Are Canon "L" lenses "too good" for the Letus

[Dan Keaton]

I am trying to build my set of primes for the Letus35 XL.

Will the best Canon "L" lens be a waste of money, in other words will the features/resolution/etc. of the "L" lens be wasted since the image will be focused on ground glass?

Or will the superior sharpness at full aperture of these lenses be worth the extra cost?

I am considering the purchase of:

Canon 50mm 1.2 L and
Canon 85mm 1.2 L

[Bob Hart]

My personal preference is the sharper the best. Night vision tubes, although the diffusion effect is a different mechanism are an even more effective demonstrator of an amplification effect of relay diffusion on existing deficiencies of a lens image namely apparent sharpness and contrast.

Subjectively and an illustration only, to my eye, a notional 1 : 1 would define a perfect relay. If 0.85 : 1 represented a good sharp lens, then 0.65 : 1 would represent the performance of say an average zoom lens or lower quality prime. Direct to camera film and printed, they might all appear to be in the ballpark of 0.85 : 1 but the contrast and colours of the sharper lenses would be better.

This is only meant to be a word picture of what I see. There is no science in the numbers I have used.

[Dan Keaton]

Thanks for the information.

I am looking forward to using the Letus35 XL in a controlled shoot.

[Ben Winter]

I have tried both a regular 50mm 1.4 and a 50mm 1.2 'L' series back-to-back and can't see how anyone justifies the price difference. I suppose if you're real nit-picky about your image, but ultimately I'm perfectly satisfied with my current set of standard primes.

[Dan Keaton]

Dear Ben,

Thanks for sharing your experience.

[Bob Hart]

Ben has a valid point. Depending on how much value you wish to add to your finished product, it could be a case of over-capitalising if you buy in a more expensive lens for what would really be a very small percentage gain.

No doubt you will know more than myself from a practical standpoint in camerawork.

If you have control over lighting, my personal preference would be to light for base settings of f5.6 on the Letus, 0db gain, 1/50th-1/60th sec shutter and f4 to f5.6 on the camcorder.

Then in order of priority, shoot say f2.8 - f3.5 on the Letus unless you want the shallow depth of field to be extreme or you have a static subject. (This might seem counter to the objective of shallow DOF but f1.8 can be difficult to manage on a moving subject and tighter iris keeps you in the sharper region of lens performance.)

Then manage your exposure levels with the camcorder's own controls, which include that -3db which my own does not have, then remove some lighting if things still roast.

Don't close aperture on the Letus (late correction - this should read SLR lens on front of Letus) more than f4. This may bring up soft fixed pattern artifacts which will become apparent on zooms and pans. There is a bit of an inrteractive slowdance with most groundglass based relay devices between light available, aperture setting and light required.

The final boundaries are the point at which groundglass artifacts appear due to the tightness of SLR lens aperture when bright light is to be controlled and the point where camera gain causes video noise when low light requires gain to be ramped up.

Gain noise from a MiniDV camcorder is an aesthetic consistent with grain from high speed film used under low-light conditions or push processed. Gain noise is not desirable with HDV as it introduces softness due to the codec attempting to cope with fine detail variations between frames.

If you can get hold of a monitor screen to use on location, I strongly recommend you do so. You wont detect soft fixed pattern artifacts without it. Actively search for these artifacts by panning across soft large-area objects at various SLR aperture settings when doing each setup.

Manually white-balance each setup on the subject.

I'm no doubt suggesting things you already know and have decided about, however there's the telling anyway.

[Dan Keaton]

Dear Bob,

Great Post! Your information will help me get started in the right direction.

It would have taken me quite some time to figure out what works best.

I will have controlled lighting and I will be shooting in a controlled environment, like on a film shoot.

Bob, I know I will be coming back to you post many times to ensure that I get all of the tips that you offered.

[Bob Hart]

Dan.

Don't put too much stake in my word stuff. I am not an industry professional as such and with the Letus35, basically I have kicked the tyres, repaired one and shot roving cam on an XL2 for a music video with it.

I like that it can be operated agile-portable with the XL2 but the weight on the lens mount makes me a bit nervous. Compared to my own device on a FX1 or PD150, I did not feel like I had laid down in front of a truck and let it run over me at the end of a 1/2day of operating the combination.

In composing and lighting your shot, where there are intended blacks or deep drop-offs there is no problem, just create and control them with 0db to -3db gain and lighting.

If there are pinpoint highlights or small patches of overbrightness, the groundglass will help your camera cope with them. Larger areas will burn out but less savagely than direct-to-camera.

Vertical bars from overbright pinpoints, the signature of CCDs, tend to be reduced, if not eliminated by the groundglass and replaced by lens iris stars.

In the areas where you want to paint visual information into the image, it is helpful to try to limit the light return off all the intended highlighted subjects and backdrops to within a three or four f-stop range, ie., try to limit the contrast across them.

You may find you need to use subtle colours in lighting to set one subject off against another if they are similarly coloured. This seems to be a modern lighting style in any event.

You will find that groundglass based imaging seems to perform better in artificial lighting environments. It is the nature of this lighting that there is less contrast across the items which matter.

Outdoors can be a very different beast. High light bright overcast conditions with overhead sun coming towards the camera and use of an f4 12mm - 24mm lens provokes a severe groundglass artifact on my own home-made device, especially with browns and reds.

