The objective of this post is not to determine what is the best camera for underwater photography, as that is simply the best camera with the best housing and the best strobes and lenses. All needs to be seen as a system in order to take stunning images.

The purpose of this article is to provide some clarity and eliminate common misconceptions that seem to be hindering the decision making of a person wanting to take underwater photos. There is always a vested interested of camera manufacturers to drive sales as well as underwater photography equipment shops to push users to upgrade their gear as frequently as possible as that generates value to them, however this will not necessarily generate value to you the consumer, the only person injecting cash in this network.

I recently posted on WetPixel a discussion that to generate a debate about the gap between APSC and MFT cameras. This in turn made me do some more research on camera sensor and I found some information that is very insightful and confirms some of my suspicions I had years ago when I attended a workshop in the Red Sea with Alex Mustard. In that occasion I was the only user on the boat with a compact camera but managed to pull some decent shots and this made me realise that there are circumstances that equalise your equipment and make the gap in the image quality smaller to the point that a compact camera picture in some cases looks similar to a much larger sensor camera. Although I shoot micro four thirds underwater I have owned and shot DSLR full frame and cropped, film and digital, I have also had an array of compact cameras, so what you are going to read is not focused on one format being better than another.

Let’s discuss some of those misconceptions in more detail.

For those that do not understand optics of dome ports underwater the reason you need to stop down the aperture is NOT because you are looking for depth of field, in fact on land you would shoot a wide angle lens wide open and it would have plenty of depth of field. The reason to stop down the lens is the field of curvature of the dome which makes the areas off centre and on the edges soft this can only be fixed by stopping down the lens. So before you think I can shoot at f/4 on a APSC so what think that your pictures will be mostly blurry on the side and besides each format has got fast lenses so this is not a main consideration for what you are going to read.

Myth 1: Larger Sensor -> Better SNR

Signal to Noise ratio is an important factor in electronics as it allows to distinguish information from noise. Contrary to what most people think SNR is not related to sensor size.

There is an in depth demonstration on this website https://clarkvision.com/articles/does.pixel.size.matter/#etendue

The comprehension of some of the concept may be too hard for many so I will attempt a simplification. What R.J.Clark says is that you need to balance the amount of light hitting the sensor before drawing conclusion on SNR. For example assume a camera with a lens of 16mm on a full frame sensor and compare this with a camera with a lens of 8mm on micro four thirds, I am using MFT as crop factor is two and makes examples easier.

An exposure of f/8 on a 16mm lens on Full frame camera is equivalent to an exposure of f/4 on a 8mm lens on MFT. Those will send the same amount of light to the sensor at equivalent exposure. However the smaller sensor will have the same amount of light distributed on a surface that is 1/4 of the larger sensor and therefore if we equalise everything we have a situation whereby the exposure value are balanced and the SNR is pretty much identical because the gain or ISO value necessary was 1/4 of the larger sensor. This SNR 18% graph on DxOMark gives an idea. I have chosen 3 cameras with the same megapixel count to remove megapixels from the discussion.

Once exposure is equalised the larger sensor has no longer a benefit this is due to the fact that the components of noise shot noise and read noise do not depend on sensor size.

However an important consideration is that ISO 100 does not actually mean the same gain in all systems and in fact a larger camera will have more photons than a smaller one at the same ISO level, this means that at the so called base ISO the larger sensor camera will have an advantage as the smaller sensor can’t decrease the ISO anymore and will need to close aperture. It also means that ISO 100 does not mean the same SNR amongst different formats. So when we compare two shots at the same ISO larger sensors will have more signal than smaller ones. This is the reason sometimes you hear things like why is my shot on my compact camera so noisy at ISO 400 compared to a full frame that looks so clean at ISO 400 but those ISO are not actually the same thing and the smaller sensor has much less photons at that identical ISO number.

Another consequence of this is that as the camera in questions have the same megapixel size larger pixels do not yield better SNR.

However with larger pixels holding more signal it is possible to extend the range of an amplifier to higher value of gain therefore larger pixel camera (less megapixel on the same size) will be able to work at higher ISO levels. This is the reason why MFT camera have a lower maximum ISO than full frame at same megapixel count.

Underwater we use strobes to counter colour absorption and never reach those high ISO levels. If you were shooting at night on land without a flash you may easily reach high ISO value like ISO 25800 or 64000 with strobes however we rarely reach even values like 1600.

Myth 2: Larger Sensor -> Better Dynamic Range

The characteristic that drives dynamic range is not actually sensor size but pixel size however at some point DR no longer grows with very large pixels.

