Hey guys!

Right now I'm trying to get a decent image with my DVX100B, a homemade adapter and two 58mm Canon 250D´s (with a stepring), but when I zoom in on the groundglass, I noticed the corners gets soft. When I zoom out everything is sharp and dandy.

Does anyone know why this happends?

Any help appriciated!

Do the corners get soft or do they darken a bit? Not sure why they would get soft. The 250D's use two elements, so they should be pretty good edge to edge. It could be that you're zooming in too far because the 250D's aren't strong enough. Maybe...

Hey Mike!

I've uploaded some footage so you can see for yourself! :)

http://martin.cmplete.com/adapter/

Compare the two pictures. Look at the green LED on the screen, it gets kinda distorted in the corner. I've managed to minimize the effect, but I want to know if I can get rid of it complete.

The funny thing is that I can get the corners into focus but that makes the rest of image out of focus. Whats up with that?

Anyone here with a DVX100B with better results than this? Please share :)

Interesting. Rather interesting. I've got a DVX100A, so I'm in a similar boat.

Seems like you're getting a hotspot in the middle there. Are you using a condenser of any sort? That may be part of it, but I doubt it. Edge blur is usually caused by the macros (or achromats in this case). Could be the stacking that's causing the problem. I've seen that happen with cheap macros, but the 250D's are achromats. Hmmm...hope someone else jumps in. I'm inclined to think it's the achromats. How far are you zooming in the DVX and what's your MF at?

Yes, I'm using one condenser (+12) close to the GG. I tried two but that gave me too much chromatic abberation. But the hotspot isn't that bad, it's just that it's more prominent in lowlight. :)


Yeah, stacking macros is not very recommended, i know. I thought I could get away with it by using good achromats, but apparently not :)

My zoom is 77 and my focus is 23.

What kinda setup do you have, Mike? What achromat are you using?

Two stacked achromats may well cause you problems, in combination with having the stepdown arrangement.

Does the stepdown ring place the achromat furthur forward of the camera? Some I have seen add as much as 4mm.

I think you may find that if you offer the achromat up to the camera hand-held without the step ring and get as close as you can to the camcorder lens without scratching it, you may get a small improvement as your zoom-through will lose a little of the edge abberation.

A single higher power achromat may be the only real solution.

7+ and nearly full zoom with the DVX100 (a?) gets you about 22mm x 16mm or slightly larger off a GG at about 120mm?

This was only with a handheld offering of my complete erecting AGUS35 for PD150 up to a DVX100 with a stepped down 58mm +7.

My stepdown ring does not place the achromat any furthur forward.

There was no condenser in the path.

The DVX100 (a?) was a nice image.

Guys, i'm confused here! I tought that dvx100b(and later models like hvx etc) have so small minimum focusing distance that You don't have to use any condenser. What i missed here??

Frank, the condenser is required to even out light distribution to (or from) the GG. The macro is another issue. Martin, pmail me on your achromat issues. I may be able to help you out :-)

Bob!

I am actually using three steprings, so that gives me about 1 cm distance between the DVX lens and macros, so I tried without the steprings, holding the achromats close to the lens, but I got the same soft corners. bummer :(

Well, a strong, single achromat might be the only way. It´s too bad I can't find any of these here in Sweden.

Frank!

I'm not really following you. Sure, the DVX has a great macrofunction, but that doesn´t mean you don't need a condenser? Perhaps you meant achromat ? Well, I can't really get that close to the GG. I have to zoom in, and when I do that without an achromat, it gets all blurry, and I can't focus on the GG! Am I making sense? :)

... condenser is required to even out light distribution to (or from) the GG...
and
Well, I can't really get that close to the GG
That is why most of sample footage seems always to me much with larger FL as lens given FL. You guys use actually smaler area than 24x36?

Martin, I think that using that strong of a condenser along with the stacked achromats may be the problem. Is the condenser placed next to the GG with the flat side against the GG? If that is the case then it's probably the achromats. If it is reversed (with the flat side NOT against the GG) I'd probably blame the condenser. +12 is pretty powerful, and assuming that it's a PCX lens that may be the trouble. Hmm...don't you love all the problems these things create? =)

Martin.

Email Dennis. As I understand things,

Achromatic dioptre - essential.
Condenser lens (es) - optional.

Frank!

No, I'm using the entire 36x24 frame.


Mike!

My condenser is very close to the GG indeed, and the flat side facing it. I just tried without it, but no, the soft corners are still there.

