Why no 40mm pancake (or similar)?

Started 10 months ago | Questions thread
David 247
David 247 Veteran Member • Posts: 3,289
Re: Conclusion: I seem to be the only one ;-)
3

mkaresh wrote:

See my comment below. Whatever was done to enable the Pentax 70mm should enable a 40mm, 45mm tops m43 pancake.

The Pentax is a collapsible lens for one thing.

Another factor that must be considered, is volume of sales. Canon, Nikon and yes even Pentax have had some of their designs around for years. Pentax has even recycled many older designs, just using more modern techniques and materials. The cost of lenses is directly proportional to the volume of sales. Hate to say this, but in the MFT market, while there is some demand for compact primes, it is a small demand. The cost of producing in low volume would dramatically increase the cost, and for a product that may never generate a profit threshold. MFT is still in it system infancy, in terms of the big marketing picture.

I do wonder what is given up by having fewer elements within the lens, 6 in the Pentax vs. 9 in the Olympus 45mm and 14 in the 45mm Pro. For a sense of what can be fit within a collapsible pancake, the Panasonic 12-32 does have 8 elements, so nearly as many as the Oly 45mm.

It is usually a matter of sharpness and image quality. Keep in mind that IQ is not a single factor but many design factors that must be balanced. resolution, flare, contrast (especially micro contrast), distortion, Chromatic Aberration, and vignetting (brightness from center to corners, and of course lens speed.

For now, the primary demand for MFT lenses centers around normal zooms, high quality (expensive) zooms and fast primes. The demand for compact lenses is in the low production cost range of wide compacts, which are easy and cheap to design and produce. Try to take a Canon, Nikon or Pentax lens and use it on an MFT, it will require a converter/extender. Again. Lens Flange distance is a major factor. MFT is built around a short lens flange by design.

Now there are physical limits related to the lens flange. Sheesh I don't believe I am going to try to explain this, but here goes. It has been more then 40 years since I studied this in college.

This is basically what a typical camera lens optical path looks like. This is a very simplified drawing.

Note that the light passes through the lens, is redirected by lens elements, reaches a "flip-over" point, where the light rays cross over and in turn project the image of the subject "upside down" on the film or sensor surface. The focal length is measured from that optical flip-over point to the recording surface and may be internal to the lens or in some cases behind it. Focal length can never be greater then total lens length plus flange length and usually must be shorter.

Pancake lenses are very simple designs. In the real world of high resolution optics, zooms and variable distance lens elements it may look more like this.

In fact in the highest quality and most sophisticated lenses, designs will even vary by the frequency of light for different colors. Pro lens tend to not just compensate for light in general but also for specific frequencies in order to get them to all focus on the recording surface equally. CA or Chromatic Aberration occurs when different light frequencies (colors) focus at different distances. That is why PRO lenses are so much more expensive, both due to the optical and mechanical complexity as well as the need to correct for various light frequencies.

With most lenses, the flip over point, which is the optical center, is within the lens. In some cases such as pancake lenses, the flip-over point or optical center may occur behind the lens, with the lens elements. The Olympus 9mm body-cap I think is a good example. Focal length of the lens or the point from crossover of light rays. occurs behind the lens within the lens flange itself. (think of a magnifying glass). This happens because the lens flange is greater then the optical focal point. (The Olympus 9mm lens by the way is a very complex optical formula with 5 elements in 4 groups, including 2 aspherical lens element.). in the case of the 9mm, the light rays coming out of the lens are focused at a point, 9mm in front of the sensor but behind the physical lens.

And yes, the term flip over is not the correct term but my memory is failing me on the correct term, but it is the point where light optical path crosses and is the optical center or focal length of the lens. (optical center is different from physical center of a lens).

Member Chas2 explains it some in this post, and is referencing Pentax lenses.

https://www.dpreview.com/forums/thread/3605036

Essentially, Canon, Pentax, Nikon, etc, have flange lengths much longer so they have more leeway in design. To create a 70mm pancake lens for the is not possible because the lens flange plus the lens length for a pancake lens is less then the focal length of the lens. There must be room within the lens or behind it for the focal cross-over.

The 20mm Olympus lens has a depth of about 25mm, plus the flange length is no more then 55mm total distance from sensor. The cross-over must occur either within or behind the lens. 20mm focal length is already, pushing the limits for MFT flange length. You can create a 70mm focal length lens but it cannot be a pancake lens. It can be done with camera bodies with longer flange lengths or by using an extension tube to extend the flange length but then it is no longer compact or a pancake. I would speculate that the longest theoretical pancake lens for MFT would be 30mm and that would probably be very complex, difficult and expensive.

Pentax, canon and others can make longer pancakes because they are already working with at least 55m of flange length so a 40mm pancake is relatively simple. A 60mm might be doable but I doubt it. The 70mm Pentax expands to increase its length to allow room, and is only compact. when collapsed.

Not the best explanation, and others can do a much better job but maybe it conveys the general idea and limitations. It all comes down to how far the optical center of the lens must be from the recording surface and. lens flange length is a factor. You cannot create a lens where the optical focal length is greater then the lens depth and flange length.

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 David 247's gear list:David 247's gear list
Panasonic FZ1000 Olympus E-M5 II Olympus 12-40mm F2.8 Olympus 9mm F8 Fish-Eye Body Cap Lens Olympus 40-150mm F2.8 Pro +5 more
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