Want a better understanding of ISO?

[KmH]

What is the best ISO for your DSLR for astrophotography?

 

[table1349]

Good stuff. Thanks.

 

[Ysarex]

He starts off good, but gets a little confused further in and ends up off base with the 1/3 and 2/3 stop claim. That 1/3, 2/3 stop phenomenon he describes is real and does apply to specific model Canon cameras -- it was/is a unique Canon engineering implementation. It is in no way universal or applicable to other brand cameras.

Some confusion: Quoting the article; "If you increase the ISO, you’ll increase the noise." No. Typically increasing ISO reduces noise, it certainly doesn't increase noise. If you decrease exposure you'll increase the noise. Get the cause and effect right. I know he's assuming that all cameras are set on some form of auto exposure and when you raise the ISO the camera exposure system will decrease the exposure and so the noise increases due to the exposure decrease. That's a poor assumption to make when all the novices out there are reading Bryan Peterson and wishing they could become real photographers by shooting in manual. It's easy (manual exposure) to increase the ISO without altering the exposure and the noise will not increase. The noise we see in digital photos is fundamentally a function of exposure.

Joe

 

[Derrel]

I had not read the linked article; today I stopped by and read Joe's comments on said article, and am glad I missed it. I'm not into astrophotography, so the visible link really didn't appeal to me much. Seems like there's a lot of half-accurate information available these days on the world wide web.

 

[Ysarex]

I had not read the linked article; today I stopped by and read Joe's comments on said article, and am glad I missed it. I'm not into astrophotography, so the visible link really didn't appeal to me much. Seems like there's a lot of half-accurate information available these days on the world wide web.

Except for the Canon camera mistake it's OK and I certainly recommend it to folks who unfortunately have learned that ISO increases the light sensitivity of the camera sensor. The author starts off claiming that nothing in digital photo is more misunderstood than ISO. Well if that's the case all the more reason to be very careful with what's said in a publication. I'd bet that a quick discussion with the author would reveal a proper understanding. This is one of our internet information age maladies (guilty myself) where we're all publishers if we want to be and we can publish at little or no cost and right now! Editor? what's an editor?

Joe

 

[KmH]

I vote for DoF as being the most misunderstood concept of digital photography.

 

[Ysarex]

I vote for DoF as being the most misunderstood concept of digital photography.

And bokeh! I need more bokeh!!!

Joe

 

[bogeyguy]

Is there an ISO for Dummy's available out there?? Please.

 

[Ysarex]

Is there an ISO for Dummy's available out there?? Please.

Sure:

On a digital camera turning the ISO knob or pushing the ISO buttons does two things:

1. It puts a spin on the camera's metering system. So a one stop ISO change (100 to 200) causes the meter to calculate a 1 stop reduction in exposure ( eg. 1/125 sec. to 1/250 sec. or f/4 to f/5.6). A two stop ISO change (100 to 400) causes the meter to calculate a 2 stop reduction in exposure -- and so on and so on. The most common reason we do this is to be able to take hand-held photos in increasingly dimmer light. If at ISO 100 the meter calculates an exposure of f/2.8 at 1/15th sec. we know we can't get a good photo hand held. A faster shutter speed is needed. Setting ISO 1600 would cause the meter to re-calc that exposure as say 1/125th sec. at f/4 -- a four stop difference. If we're going to use the camera's internal metering system then increasing ISO results in decreasing exposure of the camera's sensor. We're underexposing the sensor. Common misunderstanding: The sensor does not get more light sensitive. It just gets underexposed.

NOTE: Our camera sensor's do not have an assigned ISO. Common misunderstanding: The ISO values on the camera apply to the light sensitivity of the sensor. They do not. These common misunderstandings carry over from the film experience. The manufacturers of digital cameras know how they work but it was just easier to let everybody think, "Don't worry it's just like it was with film." Confusion has resulted. The ISO rating for our digital cameras applies to the camera's JPEG output and not to the sensor. They re-wrote the specs for digital cameras. For film ISO is a rating of the film's light sensitivity. For digital ISO identifies a standard brightness level in the camera's JPEG output. Not telling everybody about this change has caused some confusion when some of the more tech oriented have tried to test their cameras and evaluate ISO performance looking at raw files -- raw files don't really have an ISO value.