"don't you love all the problems these things create? =)"

Haha, I hear you! Sometimes I just wish I could forget all about these technical stuff, and just go out and shoot something instead. That´s what really matters in the end. :)

Bob!

I've emailed Dennis. :)

No, I'm using the entire 36x24 frame.
Why condenser if DVX100B can zoom given 24x36 frame?

Frank,

The condenser lens is for eliminating the hotspot/vignetting only and sits againsts or very close to the GG. It's the macro/achromat that helps with zooming in and focusing. They have 2 different roles.

Oh i'm sorry i am messing here with terms. But anyway i don't quite follow how You eliminate vignetting with condenser. My point is if You try more even distribute light with codenser then what is image itself on the GG. It is just bunch of lightrays as any other. How can condenser give light of well lighted object in center of frame, to object that are in edge of the frame? The only way bring that light to edge is bring object desired place(sound logically enough?).
I think there is another misunderstanding with terms. All the talk here goes about hotspotting not about vignetting. Difference is source of lightwaves. Lightwaves which are part of vignetting start from GG. But lightwaves which create hotspot come out directly from photolens last lens. So can condenser distort hotspot differently from image on GG. But never differently vignetting.

Frank,

Best way I can explain what the condenser is doing is by holding the GG in 1 hand and lens in the other, and project the image on the GG. Look at the image on the GG down the optical axis about a foot away from your eye.

Now, you notice the edges are darker then the centre. Look at the left dark edge, and move your head left so your eye is 45 degrees off the optical centre. What happens? That edge now appears brighter. Do the same for the right, top bottom.

The condenser lens is basically letting the camcorder view the GG from the best possible angle for any given point on the GG.

Ofcourse optically this is an incorrect explaination, but its one I like to give for visualizing.

Exactly such experiment made me think a few years ago thats not vignetting. Because vignetting is part of the image thatsway You just can't avoid it with any light redistibution. For instance You can print out vignetted image(place it where goes GG), place Your eye and lens as You described. What You see? Really artifacts are gone? I guess no. You'll see exactly the same image! Where the difference then? Actually You can get same result as with GG. Just take flash light and place it behind(where should be Your lens back) Your image. Without condenser You'll see now spot in the centre of picture. Use condenser now and Voila! Spot is gone.

The point is that condenser just redistributes light what goes from distance(as magnifier lens) but lives the picture itself alone because it's close enough.

So the bottomline is it's not vignetting but hotspot what can be "hidden" with condenser. Just because vignetting is part of the image and needs different kind of compensation but hotspot is direct unprocessed(not diffused) lightrays coming directly from lens.

Wayne's quite correct

Very approximately, something like the attached is happening.
http://www.physics.ox.ac.uk/users/smithdmd/intensity.html
The further the camcorder lens is from the gg, the less light is incident on it's lens. (It subtends a smaller (solid) angle relative to a given point on the gg). This is more dramatic for the gg corners.

The condenser affects those light rays going away from the axis more than those closer to it. Net result, it redirects them closer to where we want them to be.

So, like Wayne said, with a condenser in place, the image will be brightest down the axis of the device, and fall off rapidly away from it.

I've been experimenting with two condensers, one each side of the gg and this seems to have an even more dramatic "condensing" effect.


Frank - I think I see your point. You're saying that if a film cell was where the gg is then a vignette is darkening of the corners on that image (were it to be developed etc). Any effect of the redistribution of light through interaction with the gg/condenser etc is a hotspot. I.e, if the adapter itself affects the image in such a way as to get dark edges that's a hotspot, and if the actual picture incident on the gg has dark edges, that's a
vignette.

Martin, most lenses (unless spec'd otherwise) are not designed to utilize the outer ~5% of their periphery. The differential corner focus you are seeing is likely a result of spherical aberration at the edges of your achros.

Dennis, I'm sure you know tons more about this than me. But if he is using an achromat and zooming in, wouldn't he also be zooming past that 5% periphery? Just a thought. I'm sure you know more than me on this subject, I just didn't quite understand so I thought I'd throw this out there.

Yes, David. Just vignette and hotspot look in most cases so similar that it's easy to get confused.

Mike, I'm not necessarily any kind of expert :-) However, if you CAD up the 72mm DVX lens, 58 mm achro rim, and then GG, I suspect you would find that the 58mm periphery is an issue. That 5% could very well be 10%. I believe that the same issue comes into play when shooting with many fast 35mm lenses with their aperture wide open.