Why not a 12-35 F1.8 - F2.8?

Started Apr 2, 2014 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: How equivalency works.

Klarno wrote:

superstar905 wrote:

Klarno wrote

Probably size and cost concerns. The f/2.0 FT zooms were generally criticized for their being the same size as, and more expensive than, FF f/2.8 glass; enough so that, looking at the systems separately, you'd actually save money and gain capability going for an FF system with a 24-70 and 70-200 over an FT system with a 14-35 and 35-100. While they may have been marvelous pieces of glass, they didn't by any means play to the system's strengths.

You think its due to size concerns? How is the Sony 28-70 able to be so small then and priced well too?


The combo comparison size and price are almost the same, although its well known the quality of the Panny lens is superior to the Sony.

The bigger your sensor, the longer focal length you need to get the same angle of view. Conversely, the smaller your sensor, the shorter focal length you need, and the more retrofocus lens design you need to get the job done.

It has to be more retrofocus only if the ratio of the flange distance to the diameter of the image circle is greater. If you compare MFT with FF DSLRs, that ratio is slightly smaller for MFT rather than slightly greater.

It might just be easier to design a typical full frame lens just because it's been done so much longer and they simply don't have the same resolution and precision demands as smaller, newer formats.

The increased resolution requirement (per millimeter but not per image diagonal) is satsified automatically if you take an FF design, say a 50/1.4, and just scale it downwards in all relevant dimensions to so as to make a 25/1.4 for MFT. The two lenses will perform identically at the same f-stop. This holds true as long as resolution is limited by optical aberrations only. Once diffraction is taken into account, the MFT combo will do worse at the same f-stop, increasingly so as you keep stopping down.

As to precision demands, it is true that the tolerances are smaller for the smaller format if expressed in absolute terms (e.g., microns). On the other hand, it may be easier to achieve the same  tolerance in absolute terms the smaller something is. Practically speaking, I find it hard to think that this is much of practical obstacle given how well far smaller units can be made to perform these days.

One should additionally keep in mind that the smaller format offers certain advantages in terms of design. High-quality glass costs something and MFT needs smaller amount of it for a lens with the same AoV and the same max aperture (as expressed in f-stops).

However, if the task is to make an MFT lens that is equivalent to its FF counterpart, i.e., has the same max aperture diameter and thus twice the f-ratio, then the MFT lens is likely to be bigger, heavier, and/or perform less well at equivalent apertures. It is a far more demanding task to make, for example, a 25/1.0 for MFT than to make a 50/2.0 for FF.

Also the Panasonic achieves 24mm EFL at the wide end, which because of retrofocus design makes quite a difference size-wise. Heavily asymmetrical retrofocus design is less necessary on non-SLR cameras, but still remains necessary on any camera that has a fixed mount size. Both MFT and the Sony E-mount full frame system are using computational distortion correction, so they're in the same boat there.

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