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Old February 13th, 2005, 12:30 PM   #1
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Hotspots - not opaque enough?

I have just discovered that my GG isn't as good as I thought. Here are some still shots:

With explanation :
http://dvstuff.250free.com

Without the explanation:
http://dvstuff.250free.com/hotspot_web1.jpg
http://dvstuff.250free.com/hotspot_web2.jpg

Now, am I right in assuming that if the CD/GG is not opaque enough, the Hotspots (if there are 'hotspots'??) will cause the middle of the shot to be too bright? When I use the frosted CD that came with the DVD pack, it is better, but still noticeable. If I look through my DOF machine with MY GG, I can see the end of the SLR in it, which I guess means that it is not projecting onto the GG and going straight thru, right? (too transparent?)
Will a fresnel get rid of this?


Well, I am getting frustrated again because it seems when I make strides, I get pused back again. Undetered, I am still trying, but I can see why it would be easier to give up and wait for James's Micro35 to come out - but it isn't me I guess. I have to see this thing thru till the end.

Once I get my Macro, I can start bringing the camera closer to the GG, but I still want to fit in the Fresnel (if that gets rid of hotspots).
Comments please!
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Old February 13th, 2005, 05:47 PM   #2
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It's just one problem, easy to solve. James's Micro35 is much more trouble, you have to wait, email, wait again, email again, sent money, wait again, sent some emails...

Either the GG is too transparent, which include unusable materials that seem to be good, but arenít. Or you need a condenser lens, which is nothing more than a magnifying glass. The thicker, the better, unless it gives visible artifacts. Better not use a fresnell for this.

Put that condenser lens (unless you already have one) right behind the GG, so not visible for the DV camcorder. As close as it can be, about 2 or 3 mm, without touching the GG of course.

To check your GG, try the stuff I'm using, tracing paper (I have Canson Tracing Paper 90 g/m2) You can find it at art supplies stores.
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Old February 13th, 2005, 08:01 PM   #3
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Mandy

did you check the brightness tests I have done?

They are posted on my site here:
http://dandiaconu.com/gallery/Brightnes-tests

The spinning whatever will only help you get rid of the grain (somewhat) Hot spot is still there on any GG (no matter how fine or waxed or holographic etched) It is only less obvious if you use a small section 18/24mm in the centre as P+S adapters (but then you only use straight light coming trough the lens, without optical dispersion towards corners) and you do not get the "whole frame"24/36mm from an SLR lens.

A normal 50mm lens can fill easy a 45/45mm frame WITHOUT A HOT SPOT!!
Watch this two clips and you will SEE...

http://dandiaconu.com/gallery/albums...s_test_avi.wmv
http://dandiaconu.com/gallery/albums...ur_screens.wmv

PS
I can bet within a year, P+S will launch a "new generation" of converters/adapters using FRESNEL focusing screens!!!!!!!!! Read my lips.
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Old February 14th, 2005, 08:07 AM   #4
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DAN : Ok, Dan. I watched the videos and they are quite remarkable - but what you are saying is unclear to me. It looks like the Beattie screen has the best colour and clearest picture - yet you made the statement :

"I can bet within a year, P+S will launch a "new generation" of converters/adapters using FRESNEL focusing screens!!!!!!!!! Read my lips."

Wouldn't they be better off with a Beattie screen (rather than a Fresnel), because of how well it performs (accroding to your pictures). I am assuming from your posts that you don't like the spinning GG and you have a way of oscillating your GG in any case, which is cool.

OSCAR : when you said put the magnifying lens in the mix, should it go here:

ACTION >> MAGNIFYING LENS >> GG >> MACRO >> DV CAM

Thanks
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Old February 14th, 2005, 09:52 AM   #5
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Fresnel is a type of lens. Beattie screens (like any other SLR focusing screens) are: Fresnel on one side and matte on the other. I used "Fresnel" as just generic name for the same focusing screen made by... "no name".
Spinning CD (or any type of GG) has its limitations: you cannot get a BIG BRIGHT picture. If you are happy with an 18/24 (at best) and you are willing to spend all those hours (and$) to get to that, is up to you. My question is can you SELL THAT FOOTAGE? If yes, go 4 it, if no...? That's all.
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Old February 14th, 2005, 10:09 AM   #6
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I agree with you Dan, but it depends on your perspective - or anyones.
I am doing this out of curiousity and a challenge. I also want to film a short using the technology to showcase my talents as a director. Selling shorts really didn't enter my mind - or even selling a feature for that matter. Maybe others are thinking much farther ahead and want to sell, more power to them.
I am still a little confused at your 18/24 comment. I have used a bolex 16mm in the past and I really didn't see much in the way of big bright picture even then, so if this can emulate, it is pretty good to me - but if you are referring to the fact that todays DV camera can capture so much more and we are narrowing it down to a small field, then yes, it has it limitations, definitely.
I am also guessing that your little device does not have these limitations?
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Old February 14th, 2005, 10:43 AM   #7
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Mandy,
there is nothing wrong with the spinning CD (or other type of GG instead)
P+S system uses a GG in both mini and Pro and they sold quite a few and some people shot footage and they thought "Jesus came back"!!!
What I was saying is :
an SLR lens can give you 45/45mm of usable frame (if you use a Fresnel type of focusing screen) That is a HUGE image to capture on the CCD. A huge, bright, hotspotless picture in which the grain becomes so small that you can hardly have to move it!!!!!!!
If is only a GG (no matter how good) you loose most of the available picture due to vigneting. End result: you have to zoom-in until you see something acceptable (frame size) in which the vigneting is not that obvious (at the limits of what the CCD can "live with" for its already limited dynamic range)
A 50mm lens will look like an 85-100 mm. Why? Because you can not capture the whole frame. Why? Because of the vigneting.
For fun is OK. I guess I am looking for a bit more though.
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Old February 14th, 2005, 10:54 AM   #8
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Ok, so the trick is to get a moving frensel in there, right? I heard they were selling those at Walgreen for like $5.00 in a thread I had been reading a few weeks ago - I wonder if that would do it, but then you need a way to make it move!
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Old February 14th, 2005, 10:59 AM   #9
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Mandy,

The thickness of the diffuse surface (intermediate image area) has a lot to do with how much hotspot you get. If it is too thin, you get concentrated light passing through the center of the ground glass, giving you the hotspot.

