Perhaps the main points had been covered?augusthouse wrote:Why has this thread gone quiet?
I see that one of lauriek's questions remained unanswered:
Perhaps a better way to look at this issue is that a substantially larger sensor will require a proportionally longer bellows to get essentially the same benefit.I have a feeling that to get any benefit from a bellows much longer than that may require a substantially larger sensor
The basic rules are simple. You want to fill the sensor with the subject field you care about. You also need to limit the angle of view, because lenses only deliver a sharp image within some cone that defines their coverage. Typical coverage for a good macro lens at low power might be 20 degrees; for a microscope objective it's more like 10 degrees.
Suppose you want to shoot a 10 mm subject.
Then putting the rules together, assuming 20 degrees angle of view:
- a "1.6 crop factor" sensor will require 2.3X with the lens at least 65 mm from the sensor;
- full-frame 35 mm needs 3.6X and at least 102 mm;
- 4"x5" sheet film needs 12.7X and at least 360 mm; and
- 8"x10" film needs 25.4X and at least 720 mm.
The lenses that go along with these widely different extensions are actually quite similar. They range only from 20 mm for the APS-C to 27 mm for the 8"x10".
Back in the old days, it was common to use large format film for shooting small subjects. This involved putting short lenses on long extensions. The main benefit of that approach was greatly improved gradation, compared to using available small-format films. There was not a corresponding improvement in resolution, because the small effective f-numbers in the large-format setups introduced correspondingly large amounts of diffraction blur. With the lens properly set, diffraction blur in the final image is essentially independent of sensor size.
These days, digital sensors of almost all sizes provide the smooth gradation that used to require large format film. But the digital sensors are much smaller, so the need for long bellows has dropped off accordingly.