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

Started 5 months ago | Discussions thread
Anders W
Forum ProPosts: 17,888Gear list
Re: How equivalency works.
In reply to Klarno, 5 months ago

Klarno wrote:

Anders W wrote:

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.

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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.

Of course, the FF camera compared above wasn't a DSLR-- the Sony A7 with the 28-70 runs the E mount, which IIRC has a shorter flange distance than MFT. MFT is at a retrofocus disadvantage here, simply by way of 28mm on Sony E mount being longer than the flange distance, and 14mm on MFT being shorter than the flange distance.

Sorry about that. It temporarily slipped my mind (though I don't know quite how ) that it was an FE-mount lens we were comparing with.

Retrofocus is really a fact of modern lens design. There are a few manual focus rangefinder lenses that have rear elements that protrude significantly back into the camera, but what's the last AF lens you've seen that does that?

The Sony RX1 whose rearmost element sits just a couple of millimeters from the sensor. But yes, symmetric WA designs aren't exactly common these days even when they would be possible (mainly because of problems with vignetting of one kind or another, I would guess). Interestingly, the Sony FE 28-70/3.5-5.6 appears to be a bit retrofocus although it wouldn't have to be in view of the flange distance. See here:


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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