rating asa for cam+adapter

[Amos Kim]

Anybody know a way to rate a camera at a given gamma setting plus a 35mm depth-of-field adapter?

[Bob Hart]

The following info refers to P+S Tecnik's Pro35 and comes copy and paste from another website.


CINE LENS STOP LIGHT LOSS LIGHTING LEVEL

T1.9 -1&2/3 STOPS needs T3.5
T2.0 -1&2/3 STOPS needs T3.6
T2.8 -1&1/3 STOPS needs T4.5
T4.0 -1 STOP needs T5.6
T5.6 -2/3 STOP needs T7.2
T8.0 -1/3 STOP needs T9.0
T11 -1/3 STOP needs T12.7
T16 NONE same T16
T22 NONE same T22



(Source of the data used in this table is courtesy of Clairmont Camera in Hollywood)

In practical terms, my own homemade adaptor/camera combination seems to come back to about 100ASA daylight at worst. The notional rating for the camera is apparently 320ASA.

[Amos Kim]

bob, how did you measure the asa for your cam+adapter?

[Bob Hart]

Amos.

"bob, how did you measure the asa for your cam+adapter?"

Not very scientifically I am afraid. ASA320 has been published a few times on Dvinfo as the film equivalent of my camera, the PD150 so I just took that on faith alone.

I framed a subject in lighting conditions which brought me to a f5.6 manual aperture on the PD150 direct-to-camera for best apparent image with the zebras, then added the adaptor, reframed and chased the zebras again with manual aperture.

f5.6 in this instance was just a co-incidence and does not link in any way to the published f5.6 preference as the tightest aperture for groundglass relay.

I then used the ASA scale setting ring to re-adjust the readout on an old light meter to match the f-stop numbers I had to drop on the camcorder and arrived at ASA100. The little marks on the scale are very thick so it could be out ASA25 either way.

My adaptor is a one-off design so the numbers are probably not applicable to other adaptors especially Dennis Wood's which has three choices of groundglass, each passing a different amount of light.

The light transmission by my device seems to be a little gloomier than the P+S Technik Mini35-400 and about the same for the Letus35 flip for XL.

There's a new Mini35 version coming out to a product night here in June called the "Compact" and this apparently has a brighter groundglass.

I wonder if this development has been driven by the published performance of the alternative devices.

ASA numbers and light metering are probably not as convenient and effective as using the zebra system in the camera, but knowing the performance is handy for lighting and hanging onto best colours.

I am not a cinematographer's bootlace so do not attach too much validity to my comments here.

[Amos Kim]

I was thinking about a way to arrive at asa of my cam+adapter and I came up with this: How about setting the aperture and zoom of the stock lens that you will frequently use, which we will establish as baseline/normal setting (or atleast with my cam because the stock lens shows chromatic abberation at certain apertures so I have one stop that I like to shoot at). Then setting zebra at 100 and attach slr lens. Put a given amt (in fc) of light onto a greyscale chipchart in front of the camera. Adjust the slr aperture until the white chip has zebra stripes. Take note of the aperture. Then open up the trusty asc manual to the normal key:film speed:aperture chart. Plug in the footcandles and aperture and find the equivalent asa.

Any comments? Will this work theoretically?

[Amos Kim]

Will this work atleast for a given stock lens aperture and zoom setting?

[Bob Hart]

Amos.

You are likely more able and qualified than I in this arena so I am actually waiting to learn something I don't really know for myself.

I am note sure how the two irises in the path interact, like whether it is simply additive between the two or do they more or less steal the same amount of light if they are closed to the same number.

My personal preference is to stay around 1/50th sec shutter, 0db video gain and the alleged sweet spot for 1/3" CCDs of about aperture f5.6 on the camcorder.

If this is established as the baseline for a given camera, I would be inclined to attempt to measure the light performance with these fixed settings and use the SLR lens iris to control light in for the measurement.

[Amos Kim]

yes, my thoughts are exactly as yours. Now if we can get an expert or someone qualified to comment on this... anyone? :)

[Chris Leong]

The way I "rate" my camera is to set it to auto (preferably a spot meter auto) and point it at a lit 18% grey card.
Turn the gain off and take note of the aperture against shutter speed.

Then take a spotmeter to the same card and reverse-dial in the shutter speed with the aperture and see what ASA/EI I get.
That's the "film speed" of the camera.

I then double check by changing the subject (say, a white wall) and anticipate what the shutter/aperture combinations should be using my theoretical "film speed" in my spot meter, and see if the camcorder gives me a like setting.

The main reason for doing this is actually aesthetic. What I want to do is to capture or set a mood in my first shot, then maintain and retain that mood in the subsequent shots in that scene. The way that most people do that is to run the scene at a constant aperture, which yields comparable DOF over comparable zoom settings/focal lengths, or, put another way, similar mood.

