As we move in even closer beyond life size reproduction on the sensor we enter the realm of Photomacrography.
The higher degree of magnification required is achieved by moving the lens further way from the sensor and then moving the whole assembly closer to the subject to bring it into sharp focus
So what do you need to get even closer than 1:1
Extension tubes and/or Bellows and a Focusing slide are the main tools required.
There are specialised lenses like the dedicated Macro Photo lens that Canon produce the MP-E65mmƒ2.8 1-5x, that in effect have a helical tube arrangement built in, by turning a ring on the lens barrel the lens moves further away from the sensor to achieve the desired Magnification.
Once you have set the Magnification you wish to use then move the lens closer to the subject, to bring it into the zone of critical focus. A Focusing Slide will enable you to accurately position the Principal point or rather Principal Plane of Focus (PPF) about which the zone of sharp focus is equally distributed (ref CoC notes).
At high magnifications this is difficult to achieve without the use of a geared Focusing Slide as at 1:1 (1x) Magnification the depth of Critical Focus is only in the region of 2mm at ƒ11 and at 3:1 (3x) it is less than 0.5mm and the higher the magnification the correspondingly smaller the zone of CF.
Photo 1. Shows the equipment I used in my film days to venture into the Macro region (1:1) and higher magnifications.
A set of Auto Bellows with an integral geared Focusing Slide in this case rigged for use with a 50mm FD lens and a Canon F1n film Camera back the standard pentaprism has been replaced by a 6x Waist Level Focusing magnifier this will easily get me down into the 4:1 4x region and beyond, with lenses like the 35mm ƒ2.8 and the 20mm ƒ3.5 Macro Bellows lenses corrected for use at high magnifications up to 20:1, for higher magnifications the bellows stage and camera can be attached to a Microscope as shown in the Differential Interference Contrast system I used.
I can still use the bellows in this digital age I just need to replace the F1n Film camera with a Digital Camera using a suitable adaptor ring unfortunately the WL Finder cannot be used but I can view the image on a laptop to aid critical focusing.
You can also use Extension Tubes and a manual lens in this case a 35mm ƒ2.8 Macrophoto Lens employing a Canon 7D Digital body as can be seen in the following photographs P.2 - P.5 the focal length of the lens used and the amount of extension determining the magnification. Focusing is achieved by moving the assembly relative to the subject hence the need for a Focusing Slide.
Illumination becomes increasingly problematic as you increase magnification both in terms of the increasing amount of illumination you require and physically getting light onto the subject due to the decreasing amount of space between lens and subject. Fortunately Ring Flash MR-14EX just happens to fit quite nicely over the tubes.