Disappointed in S4 EDLT

[TJCornish]

I picked up a 36° S4 EDLT lens tube for gobo projection. My gobo is a B-sized glass gobo with only text.

As you can see from the picture, with the middle in focus, the outside edges were fairly out of focus, even when using a donut.

I guess I'm used to my movers that have better optics, but I thought an EDLT tube would at least get me close.

Am I doing something wrong? Are my expectations too high?

By the way, the fixture was about 20' from the curtain shooting almost perfectly perpendicular to the curtain.

 

[JD]

How wide is the projection? The problem is often caused by the fact that the center of the image (by nature) is closer than the outer edge of the projection. In other words, you could focus the outer edge and the center would then be out of focus. About the only way around that would be to move the fixture back significantly, and use a tighter beam angle.

 

[rsmentele]

Have you checked the bench focus?

 

[TJCornish]

How wide is the projection? The problem is often caused by the fact that the center of the image (by nature) is closer than the outer edge of the projection. In other words, you could focus the outer edge and the center would then be out of focus. About the only way around that would be to move the fixture back significantly, and use a tighter beam angle.

I understand the physics of what you are saying, but this can be corrected - most camera lenses are optically corrected to create a flat focal plane. If the EDLT doesn't do this, then what's the point of EDLT if it does the same thing that regular tubes do?

EDLT is designed for gobo projection, and the fact that 36° and 50° EDLT tubes are available presumably means that people want to project gobos where these beam angles are needed. It's not practical for me (or I suspect anyone) to go 200' back with a 10° tube.

 

[TJCornish]

Have you checked the bench focus?

I'm not a bench focus expert so I'm not ruling out doing something wrong, but I did bench focus the fixture in the air as part of troubleshooting this issue. It does affect the image somewhat - both focus and brightness, but I never got a great result.

 

[MNicolai]

Are you using the gobo slot or the accessory slot?

Is the gobo holder firmly in place, straight in the slot?

Do you see this same symptom with steel gobos?

Is the surface of the glass gobo clean?


As for bench-focusing, since you're already perpendicular to the cyc you can pull the gobo, make sure your shutters are wide open, and adjust the bench focus knob until the pool of light appears flat in brightness as opposed to having a giant hotspot in the center. If in doubt, back it off and try again. You should see when it's in the ballpark. Flattening out the bench focus will also help preserve your gobos by spreading out the heat so it doesn't scorch your gobo and burn it up quite as quickly.

 

[CrazyTechie]

+1 for checking the things that @MNicolai suggested. It sounds like something could be misaligned somewhere, and it doesn't take much of that for the focus on templates to be wonky. Since you are using a glass gobo I'm going to assume you are using the accessory slot. I've found that those holders can have a little bit of play in them depending on the template, which could be enough to throw off your focus. If you - gently - wiggle the light, does the focus on the pattern change?

I would also add to the list-of-things-to-check is to make sure there isn't any play with the pattern when it's in the template holder to rule that out.

An easy way to verify if the lens tube is or isn't the issue would be to put in a regular 36deg tube. I suspect your results will be the same in terms of focus ability regardless of which lens tube you use.

 

[TJCornish]

I appreciate the thoughts.

The picture I attached isn't perfect, but the problem is not an issue of the gobo not being planar to the curtain which would be seen as one side being out of focus, but rather that both sides of the gobo are out of focus compared to the center. Adjusting the lens tube in and out makes either the center in focus or the edges, but not both - the EDLT tube seems to have a focal plane that is just as convex as a regular tube. It was my understanding that the EDLT tube compensated for that much more than it apparently does.

I bench focused the fixture to flat field. I left some ambiguity because I wasn't sure if the fixture needed to be bench focused differently for gobos, but if a flat field is the goal, I have it.

When life slows down I will put the fixture on a floor base and play with it more in depth; I guess I am looking for general experiences with the EDLT tubes. Is what I'm seeing typical? Based on a picture of this same event done with the same gobo and a conventional tube, I'm not sure the EDLT is any better than the regular "J" series tube I used before.

 

[MNicolai]

Paging @starksk @Jim Uphoff @JJBerman

 

[JJBerman]

Jim I'm assuming is very busy at LDI and will respond as soon as he can.
Kirk or @HansH will respond as soon as they have a free moment.

I can't say either way as I haven't worked with EDLT tubes.

 

[dkinzer]

I picked up a 36° S4 EDLT lens tube for gobo projection. My gobo is a B-sized glass gobo with only text.

As you can see from the picture, with the middle in focus, the outside edges were fairly out of focus, even when using a donut.

I guess I'm used to my movers that have better optics, but I thought an EDLT tube would at least get me close.

Am I doing something wrong? Are my expectations too high?

By the way, the fixture was about 20' from the curtain shooting almost perfectly perpendicular to the curtain.