So with varying lighting conditions, I'll put ND filters on or add light in order to maintain a constant aperture over the shots in a scene. Yes, my lens does have a "sweet spot", but there are other considerations, usually script driven, than may call for a different look.
That's when the meters and ND/grad/filter sets come out, and when you know the "film speed" of the camera (film or video), you can get there a lot quicker than otherwise.

[Amos Kim]

Chris, your reasoning is good, but when you attach a 35mm dof adapter, it gets a little hairy as the adapter loses light, so asa of the cam+adapter is different than just cam alone. Then you factor in the two sets of apertures. Like I said, I'd like to keep a baseline/regular cam aperture and zoom to get the best image and specific FOV. I would be adjusting slr lens aperture to suit the light requirement of a given scene.

[Chris Leong]

Amos

When I have my DOF on my camcorder end stays fixed, at constant zoom, wide open (f/1.6) and 1/50 shutter speed.

So I control the exposure via ND filters on the front of the stills lens, or gain in the camcorder, or lighting.

The aperture of the stills lens remains constant too, I adjust the lighting to get the correct exposure.

When the DOF adaptor is on, I use the histogram function to get the "T stop" of the entire optical chain, exactly like I would when calibrating a very long and complex zoom lens.

(Unlike the F stop, which is a direct mathematical ratio of the widest aperture to the focal length of the lens, the T or Transmitted/True stop of a complex lens takes into account, and compensates for, the actual construction of the lens. So here, with the DOF adaptor, what we have here is simply a more complex lens chain, with a greater F to T stop discrepancy. Either way of thinking about it, you have to open up a stop or two to allow for the DOF adaptor.)

[Amos Kim]

hmm... I'm trying to follow what you're saying but I still don't understand how your previous post allows me to rate my cam+adapter's equivalent asa. You say that you keep the cam lens and the slr lens at constant, but using auto (as mentioned in your first reply) will change cam lens.

[Chris Leong]

Sorry, I wasn't being clear.

Okay, first calibrate the camcorder without the DOFbox on.
Set the camera up on a tripod in front of a grey target lit evenly to a light level we'll call setup A.
Get a base reading for the camera's "film speed" in ASA/EI.

If your camera has a histogram reading or a zebra reading, make a note of where it is (what it looks like) when the camera's auto function is on.
This is where your camcorder's display reads as having a "good exposure"


Okay, then once that's done and you have your base ASA, turn the camcorder's auto off and go to manual.


Change your object's lighting setup. Add a couple of lights, take one away, whatever. Change it by at least a stop or two.
Call this lighting setup B.
Don't change the target card, though, you'll want the eventual histogram/zebra readings to be as close to setup A's as possible.

Now adjust the camcorder's iris and/or shutter speed to get the zebra/histogram to look as it did when the camera was on auto.
Take a light reading with your light meter and compare the shutter speed/iris settings.
They should be very close.

If not, your base ASA is off:
go back to setup A and repeat the above in a loop,
refining your deduced base ASA untiltil the readings off the histogram and the light meter agree to within a half stop or better.
It takes some time to read a histogram/zebra setup if you're not used to it, but with practice, you can get it pretty darned close.

This histogram/zebra reading should (eventually) yield an iris/shutter speed setting that's the same as the light meter reading of your new lighting setup.

Double check this the other way too:
use the light meter, get a reading, and manually set the iris and shutter speed to that reading.
You should see an almost identical zebra setting or histogram reading as before.
Note this "good exposure" zebra/histogram reading.

When this is under reasonable control, move on:

Put on your DOF box and stills lens, spin or move the GG if it's that type, and use the same target and lighting setup B as above.
Adjust the stills lens aperture and/or your camcorder's shutter speed until your zebra and/or histogram readings indicate "good exposure", then deduce your new ASA setting with the DOF box on.
Or simply figure out the difference in F stops between a "good exposure" with the DOFbox on and off for an F-to-T stop compensation (or "filter factor"), it's all the same deal in the end.

Same deal - change back to setup A and use your light meter to set up your DOF box.
If everything works as planned, your histogram/zebra readings should now read good.
If not, adjust until it slides into place.

HTH

[Amos Kim]

i'm assuming that the object you are using the histogram on is the grey card? And I'm assuming your histogram function (I wish I had this!) tells you that the grey is at 50ire? Because my zebras don't go down to 50. I should just get a waveform monitor to do this. :) And how much light (in fc) are you putting on the grey card? and at what angles? Thanks for your efforts to help me, chris. I appreciate it.

[Chris Leong]

Amos
Yes, the grey card, plus something else to give you a better histogram if you'd prefer.
Have mine set to 70 but I can see that if you have no histo and only 100 zebra then things could get interesting.
Maybe then overlight to 100 and use that as a reference? All you really need is consistency - it doesn't matter if the reference point is offset.
Calibrate to where the 100 zebras pop on, then back it all down afterwards.

I typically use two 100W LTM Peppers at around 3-4ft away, full flood, 45 degree copy-stand style lighting. I'll set it up and get you an fc reading.