Letus Extreme...Can I use a Nikon Zoom?

[Paul Frederick]

I have ordered but not received my Letus yet. I'm wondering if I can use a Nikon zoom lens on it? I have not heard of many doing this but to me it would be a good solution to changing lenses all the time to re-frame your shots.

Can't wait to get it and start shooting!

[Aaron Pinto]

Yes, you can use zoom lenses.

[Bob Hart]

You can use a zoom lens with groundglass adaptors, however you may need to set the lens at its correct position relative to the focal plane for the backfocus to be correct. If this is not correct, the lens will not hold sharp focus through zoom movements. Methods for doing this are published at the Letus website.

Zoom lenses also tend have available wide apertures of no more than f2.8 and many are not of constant aperture through the zoom range and tend to become darker at the higher zoom numbers unless the camera video gain controls are left on auto to allow the camera to compensate.

Todays zoom lenses are often good performers resolution-wise but it is a general rule-of-thumb that prime lenses of equivalent build quality will perform better in terms of contrast, sharpness and available wider apertures.

Finally, the use of groundglass adaptors is intended to add value by enabling the depth-of-field and field-of-view effects of 35mm motion picture imaging.

The use of a zoom lens can tend to be only as a framing aid to avoid picking up the camera and tripod and moving it to the best position for the composition of that particular shot.

I am not saying this is your own practice but the convenience of zoom lenses can be addictive, especially if time is running short.

Another detail you will encounter is the tempation to shoot everything with the lenses wide-open to soften backgrounds and foregrounds. Have faith, for this tempation will eventually pass in favour of creatively selected apertures for narrower or deeper depths-of-field, driven by the subject itself.

Enjoy the zoom but treat yourself to a good f1.8 or wider prime lens when you are able. You will retrospectively discover that your efforts to add value to your work thus far deserve it

[Paul Cascio]

Is there any way to control the aperture on the newer 35mm Nikon lenses?

[Paul Frederick]

Thanks for the responses. Bob can you elaborate on this statement?:

"however you may need to set the lens at its correct position relative to the focal plane for the backfocus to be correct. If this is not correct, the lens will not hold sharp focus through zoom movements. Methods for doing this are published at the Letus website."

Are you sayying if I change my framing with the zoom lens from say one end of the zoom to another, wide to a tight shot, then I may have to tweek the back focus? I'm not planning to be recording during the zoom movements, just to re-frame between shots. Why would the focal plane change by just changing the zoom composition?

I do suspect you are right about wanting to shoot everything wide open! I can't wait to get that depth of field that I've been sorely lacking since moving to small format cameras.

Thanks again for the info...like I said I don't have the adaptor yet, so maybe some of these questions will answer themselves when it arrives.

[Bob Hart]

Paul Frederick.


Backfocus or collimation is the process of making a very fine adjustment of the lens mount or an internal adjustment in the lens itself to make sure the lens image falls exactly on the focal plane, in this case the textured surface of the groundglass.

In layman's terms, the deeper the depth-of-field on the subject side of the lens, the narrower the depth-of-field at the focal plane end. Wide lenses have inherently deep depths of field at the subject end and correspondingly very narrow depths-of-field at the focal plane.

At the long end of the zoom, the subject depth-of-field becomes narrow and the depth-of-field at the focal plane deepens.

There are other wrinkles in the process which make this simple description more complicated which I won't go into as I don't understand them myself.

The lens can be made to focus sharply on the focal plane but the witness marks on the barrel will not tell the truth and you will not be able to rely upon them to set focus without checking the camera viewfinder.

On the long end of the zoom it won't matter to much but at the wide end, if the back focus is out too much, you may never get the sharpest image if infinity focus cannot be achieved before the lmits stops on the focus ring are reached.

For the zoom to stay sharp throughout its excursion, then the backfocus must be spot on.

Why the backfocus must be spot-on is that the zoom lens cleverly changes its focual length during the excursion and widens or narrows its field-of-view to your choice. "Cleverly" defines its ability to do all this whilst maintaining the image in sharp focus on the focal plane.

Another more corrrect name for the zoom lens is "varifocal lens".

Move the relationship between the lens itself and the focal plane and the lens can no longer do the cleverness its engineers intended.

Unless you are doing dynamic zoom movments during the shot, it does not matter so much. Focussing sharply at the wide end is always difficult due to the poor resolving ability of camcorder viewfinders and you may find yourself wishing the witness marks were true as a backup check.

Video cams such as those used in ENG work with removable lenses have a backfocus adjustment built into the lens. The adjustment mechanism is also is used as a "macro" function with another small control lever.

With 35mm format still-image cameras and their lenses, precise build quality and robustness alone are used to establish and maintain backfocus.

Because the moving groundglass is a dynamic part and many adaptors providing a variety of lens mounts, practicality demands that the lens mounts can be adjusted where the mount attached into the adaptor body.

The P+S Technik Mini35 is alone among adaptors in that it uses build quality and robustness alone to establish and maintain correct backfocus. One would expect no less from former ARRI engineers. Of course there is a piper to be paid and that is reflected in the cost.


Paul Cascio.

With the newer digital Nikons which do not have a user adjustable aperture ring, you have to wedge the little lever in back of the lens open.

By making different sized precise plugs to shove into the space between the little lever and its end limit, you can preset the aperture of the lens but cannot ever do it on the fly.

I can't speak for other Nikon digital lenses but by this method I can get good results with the Nikon f4 12mm-24mm zoom. I wedged this one wide-open as it tended to vignette on tighter apertures.

There is only one stop of headroom before the generally accepted f5.6 limit for groundglass work is reached. With the backfocus correctly set, this lens is quite sharp but the backfocus must be absolutely spot on. On the older Letus35s you are one stop in the bad zone for certain lighting conditions.

On the new "Extreme", things may be different as the artifact is related to the coarseness of the groundglass texture versus the narrow section of the elliptoid groundglass excursion.

The "Extreme" has a finer groundglass texture and more transmissiveness. Better results than the original Letus35 have been reported on this forum.

This clip was done using a homemade AGUS35 with backpolished groundglass which closely approximates what I understand the LETUS EXTREME groundglass coarseness to be. The colour rendition is muddier than the LETUS will be.

http://www.youtube.com/watch?v=WokhLQv7fQE

I hope these comments help but don't regard them as gospel as I am not an optical engineer.

[Paul Frederick]

Bob,

Your comments do help...Alot...Thanks!. I'm sure I'll have more questions when my adaptor arrives later in the week, but you sure answered a bunch in your detailed response.

[Mark Kenfield]

Paul, as Bob said you have to be careful with the Nikon zoom lenses you choose. In Nikon's literature their "aperture ring-less" lenses are called "G lenses". Avoid these lenses, because not being able to adjust your aperture on the fly will be incredibly frustrating.

Also if you do go for zoom lenses, only get constant aperture zooms (ideally the faster f2.8 ones). my recommendations would be the 28-70mm f2.8, and the 80-200mm f2.8 - there are newer versions of both these lenses (a 24-70mm and a 70-200mm) but both of the new ones are G lenses without aperture rings.

Hope that helps.

[Paul Frederick]

Mark,

The one I currently have is the Nikon35-70mm 2.8 lens. It's an older model so I hope it will work OK. I'm expecting my Letus today! I'll report my findings, (and ask more questions!) when I can get it set up and working.

Thanks for the info guys!