8mm Forum: 3.5m throw - what's my image size going to be?

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Posted by Stuart Reid (Member # 1460) on January 21, 2017, 02:21 PM:

Chaps, next weekend is the big move, and I've learned that due to planning regs my proposed garden cinema is going to be slightly smaller than anticipated at approx 5m x 3.5m (16ft x 11.5ft) I'm going to put in a partition wall creating 2 rooms, 1.5m (5 feet) and 3.5m. The 1.5 m room will be my video edit suite and projection room housing 16mm, Super 8mm and video.

The 3.5m room will have a couple of sofas and a permanent screen with movable masking fixed at the far end (I was hoping for acoustically transparent screen material with speakers behind the screen but with the space I now have I don't think that's going to be realistic unless I can find some hi-fidelity super-flat speakers).

My question is, what's my image size going to be running films? It'll be my Beaulieu 708 with original zoom lens for Super 8, and a 16CL Xenon for 16mm using a 38mm lens (VP will use a Mitsubishi HC5500). I'll also be wanting to run 'scope too. As mentioned before I'd like to keep a constant image height throughout, but want 'scope to have that real widescreen impact.

Oh - and one more thing; Portholes. Should I have a particular sized window between the two rooms, and should it be a certain type of glass, and should it be angled?

Thanks all.

Posted by Maurice Leakey (Member # 916) on January 21, 2017, 03:00 PM:

Stuart
The 16mm projector with a 38mm (1.5") lens at a throw of 3.5m (11.5') will give a picture 90cm (36") x 60cm (24").
Use plate glass which must be tilted vertically to throw the reflected picture away from the projector and projectionist.
For portholes use the smallest sizes possible, this is to prevent any stray light passing through to annoy your audience.

Posted by Stuart Reid (Member # 1460) on January 21, 2017, 04:19 PM:

Thanks Maurice, that's not a big image is it? I think I'll need a different lens.

Posted by Dominique De Bast (Member # 3798) on January 21, 2017, 05:43 PM:

Following the math given in the Beaulieu instruction book, you should have a size, with the zoom at 11 and distance of 3,5 mt,
about 1,7 mt (that's the width, they don't explain how to calculate the height).

Posted by Lindsay Morris (Member # 3812) on January 21, 2017, 06:02 PM:

A little formula that is very useful for ALL film projectors.

Image size = Aperture Width of image on film (either mm or inches) x Throw (either M or Feet) / Lens Focal Length (mm or inches)

Either Metric or Imperial are used but naturally you cannot mix the two systems. I stick to metric as far easier.

In your case Stuart the width of the 16mm aperture image is around 9.4mm.

Thus 9.4 x Throw of 3.5M = 32.9/ Lens Focal length of 38mm = 0.84M.

There is a fair bit of variation in 16mm aperture sizes from projector to projector Eiki (Elf) & Hokushin tend to be almost 9.5mm wide but B&H are closer to almost 9.3mm. So results are rubbery but close enough.

Can be applied equally to 35mm & 70mm machines and for 35mm use 21mm as the aperture size.
Have not measured an 8mm aperture as never needed to as all my 8mm units have zoom lens, but would apply equally as its all a product of the image size & the relationship of the lens magnification & throw.
As with all formulas if you have two of the known values the third unknown can be determined by manipulating the formula.

Posted by Dominique De Bast (Member # 3798) on January 21, 2017, 06:20 PM:

This is the formula given in the Beaulieu manual, Lindsay :-) They give the figure 0.536 (cm) for the super 8 projector aperture image.

Posted by Winbert Hutahaean (Member # 58) on January 21, 2017, 07:20 PM:

quote:
about 1,7 mt (that's the width, they don't explain how to calculate the height).

Dom, that will be proportional for the height as flat projection is 4:3.

So if the width is 1.7 (= 4) then the heigt is 1.7 * 3 divided by 4 = 1.275m

Stuart,

3.5 throw distance is very short and you will get small image. With Maurice calculation that is eqv with 43" screen (4:3)...not that big.

