Is ISO same across all formats?

[timor]

Even then, there are variances between "iso-settings" from one digital camera to the next.

Yes, Sekonic is making a special kit for its spotmeters to pin point real ISO setting for any particular digital camera as this may vary from sensor to a sensor.

 

[compur]

Okay so lets say I'm in my studio. I got my subject lit and ready to go. I have my Canon AE-1 loaded up with a roll of Kodak 400 TMAX Black and White film. And then I decided to bust out my 4x5 view camera, set it up to achieve the exact same composition loaded it up with (you guessed it) a sheet of 4x5 400 TMAX Black and White film. Both cameras have the exact same settings amdist the exact same lighting setup. Will there be any change in exposure (other than the reduction in depth of field due to the larger format)? Will my image from the 4x5 film be darker or lighter than the image from the 35mm film? Are they the same for every company as well or is ISO 400 on Kodak film different from ISO 400 on Illford film?

Aside from the film ISO ratings there is the factor of shutter speed accuracy that may have a noticeable effect on real world exposure settings in your scenario of AE-1 vs 4x5 camera.

If, for example, you're comparing a 4x5 camera with a mid century mechanically controlled leaf shutter vs. a 1970s-80s electronically controlled focal plane shutter you may find that using the same settings on both will result in some degree of overexposure with the 4x5 due to it's tired timing. Old leaf shutters tend to slow with age and their top speeds were probably not all that accurate as today's shutters to begin with. When I shoot with an older shutter I try to bracket my shots with the expectation that my actual shutter speeds are going to be a bit slower than marked.

 

[Derrel]

Okay so lets say I'm in my studio. I got my subject lit and ready to go. I have my Canon AE-1 loaded up with a roll of Kodak 400 TMAX Black and White film. And then I decided to bust out my 4x5 view camera, set it up to achieve the exact same composition loaded it up with (you guessed it) a sheet of 4x5 400 TMAX Black and White film. Both cameras have the exact same settings amdist the exact same lighting setup. Will there be any change in exposure (other than the reduction in depth of field due to the larger format)? Will my image from the 4x5 film be darker or lighter than the image from the 35mm film? Are they the same for every company as well or is ISO 400 on Kodak film different from ISO 400 on Illford film?

Aside from the film ISO ratings there is the factor of shutter speed accuracy that may have a noticeable effect on real world exposure settings in your scenario of AE-1 vs 4x5 camera.

If, for example, you're comparing a 4x5 camera with a mid century mechanically controlled leaf shutter vs. a 1970s-80s electronically controlled focal plane shutter you may find that using the same settings on both will result in some degree of overexposure with the 4x5 due to it's tired timing. Old leaf shutters tend to slow with age and their top speeds were probably not all that accurate as today's shutters to begin with. When I shoot with an older shutter I try to bracket my shots with the expectation that my actual shutter speeds are going to be a bit slower than marked.

Yeah...mechanically-timed shutters often have slower-than-marked high speeds. I used to shoot Kodachrome 64 in my Nikon FM, and once I had the speeds tested...the top speeds were about 50% slower than marked, as I recall, and I used to assign KR 64 an EI of 80, or even 100, and it worked out well. Speaking though of view cameras and leaf shutters: who here can touch upon the difference in exposure efficiency between focal plane and leaf shutters??? I have a vague, fuzzy memory of some articles written discussing the differences in the way a leaf or "interlens" shutter exposes the film in a somewhat different manner than a focal plane shutter...perhaps it had to do with the relative efficiency is stopping motion at the same marked speeds, with the FP shutter and the leaf style having somewhat different "abilities"??? Am I on the right track here???

 

[amolitor]

Film ISOs were actually defined by a more or less well defined process. There are some variables in play, as Helen pointed out (developers and so on) but at least ISO 400 means *something* well defined for film.

This is not the case for digital cameras, apparently. ISOs there are simply defined by the manufacturer, and will generally be pretty close to the equivalent film speed, but will vary from manufacturer to manufacturer, and model to model. Ctein has a pretty detailed article or two on ToP about this: The Online Photographer: Why ISO Isn't ISO is a good starting point, if you like, but there's a recent article on raw that he wrote up that's appropriate as well.

Not MY cup of tea, but if you're interested in these details, they're very interesting.