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After-market OLPFs for canon DSLRs


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#1 Keith Walters

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Posted 19 June 2010 - 06:09 AM

One of the drawbacks most often mentioned by certain people about cameras like the 5D and 7D is that, because their optical low-pass filter is optimized for full-resolution stills mode, it performs poorly when the sensor is subsampled to produce 1080p HD.

It seems to me that the obvious solution is to have another OLPF that can be manually clipped on top of the original one when the operator wants to shoot HD. Since the original filter's cut-off characteristic would be further out than one designed for HD, the effect of leaving it in should be minimal.

(Ideally you would want to be able to completely replace the original filter of course, but that doesn't appear to be practical, and is almost certainly something that Canon would never have anticipated).

Does anybody know if such a thing is available?

I'd be prepared to put a few dollars on Canon coming up with something like this themselves, along with SDI component and composite genlockable PAL and NTSC (and yes there is still a hell of a lot of that being used, in fact most of the world still can't afford to re-equip its studios for digital operation).

That would be an absolute killer product! It would make the Scarlet look like vaporub :P
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#2 Phil Rhodes

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Posted 19 June 2010 - 07:42 AM

Usually the efficacy of an OLPF is highly contingent on its being the right distance from the sensor, so you're potentially adding a lot of complexity to it by demanding it can be thrown on top of the existing one.

That and another chunk of glass in the way, oh joy...

P
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#3 Keith Walters

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Posted 20 June 2010 - 01:06 AM

"It would make the Scarlet look like vaporub"

That of course, was meant to be "vaporware".
Ah spell checkers, what would we do without them and their helpful suggestions...
Actually, rubbing a bit of vaseline on the existing OLPF might do the trick :lol:


Usually the efficacy of an OLPF is highly contingent on its being the right distance from the sensor, so you're potentially adding a lot of complexity to it by demanding it can be thrown on top of the existing one.

That and another chunk of glass in the way, oh joy...

P

Yeah, but I'm sure it could be designed to take that into account, and it wouldn't necessarily have to be as precise as the stills one.

The ideal scenario would of course be a single piece of glass with different OLPF characteristics on the top and the bottom halves and some sort of motorized system to slide the appropriate one into place.

Considering the Canon HD video thing was very much an amazingly successful afterthought, a certain amount of compromise is allowable.
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#4 John Sprung

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Posted 22 June 2010 - 06:53 PM

As a practical matter, you might just experiment with various existing diffusion filters in their normal position in front of the lens. See what seems to be the best "OLPF Helper".





-- J.S.
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#5 jacob thomas

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Posted 23 June 2010 - 12:16 AM

As a practical matter, you might just experiment with various existing diffusion filters in their normal position in front of the lens. See what seems to be the best "OLPF Helper".


-- J.S.


The problem with the external filter solution discussed here Caprock Filter discussion on Cinema5D.com is it looks like the solution is throwing away even more detail.

If only canon would put out a camera with a 3k/4k sensor that downsamples to 1080p (or even better outputs some kind of raw).

Edited by jacob thomas, 23 June 2010 - 12:17 AM.

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#6 John Sprung

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Posted 23 June 2010 - 01:08 AM

Yes, of course, every OLPF throws away some detail.

You have to test a bunch of add-on filters and pick your subjective tradeoff between aliasing and sharpness.

Because these filters are optical, you can't have negative coefficients, which means you can't get the theoretically optimal (Sin(x))/x. So, as far as perfection is concerned, we're already doomed from the outset. But I bet that it's possible to make OLPF Helper filters that will be significantly better than not using anything. Better is the enemy of good enough.




-- J.S.
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#7 Phil Rhodes

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Posted 23 June 2010 - 08:16 AM

Or, y'know, you could just throw it very slightly out of focus.

At least the aliasing wouldn't be a part of the image you wanted people to look at!

P
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#8 Keith Walters

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Posted 25 June 2010 - 05:56 AM

Well I'll be buggered; Panavision Australia (AKA PAnavision Asia) are now renting 5D packages (See about halfway down the page).

If anybody was in a position to experiment with replacement OLPFs it would be them, although I don't know if there's anybody left there could think of anything that sophisticated. :lol:

They also rent RED packages, for somewhat less than the rate for a decade-old F900... Posted Image
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#9 Keith Walters

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Posted 25 June 2010 - 06:13 AM

As a practical matter, you might just experiment with various existing diffusion filters in their normal position in front of the lens. See what seems to be the best "OLPF Helper".
-- J.S.

Not really the same thing. Diffusion filters are usually just thin layers of glass with tiny bubbles in it.

Optical Low Pass Filters actually made out of thin films of birefringement materials such as calcite or lithium niobate, mounted on a sheet of glass for support.

If a diffusion filter is considered to be analogous to a audio treble control with the treble knob turned anticlockwise, an OLPF is more like a graphic equalizer with all the sliders above say 10KHz turned fully down and the rest left in mid-position.
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#10 John Sprung

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Posted 25 June 2010 - 02:22 PM

Not really the same thing.


True, but it's still worth a try -- mainly because it costs so much less. It could be a reasonable price/performance choice in some cases. If it works for the look you want for a particular picture, it would be vastly superior to softening things in post, where the aliasing is already baked in.

It should also be possible to do the equivalent of that steeper rolloff in the matte box, rather than right next to the chip. Again, there would be compromises, but it's well worth some testing.





-- J.S.
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#11 Ram Shani

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Posted 06 July 2010 - 03:00 PM

maybe you can find old 2-3 megapixel still cam( the problem is to find dslr full frame or the same size of the 7d)


and take off the OLPF and use it??
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#12 Hal Smith

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Posted 28 July 2010 - 09:44 PM

Some people are having success using black promists to reduce moire. Obviously they have to use a different grade for different focal lengths. I had a chat with Tiffen about the subject and they allowed as how there is active research in this area.
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#13 Ram Shani

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Posted 29 July 2010 - 02:08 AM

btw
i work with caprock no-2 on my 17-50 2.8 tamron and 90% of the time i don't have problem with moire and aliasing
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