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Wetpixel Live: Exposure Modes for UW Photographers

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@Alex_Mustard and @adamhanlon discuss the PASM exposure modes that are available on most cameras. They chat about when they use the various modes to achieve different results and how they use them to capture different types of underwater photographs.

 

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Good job! I learned something new. I have only ever shot in (fully) Manual mode. I had never thought through the difference between using S and using A modes (with strobes). One of Alex's comments gave me pause to think it through and realize that using A mode does mean just what Alex said - you can set the A that you want and you can adjust your strobes to the level you want and leave them there. Whatever SS the camera chooses only affects the exposure of the background. It doesn't really affect what is illuminated by the strobes. Interesting thought.

One thing that Alex mentioned is that newer cameras have an AutoISO option. That is good. I use that frequently for land photography. But, one phrase I never heard mentioned through the whole talk (though I could have just missed it) is "TTL".

I realize TTL doesn't need much talking about for the camera exposure settings, really. But, even newer than AutoISO (I think?) is the ability to use TTL solely to control the strobes, via fiber optic cables (as most people use who would be the target audience for this webcast).

I don't have much experience with it yet (though my initial trial seems promising), but it seems like the ability to use TTL control for just the strobes, while shooting with the rest of the exposure settings on Manual could be very handy. You can use M mode to set SS, A, and ISO, to get the background color you want, the DOF you want, and the lowest ISO that permits those other things, and then use TTL for flash control to let the camera meter the intended subject and control the strobes for the "right" exposure. Especially is you use Spot Metering and lock the metering spot to the focus spot.

My suspicion is that when you have a subject like a shark swimming past you, letting TTL control the flash to burn longer when the shark is further way, but then a shorter burn time when it has gotten closer will substantially increase the "keeper" rate.

I would definitely be interested to hear your thoughts and experiences on this subject.

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22 minutes ago, stuartv said:

 But, one phrase I never heard mentioned through the whole talk (though I could have just missed it) is "TTL".

I realize TTL doesn't need much talking about for the camera exposure settings, really. But, even newer than AutoISO (I think?) is the ability to use TTL solely to control the strobes, via fiber optic cables (as most people use who would be the target audience for this webcast).

I don't have much experience with it yet (though my initial trial seems promising), but it seems like the ability to use TTL control for just the strobes, while shooting with the rest of the exposure settings on Manual could be very handy. You can use M mode to set SS, A, and ISO, to get the background color you want, the DOF you want, and the lowest ISO that permits those other things, and then use TTL for flash control to let the camera meter the intended subject and control the strobes for the "right" exposure. Especially is you use Spot Metering and lock the metering spot to the focus spot.

My suspicion is that when you have a subject like a shark swimming past you, letting TTL control the flash to burn longer when the shark is further way, but then a shorter burn time when it has gotten closer will substantially increase the "keeper" rate.

I would definitely be interested to hear your thoughts and experiences on this subject.

TTL to only adjust strobe exposure has been around for a long time, it's my standard approach for landbased macro shooting with strobes and has been for as long as I've been shooting digital and the ability to this is there UW as long as you have a TTL converter or an on board flash with an S-TTL (or equivalent TTL mode), which have been around for sometime as well.

But underwater is a different story, the conventional wisdom is that TTL UW doesn't work for wide angle. much of the time the camera chooses to turn down the strobes or reduce shutter speed to pull more ambient light in.  The important thing to note is that it is the camera that chooses what to do in TTL and has little to do with what strobe or TTL converter you are using.  The strobe and TTL converter are only important in correctly interpreting what the camera has chosen to do. 

I am starting to see reports that some newer camera models are doing a reasonable job using TTL on wide angle shooting and this seems to be camera specific, which to me makes sense.  The camera is what decides the ambient/flash balance.

One of the bigger issues with TTL is repeatability - each shot the camera calculates a balance between the ambient and flash exposure and it may change with each frame if the subject size, orientation and framing change from one exposure to the next.  With Manual exposure each frame gets the same flash exposure and there is a reasonable tolerance for distance.  If distance changes too much maybe you also should have adjusted strobe position a little as well?

