Effective F-Stop with 35mm Adapters
Posted 21 October 2007 - 12:48 PM
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?
Posted 21 October 2007 - 07:23 PM
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.
Posted 21 October 2007 - 07:32 PM
Posted 21 October 2007 - 07:34 PM
Posted 21 October 2007 - 07:54 PM
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.
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.....
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!
Posted 23 October 2007 - 03:43 PM