f stop range

[trez]

I'm afraid there's no fixed IRE-fstop relationship. One measures the video signal voltage on the linear scale while the other is logarithmic... There are other variables too, such as gamma etc.
For testing, use your in-camera gain to quickly check the IRE one stop higher/lower from any initial value. At the settings I use the most, when pointing the camera at evenly lit grey card that reads 50 IRE I usually find that adding 6dB gain (which should be equivalent to opening up by one stop) results in ~70-71 IRE.

 

[Shooter]

I'm afraid there's no fixed IRE-fstop relationship. One measures the video signal voltage on the linear scale while the other is logarithmic... There are other variables too, such as gamma etc.
...........

Exactly....as I thought . This brings into some question the claims as tested, demonstrated and stated as regards the f-stop factor.


In a previous life I owned and ran a Motion Picture processing laboratory. We used a densitometer to get precise and accurate readings from grey scales shot on negative. We interpolated density readings into f/stop factors. Very valuable for testing lenses, meters and cameras. We were never able to establish any useful correlation with waveforms (via Telecine).

 

[jenningsp]

Thanks for your recommendation Patrick, I did so some five years ago. Nikon 35/1.4, 55/1.2 and many others...
(the primes I wished for was for my SD10, btw)

I got (back then) when using those primes (with an adapter) the same results as you posted today, so.. I guess we just endorsed somehow each other's findings.:beer:

i was just using you and my recommendation as a generality for the benefit of the newbies. i know you're a dvxuser dinosaur and i'd like to think i am one too - because i know what your avatar pic is of :beer:

as for waveforms. you want to do the 6dB gain thing in the area that you're testing - because one stop might be ~20 IRE in the 70-100 range but it'll be somthing like ~10 IRE in the 30-50 range. but it all varies depending on the camera and gamma. - this is why i got out the spot meter

 

[Dennis Wood]

We shot the light loss clip linked in post #10 18 months ago or so to end a debate regarding the Brevis35 and our light loss observations. It surely did that, and to my knowledge, we're the only adapter company who's been willing to provide this information. We have invested a pile of $$ into a collection of DSC labs charts including their AMBI2 rear-illuminated system. We often use this system for light tests as it can be calibrated to be evenly illuminated (no chart installed) to better than 1/10 stop edge to edge. We also test with their rear-illuminated 72db OSG target which provides a calibrated 13 stop pattern..handy to see what's happening from 0 to 100 IRE on each camera. These tools are pretty much essential as we continue to refine the Brevis35 system and test out a growing pile of optics.

Every camera we've tested (8bit) has shown about a 20 IRE drop that correlates to one f-stop on the camera. This is when the baseline is at 50 IRE which we typically get by setting f-stop on the camera at f2.8 and adjusting shutter speed so that the waveform monitor shows 50 IRE. Typically it ends up at just under 70 IRE when one stop (f2.8 to f4) is dialed in. I can tell you that on the Sony EX1, Canon XH-A1, Canon HV20/30, Canon HG21, and Panasonic HPX500, the results are fairly consistent. Given that the flat response (no gamma curve) of the cameras is about 5 stops, then approx 20 IRE per stop makes some sense. The reason we start at 50 IRE though is that the camera's gamma curve makes the response non-linear as you approach the extremes so that the 5 stops end up something like 8 (and approaching 10 with the EX1). I'm no Adam Wilt when it comes to gamma curves etc, but our results are pretty consistent with what I've described. It always makes for a lively discussion

In another one of our clips in Cinevate's Video University, we show the light loss using CF1,2 and 3 which is likely the only adapter test using 3 different imaging elements. That clip shows bokeh differences and waveform monitor tests with each. You'll get an idea of linearity in that clip. With regard to lenses and speed...there's no question that fast lenses tend to be better with regard to edge to edge illumination, sharpness and contrast due to better design and better MTF values. They also give more range when elements like our CFX and CFXL are used. The Zeiss lenses that we sell are hands-down our favourites and that's comparing them to a large collection of glass. This may seem like overkill if you're talking adapters but in our Zeiss/EX1 lens test segment we show the difference between a Zeiss F1.4 50mm and a less expensive Nikon f1.8 50mm side by side. On a big screen using the raw footage, the difference in contrast is startling. Slap the manual Zeiss optics on a Canon 5D MK2 and they're downright cool. Good glass never goes out of style

Cheers,
Dennis Wood.