Downside of using below native ISO ?

molmi

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Hi guys,

Recently i taked some photos at 100 ISO with body that came with 200 native ISO. and the result was good.
My question is what are the real (or practical) downside of using undernative ISO ? has anyone of you ever had a problem or limitation using a lower than native ISO?

thanks
 

bargainguy

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I don't believe you get any image benefit with undernative ISO. Where you could get benefit is a more optimal shutter speed / aperture combo for any given light source.

Every time you add amplification to the digital signal by increasing the ISO, you also introduce noise. That noise will not be present either at native ISO or undernative ISO. That's my understanding, anyway.
 

pdk42

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The bottom line is that extended low ISOs won't normally give you any IQ advantage that you couldn't gain by over-exposing at the sensor's base ISO and then pulling the shot back in post processing - since in effect that's what these low ISOs are doing. The risk of course is that in both cases (over-exposing at base ISO or doing a "normal" exposure at extended low ISO) you may end up over-exposing parts of the image - so-called "blowing the highlights". So, it's something that might help in scenes with low contrast (so no bright highlights), but the risk is always that you'll overcook it. IMHO, nothing looks worse than blown highlights!

EDIT - of course, the option of over-exposing and then pulling back in PP is only something you can effectively do if you shoot raw. If you shoot JPEGs then it's a technique that won't work too well.
 

pdk42

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A bit more of a technical explanation:

- A sensor works by turning incoming photos into electric charge. As photons arrive at the sensor sites (sensels), they liberate electrons which are then accumulated in a charge well - one for each sensel. At the end of the exposure, the sensels will be filled with a certain charge and this is then turned into a digital value by various electronic elements - the final one being a Digital to Analogue Converter (DAC).

- A certain number of photons will completely fill the well and this will be turned into the highest numerical value for that sensel. If we have more than the necessary photons to fill the well, then they will be ignored since once the well is full, it can't be filled any more. If this happens, we get "blown highlights".

- At low light levels, the charge accumulated will be small and various noise elements will be visible as well as the signal from the light. These noise elements will include those from random photon effects (shot noise) through to noise introduced by the electronics (e.g. read noise).

- The DR of the sensor will be in effect the range of values between the point at which noise is not problematic and when we get blown highlights. The definition of "problematic" varies a little so DR is somewhat of a subjective measure according to who is measuring it.

- The efficiency of turning photons into charge is a feature of the sensor alone and cannot be changed by controls on the camera (not even the ISO control). Camera manufacturers will express this sensitivity in terms of the camera's base ISO. The base ISO represents an equivalence to the old photographic "speed" rating we all know and love.

- In principle, a scene exposed using an ISO-calibrated light meter at the same ISO as the sensor will generate an image where the final output has the "correct" brightness. If the contrast is not too high, this means that the sensels won't be overfilled and we won't have any "blown highlights". Likewise, there will be enough light in the dark areas that preserve some detail and don't lose it to the noise.

- If we underexpose the sensor by not giving it enough light then the wells will be underfilled and the photo will look dark unless we do something. We do that by increasing the ISO dial on the camera. But that's not going to change the sensor sensitivity - all it does is amplify the signal. The amplification could be done before the DAC (so an analogue amplification), or after (so a digital amplification). Amplification will brighten the image, but it'll also brighten the noise so we'll see more noise as the ISO increases.

- But what if we decrease the ISO dial below the base ISO? What's happening there? Well, what's happening is that the camera is exposing the sensor as if it's less sensitive - i.e. it's boosting the exposure. It records this fact in the information with the image and then when the image is rendered for viewing, it pulls the brightness back to compensate for this over exposure. This pulling back will happen in-camera for JPEGs, but in your favourite raw editor if you use the raw. The camera is telling the raw processor what the mid-grey numerical value is on the image and the processor will do as it's told. In effect, it's negative amplification.

- You can do your own negative amplification at base ISO by doing your own over-exposing and your own pulling back - the effect will be the same. It's called ETTR - Expose To The RIght - from the effect on the histogram of moving it to the right.

- This negative amplification trick works fine, and it will reduce the noise in the image - which is great. But, the over exposure that's at its base risks over-filling the wells and giving blown highlights. So, it's a technique that only works on some scenes. In fact, you can see this yourself very easily by looking at the histogram or the over-exposure indicators when you choose a low ISO setting since you will see it showing over exposure more readily than at base ISO.

- So, overall it's just a trick - not at all the same as the camera having a lower base ISO.
 

SpecFoto

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Moimi, I have been using ISO 100 since I first got my EM5 back in 2012. I first read about the EM5 sensor actually being a true 145 ISO +/- from some test labs reports published when it first came out. My EM1's bought the next year to replace the EM5 used the same sensor and that sensor was tested out about the same true ISO, at 148 +/-.

I shoot a lot of portraits in the very bright (and hot) desert sun and use the ISO 100 to help with reducing the amount of ND filter I need to get the shutter speed down to the flash sync speed of 1/320 with the EM1, or now 1/250 with the new EM1.3. Never had any issues with shooting ISO 100. As some mentioned, yes you can blow out clouds if you expose too close to the right of the histogram, but I usually shoot EC 1/3 stop under anyway and it has never been an issue for me.