With microcrystalline wax, the hotspot is a lot less evident. Here is some footage + framegrabs shot with a CAMERA+MACRO+WAX-GLASS+35MM LENS arrangement:

http://209.214.235.122/mwtest/

Since that test, I have added a condenser between the macro and wax-glass (flat side against the wax-glass) to fix any vignetting.

I haven't tried but I imagine that if you add a fresnel type lens (w/ rings - to concentrate / distribute the light evenly) to the front side of the ground glass (flat side facing glass / ringed side facing 35mm lens), that should help out a lot. You shouldn't see the rings, since you'd be focused on the ground glass, and you should get a more even brightness in the picture. Moving a fresnel makes no sense because it is designed to stay in one place to focus the light properly. The ground glass would be the only thing that needs to be oscillated/shaken/spun, and that's just to remove grain.

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Old February 14th, 2005, 11:23 AM   #10
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Frank,

Thanks for the reply! Where did you get the wax-glass from? Is this an easy procurement or something to special order? There is a glasssmith in my town, I wonder if he/she can make up some of this stuff?

Also alot of peopel are talking about condensers - what exactly are they?

I don't remember Agus having these problems with his system way back then, I wonder how he overcame the hotspot problems...
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Old February 14th, 2005, 11:25 AM   #11
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Generally speaking a Fresnel isn't very good for imaging quality. I try not to use them if possible.

That said the beattie screen looks fine :)

As for the hotspot: Yes, the problem is precisely the fact that the surface is not opaque enough. The more diffuse the material the less of a hotspot you will end up with. In fact it may be theoretically possible that the more diffuse a surface you use the brighter your screen should be (for instance, note that the OptoSigma ground glass was brighter than any other ground glass [literally, ground glass, not focusing screen] and this was due to it's greater diffusion).

The problem with condensers is that they tend to have really bad imaging quality when used as part of *an imaging* system. They're really, really fast lenses but because of this they have lots, and lots, of aberration.

What you can do to eleminate this though is place the condenser between the 35mm lens and the ground glass. Place the flat side right up against the ground glass. This will act as a focusing assembly rather than an imaging system so you won't have any problem with aberration and you should get a brighter image to work with.
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Old February 14th, 2005, 11:36 AM   #12
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When I say "wax-glass", I'm talking about a layer of wax sandwiched between two pieces of glass. Sorry about that! :-)

Here's a thread where more info can be found:
http://www.dvinfo.net/conf/showthrea...threadid=33489

To make one of these wax adapters cost me around $2. (I'm using glass from cheap picture frames, and I'm working with a 1 LB sample block of microcrystalline wax.) And while it is faster than grinding glass, you often have to retry, retry, retry due to having either debris or bubbles inbetween the glass pieces.

A condenser lens is flat on one side and curved on the other. I have tried a few types. One (not completely flat on one side, but it still worked) was taken from a teleconverter. Currently, I am using some lenses taken from a set of magnifiers that you use when making fly lures. The quality is a lot better than I had expected. The lenses are the perfect shape / size for me also.

Since I have two of them, I have tried two different arrangements: First, with both lenses on either side of the wax-glass (flat side against it), and second, with just one lens against the wax-glass, curved side facing the camera. I didn't really see a big difference between those two setups, but I didn't do any thorough tests.
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Old February 14th, 2005, 12:42 PM   #13
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Would a Magnifying glass do the same thing? I have been experimenting with a magnifying glass in between the SLR and the GG, but it doesn't seem to help. The hotspot is still there. I am going to phone the GlassSmith today and find out if he/she can make up a GG or even a condenser lens.

I am still unsure what exactly a condenser lens does - 'fast fast fast lens', doesn't really make any sense to me considering I am not an optics person.

Even when I am using the Frosted CD from the DVD pack I am getting a hotspot, so I am not sure why this is - because I would have thought it was pretty opaque!

I just went out and purchase some tracing paper and a glue spray. Going to try this out on a CD and see what happens.

Anyone have a good/fast method of rubbing off the silver layer from a CD to make it clear?
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Old February 14th, 2005, 12:57 PM   #14
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Mandy,

I'm not an optics expert myself, but basically what a condenser would do is 'spread' the light more evenly across the surface. The rays of light coming from the 35mm lens on the outsides would be collected and focused/concentrated better, as far as I know.

Quote:
Anyone have a good/fast method of rubbing off the silver layer from a CD to make it clear?
You can take something thin and flat and scrape (don't make scratches too deep to grind over, though) part of the silver away. Then take some duct tape (or some other really sticky tape) and stick it to the silver part where you've started the scrape and pull it up. Once started, it should lift a majority of the silver away from the plastic disc.
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Old February 14th, 2005, 02:48 PM   #15
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Well, I have uploaded some new pics and footage. I think I am going to need the condenser to spread the light evenly, but maybe not:

http://dvstuff.250free.com

http://dvstuff.250free.com/hotspot_web3.jpg
http://dvstuff.250free.com/hotspot_web4.jpg

There is also a video there that the pics were taken from.

I have to say, I am getting happier with the footage as I go. I still have alot of little things to fix, but it is getting there!
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