For example a side on shark shot from below shows a lot of white belly.  Another shark swimming past at a 45° angle and slightly lower shows mostly a darker back.  TTL will change the exposure based upon reflected light when maybe it should have left the exposure constant. Sure there is some tolerance there to tweak the exposure in post but that's also there to a certain extent with a manual shot a touch further away.

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At the risk of pushing Wetpixel Live a bit too hard...here are our thoughts about TTL:

 

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9 hours ago, stuartv said:

My suspicion is that when you have a subject like a shark swimming past you, letting TTL control the flash to burn longer when the shark is further way, but then a shorter burn time when it has gotten closer will substantially increase the "keeper" rate.

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The issue that is not mentioned enough - relating to this - is that the strobe position for both of these scenarios is very different.

Just having the right strobe power is not all you need to change. We need a TTL-auto-strobe positioning control, that looks for uneven lighting or too much backscatter and moves your strobes as well as changing the powers!

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10 hours ago, ChrisRoss said:

I am starting to see reports that some newer camera models are doing a reasonable job using TTL on wide angle shooting and this seems to be camera specific, which to me makes sense.  The camera is what decides the ambient/flash balance.

One of the bigger issues with TTL is repeatability - each shot the camera calculates a balance between the ambient and flash exposure and it may change with each frame if the subject size, orientation and framing change from one exposure to the next.

 

Exactly the point. Newer cameras - possibly in conjunction with the latest TTL-capable flash triggers - may possibly do a much better job than older cameras. 

I don't know about other or older cameras, but I know that my camera has a Spot metering mode and a setting to lock the spot where it meters to the spot where the camera is focused. So, in theory, I can use a Focus Area of Spot w/Tracking, and lock onto the shark's eye. Then, the camera can use TTL control for the strobes to meter the exposure specifically for that area of the shark's head.

On paper, that seems like it would work pretty well.

As for repeatability - I'll flip that one back to you. The point is having strobe metering for a dynamic environment, where the conditions ARE changing from moment to moment. So, I would not want repeatability in the sense of the same exposure and strobe settings on every shot. I WANT the strobe power to change (in a "smart" way) as the subject distance/size, orientation, and framing changes.

 

1 hour ago, Alex_Mustard said:

The issue that is not mentioned enough - relating to this - is that the strobe position for both of these scenarios is very different.

Just having the right strobe power is not all you need to change. We need a TTL-auto-strobe positioning control, that looks for uneven lighting or too much backscatter and moves your strobes as well as changing the powers!

Totally understood. But, I am much more likely to be able to reach out and quickly tweak a strobe's position, while maintaining my ability to capture the moment than I am to reach out and quickly adjust my strobes' power quickly.

in other words, it's clearly not ideal to use TTL to control strobe exposure, for shooting a subject swimming by - from far to near to far again. But, it also seems much better (at least, potentially) than trying to shoot a subject swimming by using fully manual strobe control.

Have any of you actually done any experimenting personally with shooting with TTL for strobe exposure control, using fiber optic triggering? (I mean to sort of leave out discussion of older systems and older tech, and try to concentrate on current stuff that the average enthusiast might buy today)

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4 hours ago, adamhanlon said:

At the risk of pushing Wetpixel Live a bit too hard...here are our thoughts about TTL:

 

 

Thanks, Adam. Having watched that, I have a couple of comments.

One, it seemed like you and Alex went through that whole webcast talking about TTL with the unspoken presumption that you're specifically talking about TTL for strobe exposure control. To the new u/w photographer who doesn't really know about TTL and is trying to learn, I'm not sure it would be clear that the term TTL also applies to camera (vs strobe) exposure control when shooting in P, A, S, or even M, if you're using AutoISO. So, this webcast really does not give them a complete understanding of TTL in u/w photography, but only a view into using it specifically for strobe exposure control, with full manual control of all the other exposure settings.