I have that very problem and in this below link is telling how I deal with it:

A lens you need for short throw problem

O yeah, the above link is a cheap method, I don't need a lesson to buy an expensive zoom lens.

Yeah, I knew it... [Wink]

Below is the screen shot with my body (height 175cm) for comparison.

Cheers,

Posted by Lindsay Morris (Member # 3812) on January 21, 2017, 09:05 PM:

Out of curiosity I grabbed a couple of old Std 8 fixed F/L lens machines & a Noris aperture measured 3.6mm & a Cinekon measured 3.9mm.
So the figure Dominique quoted of 5.36mm is possibly about right for S8 frames.
My Elmo ST1200HD looks to be about 5.4mm wide as best I can judge as not possible to get a vernier caliper right into the gate to really accurately measure...the lens will not swing right back to allow access.

When you go into scope mode Stuart on the 16CL your image will be twice the width of what you get with just the 38mm lens in use.
With a Zoom S8 machine you can zoom back & forth to find the best spot to fill your 16mm sized screen with a full height S8 image.
At that short viewing distance both S8 & Std 16mm should look good but scope on 16mm will be VG & similarly on S8 if you go to scope on that unit.
I have a 25mm lens for my Elmo 16CL xenon but rarely use it & a lens that size FL will give you a 1.3M wide Std 16mm image & 2.6m wide for scope.
So there were 25mm lens made to fit the Elmo 16CL series you just gotta find one.

Posted by Stuart Reid (Member # 1460) on January 22, 2017, 12:58 AM:

All very useful information. Thank you all.

Posted by Maurice Leakey (Member # 916) on January 22, 2017, 02:28 AM:

Stuart
As you are going to spend time, and money, on your project, I do think you should reconsider reorganising your internal layout, perhaps just the one all purpose room.
3.5m is really too short a throw for a decent picture.
Why not use the 5m (16ft) length for your throw?
As space is limited, perhaps you should not have a separate projection room.

Posted by David Skillern (Member # 607) on January 22, 2017, 03:53 AM:

Hi Stuart,

my screening room is 18ft by 8ft with a 6ft by 4ft screen - no partition - but sofa acts as a partition for my bookcases and unit where I keep all films and projectors - 8mm,16mm and NEC LCD projector - which is linked to a Dolby DTS Yamaha sound Processor - the "Shed" that's in the garden is for my daughter and her Drum kit - so at least I get my Screening room back

Posted by Stuart Reid (Member # 1460) on January 22, 2017, 04:40 AM:

Maurice, I think you're right. Would have been nice but it's just not feasible is it? That extra 1.5m would really help the throw wouldn't it?

Posted by Steven J Kirk (Member # 1135) on January 22, 2017, 05:02 AM:

My throw is 3.5m. Use a 25mm lens on 16mm for a good-sized 4:3 image but if you can arrange a longer throw do it. On scope you may find you have to use 38mm as I do to give a 6ft wide screen image. A 25mm may not clear the inside of the barrel ( vignetting ) of a scope lens. If you have the choice of a longer throw for 16mm you will avoid much hassle. It can be done but keeping the same vertical height for all aspect ratios I have not found possible.

Posted by Maurice Leakey (Member # 916) on January 22, 2017, 05:13 AM:

Stuart

A throw is measured from the picture gate to the screen surface so your maximum available internal length of 16ft is, more or less, reduced to 15ft.

A 16mm projector with a 1.5" lens at 15ft will give a 4x3 standard ratio picture of approximately 44" x 33". A cinemascope picture with the same 1.5" backing lens would be twice the width, but still the same height.

The Beaulieu's zoom lens can adjust to fit the 16mm picture.

I hope this gives you some help with your decision.

Posted by Stuart Reid (Member # 1460) on January 22, 2017, 05:35 AM:

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