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07/08/2011 09:08:49 PM · #1 |
Recently I have read several articles suggesting that when taking photos at high ISO levels, one should always try to overexpose the image slightly in order to reduce the amount of noise. I have experimented a bit with this and the trick absolutely works, and it is also very visible in the examples in some of the articles - such as this: //static.foto.no/linkeddata/articles/images/30150_1024x768.jpg
However, I can't help wondering if it's rather pointless to bump up the ISO to get a shorter exposure time if you are going to compensate by overexposing the image and thus making the exposure time longer again... In the linked example, the best quality is obtained at +1.3 EV at 3200 ISO. That means one could just as well have shot the image at 1250 ISO and +/-0 EV with the same shutter and aperture setting, so why bother going up to 3200 if that is the case? Is there sort of a "sweet spot" in the tradeoff between higher ISO and overexposing?
Edit: Here is a link to the article (in Norwegian) in case anyone would find it interesting: //foto.no/cgi-bin/articles/articleView.cgi?articleId=42642
Message edited by author 2011-07-08 21:12:30. |
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07/08/2011 10:28:45 PM · #2 |
With some cameras there may be an advantage to avoiding shooting at particular ISOs, namely those that are not full stops apart form the base ISO. This is because of the way the intermediate ISOs are achieved with these cameras. Beyond that, there's no real noise advantage to, say, shooting at 3200 and overexposing by a stop vs. shooting at 1600 and exposing normally. The risk, of course is that you blow highlights you can't recover.
Exposing to the right is always a good practice, just don't take it to the extreme that you give up highlights you later find you needed to hold. Bottom line, if you find that you are typically reducing exposure during RAW conversion and rarely increasing it, while using highlight recover sparingly, you probably have a near-optimal practice with regard to exposure. |
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07/09/2011 11:43:02 AM · #3 |
Thanks for the answer. You got me a bit curious on the part about using the "non full-stop" ISO values. What exactly is special with the way these are achieved / implemented? I always envisioned ISO values in digital cameras as simply stretching or compacting the range used in the A/D conversion of the voltages from the sensor...
When it comes to exposing to the right, are you saying that as long as I don't blow out the highlights, it basically always makes sense to step up the ISO and overexpose rather than exposing for "correct" exposure at a lower ISO?
IE, say I want to take a picture of a given scene at f2.8 and 1/100s. At 160 ISO this might lead to a "correct" exposure, while at 200 ISO it is overexposed with about 0.3 EV. I should generally go for the latter? |
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07/09/2011 12:49:47 PM · #4 |
Originally posted by kirbic:
Beyond that, there's no real noise advantage to, say, shooting at 3200 and overexposing by a stop vs. shooting at 1600 and exposing normally. |
Doesn't this depend on whether ISO is applied to the analog signal or the digital signal? I believe different cameras handle this in different ways. If the latter, I can understand how the two would be equivalent, as you suggest, provided you're shooting RAW. And this gets back to the OP's original point. But if ISO is a directive to amplify the analog signal by some multiplier (x2 for ISO 200, for instance) before A/D conversion, then it's not so simple. I don't know a lot about these things, but my understanding is that this approach produces less noise. For example, quantization error isn't subject to the ISO amplification since it's applied before ADC. So there might be an advantage?
Sorry to muddy the waters. |
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07/09/2011 03:44:04 PM · #5 |
| Regarding Canon, the best ISOs are full stops starting at 160. Link with a grain of salt . I don't think it matters to be honest. |
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07/09/2011 09:24:21 PM · #6 |
Interesting stuff, bspurgeon.
I did read through the comments to the article though, and it was suggested that in fact the full stops are 100-200-400-800 etc. According to commenter "Luis", the reason why 160 produces better s/n ratios is that it is in fact using ISO 200 but using exposure settings as if it were 160. That means the picture gets overexposed with 0.3EV and the camera simply pulls it back 0.3EV for you after exposure.
This is also backed by the article linked to by one of the commenters (translated from spanish): Here
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07/10/2011 02:32:09 AM · #7 |
| Tried it an I think it produces some pretty amazing results. |
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07/10/2011 03:40:34 PM · #8 |
Originally posted by wiesener:
When it comes to exposing to the right, are you saying that as long as I don't blow out the highlights, it basically always makes sense to step up the ISO and overexpose rather than exposing for "correct" exposure at a lower ISO? |
Nope... basically, I'm just telling you to always use the greatest exposure you can, without blowing the highlights. As far as ISO, use the lowest ISO that your required shutter speed will allow. |
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