This graph shows that the Panasonic GH5 has a respectable DR at low ISO however it drops faster than the D500 and 1DX MkII. Surprisingly for some the D500 has more DR than the larger pixel 1Dx MKII.

If we look at the maximum possible DR and the ISO at which we would still have 7 bits colour and at least 10 stops of DR we have the following values:

Camera	Max DR	Highest Usable ISO
Canon 1DX MKII	13.5 Ev	3207
Nikon D500	14 Ev	1324
Panasonic GH5	13 Ev	807

Although the larger pixel camera does not hold the highest DR it is able to shoot at higher ISO and still keep a decent DR and color tone.

If we calculate the Ev between the ISO value we see that the MFT sensor is 2 Ev away and the APSC is 1.3 Ev away from full frame, this is pretty much in line with the crop factor and therefore once we equalise Depth of field there is no benefit between the various formats at same megapixel count, though the Nikon D500 is the camera that has the highest DR in absolute value. So if you have an extremely high amount of light the D500 would be able to product a high DR image. Underwater however this is rarely the case underwater so the conclusion is that if you are after a 20 Megapixel camera there is no material difference among the various formats in practical underwater use.

Myth 3: Larger Pixels are Better at equal sensor size

Although larger pixels are better at sustained dynamic range, for example in low light, evidence shows that as long as the camera is not limited by diffraction more megapixels are better.

I am comparing here 3 Nikon full frame cameras that have respectively 24, 36 and 47.8 Megapixels.

SNR is not impacted by the sensor resolution and this is due mostly to the fact that at similar size downsampling equalises the smaller pixels.

Looking at Dynamic range the situation is the same and actually the camera with more megapixels has an edge until ISO value become very high.

Finally the graph for color sensitivity, an important metric for shots with strobes and portrait work, confirms that more megapixels also bear better results.

Please note that this data is limited to sensor analysis and does not take into account the effect of very small pixels on diffraction and sharpness that is a topic on its own.

Choosing a Camera for Social Media

Today majority of people do not print their images and post them on social media or website. Those typically have a low resolution frequently less than 4 megapixels. Screens usually have low dynamic range, and JPEG images are generally limited to 12 Ev Dynamic Range this is a value that is at reach of any camera today starting from 1″ compact cameras but is unreachable to majority of computer screens or phones.

My suggestion for users that post on social media is to find the best camera that fits their budget and ergonomics and worry less about sensors, invest in optics either wet or lenses and port and strobes, as those will yield a higher return.

Today most cameras have a port system anyway so an advanced compact such as the Sony RX100 series or a Micro Four Third camera of small factor (Panasonic GX9 for example) are more than enough.

Choosing a Camera for Medium Size Print

I print my images typically on 16″x12″ or 18″x12″ paper or canvas.

Generally I want to have around 300 dpi so that means I need a 20 Megapixel camera as a minimum. This cuts out a large part of the smaller MFT cameras and also the compacts because the real life resolution is far from the declared pixels.

In my opinion, if you are a user that prints medium formats, a pro grade MFT or an APSC camera is all you need, besides the latest winner of UPY shoots an APSC with a Tokina lens and plenty of winners don’t use full frame.

For those who just want the Best

The best image quality today is produced by high megapixel full frame cameras there is no doubt about it. Full frame cameras however are subject to depth of field issues and as we have seen once you shoot at equal depth of field the benefit is for most eroded.

To get the best outcome of a high megapixel full frame camera you need to be able to shoot at the lowest possible ISO, this is almost impossible if you are shooting a fisheye lens behind a dome as your aperture of f/11 means very little light is hitting the sensor so your ISO will most likely hit 400 many times and at that point the benefit of full frame is gone.

I have looked at all technical details of Alex Mustard images on his book and nearly all shots taken with a full frame camera have at least ISO 400 or higher, with very few exceptions at 200 or lower.

So how to do you manage to shoot at the lowest possible ISO on full frame? You need to be able to shoot at wider aperture and this today means optics like the Nauticam WACP that have two stops benefit on a wide angle lens and three on a rectilinear lens behind a dome on full frame.

The WACP however has a field of view of 130 degrees and therefore is not as wide as a fisheye and unsuitable for close focus wide angle, recently Nauticam has released the WACP-2 that retails at $7,460 and can cover 140 degrees.

My consideration is that, if you are not prepared to spend money for a WACP like solution, then there is no point investing in a full frame system as the benefit goes away once you equalise depth of field.

Conclusion

Underwater photography is an expensive hobby and every time I am on a boat and see how much money goes into equipment to product average photos this saddens me. While improving technique is only a matter of practice and learning, making the right choice is something we can all do once we have the correct data and information at hand.

I hope this post is useful and helps your decision making going forward.