Fundamental question about DSLRs vs. film cameras

[Muse]

I used to be pretty into film cameras, built my own darkroom, bulk loaded and shot a lot of B&W using a Rolliflex (still have) and a SLR Nikon F. IIRC, ISO referred to the speed of the film I was shooting.

A DSLR doesn't use film. It has a sensor and a maximum megapixel rating, which I believe represents the maximum resolution you can achieve and translates in the largest size digital image file achieved for a given photo.

When people refer to different ISOs used with a DSLR (e.g. 800 or 1600) it does not refer to the speed of the film! Just exactly what does it mean? Thanks for your answer and referrals to an online resources appreciated.

 

[fuzzybabybunny]

Sometimes i think "speed" is a strange term for film. i prefer to use "light sensitivity." A higher ISO film or sensor will be more sensitive to light, meaning it'll capture more "light information" in the same amount of time as a lower sensitivity film or sensor.

 

[ElFenix]

sensitivity, just like it does on film.

rather than changing film, a digital camera turns up the gain on analog amplifiers while reading out the signal from analog amplifiers.


i suspect that any time you see 'hi' 'hi2' or 'lo' as the indicated sensitivity, the amplification is not being done entirely by the hardware amplifiers, and software is pushing/pulling the exposure in camera.

some sensors, for example, have a native sensitivity over 100, so the lowest native sensitivity is 200. though that is probably slightly amplified. when set to the equivalent of ISO 100 sensitivity, (L1.0 on the D300, for example) they lose highlight range.

 

[Muse]

Originally posted by: fuzzybabybunny
Sometimes i think "speed" is a strange term for film. i prefer to use "light sensitivity." A higher ISO film or sensor will be more sensitive to light, meaning it'll capture more "light information" in the same amount of time as a lower sensitivity film or sensor.

This stuff I knew. It pertains to film. Well, you mention sensors, so you are speaking to digital too. But in a film camera you swap out the film to achieve different ISO performance. In a digital camera, no such thing.

I presume, based on ElFenix's comments that boosting the ISO with a digital camera has the effect of introducing more noise to the results. I can't think of anything else, which doesn't mean I'm right. Noise is a term I'm seen mentioned in the threads here and I presume it means artifacts (or other forms of aberrations) in the image.

 

[Madwand1]

It's just like with film -- you get greater light sensitivity by dialing in a higher ISO, but you also get more noise. The noise is also somewhat similar, in that it looks something like film grain. The effects and degrees vary greatly with particular models, just as they did with film. For this, you should look at test results for the sensors of interest to start. Any decent digital camera review will contain this information, and show it visually.

E.g. http://www.dcresource.com/reviews/canon/eos_5d-review/

As with film, the effects of noise will also vary according to the lighting conditions -- you'll see more noise in the shadows.

The noise in a DSLR is at least an order of magnitude lower than it is in film, but it can be more bothersome when it's there, so you still need to take it into account. Perhaps it's just because we've become more accustomed to having essentially noise-free digital images (whether through low noise sensors or aggressive noise reduction), and conversely the presence of film grain in film sources. Perhaps it's because the character is different. In the end, much of this is context-sensitive and subjective -- one person's "usable ISO 800!" is another person's "yuck!".

 

[ElFenix]

Originally posted by: Muse

I presume, based on Elfenix's comments that boosting the ISO with a digital camera has the effect of introducing more noise to the results. I can't think of anything else, which doesn't mean I'm right. Noise is a term I'm seen mentioned in the threads here and I presume it means artifacts (or other forms of aberrations) in the image.

noise is just like you would get on your tv if you set it to a channel that isn't broadcasting in your area. that's all noise, and no signal. on a digital camera, unless you're taking a picture of the inside of your lens cap, you'll have some signal. so noise is color blotches.




with both film and digital exposure means how long and at what light intensity the medium is, well, exposed to the light coming out the back of the lens. exposure is just f-stop and shutter speed. it doesn't have anything to do with sensitivity of the recording medium, though it is expressed at a base sensitivity of ISO 100.

apparent exposure is what you get when you modify exposure by the recording medium's sensitivity. if you use a medium with 3 stops more sensitivity than base, ISO 800, you can change either part of the exposure calculation by 3 stops. but your ISO 800 film usually has larger grains which are more noticeable than the grains in ISO 100 film, especially when blown up. so you don't get as much detail, and any grains that don't work properly or receive inconsistent exposure due to gradations or edges stick out in the image.


with a digital sensor you can't actually change the sensitivity of the photosites. for example, let's have a night scene. you could either expose on a tripod with your sensitivity at ISO 100 and a long shutter speed, or you could expose at ISO 1600 with 1/16 the shutter speed. for the example, let's say the noise floor for the sensor is 10 photons per photo well; anything at or below 10 is just not enough to get a signal.

at ISO 100, you may be capturing 200 photons per well for the shadows of the scene. you have a 20:1 signal to noise ratio. you get smooth shadows without color blotches.

at ISO 1600, with 1/16th the shutter speed, you're capturing 12.5. that's barely above the noise floor. to correct the exposure, the camera employs it's amplifiers amplifying both the signal and the noise 16x. so now you've got a signal of 200, just like in the ISO 100 example, but you've got a noise of 160. so you get lots of blotches and not much detail.

there is actually another step before a RAW file is made that adds more noise, and that explains why you get worse results pushing an ISO 100 exposure 3 stops to 1600 in post process than simply shooting at ISO 1600 anyway. (though, as i mentioned above, i suspect various cameras push or pull in camera to achieve 'hi' and 'lo' ISO settings, so post process software should give a similar result.

 

[soydios]

On film, ISO sensitivity is a function of the size of the silver halide crystals. A more sensitive film has larger crystals, which results in "grain" when enlarged to print size.

In digital, ISO equivalent sensitivity is a function of the gain on the analog amplifiers at the sensor output. A higher gain increases signal strength, but also increases the intensity of underlying noise. This noise manifests itself as less accurate digital readings, which are especially apparent as blotches in shadow areas, where the ratio between signal and noise is low to begin with.

A sensor has a base sensitivity, where the signal-to-noise ratio and dynamic range are greatest. Any gain amplification or reduction above or below this point will result in reduced dynamic range.

 

[Muse]

I now recall that film sensitivity used to be measured in ASA rating. Is this the equivalent of ISO?

 

[ElFenix]

Originally posted by: Muse
I now recall that film sensitivity used to be measured in ASA rating. Is this the equivalent of ISO?

the ISO scale corresponds with the old ASA scale.

ISO is the same ISO that does the ISO 9001 regulations. international organization for standardization.