Auto ISO#16

Ability to have auto ISO in manual mode. It might sound contradictory to have an auto setting in manual mode, but this is a great feature supported by my Nikon DSLR.

We would love to make ISO and Shutter Speed completely independent of each other (so that you can set ISO to “Auto” and Shutter Speed manually), but I believe this isn’t possible on the iPhone. Apparently, as soon as ISO or Shutter Speed are set manually, the other also becomes manual by default and can’t be set to “Auto”. If I’m not mistaken, this is also the case in other camera apps like ProCamera, Moment, and Halide.

@Florian - can you confirm whether I’m right about this limitation?

It is possible to have shutter AE. The apps Firstlight and 645 Pro both have auto iso with selectable shutter speeds.

@Mark Robertson Thanks for the pointer - you’re right, I just tried it in Firstlight! We’ll investigate.

This may help

the Ricoh GR has this feature build in. I loved it. A kind of semi-automatic. You could fix for example shutter speed and ISO and aperture adapted automatically.

ProCamera has it

Manual EV with auto iso would be something nobody else has. Then every control has intent. You set EV to fix scene brightness without metering, aperture for depth of field and shutter speed for motion control. ISO is computed automatically to maintain the fixed EV as shutter And aperture change. Current auto iso involves metering which is annoying because it shifts the exposure as you recompose. Manual ISO is annoying because every change to shutter or aperture requires a compensating change to ISO to maintain scene brightness. This solves both.

Bigolsen, let me know if I’m translating properly. First the ground work - there is no aperture control on a phone camera, only post processed simulations.

Given that I think I’m hearing you would want shutter priority with an exposure lock button on the meter reading and an EV wheel that adjusts it relative to the initial meter reading.

If you increase the shutter speed one stop you want the iso boosted one stop. If you drop the ev one stop the relative iso would get dropped one stop. If you decrease the shutter speed on stop after then iso would go down one stop. And on and on…

Now down to implementation. Apple probaly doesn’t expose a raw meter reading, but you don’t need it. When the exposure lock button is clicked A pin is put on the current shutter speed and another on the iso. The ev setting is just an adjuster on the iso. The shutter speed is also an adjuster on the iso.

if exposure lock is on you follow this formula:
New iso = old iso + ev adjustment + inverse of shutter speed adjustment

if exposure lock is off you follow this formula for shutter priority:

pull shutter speed and iso from auto exposure
new iso = auto iso + (auto shutter speed - target shutter speed)

Might have a plus or minus off but the gist is calculate the difference in the shutter speeds and then do the opposite for the iso to maintain the same exposure. This will accomplish shutter priority without exposure lock.

^^ This is Ryan Filgas speaking, just not logged in.

good point on the aperture! This is a feature on wanted on my regular cameras too :-). You’ve described it correctly, it could be considered an EV lock with a relative adjustment. I was imagining the EV display would be absolute in EV100 scale: https://en.wikipedia.org/wiki/Exposure_value#Tabulated_exposure_values

Which would be nice if you want your exposure to match over different sessions, because EV100 = 13 is an absolute exposure/sensitivity level that’s always the same.

So workflow would be, switch to EV100 mode, it shows current metered EV100 level like 14: Hazy Sun (text brief or optional) and then works exactly as you described where bumping to EV100 15 (which is really lowering ISO!) causes ISO to drop 1 stop. Since the view will darken, that should be a downard swipe or arrow to make the value larger. increasing shutter speed would leave the EV alone andd as you said, bump the ISO up so that EV100 value is unchanged.

ISO is just sensor gain which has no mapping to photographic intent. It’s kind of never made sense, it just used to be a necessity before digital when you had to change film stock to alter it.

it seems like both the analog and digital gain values for the sensor should be managed by the camera to achieve the deisired brightness with maximum dynamic range and minimum noise. I don’t know if the iphone manages digital gain secretly or really adjusts the analog sensor gain for all iso values.

short version: YES! you got what I meant!

Got it! So the EV value you want to adjust isn’t a baseline from a meter reading, it’s an absolute measure of how much light hits the sensor (and level of gain for the iso).

So one user experience would be to set EV 100 mode as you describe and exposure lock is turned on at the first metered reading (or whatever was last set by user if preferred)

On the ev wheel/scroll you would see the absolute values you expect. Instead of a -2EV from the base exposure you would see the 14.

This becomes a bit complicated and has caveats. Phone cameras have a fixed aperture so to calculate this you have to first get the phone model, then the lens in use, and then look up the aperture size from a lookup table (after you’ve gathered the data). You can then take the shutter speed and iso to do the math needed.

For consistency you have to have assurances that iso 100 is the same on every phone model. So the first supported phone would serve as a baseline as the source of truth for how bright iso 100 is. For each new phone you have to apply a multiplier to the ISO to make sure the exposures are consistent. That is EV 14 on one phone is the same as EV 14 on another model.

That last bit is a bit extra though (just polish), for a minimum viable product you could get away with an exposure lock button and then a config in the menu that switches the ev values to the EV100 view.

You should open up a separate issue! It’s not the same as shutter priority/ auto iso, although it is indeed dependent on it.

Sure! I can open a different issue! On the per phone difference, that’s actually the definition of ISO 100 is a particular standard sensitivity.

so the camera ISO values are supposed to be calibrated to match this: https://en.wikipedia.org/wiki/Exposure_value#EV_as_an_indicator_of_camera_settings which is the table for ISO 100.

so EV 14 at f/2.8 and ISO 100 means shutter speed is 1/2000 (see table). That’s true for all cameras, because ISO is a standard. Of course the required ISO for a different shutter speed can be computed from there.