What kind of lighting are you using? I think that is where there is great potential for improvement. With textured subjects, the angle (s) of your light source (s) with respect to the optical axis of the camera can make a significant difference in the contrast and the amount of detail you can see.
The fact that their subjects are usually inside aquariums presents additional challenges, but they are not insurmountable. Much of what is discussed about lighting in this question about fish photography in an aquarium would also apply to your situation.
Its 12 MP APS-C D90 has the same pixel density as a 27 MP FF sensor, so it would actually be losing some ground with the D610 at 24 MP, but the 6% linear difference would be negligible. I would simply get more in the frame with the same lens. The D810, on the other hand, has a 36.3 MP sensor. In terms of linear resolution, that is a 23% increase over 90D that could be significant if your lens is up to the challenge.
Which brings us to the choice of the lens, which is where all macro photography should begin.
We should probably pause here to define some terms:
- MFD: The minimum focus distance is measured from the subject to the image plane. That is, the focusing distance is measured from the film or the digital sensor to the subject.
- WD: the working distance is measured from the front of the lens to the subject. The working distance can be defined as the minimum focusing distance minus the distance from the image plane to the front of the lens when the lens is fully extended in MFD. If a lens has an 8 "MFD and is 5" from the sensor to the front of the lens when the lens is focused on MFD, the WD would be the remaining 3 ".
- MM: the maximum magnification is the largest that the lens can project a focused image of the subject on the film or the sensor. If a lens has a 1.0X MM, you can project an image of the subject of the same size on the sensor as the subject in real life. If a lens has a 0.5X MM, you can project a medium-sized image of the subject on the sensor. This increase is measured on the sensor.
- RR: the reproduction ratio is another way of expressing the maximum magnification amount of a lens. It is the ratio of the size of the projected image compared to the size of the subject. 1: 1 is the same as 1.0X MM. 1: 2 is the same as a 0.5X MM, 5: 1 is the same as a 5.0X MM (very specialized lenses), and so on.
- Magnification ratio: when we see images of most of our cameras, we enlarge the resulting image far beyond the size of the sensor or film. If we have a full-frame camera (sensor or 36×24 mm film) and a lens with a 1.0X MM, when we see the image even at 4×6 inches, we have used a magnification ratio of approximately 4.25X (linear), then the subject will appear 4.25X wider and taller in a 4×6 inch print than its actual size. It seems that 8.5X is the actual size for an 8×12 inch print, and so on.
Most macro lenses are only capable of reproducing 1: 1 at their minimum focusing distance. If you have to shoot from further away, you lose part of that increase. The further back you have to shoot, the smaller the subject in the frame will be.
- If you are using the AF Micro-Nikkor 60mm f / 2.8D lens, it has an 8.7 "MFD and a 3.56-inch working distance.
- If you are using the AF-S Micro-Nikkor 60mm f / 2.8G ED, Nikon lists the MFD at 7.2 "which leaves a working distance of only 2 inches when the lens is extended to MFD.
The density of the glass through which you are firing will affect this in some way, and the density of the water on the other side of the glass will affect it even more. Refraction due to the air / glass / water interface will increase the magnification, which will also slightly reduce the MFD.
In order to use the maximum magnification with subjects that are more than 2-3 inches from the side of the aquarium, you will need a macro lens with a greater focal length than your current 60mm lens.
The following working distances are for each lens with a maximum magnification of 1.0X (1: 1):
Tamron 90mm Macro (there have been several well-considered versions, I am using the specifications of the Model F017) has a working distance of approximately 5.1 "
Nikon AF-S 105mm f / 2.8G Micro has a WD of approximately 5.3 "
Sigma 150 mm f / 2.8 EX DG APO HSM Macro has a WD of approximately 7.3 "
Sigma 180mm f / 2.8 EX DG OS HSM Macro has a WD of approximately 10 "
Other macro lenses in each of the focal length ranges are similar. The longer the focal length, the greater the MFD at 1: 1 and, assuming that the lens lengths are relatively proportional to their focal length, the greater the working distance.
There are many other questions / answers here regarding the technique of doing macro work, but we will mention a couple of things that many new people in macro work can easily overlook:
- For maximum magnification, the lens must be set at the minimum focusing distance. Usually, one manually sets the focus of the lens to MFD and then moves the camera forward or backward until the subject is focused.
- For maximum detail, a sturdy tripod or other solid camera mount is required. When using the MFD technique mentioned above, a macro rail between the tripod and the camera can be invaluable. Macro rails can vary in price from around $ 20-30 to several hundred dollars, but you can find very good ones for less than $ 100.
¹ There are some macro lenses that have higher reproduction ratios than 1: 1. With such lenses, the highest reproduction ratio is always in MFD, and 1: 1 would be at a focus distance greater than MFD.