A1 and Light Meter Calibration

B

blissgirl

New member
Hello Everyone!

I am hoping someone in this incredibly helpful forum might be able to enlighten me.

According to Canon's customer service, because of a variety of factors they don't give an ISO rating for the A1. I have heard some say it's rated at an ISO 320, but I haven't seen any info anywhere to back that up... plus is the 320 at -3dB, or +/-0dB?

So... I am trying to calibrate a Sekonic 758C with the A1 without using a waveform monitor... has anyone accomplished something like this? I am using the 18% grey card method, 24f, 180 shutter angle. My ISO readings on the Sekonic are all over the place.

The issues seem to be:
1. There is only spot metering in Card mode on the A1 (I think)
2. I am unsure of the metering in Tape mode ... if it is evaluative, then will I be able to get a proper calibration if I light my 18% grey card very evenly? This doesn't seem to be working so far.
3. If I use Card mode (which I believe is just for stills) will the calibration work... do the stills use the same custom functions and settings video uses (which factor into a proper ISO for calibration)?

Any input is greatly appreciated.

~Rina
 
There's no point in using a light meter to measure for exposure for a digital camera. The different gamma curve can easily cause you to expose improperly if you rely only on the meter. The only way you would be able to do it is if you run exhaustive trial-and-error tests to see how the reading on the meter corresponds to the camera, but if you just go off a value like 320 ISO you're just asking for trouble.
 
Huy Vu said:
There's no point in using a light meter to measure for exposure for a digital camera. The different gamma curve can easily cause you to expose improperly if you rely only on the meter. The only way you would be able to do it is if you run exhaustive trial-and-error tests to see how the reading on the meter corresponds to the camera, but if you just go off a value like 320 ISO you're just asking for trouble.
Click to expand...

Thank you for the reply. I am calibrating my meter for my current camera gamma curve and settings, and I realize that if i change those settings I will have to re-calibrate. I should have been more clear about that.

I did wonder if there is some ISO equivalency for the out of the box default settings that Canon could give, that I could use for comparisons or a starting point (which is why I mentioned 320 that I have seen mentioned in various forums). But I haven't found any info on that.

I am wondering if you or anyone has calibrated a light meter to their specific camera settings successfully without using a waveform, and how they went about it? I know it is possible to do for other video cameras with a spot meter function ... there is a great video on FreshDV about it.

:dankk2:
Rina
 
I've done a handful of grey card tests for the XH-A1 (for the purpose of calibrating a light meter to be used with a cam+adapter combination) and I've found Canon's rating of 320 ISO to be pretty close. I think the last one I did I got 240 ISO @ -3db gain, which is like half a stop of light or so. I didn't run the test at +0 db gain because I almost never shoot at that gain setting.
 
Out of the box settings at -3 db gave me readings of 160 when I am shooting between f4 and f8.
 
Finnegan said:
I've done a handful of grey card tests for the XH-A1 (for the purpose of calibrating a light meter to be used with a cam+adapter combination) and I've found Canon's rating of 320 ISO to be pretty close. I think the last one I did I got 240 ISO @ -3db gain, which is like half a stop of light or so. I didn't run the test at +0 db gain because I almost never shoot at that gain setting.
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Thank you Finnegan, that helps a lot. Yes, we will be setting up with a Letus adapter, so we will take that into account. Did you use the A1's metering in Tape mode? I wasn't sure if that was giving me a proper metering because I don't think it uses spot... maybe evaluative or matrix.

It seems strange to me that Canon customer service says they can't give me a native ISO rating. So I guess that 320 number is based on peoples' individual testing and other cameras with similar chips?
 
cyclone said:
Out of the box settings at -3 db gave me readings of 160 when I am shooting between f4 and f8.
Click to expand...

Thank you cyclone! :thumbsup: Did you use a grey card, and were you metering in Tape mode on the A1? Didi you use a light meter to determine these readings? Sorry for all the ?'s, I am very curious.

Rina
 
Hey everyone, here is an update:

We did some tests before putting on the Letus and consistently got:

ISO 125 at -3dB
ISO 160 at +/-0dB

Lower than I expected, and only 1/3 stop difference?

This was with an evenly illuminated grey card which read f/5.6 from corner to corner.
We tried using the CINE.V, CINE.F and VIDEO.C presets and saw no difference in exposure. I was sure that changing these presets would affect the ISO, but they did not! Does anybody know why?

I will post updated numbers when I test with the Letus.

Cheers :beer:
 
Yes, I used a grey card with an incident light meter in the tape mode. Glad you were also able to do a test. Thanks for posting results.
 
That's so strange that some people seem to be getting lower ISO ratings than I did. Makes me wonder if I did anything wrong, but I'm 99% sure I didn't. A couple of the times I was supervised by John Hora, ASC, and he confirmed my tests. I guess I'll have another run - like I said I didn't run a grey card test at +0 db gain because I never use that setting, I almost exclusively shoot at -3 db gain just so I am clear of noise. And like I said before I got around 220-240 ISO for -3 db gain with the Panalook 2 preset.

Come to think of it, I suppose the difference in our ISO ratings could come down to settings, preferences, and presets. There are a number of presets that are geared toward low light performance and other such things that would likely affect the outcome of the test itself. I guess its just important to remember that when you're dealing with irregular gamma curves like you often do with video cameras, the ISO tests you do are valid only for that particular camera setting - once you change gain, preset, or add an adapter its all for not.
 
Sure thing Cyclone, thank you for your help.

Finnegan, I also wonder if we might have been doing something wrong, but we kept double checking everything. I think you are right about the different presets/settings. We didn't try any other settings besides the presets already default in the camera, not the Panalook 2, although I am curious and would like to give that a try if we have time.

With the Letus attached we confirmed the 1/2 stop light loss, as -3dB was at ISO 80.

Thanks everyone for your input, if anyone else has any test results I would be interested in them :)

Rina
 
Hmm, when i did my test with the letus, i remember rating the camera at around ISO 50 or under. The adapter does only lose 1/2 stop, but then the camera goes from F1.8 to something like F2.6 to F2.8 when zoomed in enough to get academy frame. That's a total of .5 stops of light loss from the adapter, and another 1.5 stops of light loss from the zoom-in factor. So if you rated the camera at ISO 160, then really the camera is at 40 ISO. ( again, depending on the preset, but i never bumped it away from Gamma 2, any other gamma made too much contrast )
 
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