ISO & aperture

I was reading an article on Underwater Photography Guide's website on wide-angle and read a sentence that threw me.  On this link:  https://www.uwphotographyguide.com/wide-angle-photography, the author makes this statement: 

Quote

If I am shooting in low-vis water, or subjects further away, I may increase my aperture to ISO 200.

Aren't ISO and aperture two completely separate things?  I thought aperture is a measure of how wide the shutter physically opens up, whereas ISO is a measure of digital amplification of light that reaches each pixel on the sensor.  Am I misunderstanding something?  Or is that just a typo on the website?

They are indeed totally different. The third corner of that little triangle being shutter speed...

Yep, I agree with Stu.

I think I've come to see the three elements (shutter speed, aperture and ISO) in underwater photography use as:

- Shutter speed: to vary the blueness or blackness of the image background especially for macro. The faster the blacker. Fires the strobes up to a certain point (maybe 1/250 depending on system)  and higher using HSS. Freeze motion when strobes not in use. 

- Aperture: controls depth of field so a large element of creative control. 

- ISO: increase when the shutter speed has got especially slow to help avoid motion blur or camera shake. Higher ISO helps with motion freeze when strobes not in use. 

These may not be entirely classic photography uses - but I find these work for me underwater.

In addition to the above on ISO. As long as image quality doesn't degrade, given an aperture and distance, a higher ISO requires less strobe power, so you get faster refresh and more shots off the battery. This has the side effect of pushing up natural light, if that becomes a problem it may need a shutter speed increase to compensate.

There are two schools of thought, (1) most people are using the exposure triangle for ambient light photography and (2) using the exposure diamond for flash/strobe photography.  There are a lot of people who are afraid to raise their ISO for fear of image degradation.  I have used ISO as high as 8,000 on my Olympus EM1-3 without fear, this past new years eve my kid was playing with his cousins and the dogs in the backyard and I needed to use a high iso to freeze the motion and guess what I was able to get some pretty good images for family use.  I use On1 Photo Raw to edit my images and it has a noise reduction plug in that works really well. 

3 hours ago, JohnLiddiard said:

In addition to the above on ISO. As long as image quality doesn't degrade, given an aperture and distance, a higher ISO requires less strobe power, so you get faster refresh and more shots off the battery. This has the side effect of pushing up natural light, if that becomes a problem it may need a shutter speed increase to compensate.

I hadn't thought of that dynamic before, but that totally makes sense!

Out of curiosity, what ISO do you guys normally shoot at underwater?  Most of the reading I've done seems to say keep it at 100, maybe go to 200 if you have to.

1 hour ago, hoekma said:

Out of curiosity, what ISO do you guys normally shoot at underwater?  Most of the reading I've done seems to say keep it at 100, maybe go to 200 if you have to.

Yep, that's what I use

It looks like you probably found a typo in the Underwater Photography Guide article.

-Tinman

On 1/18/2023 at 7:57 AM, hoekma said:

Out of curiosity, what ISO do you guys normally shoot at underwater?  Most of the reading I've done seems to say keep it at 100, maybe go to 200 if you have to.

The base ISO is 200 for my camera. ill probably set it to 200 - 400 and then leave it there for the day.

On 1/18/2023 at 8:57 AM, hoekma said:

Out of curiosity, what ISO do you guys normally shoot at underwater?  Most of the reading I've done seems to say keep it at 100, maybe go to 200 if you have to.

IF I am shooting macro with strobes, then I use the base ISO on my camera (ISO 64: Z9, D850, D810) for maximum dynamic range.

I also tend to work with the idea of 'ISO invariance', even if I see that my strobes are underpowered for the subject.   (This is a matter of range to subject when shooting macro, but it comes up much more often when I'm shooting wide angle.)

With ISO invariance, the idea is that it is the same to push files in post (say to boost by 2 stops) as to raise ISO (by 2 stops) when shooting.   It also tends to help to not blow out highlights, which is a strong consideration on some highly reflective or radiant subjects.

At base ISO on the Z9 or D850, I have massive capability to boost shadows and the highlights are pretty protected as well.   By ISO 400 most of that 'throw-away dynamic range' is used up, meaning I no longer can boost shadows much and highlights may be right on the edge.  This is just a consequence of boosting image values by raising ISO on camera - same as if I'd already boosted shadows 2 stops to get to ISO 400.

There is a wrinkle in that when going from ISO 400 to 500 as that is where the 'dual gain' nature of ISO performance happens on the Z9/D850 cameras.   That is, at least in-camera ISO 500 has more dynamic range than in-camera ISO 400.   Not sure how this affects ISO Invariance and post-processing, but when shooting I keep in mind that with ISO's over 400 I better have the basic exposure correct as I can't simply boost shadows for underexposure at that point.   But this is when I'm mostly shooting ambient or mixed strobe/ambient and I want to see the results clearly in my viewfinder anyway.  (For macro I have a focus light.)

Interesting -- I didn't realize the ISO had a specific effect on shadows in post-processing that way.  Thanks @phxazcraig!  Seems like 100-200 is the slot generally from what I'm gathering from this conversation, and then up to 400 in special circumstances. 

BTW, the article on the UW Photoguide has been corrected:

If I am shooting in low-vis water, or subjects further away, I may increase my ISO to ISO 200 or even 400. 

9 hours ago, hoekma said:

Interesting -- I didn't realize the ISO had a specific effect on shadows in post-processing that way.  Thanks @phxazcraig!  Seems like 100-200 is the slot generally from what I'm gathering from this conversation, and then up to 400 in special circumstances. 

I don't know I would call a specific effect, every camera has a base ISO where the dynamic range is the maximum and the dynamic range steadily decreases as you raise ISO.  The effect of this is that raising shadows makes progressively more noise.  There's nothing actually wrong with ISO400 if you need to use it's just that recovering from under exposure creates more issues.  Dynamic range is often not that important in UW imaging except in specific circumstances like sunballs or split shots.

On 2/14/2023 at 10:54 PM, ChrisRoss said:

I don't know I would call a specific effect, every camera has a base ISO where the dynamic range is the maximum and the dynamic range steadily decreases as you raise ISO.  The effect of this is that raising shadows makes progressively more noise.  There's nothing actually wrong with ISO400 if you need to use it's just that recovering from under exposure creates more issues.  Dynamic range is often not that important in UW imaging except in specific circumstances like sunballs or split shots.

Modern cameras are now dual-gain in many cases.   Some special sauce kicks in to boost dynamic range at some ISO value.   For some Nikons you actually have greater dynamic range shooting at ISO 500 than ISO 400.   Pays to know your camera.