Hello TJ. I am the designer of the EDLT lens tubes, so perhaps I can answer your question. The inability to focus on both the center and edge of the field is called field curvature, and I'm not surprised that you're seeing some with the 36 EDLT. The best suggestion that I can offer is to compromise, and focus on a point midway between the center and the edge of the gobo. The 36 EDLT is superior in sharpness and contrast to the standard 36 degree lens tube at all points across the field. To eliminate field curvature would require a more complex projection lens design with more elements and expense. I hope that this helps.

 

[TJCornish]

Hello TJ. I am the designer of the EDLT lens tubes, so perhaps I can answer your question. The inability to focus on both the center and edge of the field is called field curvature, and I'm not surprised that you're seeing some with the 36 EDLT. The best suggestion that I can offer is to compromise, and focus on a point midway between the center and the edge of the gobo. The 36 EDLT is superior in sharpness and contrast to the standard 36 degree lens tube at all points across the field. To eliminate field curvature would require a more complex projection lens design with more elements and expense. I hope that this helps.

I appreciate your explanation, but I am left wondering what the point of EDLT's "superior sharpness and contrast" is if I can't focus a gobo? Gobo projection is listed as a primary application for EDLT on your product page: http://www.etcconnect.com/Products/...anced-Definition-Lens-Tube-EDLT/Features.aspx

The EDLT datasheet has this picture (which actually looks even sharper than this before the forum software compresses the crap out of it), which unless the output below is the middle 25% of the field is false advertising, or at least not true of the wider tubes:

Am I the only person who uses gobos and expects them to not look like garbage?

@Ford @Ben Dickmann Do the Ovation tubes have a flat focal plane?

Anyone want to buy a slightly used 36° EDLT?

 

[TJCornish]

Here's another ETC page proclaiming "The optical system is highly optimized for sharp focus of intricate gobos with minimum distortion and maximum lumen output."

 

[josh88]

Because its a large step up from what the standard is, therefore it does the job much better than a regular lens tube. Different degrees are more susceptible to the the curvature distortion, it sounds like 36 especially. Minimum distortion doesn't say NO distortion.

If I have something that I really really need clear and focused well I'm going to use a projector. The "high def" tubes have always been clear enough for us to use text clearly. Above that we're not using a gobo. That being said I know we've hard sharper results out of ours that the image you posted.

 

[sk8rsdad]

The OP may have an A-size gobo. a B-size gobo would use a smaller portion of the available beam and likely be less susceptible to curvature distortion.

 

[TJCornish]

Because its a large step up from what the standard is, therefore it does the job much better than a regular lens tube. Different degrees are more susceptible to the the curvature distortion, it sounds like 36 especially.

I presume it gets worse with increasing beamwidth.

Minimum distortion doesn't say NO distortion.

Well, that is subjective, I guess. I'm comparing against my VL770 and Chauvet Maverick MK2 spot fixtures, both of which are significantly better than the EDLT throughout their zoom ranges, though admittedly they cost slightly more.

If I have something that I really really need clear and focused well I'm going to use a projector. The "high def" tubes have always been clear enough for us to use text clearly. Above that we're not using a gobo. That being said I know we've hard sharper results out of ours that the image you posted.

By projector, are you referring to a video projector or a gobo projector? Most of the gobo projectors I'm aware of are fairly small and low output. Is there a S4-level brightness fixture out there?

 

[TJCornish]

The OP may have an A-size gobo. a B-size gobo would use a smaller portion of the available beam and likely be less susceptible to curvature distortion.

I have a B-sized glass gobo. I don't know what would be worse - an A-sized gobo in a 26° fixture or the B-sized gobo in the 36°.

I am absolutely swamped for the next couple weeks, but after that I will do a side-by-side with EDLT and non-EDLT S4 tubes - I have both "Rev J" and unmarked tubes, and if @Ford or @Ben Dickmann are willing to send me an Ovation tube, that as well.

 

[Ford]

Hi TJ,
We’re at LDI right now, but if you give me a gentle reminder, next week, we can absolutely get you a lens tube for your comparison.

 

[TJCornish]

Hi TJ,
We’re at LDI right now, but if you give me a gentle reminder, next week, we can absolutely get you a lens tube for your comparison.

Thanks Ford. Looking forward to connecting with you soon. Hope you enjoy the show.

TJ

 

[josh88]

I'm comparing against my VL770 and Chauvet Maverick MK2 spot fixtures, both of which are significantly better than the EDLT throughout their zoom ranges, though admittedly they cost slightly more.
By projector, are you referring to a video projector or a gobo projector? Most of the gobo projectors I'm aware of are fairly small and low output. Is there a S4-level brightness fixture out there?

I think thats the point, its more expensive, but still cheapish. There are more expensive ways to get it clearer. Its the middle ground. Better optics than a regular tube, less better than more expensive options.

and yes. I mean an actual projector. If I want something I know will be completely clear we'll switch to a projector and throw a slide up or map something. I know thats not viable for everybody, but thats my next step up at that point.