Author Topic: EV Tables?  (Read 195 times)

Electra

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EV Tables?
« on: July 06, 2017, 12:04:07 AM »
Because this is a non-standard sensor/application, would it be an idea to start working out exposure/shutterspeed/f-stop tables?   ( https://en.wikipedia.org/wiki/Exposure_value )

I'm very new to 'real' cameras, this is be my first one that isn't a point-and-shoot or a mobile phone so I've been asking around and trying to get an idea of how different factors interact. (I am finding out about the problem with depth on field and low F-stops.  *grumble*)

There seems to be a way to combine  light levels, your lens's f-stop and the sensor to give what shutter-speed(And thus framerates) you can expect/should use.
Or the other way, where given a lighting condition and desired shutter speed, what f-stop lens you would need to give enough light to have this work

I know these are meant to be ball-park figures and your final settings are done on the day, but it would be nice to have at least a basic chart showing roughly what you can do with what lens at what lighting level and to at least give you somewhere to start when playing with the camera.

Or is this all basically irrelevant in this application and it's purely a try different lenses until you find one that you like?

BiduleOhm

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Re: EV Tables?
« Reply #1 on: July 06, 2017, 03:56:19 AM »
It depends on the light level, not the lens (a lens A with with the aperture (f-number) set to 1.2 will let the same quantity of light enter as another lens B with the aperture set to 1.2), so if you want to make a table you need one table per light level which is not very doable (maybe make only one table with the light level of the sun at noon as it's a good standard?).

I guess the best thing to do is tables of depth of field for each lens as a function of the aperture numbers (and zoom levels if it's not a prime lens) so when you want to shoot something you know the lowest aperture number you can set given the lowest depth of field you want; then you set this aperture and you can then set the others parameters after that. This is assuming you want the lowest aperture number possible (which is pretty much always the case with HS cams as you always want more and more light so you can have a decent exposure and shutter speed...).
« Last Edit: July 06, 2017, 03:59:05 AM by BiduleOhm »

Electra

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Re: EV Tables?
« Reply #2 on: July 06, 2017, 07:55:16 AM »
At least for basic light levels, this chart looks handy: https://en.wikipedia.org/wiki/File:Exposure_Value_Scale_Visualized_as_Circles.png
Full daylight / outdoor cloudy(some shade) / indoor(normal lights)  is enough steps, but more would be better. It doesn't have to be super detailed and accurate, just a quick lookup table to work out rough numbers so you have some ideas where to start and how things compare. The tables also look predictable so once you have them made up you can guess between the lines.

I know that the same F-stops give the same level, I'm looking for a way to compare, for example, what sort of shutter speed(And thus what frame-rates) I can expect with the  Zenit  300mm f/4.5 lens I've been given compared to, say, the Computar 12.5-75mm f/1.2.

It would be handy to be able to have  rough lookup that lets you go one way or the other. To know where to expect your shutter speed to be what 'x' F-stop and 'y' light level or  'Can I get away with '2x' fstop and a beter depth of field instead. So you know what sort of lens you after.

I was under the impression at the start that anything under abour F2 or 3 was unusable on this camera unless you had some really big arc lamps, this has been proven quite wrong and such a chart would have been handy.  Even if it's just for the range of lenses that are sold with the camera.

BiduleOhm

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Re: EV Tables?
« Reply #3 on: July 06, 2017, 12:05:44 PM »
Ok, so if we take the minimum ISO for the color camera (ISO 320) and this calculator: http://www.scantips.com/lights/exposurecalc.html in manual mode we see you'll have a 3.8 EV difference between the two lens. We also see that with 4.5 you'll need 16.3 EV (very very bright sunny day) and with f/1.2 you'll need 12.5 EV (overcast) for a shutter speed of 1/4000 (= 250 s, can shoot up to 4 kFPS). You can add gain to up the ISO and shoot at a higher FPS and or use less light, but you'll also add noise if you do that.

With the other lens you can go up to about 1/48000 (20 s, up to 48 kFPS but the Chronos max out at 40 kFPS anyway) but you'll have a very very short depth of field.

So you can certainly use a f/4.5 lens but you'll not be able to do much higher than 4 kFPS on a very bright sunny day (without adding gain) so it's not the best lens in the world but it's not unusable either :)

Electra

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Re: EV Tables?
« Reply #4 on: July 06, 2017, 11:10:14 PM »
Thank you for the detailed response, I only just understand and follow that, it's quite confusing for someone who is new to cameras.

This is why I was hoping there could be a table drawn up(Maybe it could go in the manual) and would allow someone to do this without going through all those calculations to compare different combinations of things.

For example, I thought that the ISO changed depending on your shutter speed? Or is that an independent control again? Or linked with another option?

This sort of confusion is why the 'how to use a manual camera' crash course in the manual will be so handy!  I can fumble through a lot of it asking stupid questions, but being able to look up until I get a feel for it or after not using it for a while will be very handy.

BiduleOhm

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Re: EV Tables?
« Reply #5 on: July 07, 2017, 04:11:46 AM »
Ok, so to make a very short 101 explanation:

- ISO: is the sensibility of the film (or sensor). Higher ISO means more sensible so less light needed.
- shutter speed: is the time the film or sensor is exposed to the light. Higher shutter speed means less blur on the image but also a darker image (the light has less time to expose the film or sensor).
- aperture or f-stop or f-number: is how open the diaphragm is. Higher f-number means a diaphragm less open so less light but more depth of field.

All of those are independent settings.

NB: with digital camera you only have one sensor so you also have only one ISO sensibility (you can't change the film for another one who is less or more sensible as the film is the sensor...) but you can add gain (amplify) after the image is taken, however it doesn't make the sensor more sensible, it's like the brightness setting in photoshop for example, you amplify the useful signal but you also amplify the noise (example of a noisy image: https://en.wikipedia.org/wiki/Image_noise#/media/File:Highimgnoise.jpg) so you can't add 24 dB of gain and expect a good image quality (but if you're already maxed out on the light, the shutter speed and aperture settings you have no other choice...).

NB: a rule that can be useful https://en.wikipedia.org/wiki/Sunny_16_rule

I bet there's a lot of tutorials on the web on how to use a camera in manual mode for beginners so you can read those while waiting for the crash course chapter in the Chronos' manual.
« Last Edit: July 07, 2017, 10:35:20 AM by BiduleOhm »