Letus Extreme - need your help

[Alvin Witcher]

Hi,
I test film with my canon xh a1 / letus extreme / canon fd 50mm.

Why I don't get sharp picture?

Look at

This is a password protected video on Vimeo

It short less 2 min.

password is: chp

THX

[Bob Hart]

NOTE: A little more information has been added in the last edit.

On first impression, it seems you are shooting with everything wide-open to get the light you needed, camcorder iris wide-open? Canon 50mm lens wide-open?

Most stills lenses are not in their sweet spot for sharpness and contrast wide-open. The FD could be showing its age here. My personal preference would be to light for about f3.5 and use a f1.4 or f1.8 lens set at about f3.5. Ideally there should be enough light to enable the camcorder iris to be at least f2.8 and better in the zone f4 - f6.3 which seems to be sweet for most camcorders.

This clip is with the Letus Extreme on a HDV cam, the Sony Z1 in a lower light environment with lenses on front and camcorder iris opened up wide.

http://exposureroom.com/members/DARA...253f62ecdd2be/

F3.5 would help keep your image sharper through having a manageable depth-of-field. Depending on how good the camcorder lens is, using its iris wide-open may also introduce some softness and flare.

Relay focus is also something you have to manage to the point of obsession. Trimming relay focus by eye without a larger monitor screen, just the viewfinder or LCD screen, without a focus chart to help is is hard enough. Without one, it is pretty much impossible.

The recommended method of setting relay (camcorder) focus is to set the front lens to a tighter aperture to provoke the groundglass texture into visibility with the motor stopped and open the camera iris to its widest to narrow its depth-of-field to ensure the most accurate focus, then reset your irises on front lens to wider than f5.6 and trim your camera exposure with iris and ND filters. If you can afford to buy and use matteboxes and ND filters in front of the front lens, this helps even more to manage flare and enhance apparent sharpness and contrast.

I argue against relying totally on setting relay focus by focussing on the "sandpaper" texture of the groundglass. This textured surface from peaks to valleys can be up to 5 microns deep and the ideal image plane may fall somewhere in between. A very fine focus trim to where the best image plane resides within that 5 micron zone will achieve that little be extra. Only a resolution chart will get this for you.

After an initial setting by focusing on the groundglass texture, I then trim with a resolution chart. While the LCD on the camcorder will not resolve the finest lines the camera can see, it will produce a moire pattern on the sharpest position.

How far you are zoomed in onto the groundglass will also affect apparent sharpness. The most practical groundglass texture as I understand it has a "grain" size of about 5 microns. This limits the resolution available from any groundglass. The smaller the area of it you view, the lesser rendition of detail there will be.

The ideal is to zoom back until you pick up an edge of the prism path, then zoom in a little so that the edge is just on the LCD frame edge, then zoom in about another 5% to 8% depending on camera type, to ensure the whole sensor image is clear of stuff in the optical path. Camcorder viewfinders typically show you a little less than what the camera sensor sees and you can end up with something in the edge of the image you do not see in the viewfinder or LCD.

In this clip with the Letus Extreme on an EX1, I was slightly too wide with the zoom in chasing sharpness in the image and you see the edge of the prism path which was not visible in the LCD screen.

http://exposureroom.com/members/DARA...d6b3f40a2cadd/

You were moving around in the room and not trimming focus on the Canon 50mm lens so a lot of your image was out of focus to the groundglass. What you are able to get away with in poor focus on a bare 1/4" or 1/3" camcorder you cannot with a 35mm adaptor or larger format camera like a Canon 7D or 5D

For best results with a Letus or any other adaptor, handheld agile is not the way to go without rehearsaing the shot and another person pulling focus for you.

All the value adding you aspire to in using a 35mm adaptor does not come with the adaptor alone. You need good composition of your shot and proper lighting techniques. The 50mm would be the wrong lens to use in an agile mode like you were doing. A 14mm f2.8 or 24mm f2 might be more suitable, depending on what you want to cover.

Any groundglass image is going to be apparently softer than a direct-to-camera image because it is. This aspect is what makes an adaptor take the harshness out of a video image.

However in the apparent resolution stakes, not all is as it seems. Most consumer or prosumer camcorders do not see as sharply as you may think. A lot of the detail is artificial sharpening of edges applied cleverly in the camera's processor. Because the camera does not see so many sharp edges in a groundglass image, it may choose not to massage to image as much. Therefore you may find you can improve your "apparent" sharpness by applying up to 15% sharpening in post without introducing artifacts.

[Bob Hart]

Alvin.


You might be interested in this older post.

http://www.dvinfo.net/forum/let-us-d...extreme-3.html

I don't recommmend you modify your Letus as it will likely get you into more trouble than benefit unless you are skilled in machining and have something close to a cleanroom to work in. Your camera's HDV resolution will likely only resolve the "B" and "G" blocks of the Lemac test chart shown so there is little point in tweaking to the max.

If you scroll down the page, you will find two test chart images from a SI2K and Sony PMW-EX1 side-by-side and furthur down, a panel of four images. The one of the toadstool will be most relevent as the groundglass motor was stopped.

The "grain" texture is visible and demonstrates the smaller scale of each "grain" relative to frame size when the camcorder lens is zoomed wide, hence ability to confer greater "apparent" sharpness.

By nature, the frame of the Letus Extreme may appear a little offset, one way or another due to variations in centricity of the lens optical axis of camcorders relative to the actual centre of the sensor.

There will also be variations in the optical path between different Letus Extremes due to design and impacts the heavy internal compound prism receives during shipping.

The size of the internal camcorder parts is extremely small. To achieve spot-on centricity would be possible but hideously expensive, not justifiable for an affordable consumer/prosumer product.

For most purposes, re-centering the Letus frame to match the camcorder frame is achievable by loosening the screws around the adaptor ring at the rear of the Letus, skewing the fit of the ring on the shoulder slightly to center the frame to the camera view with the camcorder lens zoomed back a little to bring the edges in, then re-tightening the screws.

For the more obsessive among us like myself, the bending of the two optical axes prompts us to think that one side of the groundglass image will be closer than the other and soft edges will be apparent. Unless the camcorder iris is set wide-open in poor lighting conditions, the inherently deep depth-of-field of 1/4" and 1/3" sensors and camcorder lens combination should normally take care of this.

I hope this helps.