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EXCERPT page containing first few paragraphs. 2023-10-03 10:55:06

Field Curvature with Two Leica 24mm Lenses

This example compares the Leica 24mm f/2.8 Elmarit-M ASPH to the Leica 24mm f/3.8 Elmar-M ASPH. The 24/3.8 Elmar-M is a newer design, and effectively a full stop slower.

This is a fantastic example comparison, because while the lenses appear similar, they are actually quite different in behavior. No one can doubt the quality of either, yet the comparison shows a large difference.

MTF hints

As seen on the MTF chart below, the 24/3.8 lens delivers much superior field flatness, whereas the 24/2.8 slopes off steadily: field shooting shows that it is due to rearward field curvature. It is hard to take a sharp image of anything planar (flat) without going to f/5.6 - f/8 with the 24/2.8 Elmarit-M.

Although the mouse-over below shows f/2.8 vs f/3.8 (not quite fair), we are not looking for absolutes , but rather general behavior. Namely, that the 24mm f/2.8 Elmarit-M shows an early and strong drop-off in contrast beginning at the middle of the frame, which will be seen as strong blur towards the edges (on a planar subject). By comparison, the f/3.8 Elmar-M has much more consistent sharpness right out to the corners.

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  • Depth of field and how to bypass depth of field limitations via focus stacking.
  • Optical aberrations: what they are, what they look like, and what to do about them.
  • MTF, field curvature, focus shift: insight into the limitations of lab tests and why imaging performance is far more complex than it appears.
  • Optical aberrations: what they are, what they look like, and what to do about them.
  • How to test a lens for a “bad sample”.

Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.

Variants Leica 24mm f/3.8 Elmar-M,24mm f/2.8 Elmarit-M f/2.8 available in full article

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