2. ISO's second function is in fact a post-processing function. It occurs after the photo is taken. If the sensor has been underexposed the camera will use one or both of two methods to boost, lift, brighten, amplify, gain (all of these words have connotation problems) the sensor signal to normalize the raw file so that the JPEG processor can do it's job and make the camera JPEG. The signal from the sensor is electrical and in fact analog not digital. To get to the digital raw file the sensor signal is processed through the ADC (analog to digital converter) -- the device that converts the electrical signal into numbers. If the sensor has been underexposed the engineers can either boost the electrical signal going into the ADC or adjust the numbers inside the ADC or a combination of both. What's necessary is values in the raw file that represent a normal exposure so that the JPEG appears normally exposed. What's happening here is not increasing any kind of light sensitivity. The bleepin' exposure is over already! The exposure and only the exposure determines the data recorded. The ISO processing methodologies determine how well the camera system manages the recorded data.

NOISE: There are multiple sources of noise, but if we're doing this for dummies then it's simple. Noise is a function of underexposure period. The noise we all see in our high ISO photos is what's called shot noise and shot noise occurs in a weak signal from the sensor. It occurs as a result of exposure. ISO spinning the meter let's us re-calc a reduced exposure and that reduced exposure creates the noise. Underexpose the sensor and you get noise. Underexpose it more and you get more noise. When the 2nd function of ISO kicks in and boosts the sensor signal it unfortunately boosts the noise along for the ride -- the noise is part of the signal. Now the fact is that in most of our cameras that signal boost we get from the ISO circuitry leaves us better off noise wise than if we skipped it and tried to do the entire job only with the digital numbers in or after the ADC (digital scaling). So the ISO function then helps and in fact reduces noise caused by underexposure. In some newer cameras the sensors are so efficient that the ISO signal boost is no longer an advantage. These are cameras now being referred to as ISOless and indeed some of the manufacturers have started either removing the signal boost or going with a hybrid system that uses both analog signal boost plus digital scaling (adjusting the numbers).

DR: The full dynamic range of the sensor is only available at base ISO (no signal gain or digital scaling). You get the full DR of the sensor with a full exposure. Underexposing the sensor effectively reduces DR in that data that you could have captured is not recorded. Applying an ISO signal boost or digital scaling directly reduces DR. Imagine a 50 cm ruler laid out on a table. That's your sensor's max DR capacity and if you expose so the highlights reach the ruler end at 50 cm you've got a max DR capture. Now reduce exposure 2 stops (ISO 100 to 400). Let's pretend each stop is 5 cm. Those highlights that were at the ruler end are now 10 cm lower down. The ISO circuitry grabs everything recorded from 0 cm up to 40 cm and lifts it up so that in the raw file the highlights are back at 50 cm. Well there was nothing down there below 0 cm and so you just lost two stops of DR capacity. And of course the low end data that's very noisy has been lifted up the ruler as well.

Editorial comments: 1. I think it's better to know how it really works than to believe in myths. 2. Noise is part of the medium just like grain was part of film. The first time a novice film shooter saw a print from a Tri-X neg they'd say, "looks like it's printed on sandpaper, can you fix that?" The photogs would tell them it's part of the medium you'll get used to it and learn to like it. But off they'd go anyway on a quest to beat the grain. Years of bigger cameras and crappy flat prints from super slow speed films and one day they'd see a print from a Tri-X neg again and have an epiphany and the photogs would say, "told you so." Our modern sensors are amazing. If you need to underexpose one by 5 stops to get the shot do it! Most exciting advancement in shooting I've seen in 40 years is modern AF applied to sports and wildlife which works best when you put the camera on auto ISO and get the shot!

