Chris Ross

HSS works by imitating a continuous pulse. The capacitor can generally only do a limited pulse length and if just fed through the normal circuit for longer would run out of charge. So it pulses and this has to be like a duty cycle controller to reduce the current drawn and so enable the pulse to run for the entire time the shutter is open. With the high frequency each line of pixels receives multiple pulses as the shutter curtain travels upwards, as the shutter curtain window narrows with faster shutter speeds each line of pixels progressively sees less pulses this is why power goes down each pixel sees a lesser number of pulses as they are uncovered for less time. For example if the frequency is 100 kHz, then there will be 100,000 x 1/320 = 312 pulses on every pixel.

Looks very Good Pavel, I would guess this streamlines your model range a little as it will fit any housing with enough space to route the hotshoe cable. Out of interest what is different about the Isotta and ikelite M16 housings that requires a different version - perhaps something about the way the o-ring seal is achieved? There was a related question on this recently.

To those wanting to fit triggers in the A series Sony housings, the new external trigger that UWT has put out may be of interest, it only needs space for the hotshoe cable inside. Assuming it works as advertised it could be a good option. There is a an announcement onsite currently here:

I think you will find this will depend on the shutter speed. at very high shutter speeds like 1/4000 sec the band crossing the sensor is quite narrow. Most often in UW work the SS will be in a 1/320 - 1/500 range and all you are trying to achieve is prevent the black band in the image and the opening that moves takes up a large portion of the sensor and everything receives multiple flashes I suspect even at the highest shutter speeds it might receive 5-10 pulses or nore depending on pulse frequency and speed of the shutter curtain.

a 5D IV is quite an old camera now, live view was quite a new feature back then. Live view as you know uses data from the sensor for AF as the mirror is up and the phase detect AF chips can't be used. The Phase detect sensors could AF the camera but the on chip AF is just not up to the task unless there's plenty of light available.

This is talking about human vision and it doesn't apply to pixels they store every photon they receive. This threshold refers to enough contrast that human vision can say it's a different shade or brightness. The object still reflects the light it just can't be detected against the background brightness - a signal to noise issue. If you can pick up the object as backscatter with a single pulse shot and it stays on the same pixel then you will also see it with HSS. I estimated that a 1/250 sec exposure sees something like 80 pulses and the object needs to move very rapidly to get to an entirely new set of pixels. If a particle sees only one pulse it is about 6 stops under exposed roughly speaking. Regarding video lights they are usually dimmed by a similar mechanism to HSS, rapid pulsing, so 50% strength comes from the light being powered on about 50% of the time, the cycling is rapid enough we can't see but can be a problem with video causing flicker.

Thinking of ways to quantify % of liveaboards in the Red Sea, so I went to liveaboard.com and they list a total of 311 liveaboards worldwide, while in Egypt they list 42. They probably don't list all options in Egypt, but likewise they probably don't list all options worldwide. They have for example 52 in Indonesia and 24 in the Maldives. I think this shows that the safety performance in Egypt is very poor compared to the rest of the world.

I'm sorry I have no idea what you are talking about. In HSS each pulse illuminates the subject and the backscatter particle with the same intensity of light. If the particle moves to a different set of pixels between pulses it won't be as bright as the pixels where it used to be don't get more light. If it stays on the same pixels it gets the same amount of light as the subject and is just as bright as it would be in a single exposure that provides the same exposure on the subject. The pulses may be short and have less light but they have to add up the same amount on your subject for the exposure to be the same. This is achieved by using more of the stored energy in the capacitor and/or wider aperture and/or higher ISO.

I think you are somewhat missing the point, this lens performs acceptably behind a dome where the centre of curvature is around 80mm behind the entrance pupil. Sure the pics in the link I posted are relatively small, but they are enough to show that they produce pics a lot of people would be delighted with. It basically shows that the lens is relatively tolerant of misplacement of the centre of curvature. If you look at the size of this port, the dome section is only about 20mm high so the lens focuses right down on the port allowing you to get very close to your subject - which makes a significant difference for CFWA, the subject becomes all the more prominent in the frame and small differences in distance makes a big impact with a fisheye lens. But this of course involves a compromise, with a small deterioration in corner performance.

Thanks @adventurer for your response, I should mention that I spent some time comparing the two different extensions with the 8-15mm though it didn't help that the image scale was different in them and spent a lot of time flicking back and forth between the two images till I thought i was starting to hallucinate. So I downloaded the two images and cropped an equivalent part of each and pasted in a single image to compare them, here it is 30mm is on top: Now I may need to get my eyes checked but the difference doesn't really set the world on fire, if anything there might be a bit more micro contrast on the collar to the right of the 6 in the serial number, but really not much in it to my eyes. To be fair there may be more to be seen in the original tifs, but based on this I think I could use either one happily. It would be interesting to see a similar comparison for a rectilinear lens.

A rectilinear lens is always going to be more sensitive to this than a fisheye lens, the way they render lines is fundamentally different to a rectilinear. It actually not possible to make a rectilinear lens with a 180 deg diagonal field of view, have a look at the formula for rectilinear lenses it falls apart when you try to solve it for 180 deg. Basically the ends of a straight line need to be at infinity to be rendered rectilinearly. If you look at the infocus field for a fisheye it's a curve not a line, almost like it was designed to be used behind a dome. I have never doubted that a rectilinear needs to be close to the ideal position. Fisheyes however are another story. I don't have the maths to describe it correctly, but experience from lots of people posting about the Canon 8-15 show it's nowhere near as sensitive to dome position. I don't doubt having the centre of curvature 40 or 50mm away from the entrance pupil will have some impact, but if you are within 5-10mm you will be in pretty decent shape. Some of it seems be down to the way the image is compressed in the corners, the image scale is a lot smaller and kind of hides aberrations by shrinking them. If you want to do these investigations, then more power to you of course, but if you don't fret about your image corners too much the Nauticam port charts seem to be pretty decent.