Even without flash, If I am in the bright sun, my EM1's and now EM1.3 are at ISO 100. I shoot with fast primes and my favorite lens, the Nocticron 42.5 f1.2 will blow out past 1/8000 ss if shot wide open in the full desert sun. Shooting at ISO 100 allows me to use it wide open in bright sun and get below1/8000ss without adding a ND filter.

Here is one from 6 years ago with the EM1 at 1/320, ISO 100 with the 12-40 f2.8 Pro @ f2.8 and a 3 stop ND with OCF. Shooting at ISO 100 with a flash sync of 1/320 gave me the extra 1-1/2 stops I needed beyond the 3 stop ND to get proper exposure.

_EM13842.jpg
Subscribe to see EXIF info for this image (if available)
 
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PakkyT

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Recently i taked some photos at 100 ISO with body that came with 200 native ISO
You actually shot at ISO 200 still. At ISO "100" the camera doesn't change ISO, it simply overexposed by one stop as @pdk42 points out then reduces the exposure by one stop in camera for the generated JPG. The advantage is more detail in the shadow areas since you changed the shutter to make it twice as long so you gather more light.

My question is what are the real (or practical) downside of using undernative ISO ?
"undernative" - great word! I like it. Anyway, the downside is since you are overexposing by one stop you are therefore reducing your headroom for highlights by one stop. So any bright parts of the photo are "twice" as likely to be blown out (maybe "twice is not the right word, but you get the idea). If shooting raw you can do the same thing yourself by leaving the ISO set for 200, get "proper" (in Olympus's opinion) exposure, then double the shutter time, take the photo, then on the computer adjust exposure by -1 EV.
 

PakkyT

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Here is one from 6 years ago
Jim, Just wanted to compliment you on your editing that you left her skin as is (or nearly so if you edited). Some editing is nice sometimes, but man, I hate looking at modern shots with all the editing that renders all models with plastic looking perfectly smooth skin. It just looks so fake, like a doll; weird. I won't candy coat it, ya her skin is not perfect, but damn it, I know it is HER skin, not some computer version. Now if you could just pan down a bit... :inlove:
 

retiredfromlife

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I have been watching a lot of the Olympus Australia training videos lately, and in all cases they repeat to use the baseISO of 200 for maximum image quality. I can see others have posted the reasond why above. Just confirming what the Olympus Australia position is as this question poppped up a lot in their live youtube events.
 

PakkyT

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I have been watching a lot of the Olympus Australia training videos lately, and in all cases they repeat to use the baseISO of 200 for maximum image quality.
The funny thing is they won't actually say that ISO "100" is really ISO 200, do they?

If anyone wants to prove it to yourself, switch your camera to full manual mode. Set the ISO* to 200 and Shutter & Aperture to whatever is appropriate. Now change ISO* to "100" using the same S & A and take a second shot. Now import the raw* files to your computer. You should have identical exposures (being careful to not allow something like Olympus Workspace to see the "ISO 100" tag and automatically dial back the exposure one stop to show the "as shot" version).

*note that ISO only relates to the JPG output. RAW files do not fall under the ISO standards.
 

Tywais

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the final one being a Digital to Analogue Converter (DAC).
Small pedantic correction. It's an ADC (Analog to Digital Converter) that follows the amplifier. Just another tidbit, ISO control sets the gain of the amplifier. Higher the ISO, higher the signal amplification including noise. :)
 

retiredfromlife

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For people like me I think lighting conditions and personal skills have a far bigger bearing on final IQ than camera parameters, and how far you want to pixal peep. I have used low ISO in certain lighting situations, can I tell the difference, no. :speechless:

Me I just use the setting I hope/think will work best at the time and press the shutter. If it works good, if not oh well...........
 

SpecFoto

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Jim, Just wanted to compliment you on your editing that you left her skin as is (or nearly so if you edited). Some editing is nice sometimes, but man, I hate looking at modern shots with all the editing that renders all models with plastic looking perfectly smooth skin. It just looks so fake, like a doll; weird. I won't candy coat it, ya her skin is not perfect, but damn it, I know it is HER skin, not some computer version. Now if you could just pan down a bit... :inlove:
Thanks, but no editing at all except exporting the LR raw file with jpeg standards where the tone is slightly adjusted and the photo sharpened a bit, the makeup was all Leann's. I agree and try to leave the skin au natural, unless the model specifically asks for pp.
 
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pdk42

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Small pedantic correction. It's an ADC (Analog to Digital Converter) that follows the amplifier. Just another tidbit, ISO control sets the gain of the amplifier. Higher the ISO, higher the signal amplification including noise. :)
Yeah, sorry. Must have had a brain freeze, I meant to say ADC, not DAC! I corrected my original post, so if anyone reads this post it might seem a bit confusing!
 

RAH

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With my E-30 and onwards, using under base iso gives noticeably less noise, and marginally less DR.

With my E-M1 MkII, the DR remains the same, but still less noise due to collecting more light for a given exposure.

That's my practical observation from thousands of shots.

Olympus Australia etc notwithstanding.
Interesting info. I just would appreciate one clarification - when you say "the DR remains the same," do you mean it remains the same as in "marginally less," or do you mean that it remains the same as in the same as base ISO?
 
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