35mm Adapters: Construction; Function; Pros/Cons

[TimurCivan]

Some people are still somewhat confused about what makes a good 35 adapter. Im going ot break down how they work, what they do and what the pros and cons of the different designs are. I am not neccesarily going to Review each individual adapter in this thread. But rather im goin to try to encapsulate how the whole process works. This is a Work in progress and will be continually updated (with better illustrations in the future.

Essentially The 35 adapter is an on the fly telecine. Youre recording an image off of a small projection. Now to better understand what exactly is happening, lets examine how 35mm film camera works. The imager, is in a film camera, obviously the Film stock. The light coming through the Lens projects an image on the surface of the film, burning and image into the light sensitve chemicals. There are a few importhant things to note here. There is a direct proportion between the Size of the imager, the focal lenth of the lens and the percieved field of View of the final image. Tiny tiny 1/3" CCD imagers like on our HVX200's, Dvx100's , XHa1's and XL2's need small short focal length lenses to produce the same field of view as their much bigger counterparts.

This is basically whats happening inside a Film camera.

Though in reality, the film goes in a downward direction... I was tired when i made the drawing... but it still serves the purpose.

The larger lens is projecting the image on a large peice of film.


Now one of the desireable side effects of having a large imager and longer focal lengths is a "shallow Depth of Field". Optically the area thats in focus is thinner spatially because of the long focal lenth. ( the apeture plays a part in this too, but lets leavethat out of the discussion for right now)

This is why in the "movies" you see thoes beautiful soft backgrounds, perfectly out of focus. it adds texture and depth to the image. Because of the kind of sensor that on the Video camera, and their repsective short focal length lenses, achieving this look is difficult to say the least.

A few years ago, some genius over at P&S Technic thought of a way to create a "live" telecine. where the Video camera was recording the image as it was produced, not from projecting film, but instead from the lens directly! The Mini35 was born.

How they accomplished this was, they created a contraption that placed a frosted piece of glass at the exact focal flange distance ( the distance that the film would have to be from the lens so the lens can reach sharp focus on the film at infinity), then having a video camera record the projected image.



Now here is where a few issues came into play. one was, how to remove the grain that was visible from the frosted glass. Their solution was to create a spinning piece of glass. however the early units spun in place, around the center of the image, creating a swirling dot in the middle of the image, they soon uprgraded to a Vibrating gg. This eliminated the grain, and created a smooth clean image, that the Video camera could record. The other problem was that most video cameras couldnt zoom in and focus close enough at the same time to fill the Video cameras sensors with a full frame image. This is when they emplyed an Achromatic Diopeter to the contraption. This enables the Video camera to focus on the ground glass. However, in most 35mm adapter solutions, the achromat is usually the culprit for poor image quality. They can vary wildly interms of optical quality. Good ones have almsot no effect on the image, and poor ones can cause vingetting, barrel distortion, resolution loss at the edges of the frame and sometimes even cause some light loss. Another quirk of the 35 adpter process is that the nature of optics, the image projected on the Ground Glass is upside down. Thus the image recorded by the video camera is upside down. Now normally in the video cameras CCD's the image projected when shooting is also upside down, but the circuitry rights the image before recording to tape/p2. P&S technic used an optical prism to flip the image over, and make it easier to work with. As of the time of this writing only the P&S Technic, Movietube, and Letus FE are prism flipped.

So now that you know how it works, lets look at the different designs more closely. There are three options for making the adapter work. The static GG, the Vibrating GG, and the Spinning GG. Each has its merits, strengths and weaknesses.

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Static Ground Glass:


This solution is the "simplest". The Ground glass is made of a wax or acrylic type polymer that has very little visible grain when the image is projected on it. I own a go35 pro, and it produces VERY sharp, very pleasing pictures.

Available on:
Ifocus 35
Go35 Pro
Cinemek 35* (never released)
Movietube

Pros:
1: The adapter can be very small. basically just a tube of metal with the components inside.

2: Sharp picture.

2: No batteries, as it require no power.

3: No noise, because it has no moving parts.

Cons:
1: Static grain. When you use a static adapter the enemy is static grain. There is an optical phenominon where at wide open apeture on the 35mm lens, there is no Static grain. but when the lens' apeture is stopped down, magically the grain appears. These adapters arent too great in HD because of Static grain. its more apparent in High Definintion.

2: Dust. Dust will Kill your image. it sticks there like a hair stuck in the gate and is sharp in focus. Always keep compressed air with you.

3: sometimes the Ground glass material is very fragile.

4: Support. Static adapters being so small often dont provide any kind of support other than their direct attachment to the front threads of the Video camera. Thsi puts unnecesary stress on the cameras fragile lens assembly. External support is required at additional costs. (excludes movietube as its a system that cradles the camera)

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Vibrating Ground Glass:



These adapters are usually VERY similar in design to the static GG adapters. however the real difference is that the Ground glass material is mounted on small springs, and has a tiny motor that oscilates the GG diffuser, eliminating that static grain that can ruin the shot. Often these adapters are very small as well.

Available on:
Letus 35 (all models)
Brevis35
P&S Technic (current model)

Pros:
1: Good balance between performance and size.