And, two, Adam, during the 'cast you had a throwaway line that "your camera doesn't know what your subject is." I don't think I agree with that. When I'm shooting birds, whether in Spot w/tracking or Wide or Zone focus area, my camera does know (or figure out) what my subject is. And it does the same underwater. So, with regard to TTL, the camera does seem to have the ability to know or figure out what the subject is and, thus, use TTL to meter to expose the subject correctly. Of course, that is provided the camera setting for metering mode is set appropriately, and that the strobes are positioned appropriately.

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I think we are confusing terminology here.

TTL in this case refers to "through the lens" flash output control. I have never heard it as being referred to in connection with Auto ISO or any of the automatic camera exposure modes, but you are of course correct that exposure metering is (all) done through the lens...even manual mode is typically achieved by using TTL metering! For example, I use my camera's light meter to determine the correct exposure, and this is established via TTL metering...

Spot metering would allow you to use your camera and strobe's TTL flash control as long as your metering point will track with your focus. Sony's will meter to the start point of the track, but will not track as the subject moves around the frame. No metering (or AF) system actually knows what the subject is (e.g. size, color, texture), but uses exposure and focus values only. 

So with a big animal it will accurately meter on the focus point (eye) but if said subject is far away, will overexpose the rest of the frame. As it moves closer, it will tend to underexpose the body of a big animal, as it meters on the eye.

It is counter intuitive, but manual exposure control is actually simpler!

I use fiber optic and electronic strobe triggering and have (modern)  optical TTL triggers. For what it is worth, I actually find that the electronic TTL is more accurate than fiber!

 


 

 

 

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Maybe worth bearing in mind that what TTL metering does (in most forms) is set up the image to create an overall 18% grey (as in the old-days Kodak grey card). It's up to the photographer to decide then whether an overall 18% grey is the right shade to achieve the exposure required and whether to use the +/- exposure compensation settings. 

A shark must be a good bit of 18% grey!

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6 hours ago, stuartv said:

As for repeatability - I'll flip that one back to you. The point is having strobe metering for a dynamic environment, where the conditions ARE changing from moment to moment. So, I would not want repeatability in the sense of the same exposure and strobe settings on every shot. I WANT the strobe power to change (in a "smart" way) as the subject distance/size, orientation, and framing changes.

Totally understood. But, I am much more likely to be able to reach out and quickly tweak a strobe's position, while maintaining my ability to capture the moment than I am to reach out and quickly adjust my strobes' power quickly.

 

 

As Tim alludes the TTL makes everything 18% grey, so the correct exposure for the shark in the example given is the same as the distance is the same but TTL would reduce strobe output with a frame full of white belly and increase it if there's not much white showing.  And if the exposure is not right it's a guessing game as to what the camera did vs what you and the subject did to cause the exposure issue. 

I've not heard the term TTL used in any context other than automating flash exposure.  Fibre optic triggering for TTL is not a new technology and has worked after a fashion for many years, mostly working better with macro.   The ultra bright LED triggers are new but all they are doing is imitating an onboard flash and only recently have they had enough light output to do that. 

 

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Sorry to dredge this up from the distant history of a week ago. I was out diving. :)

I found the explanation I was looking for regarding TTL here:

https://en.wikipedia.org/wiki/Through-the-lens_metering

Apparently, my understanding of TTL comes from back when I was in high school (in the 80s) and learned film photography and darkroom work then. It seems that, in modern parlance, TTL means the same thing it did back then, but isn't viewed the same way because of the change in capabilities brought in by modern cameras.

Technically, TTL means Through-The-Lens and it applies equally to any time you're shooting where the camera is doing metering and setting any part of the exposure - whether you're using a flash or not. If you're shooting a modern DSLR or MILC in Manual mode, with no flash at all, but using AutoISO, then the camera is still metering the scene "through the lens" and determining what to set the ISO value to. Technically, that is still TTL. The only time it is not TTL shooting is when you're explicitly setting the SS, the A, the ISO, and the flash power (if using flashes).

But, evidently, in modern usage, the way TTL works with flashes is different than it was for analog SLRs, so the usage has change connotations to be specific to digital flash photography.

Thus my confusion and point I made to clarify what was said in the video, for people whose understanding of the term TTL predates modern digital cameras.

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