A1 Iris Bloom (again) at DVinfo.net

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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old May 18th, 2007, 11:25 PM   #1
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A1 Iris Bloom (again)

I recently posted about my seeing a slight iris bloom in low light when at the end of the lens... no one replied. Has anyone else experienced this?

I had another shoot today where this happend again. It's very subtle, but it is there. Not only do I see the iris slightly bloom or shift, I see the actual numbers roll up. Today it went from F 1.6 to F 2.4 without me touching anything (beside the zoom control) - AND I'm in manual. Manual everything. At least I think I'm in manual everything.

Situation: I'm at a car dealership in the showroom. Available light. I zoom into a car to get focus (I'm at the very end of the lens) - and when I pull back out there is a subtle iris bloom or change. I see the numbers move too. I put the gain to 0 db (I typically shoot at -3 db). I zoom in again and same thing happens. The light is not bad, but it is slightly low. I check my settings and everything is in manual (including "auto gain"). I roll tape and do it again and IT GOES TO TAPE THIS WAY. I checked it back at my studio and I see the iris move. I've never experienced this before with other cameras.

Is there something else I'm missing? Should something else be "turned off"? Or is this just a function of the Canon lens in low-light situations (at the end of the lens)?

Anyone else experience this "phenomenon"?

It's not a huge deal because I try to be in good light, but shooting b-roll sometimes you're not...

Thanks for the help,
Blake
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Old May 19th, 2007, 12:41 AM   #2
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In the future, it's adviseable to not start another post with the same question. Just reply to your original.

What you're seeing has been talked about here before. But I think you calling it "blooming" had me confused & may have others as well. Blooming to me is a totally different thing.

This unfortunately is normal from what I understand with servo iris controlled lenses. The A1 will close to f3.4, if I recall, at full zoom in. This is the 1st camera I've owned that does this also... but there's nothing that can really be done about it, other than accepting it & just be aware of it.

Bill
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Old May 19th, 2007, 01:48 AM   #3
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Just a slight correction: variable max. aperture isn't isolated to servo-only lenses. Often called the "speed" of the lens, many non-servo video and stills zoom lenses have similar characteristics, and is a function of the size of the front element of the lens (among other things).

Take a look at pro sports photographers on the sidelines of games lugging around those mongo 300-400mm lenses with the giant front elements. That would be the size of glass you'd need to get a 20x zoom with a fixed 1.6f on your A1. Not to mention, the lens alone would likely cost more than the current asking price of the A1.

Anyways, to avoid the lens stopping down on you as you zoom out, determine beforehand how far out you'll zoom and make sure that max. aperture is sufficient light for the shot. Then set the iris for that aperture at the wide end prior to zooming.

HTH,
Brian Brown
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Old May 19th, 2007, 01:59 AM   #4
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The XH-A1 lens is not a constant aperture lens, which means maximum aperture changes with focal length. The lens is capable of f/1.6 at wide angle, but only f/3.5 at full telephoto. Annoying yes, but that's lens physics - a constant f/1.6 lens would be HUGE.

I've only had my A1 for 2 day, limited experience. But I think the solution is to predict the max. focal length required for the shot, note the maximum aperture associated with that focal length and manually selecting it for use through the entire scene.

Edit: This is not an echo. Was typing as Brian posted his response. :)
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Old May 19th, 2007, 11:00 AM   #5
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Thank you for the info guys! I just recently got my A1, and as mentioned, have not seen this before on other cameras. Of course I understand that when you're zoomed all the way in the light is less, but I had just never seen the iris actually move (when in manual mode).

The other cameras I have used/owned had traditional broadcast style lenses.

And sorry about the confusion on the word "bloom" - that's just what I call it when the iris moves as sometimes seen when on auto exposure.

Thanks again and I'll keep all this in mind for future shoots.

Blake
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Old May 19th, 2007, 05:39 PM   #6
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The DSR250/PD150/PD170/VX2100 do the same thing, and I think all the fixed lens cameras want more light when zoomed in too far. The lens stops down when you zoom in beyond a certain point if you're shooting fairly wide open. This usually isn't even noticed when shooting outdoors because you're generally stopped down beyond the f-stop of what the lens is at full zoom, so when you zoom in it won't change; it only changes if you're opened up beyond a 3.5 at your wide shot.

When it is a problem is, say, you're shooting an interview and want to shoot wide open to minimize depth of field. If you zoom in to change the focal length, then you have a problem. What I do is get close enough so I don't have to zoom in much for the ECU. Sometimes I may have to move the camera itself in closer or farther away.
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