The f4 leaves only one and a half f-stops of "headroom" outside of the f5.6 which seems to be the magic cut-off point and I found that changing the SLR lens and camcorder aperture only moved it about from ground to sky or a car door.

You will discover that SLR lens aperture (iris setting) and camcorder aperture, interact relating to groundglass artifacts. In practical terms I think you will find you need f3.5 or wider on the SLR lens to safely avoid any soft fixed pattern artifacts, especially outdoors.

The Letus seems to show its artifacts against blue sky or light coloured bright walls. My own device doesn't like reds. I have yet to figure that one out.

The spinning disk devices like my own seem to display their groundglass artifact as flicker or streaks moving across the screen and the onset is usually easy to detect. The Letus artifact is more subtle and harder to impossible to see in a camera viewfinder.

A rule-of-thumb might be, light for f5.6 on the SLR lens, 0db gain, 1/50th sec shutter, f5.6 on the relay lens, then open the SLR lens two or more stops and adjust the camera and relay iris to restore correct exposure, pan across broad bright areas to check for fixed pattern, re-adjust settings to eliminate fixed pattern as much as possible.

If actors or objects are to move through the shot, check to see if any soft fixed pattern is evident on them as they move through, ie., white shirts and blouses. Different costume choices may be required.

If the shot requires a pan across background to follow a move, try to choose a background which is darker, not light than the subject and preferably is textured, like forest or stony roadside excavation so any artifact is buried in the texture.

Contrary to best practice, if you have difficulty eliminating an artifact against a textured background, you may find you can use a 1/100th sec shutter. This will confer the collateral benefit of holding the subject you are following sharper. The texture or the background will be more sharply defined but also juddery.

The visual confusion caused by this will tend to bury any artifact even if it is aggravated by the higher shutter speed.

There will be an optimum panning rate for this to happen and you may need to choose a wider or narrower lens and adjust the camera to moving subject position and distance of the moving object to the background to get the motion of the background across the frame and the scale of the background texture just right.

An artifact in a light bright soft area, might only be minimised by burning it out with a wider aperture.

[Dan Keaton]

Dear Bob,

Thanks again for another great post. I am amazed how much I am learning about using the Letus35 XL prior to actually receiving it!

[Bob Hart]

Dan.

I did not realise you do not have the Letus35 in hand. You will need a fair bit of practice before you try to shoot something serious with it as it takes a while to become intuitive with the extra controls and interactions.

Perhaps a summary of functions is a better approach.

Starting from front-to-back - description of controls and functions of the assembled combination follows :-


SLR STILL-CAMERA LENS. >>

Lens focus control - FUNCTION: Primary focus of the image onto the groundglass. Does the same job as the original camera lens.

Lens aperture control. - PRIMARY FUNCTION: Between f5.6 and widest aperture selects the depth of field. SECONDARY FUNCTION: Assists control of light entering the camera when all other controls run out of range.


>> LETUS35 ADAPTOR UNIT. >>


BATTERY BOX - FUNCTION: Power switch and power supply is located here.


RELAY LENS. >>

Lens focus control - FUNCTION: Sometimes called "backfocus", correctly called "relay focus" in this application.

This adjustment enables the groundglass and the image projected on it to be focussed sharply into the camera. (This is not the focussing control for the the image on the groundglass but must be maintained at its sharpest setting to preserve resolution of that relayed groundglass image.)

Lens aperture control - FUNCTION: Also called "adjustable neutral density filter" in the P+S Technik professional products>

This adjustment is the primary controller of light entering the camera. (This control is also used interactively with the front SLR lens aperture control to compensate for changes in light entering caused by choices of depth-of-field.)


NOTE:

On the XL family of cameras, automatic focus, optical image stabilisation, camera contolled aperture and neutral density filter functions reside in the Canon lens and are removed, therefore disabled by replacement of this lens with the LETUS35 appliance.

A telltale flashing message "check lens" seen in the viewfinder is the normal operating state when non XL lenses of adaptors are fitted to the XL camera family.

In the unlikely event that the available controls cannot manage excessive image brightness, disks of ND filter gel need to be made and preferably added into the image path between the front SLR lens and the groundglass.

The simplest method is to insert them into the space behind the lens inside the mount on front of the LETUS35.

It is prudent but not mandatory to tuck a piece of gaffer or insulating tape approx 1/4" x 5/8" over the conductive pins inside the Canon XL lens mount to minimise risk of short circuit to the solid metal mount of the LETUS35 or other non XL lens mounts.

This might occur if the pins have been loosened or damaged and then stick out furthur than they should. Don't forget to hook it out with finger or soft non metallic probe when you put the XL lens back on.

Gum from the tape might need to be cleaned off the pins if the camera reports a fault after the genuine lens is fitted back on to the camera.

[Dan Keaton]

Dear Bob,

Thank you for the nice, clear instructions. I am sure many user's will be reading your instructions.

In reference to the use of best quality lenes, such as the Canon "L" series, I had an interesting insight while speaking with a friend last night.

The Canon "L" series, in my opinion, were better than the regular lenses in multiple ways. One way was that they were sharper edge to edge at full aperture.

This is interesing when considering their use with the Letus35 XL adapter in that the edges of the frame are not used by the Letus35 XL due to the 1.9 maginification factor. At least this is my theory at this time. I am open to other opinions and discussion.