That may well be but Nauticam for example suggests 20mm of extension to compensate. I don't think 2mm will make a lot of difference in your images, most port systems only go up in 5 or 10mm increments depending on the housing system you are working with.

Nauticam already has a port recommendation for a Canon 8-15 plus 1.4x in the port chart, they just add 20mm of extension and they have a zoom ring for that combination as well. From what I recall and various evidence of what various manufacturers offer on the internet for this lens it seems that fisheye lenses are not particularly sensitive to positioning the dome port correctly. A prime example is ikelite 8 inch compact dome which is quite a small segment of a sphere and the Centre of curvature is well behind the dome. It works and the image samples provided look quite OK at first glance. I'm not saying that you can't or shouldn't try to do better, but this port is a long way out from ideal placement and still provides quite usable images. Here is a post with examples. GET the Canon 8-15mm Fisheye Before It’s GONE! // Underwater Review &

The valves will both have an m16 thread, the only thing that might be different could be placement of the o-ring so that it contacts the seating surface correctly.

How big was it? If it's quite small it looks reminiscent of a triplefin, except for the mouth, but a large mouth triplefin seems close? https://www.inaturalist.org/taxa/338587-Ucla-xenogrammus/browse_photos

In general two strobes are not additive because you generally don't point them at the subject UW, rather have the edges of the beams overlap a little in the centre so each strobe lights mostly it's own side of the image and fills in the shadows created by the other strobe. Whether you need the extra power depends. At one extreme shooting a full frame camera with an MWL lens at the recommended f16 needs plenty of power, compared shooting the same scene with a 180° diagonal fisheye at f8 or even f5.6 on m43. Likewise shooting full frame big animals at f13 with a 16-35 in a big dome that don’t get that close is also power hungry. Macro on the other hand you can shoot at f22 with a single small strobe as you are so much closer - a single MF-2 or Inon S220 can work just fine.

That's a big reason I wouldn't consider them, but you've got them already so need to deal the weight. I assume you have your floats on the closest arm to the strobe. I'm not sure what you are using for floatation but ideally if you get the strobe and float near neutral on it's own and closely linked to each other it should be less prone to torquing. However if you are using a lot more than the 200-250 gr per side that is required to offset the strobes then they'll want to torque, You could transfer the rest of the buoyancy to the inner arms closer to the housing and potentially bolt some foam underneath. 10 Bar have this: https://www.divesea.shop/shop/10bar-base-tray-float-29093#attr=3681 It could work if 400 gr is not too much. The principle is that if the buoyant part is above the non buoyant part it's stable and the closer the buoyancy is to the centre of gravity the more stable it will be.

12 and 6 o'clock strobe positions is generally not recommended for a number of reasons, lighting from below by the 6 - o'clock position strobe tends to look un-natural. Plus the bottom strobe might be a lot closer to substrate and blow it out. The general recommendation tends to be put your strobes in 9 and 3 or 10 and 2 or some variation while in portrait as well. See this video at about 9:35 for the positions: https://tutorials.brentdurand.com/underwater-strobe-positioning/amp/#wide I would also add if you are getting a lot of torque like that you might have too much flotation on your arms and could benefit from less on the arms and transfer some buoyancy to the housing possibly under the base. I had this issue with my rig which had around 1.5kg in float arms and it was difficult to twist upwards. I ended up pulling the two smaller float arms and replacing them with standard ones and diving the rig a bit negative.

I once dived with a camera that was slightly positive (new configuration) and found it really magnified the tendency for the dome to want to twist upwards, was quite a pain to use on that dive. It was maybe about 100 gram buoyant at most. I'm always clipped to my camera with a lanyard.

I have seen insects on land that can depart between pre and main flash, they are mostly small flies, very fast to react, I would think water would slow most things down enough for most subjects. The time between pre and main flash is generally pretty small. Having said that With a bit of thought and practice I would think you should be able to get manual to work for you first time.

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The website linked has some nice shots, though I should note that I looked at his wide angle shots and checked the EXIF data and it shows "flash not fired"for all that I looked at - the macro shots however all show flash fired. So all of the wide shots are natural light and they do look very well balanced . The AOIs would be a great solution for macro where you need a lot less power compared to wide angle as the strobes are so close to the subject. I am thinking they might struggle a bit for wide angle.

Shooting m43 is less demanding you are not closing your aperture down so much compared to full frame. Certainly reef scenes take the most power, but the YS-D2 has enough for m43. Diving in the tropics also involves thinking about transport, The YS-D2 is near half the weight and is almost neutral UW, if you are using the AOI housing then you likely won't need any buoyancy arms, but you'll probably be wanting them with the HF-1 strobes. On the dual strobes, while you can have them different and make it work and even if you want to have uneven lighting it's less thought to go from even light both sides to unbalanced, to me it's conceptually easier to have two identical strobes as my starting point.

It's a very powerful strobe, typically with m43 you shoot at f8 for wide angle. For full frame on the same shot probably f13-16 range depending on your setup, you just don't need as much flash power. It will certainly work, but it's quite a heavy strobe and a lighter cheaper strobe will get the job done.