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Effective F-Stop with 35mm Adapters


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#1 Tim Tyler

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Posted 21 October 2007 - 12:48 PM

I understand that 35mm adapters (P+S, Letus, Redrock, etc.) claim to eat up between 1/2 to 2-stops of light. They typically measure this by photographing an illuminated white card with a lensless adapter and adjusting the camera iris so that zebras are just visible in the viewfinder. Then they remove the adapter, reframe the white area with the naked camcorder lens, adjust the camcorder's f-stop so that zebras just become visible, and note the f-stop difference.

I'm wondering how to calculate additional light loss once a lens is mounted on the adapter. So if a f/2 SLR lens is mounted to an adapter with a measured 1/2-stop light loss, what is the effective f-stop of the SLR lens when set to f/2?

For example, with the HVX200, the camera lens with an attached achromatic needs to be zoomed in to focus on an adapter's ground glass. That means the HVX is at a f/2.4 (wide open). Add an adapter's 1/2-stop to that and we're at f/2.8.

If we put a f/2 SLR lens on the adapter, are we loosing another two stops of light for an effective stop of f5.6?

Am I making sense? For example:

[codebox]HVX/Adapter SLR Lens Effective HVX Stop

2.8 1.0 2.8
" 1.4 4.0
" 2.0 5.6
" 2.8 8.0[/codebox]

So would a f2.8 SLR lens require the same amount of light as the naked HVX200 lens at f/8?
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#2 Michael Nash

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Posted 21 October 2007 - 07:23 PM

I think you're going about it wrong. If I understand it correctly, the sensitivity of the camera changes when an adapter is attached, not the "effective f-stop of the lens" (whatever that means). An SLR lens set to f/2.0 has an effective f stop of... f/2.0! The exposure compensation is a ratio between the taking lens f-stop and the camera f-stop needed to hit a target sensitivity.

If an HVX exposes an image properly at f/4.0 and the adapter reduces the camera's sensitivity by one stop, the taking lens would need to be set to f/2.8 -- when the HVX lens is set to f/4.0. But if you set the HVX to f/2.8, you'd be overexposing the groundglass image by one stop. You'd compensate by closing down the taking lens to f/4.0.

"But wait, f/4.0 on the lens, that's a zero stop loss. How can that be?" Simple -- it's not a zero stop loss; it's a proper exposure by the taking lens but a wider aperture in the camera. The HVX is now shooting an f/2.8 instead of an f/4.0 because it's lost one stop of sensitivity through the adapter.

In reality, the light-scattering quality of the groundglass makes the ratio non-linear as well. Wider taking-lens apertures "fog" the groundglass, requiring a more compensation from the camera aperture, and vice versa.
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#3 Tim Tyler

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Posted 21 October 2007 - 07:32 PM

That would mean that if the naked HVX lens' proper exposure was a f/2.4 (wide open), and I added a 1/2-stop (loss) adapter and used a f/1.4 SLR lens that I'd be overexposing the image on the sensor by 1 stop? That can't be.
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#4 David Mullen ASC

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Posted 21 October 2007 - 07:34 PM

I haven't used those adaptors on a camera with a fixed lens, but I would assume that you'd keep the camera's permanent zoom lens set wide-open and not adjust its iris, only adjusting exposure on the attached 35mm lens. And then basically you'd have whatever light loss that the set-up causes to deal with, but since you generally don't set the f-stop in video with an external light meter reading, but based on the zebras & waveforms, etc., it's not like you need to calculate a compensation -- the camera just "behaves" less sensitive to light than it would without the adaptor.
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#5 Tim Tyler

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Posted 21 October 2007 - 07:54 PM

Some back story...

I did a 1-day shoot a month ago with the Mini-35, HVX, and Ultra Primes (T1.9). We moved too quickly for me to do any tests, but I enjoyed the look the adapter created. Didn't use a meter, but I felt like I was lighting about 3 stops brighter than usual due to the adapter.

I now have an adapter on order that claims 1/2-stop light loss, and I expect delivery in about a week. I shoot a fair amount of corporate interviews with the HVX, and I usually light them to a f/2.4 with Diva lights, but I can do a 5.6 without needing more lights or making the talent uncomfortable.

I'm trying to determine the practicality of a f/3.5 zoom lens for these interview shoots since producers will expect focal length changes during the interview and lens changes won't be an option.
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#6 John Brawley

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Posted 21 October 2007 - 08:07 PM

I haven't used those adaptors on a camera with a fixed lens, but I would assume that you'd keep the camera's permanent zoom lens set wide-open and not adjust its iris, only adjusting exposure on the attached 35mm lens. And then basically you'd have whatever light loss that the set-up causes to deal with, but since you generally don't set the f-stop in video with an external light meter reading, but based on the zebras & waveforms, etc., it's not like you need to calculate a compensation -- the camera just "behaves" less sensitive to light than it would without the adaptor.



On the P+S adaptor there is an iris control on the adaptor and you also have the 35mm aperture as well. I generally set the 35mm lens to a target aperture, usually around 2.8 or 4. Then I use the secondary iris to control the exposure. The advantage of this is that you can of course maintain a consistent DOF and the secondary iris doesn't affect DOF. The camera's aperture if of course set to wide open.

I don't use meter's much on video stuff, but it *feels* like it's a couple of stops to me.....That's before stopping down to 2.8 or 4 i mean.....



jb
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#7 Michael Nash

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Posted 21 October 2007 - 08:51 PM

That would mean that if the naked HVX lens' proper exposure was a f/2.4 (wide open), and I added a 1/2-stop (loss) adapter and used a f/1.4 SLR lens that I'd be overexposing the image on the sensor by 1 stop? That can't be.


You'd be overexposing the image on the groundglass. You're shooting a "rear projected" image, not the actual scene. Take a look at the samples here and you'll see how the ratio concept works.

Look at it this way: the method of "rating" an adapter by shooting a white card with & without the adapter is only measuring the light reduction caused by the adapter itself, and only at the f-stops tested. Changing the camera's stop and the shooting stop will no doubt produce different results, so test!
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#8 Chris Keth

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Posted 23 October 2007 - 03:43 PM

There really isn't a formula because an f-stop isn't really a measurment of light or of the reduction of light. The simplest solution for you would really be to set everything up and rate the effective ISO of the system at zero gain, everything wide open. That would be a useful number to know.
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