lot more shallow. As you stated, you are underwater and are somewhat subjected to current moving you around, certainly enough to give you some soft images by moving just a little bit in relation to the subject. It's the nature of the beast.

I'll also second what Tom says about lenses and sweet spots. He mentioned you can tell that edges are underexposed vs. the center of the image and I have confirmed this while using a waveform monitor. You'll see the edges droop down on the scope until you close down to around F4, at which time the line becomes flat. As a general rule, the sweet spot kicks in around 2 stops down from wide open on most every zoom lens I have used.

-gb-
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Old October 20th, 2007, 01:13 PM   #22
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The 'everso slightly blurry' shots - these can't be blamed on dof problems or on very slight miss-focusing (i.e. it's sharper somewhere else in the shot)?

Greg - all lenses vignette the edges wide open - primes as well as zooms.

tom.
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Old October 22nd, 2007, 07:02 AM   #23
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The 'everso slightly blurry' shots - these can't be blamed on dof problems or on very slight miss-focusing (i.e. it's sharper somewhere else in the shot)?
Probably so. I think on this particular macro trip i went a bit overboard with opening up the iris to shorten the depth of field for a "film-like" effect. So I was often as wide as F2. Probably didn't need to do this as the diopter was shortening the DOF anyway.

(Will post some finished edited footage in due course)
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Old November 26th, 2007, 03:34 PM   #24
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I thought I recognised your face. Nice stroke! I especially like that slomo opening shot.

Thanks again for the explanation. This is really great stuff for someone like me who missed out on some basic theory.

So according to your explanation, large irises (low F numbers) are to be avoided and according to Sony, small irises (large F numbers) are to be avoided too (p32 of the Z1 manual refers to diffraction and I have noticed this myself). So does that mean I should be staying in say the F3.1 to F6.8 range where possible?

I did some underwater macro tests with the Z1 when I got my Century diopter. They are here if anyone's interested:

http://wetpixel.com/forums/index.php?showtopic=20474
If anyone is interested, you can email me and I will send you an Excel file which will calculate minimum focusing distance with a diopter attached.

Send email with a BOLD title that say "SCHNEIDER CLOSE UP CONVERSION".

Thanks,

Ryan Avery
Regional Sales Representative
Schneider Optics
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