Joe

 

[dosa750]

Good reading, thanks.

 

[denada]

Ysarex, so if i'm shooting raw in manual, what iso my camera is set on doesn't matter? i set it in raw editor later?

 

[Ysarex]

Ysarex, so if i'm shooting raw in manual, what iso my camera is set on doesn't matter? i set it in raw editor later?

The ISO set on your camera matters very much saving raw files or not. No matter how you're controlling the camera's exposure functions (manual, semi-auto, programmed) you still have to determine exposure. If you're using an independent hand light meter you still have to determine exposure and exposure matters a whole lot. The camera ISO setting does two things. 1. It helps determine exposure (meter). and 2. It processes the sensor signal in creating the raw file. Both functions are critical determinants of the raw file and can't be adjusted later. When shooting you can't ignore function 2. if the ISO is set above base, it will alter your raw file.

If you're camera is ISO invariant (right now that's a few specific cameras) then you have the option to set the camera ISO to base and ignore it. Post processing in the raw converter can do the same job. Exposure still matters.

Joe

 

[denada]

got it. understood point one but there are situations where shutter speed and aperture are non-negotiable so i'd been adjusting iso to get the right exposure. because of point two i will continue to do so. thanks!

 

[Derrel]

Our modern sensors are amazing. If you need to underexpose one by 5 stops to get the shot do it! Most exciting advancement in shooting I've seen in 40 years is modern AF applied to sports and wildlife which works best when you put the camera on auto ISO and get the shot!

THAT is an amazing statement, and one I totally agree with, 100%. The newest d-slrs have amazing AUTO-ISO implementations, far, far, far better than what we had in the D200 and D2x generation back in 2005. At that time AUTO-ISO was sucky, and sensor performance was often not very good above the base ISO level, and 99% of all digital cameras had an APS-C or smaller sensor in them. Cameras are different these days, but there seems to be a lot of people who are unaware of how ISO is now no longer a hugely limiting factor. I'm glad we have Ysarex here to explain that AUTO-ISO use is now a viable, productive technique.

About a year ago, I had a disagreement with the leader of a photo Meetup group. He asked me why I would ever let the camera ,"Choose my ISO for me?" I told him that the f/stop and shutter speed settings I had selected in Manual exposure mode with AUTO-ISO were more critical to the success of the shoot than the exact, specific ISO value was. Specifically, f/5.6 for depth of field, and 1/1250 for safety margin at the ocean beaches during gusty 15- to 20 MPH winds with an 80-200 non-stabilized AF zoom. Medium f/stop and a shtter speed of 6x over maximum focal length, minimum, for wind-shake elimination.

Him, being the owner of ONLY old-tech digitial, could simply not understand that a much newer full-frame Nikon would perform so,so differently than his D70, a camera from 2004. At one time, ISO was a huge, limiting, ever-present boogeyman for d-slrs.

 

[Ysarex]

got it. understood point one but there are situations where shutter speed and aperture are non-negotiable so i'd been adjusting iso to get the right exposure. because of point two i will continue to do so. thanks!

Stop for a minute and think about what you're saying there. You're not adjusting ISO to get the right exposure. ISO is not an exposure determinant. When you make the decision that a shutter speed choice and f/stop choice are non-negotiable you are at that point determining exposure. Exposure is the total amount of light that reaches the film/sensor. It is a function of a) the scene brightness, and b) attentuation through the lens - aperture, c) over time - shutter speed. Changing ISO doesn't change any of the three above factors. In a digital camera raising the ISO engages post processing electronics/software that compensates for an assumed underexposure to normalize the output image brightness.

It's a distinction that matters. Exposure is fundamentally IQ determinant. If ISO isn't altering exposure then, as a post processing procedure it isn't altering IQ as determined by exposure.

Joe