2: Not effected by stopping down the 35 lens, like static solutions are.

3: Good battery life because teh motors are so tiny.

4: Sharp picture.

5: Easy to handle hand held. If much care is given, and the rail system si removed, the cmaera can be much easier to use hand held than any other system.

Cons:
1: Because teh Ground glass is vibrating, the gg literally moves a mm in onedirection, stops, then moves back etc.... its constantly moving around, and changing direction. IF you use a high shutterspeed, like 1/120 or above, sometimes the camera can freeze the GG mid swing and a swirling alias becomes visible on screen. Under normal circumstances this is not a problem but it is inherent in the design when pushed to extremes.

2: Support. Vibrating adapters being so small often dont provide any kind of support other than their direct attachment to the front threads of the Video camera. External support is required, for safe use. Excluding P&S technic as its a system cradling the camera.

3: Halation of highlights. Some vibrating solutions, because of the vibration, and the material of the GG can cause a flaring of highlights. usually remedied by stopping down the Iris on the 35 lens, or using external ND filters.

4: Dust. Dust on the GG in a vibrating design will cause a spot of black blur on the image. Always keep compressed air handy.

5: noise. Vibrating adapter emit a low hum.

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Spinning Ground Glass:



These adapters are a whole different style. The Ground glass assembly actually spins a disc around at a very high speed. The GG assembly now sits in a box, that is mounted on 15mm rods, and is attached to the camera via a baseplate of somekind. The camera peers through the Achromat, at the GG disc. Sometimes the camera's lens need not be physically attached to the achromat like the static/vibrating solutions. Though it is reccomended to prevent the system from flexing and throwing the alignment off.

The Disc is made of a translucent material like the static and Vibrating solution. The image is projected on the disc, and the camera reads the image directly off the disc.

Can be Found on:
SGPRORev2 (and soon Rev3)
Redrock Microsystems M2
WDR35

Pros:
1: Imaging Flexibility. This system, because of the continuous, steady and VERY fast motion of the GG disc, is far less succeptible to grain, dust, and artifacting from high shutter speeds. Infact, the Iris can be closed down to F22, and the shutter speed be increased to 1/250th a sec, with certain spinning adapters, with no visible GG artifacts, other than underexposure...... obviously.... The system offers the cinematographer a more wide range of choices in the shutterspeed and apeture settings. Critical to achieving the desired look.

2: Image quality is usually high. Sharp and with little grain.

3: Sturdy. Becuase of the rail mounted design, there is far less stress on the camera lens because of the weight of the adapter/35mm Lens combo. The GG box usually bears the weight of the lenses, and it inturn is beared by the rail system.

Cons:

1: Unweildy. Because the GG box, rails and lens has to be attached at all times, the whole rig becomes large, heavy and difficult to manage handheld. Shoulder braces, like a DVrig, or Spider brace become a neccesity.

2: Noise. Sometimes spinning adapters can make a low "whirring" noise.

3: Size. The whole assembled rig becomes large and heavy. Sometimes requiring a step up in tripod head.

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All in all these are what to expect when using various adapters.

All of these adapters perform well, but the one thing that will make any adapter perform beautifully is a really good lens. Great glass is the heart of the system.

From experience, My Sgprorev2 system, is so resolution loss less, that the limiting factor on the quality of image i get on the P2 card, is the quality of the lens i put infront of the adapter. So always get good glass.... the best adapter in the world is nothign without a GREAT lens infront of it.

-T

[Attic6]

I have one quick question, i'm sorry if i ruin the value of this thread with my post but, What if instead of using the achromat to basically zoom in on your ground glass, you use the opposite to enlarge the projected image onto the GG? as long as you are not using a SLR GG or spinning cd balnk GG, you could make the GG up to 2x the usual size in theory (excluding the optics to get the projection that big) and that would be much easier to focus on with big front end cameras like the dvx, and would it not make the grain less apparent which would benifit the HD guys? It seems practical to me except i'm not experienceed enough with the back end and projection of stills lenses.

Sorry, don't mean to hijack your thread.

Jimmy

[TimurCivan]

theoretically you could, but you would lose too much light.

[Attic6]

so the 35mm film size is the sweet spot.
I'm gonna give it a try at some stage, not that i doubt you, just that i think its a challenge...

[Nathan Beaman]

Great breakdown Tim!

[4mat]

Great read Tim and well done for putting everything in context for us.
I can't help but wounder if theirs not a better material that could perhaps replace the GG element and thus solve the hole grain issue. Idealy a material with no grain what so ever but rarther a solid tanslucent material of some type.
I'm sure this has been hevaly researched already i'm just curious.
-matt

[TimurCivan]

yea thats what the Cinemek/Go35/Brevis guys are workign on. grainless diffusers. but its difficult and expensive to research.

[4mat]

I'm sure it is, perhaps the research should be made public however perhaps things have got a little too lucrative for that now.

[HVXTiff]

Thanks for this awesome write up!

[JimiK]

Good job Timur
Question
Do they give you anything else besides DOF?Claerer image,sharper......etc?