The Iris Shifting on non fixed iris lens

Hello to everyone,

I need information.

i have mounted on URSA EF a lens with not fixed F (Tamron 28-300 F3.5/6.3), when i zoom the ursa change the iris.

example if i set the F to 11 when i zoom the ursa update the F from 11 to 22.

there is a methot to block IRIS ?

thanks in advance.
best regards

Roberto, just a guess but if you:
1) zoom to the longest focal length and then set the aperture to f/6.3 or smaller, such as f/11;
2) zoom to the shortest focal length, check aperture; and
3) zoom to the longest focal length, check aperture.

Does the aperture remain at f/6.3 or wherever you set it through the second and third zooms?

As you know, if you set the aperture to f/3.5 at the shortest focal length, the aperture will ramp to f/6.3 as you zoom to the longer focal lengths.


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Yes it will, and it is the Lens that is changing the f/stop, not the Ursa Mini camera. All still camera zooms that are not a "fixed" max f/stop will change the Iris setting when zooming back and forth. Only a fixed max f/stop zoom, like the Signal 18-35 f/1.8 will stay at the f/stop you se during a zoom change, as the lens adjusts the Iris to keep a constant F/stop setting for the camera.
DS

i have tryed on 5d mkiii and when i set the F to 11 when i zoom and unzoom the F remain at 11
on ursa (not mini) the F ramps to 22 is very strange i have tryed more lens and is the same.
please help me i need to block the F for not lose the exposure thanks.

Denny, just tried it with a Sigma 8-16 F4.5-5.6 on my Canon 400D.
If I set the lens to F5.6 at 8mm and zoom in to 16mm, it stays at F5.6.

On the UM46k:
If I set the lens to F5.6 at 8mm and zoom in to 16mm, it ramps up to F7.3!

Robert, this is how Canon and Sigma gets away with keeping the cost down in these zooms down, the lens Iris control is in your Canon 400D, which sets the f/stop in the lens, and if it is a smaller stop than the min. f/stop in the range, the camera will lock on to the f/5.6 you set, and keep it there while you zoom. The camera body and lens are working together to prevent additional ramping up. On non-Canon cameras, like the BM Ursa Mini, this feature is not available, after all, Canon wants you to use this lens on a Canon camera! The new Canon EZ Cine zoom is setup the same way, Iris control is done in (Canon) camera.

Canon does not share this info on lens control, so companies like BM have to set this up by reverse engineering, and getting this level,of Iris control is not worth the extra cost for the few lenses that need this feature.

I had the same experience with a Four Thirds Panny/Leica zoom, that was a /2.8/3.5 14-50 zoom, I got to use on my AF100 for interview shooting and doing green screen (chroma key) PSAs. I would set the lens at f/4.0 on the AF 100 (not with the Iris ring on the lens) and the lens would stay at f/4.0 during a zoom change. When I put this lens on the Micro Cinema camera (both using the same Panny MFT adapter), the Micro (and the same on the Pocket), would ramp up the f/stop during a zoom change, regardless of the f/stop setting in the f/4 to f/11 range.

Apparently, the AF100 (like Canon DSLRs) was locking the lens f/stop setting, opening the Iris during the zoom to a longer focal length, to keep it set at the camera f/stop setting. I have not tried using this lens on the Micro in "auto Iris" mode, think I will give this a try, and see if the camera can hold the f/stop in this mode (camera set Iris based on shutter setting and light hitting the sensor, to keep a constant exposure).

BM cameras do not do this -- After all, BM expects you to use professional Cine Zoom lenses with a fixed max f/stop.

Edit: I checked the Panny/Leica zoom with the Micro camera set on auto Iris, and it still ramped one f/stop during a full zoom from 14mm to 50mm. It stayed at f/3.5 for a partial zoom to around 35mm before changing to f/4 and then /4.5, on an evenly lit wet grass field subject.
Cheers

Thanks, Robert. Good to know my suspicions were justified.

Best to save for a cine zoom or buy a photo zoom with constant aperture.

The most affordable new cine zoom for Super 35 is the new Zeiss T2.9-T3.9 to be released in 2017 for only $5,500 and, if memory serves me, it ramps gradually through the entire zoom range, which is a very strange way to design a cine zoom. As I've mentioned elsewhere, better the new Fujinon 20-120mm T3.5 constant aperture, but more than triple the cost for their 'budget' zoom!

My current Fujinon cine zoom is a constant T2.4 aperture from 7.8 mm through 91mm, over that up to 156mm it ramps. As long as I stop zooming at 90mm, I'm good. That's a ⅔" B4 mount lens so we're talking crop and all so a 90mm is all I ever expect to need since I'm not stalking wildlife on mountain tops.


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Yes, the Zeiss ramp is only about 1/Tstop over the entire range, and you normally wouldnot be going from one end to the other in a single zoom, live take. Zeiss is marketing it as a "Variable" focal length lens, eliminating a lens change during a shoot, so the slight ramp up can be compensated quickly with the Iris ring or shutter speed adjustment, as you are changing the focal length before rolling the camera, and not during, as was done on ENG zooms or film camera Cine zooms, where the camera would be zoomed in to pull focus and then pull back for the shot, or a slow zoom pull was made while rolling. -- all outdated techniques today, with punch in momitors to pull focus with quicker that zoom changes, and a live zoom is no longer an "in" or used technique, with the availablity sliders, inexpensive light weight dollies, etc.

I was on set on the he bow of the USS Missouri (WW2 battle ship) for the filming of a scene for a,TV mini series, shooting Roosevelt, Nimitz and McArthurs famous meeting in front of the ships "Big Guns". The crew rigged a track dolly on the ship's foredeck, fitted a wide lens and rolled from a wide shot, dolling in for the close up of Roosevelt -- No zoom lens here. All was shot on Academy 35mm film, for transfer to video in post. My first encounter with the small baby track camera dolly they used. I thought this is great tool for location shooting.
Cheers

All true, Denny. Great experience. I do appreciate that Zeiss is very upfront about their limitations and as you mention, in practical use, it may not be a factor. Maybe I should just go with the Zeiss next year! But I would need to use it as a zoom for live event tracking where a dolly is out of the question. I might be able to squeeze that in next year or definitely the year after.


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Sounds good Rick, then perhaps you could add a Jib Boom rig, and get all the great shots
Cheers

I know I'd love to have a jib boom rig or a crane at my disposal... I think I'd better just move back to Los Angeles! Say in eight years? (Just kidding, Donald.)


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