Following from my previous post I wanted to further investigate the implications of formats and megapixels on Macro Underwater Photography.
I also want to stress that my posts are not guides on which camera to choose. For Macro for example some people rely on autofocus so there is no point talking about sensors if your camera does not focus on the shot!
Macro underwater photography and fish portraits in general is easier than wide angle because is totally managed with artificial illumination, although some real masterpieces take advantage also of ambient light.
There are a number of misconceptions also here but probably on the opposite side of wide angle there is a school of thinking that smaller cameras are better for macro but is that really the case?
Myth 1: Wide angle lens -> More Depth of field than Macro
Depth of field depends on a number of factors you can find the full description on sites like Cambridge in Colour a good read is here.
A common misconception without even starting with sensor size is that depth of field is related to focal length and therefore a macro lens that is long has less depth of field than a wide angle lens.
If we look at a DOF formula we can see that the effect of focal length and aperture cancel themselvers
A long lens will have a smaller field of view of a wide lens so the distance u will increase and cancel the effect of the focal length f.
The other variables in this formula are the circle of confusion c and the F-number N. As we are looking at the same sensor the c number is invariant and therefore at equal magnification the depth of field depends only on F number.
Example: we have a macro lens 60mm and a wide angle lens 12mm, and a subject at 1 meter with the 60mm lens. In order to have the same size subject (magnification) we need to shoot at 20cm with the 12mm lens at that point the depth of field will be the same at the same f-number.
So a wide angle lens does not give more depth of field but it gets you closer to a subject. At some point this gets too close and that is why macro lenses are long focal so you can have good magnification and decent working distance.
Myth 2: Smaller Sensor has more depth of field
We have already seen that sensor size is not in the depth of field formula so clearly sensor size is not related to depth of field so why is there such misconception?
Primarily because people do not understand depth of field equivalence and they compare the same f-number on two different formats.
Due to crop factor f/8 on a 2x crop sensor is equivalent to f/16 on a full frame and therefore as long as the larger sensor camera has smaller possible aperture there is no benefit on a smaller sensor for macro until there are available apertures.
So typically the smaller sensor is an advantage only at f/22 on a 2x MFT body or f/32 on a APSC compared to a DSLR. At this small aperture diffraction becomes significant so in real life even in the extreme cases there is no benefit.
Myth 3: Larger Sensor Means I can crop more
The high level of magnification of macro photography create a strain on resolution due to the effects of diffraction this has a real impact on macro photography.
We have two cases first case is camera with same megapixel count and different pixel size.
In our example we can compare a 20.3 MFT 2x crop camera with a 20.8 APSC 1.5x crop and a 20.8 full frame Nikon D5.
Those cameras will have different diffraction limits as they have pixels of 3.33, 4.2 and 6.4 microns respectively those sensor will reach diffraction at f/6.3, f/7.1 and f/11 respectively so in practical terms the smaller camera format have no benefit on larger sensor as even if there is higher depth of field at same f-number the equivalent depth of field and diffraction soon destroy the resolution cancelling the apparent benefit and confirming that sensor size does not matter.
Finally we examine the case of same pixel size and different sensor size.
This is the case for example of Nikon D500 vs D850 the two cameras have the same pixel size and therefore similar circle of confusion. This means that they will be diffraction limited at the same f-number despite the larger sensor. So the 45.7 megapixels of the D850 will not look any different from the 20.7 megapixels of the D500 and none will actually resolve 20.8 megapixels.
So what is the actual real resolution we can resolve?
Using this calculator you can enter parameters in megapixels for the various sensor size.
In macro photography depth of field is essential otherwise the shot is not in focus, for this exercise I have assumed comparable aperture and calculated the number of megapixels until diffraction destroys resolution
Note that the apparent benefit of MFT does not actually exist as the aspect ratio is 4:3 so once this is normalised to 3:2 we are back to the same 6.3 megapixels of full frame. APSC that has the strong reputation for macro comes last in this comparison.
So although you can crop more with more megapixels the resolution that you can achieve is dropping because of diffraction and therefore your macro image will always look worse when you crop even on screen as now most screens are 4K or 8 megapixels.
For a macro image depth of field is of course essential to have a sharp shot however we have seen that sensor size is not actually a consideration and therefore everything is level.
Color depth is important in portrait work and provided we have the correct illumination full frame cameras are able to resolve more colours. We are probably not likely to see them anyway if we are diffraction limited but for mid size portraits there will be a difference between a full frame and any cropped format. In this graph you can see that there is nothing in between APSC and MFT but full frame has a benefit of 2.5 Ev and this will show.
Surprisingly for most the format that has an edge for macro is actually full frame because it can resolve more colours. The common belief that smaller formats are better is not actually true however some of those rigs will definitely be more portable and able to access awkward and narrow spaces to what extent this is an advantage we will have to wait and see. It may be worth noting that macro competitions are typically dominated by APSC shooters whose crop factor is actually the worst looking at diffraction figures.