Why are there no 200mm or 300mm pancake lens

713alan · 08-22-2007, 02:38 PM

The pancake lenses seem to have good quality and reasonable prices. If they can make a 70mm f2.4 pancake lens, why are there no longer versions. I think there would be a market for a short, light 200mm or 300mm lens in f4 or faster. Is there some physical limit to the pancake design?

AVANT · 08-22-2007, 02:48 PM

Because longer focal lengths requires greater distance between the len elements. To maintain sharpness and correct the picture at those distances, it'll requires more glass. To get larger aperatures, it requires larger glass elements... list goes on...

matiki · 08-22-2007, 02:49 PM

Originally Posted by 713alan

The pancake lenses seem to have good quality and reasonable prices. If they can make a 70mm f2.4 pancake lens, why are there no longer versions. I think there would be a market for a short, light 200mm or 300mm lens in f4 or faster. Is there some physical limit to the pancake design?

I think it'd be really hard to squeeze 20 or 30cm of focal length into a pancake. Call me crazy.

713alan · 08-22-2007, 04:02 PM

OK, I misunderstood the term pancake. Since Pentax only uses pancake to refer to three lens, I thought it was a lens design not just a marketing term. Instead all of their short focal length lenses are as pancake as the official pancake lenses.

**Peter Zack** · 08-22-2007, 05:20 PM

No not really. Not all short focal length lenses are 'pancake'. This nickname only refers to the 21, 40 and 70mm versions. The 14mm or 31mm versions would never be called pancake. What they really mean when referring to them this way is they are very flat or slim. They don't stick out from the camera body very far, unlike conventional lenses.

713alan · 08-22-2007, 07:31 PM

Peter

That was my question. If Pentax can make some lenses as pancake design that are shorter than their conventional lenses, why can't they make shorter versions of their other conventional lenses?

**Peter Zack** · 08-22-2007, 08:11 PM

I'm no engineer so I'm only going from what I've picked up over the years and read about with lens design. But I think Avant was correct. Greater distance is required between the glass elements as the lens gets longer and then there are aspherical elements to correct abberations that are thicker and so on. Also many longer lenses will have a cam with a floating lens to get better image quality at different focus distances. A 300mm lens will react differently at say 10 feet when close focused than it will at 100 feet. So there is an internal floating element that keeps the lens sharp at both distances. It needs room to move. I don't think this is as critical on a short focal length lens.

Then there is speed. Many of these short focal lenth lenses in a pancake are slower at max aperture (20-25%). There simply isn't enough room to have a large diameter set of elements in there to let more light in. So the 43mm is f1.9 the 40mm is f2.8, 70mm f2.4 vs. 77mm f1.8 and 21mm f3.2 vs. 20mm f2.8

If the 300mm FA* is f4.5 and a pancake (if it could even be done) was f 6.0 no one would want it because that lens has an entirely different function. Most users want that lens for either wildlife or sports shooting and will add a 1.7 x TC or 2 x TC much of the time to it to get that great shot of an eagle. It's enough of a challenge at f 9 to get that shot and would be very difficult at f 12 in many cases. It would just be too slow to be of any value.

Plus there's the macho factor. big ears, big feet.....the longer the lens.....

little laker · 08-22-2007, 10:14 PM

I have to agree with Peter. There's no way that I could get most of my wildlife shots with a lens that slow.

I do have a 70-200mm lens that's just a little smaller than the kit lens, and although it's as fast as the rest of my lenses it can't match the picture quality of the rest of my collection.
As a result it's hardly ever used.
A 200mm pancake would probably be worst yet

Abbazz · 08-22-2007, 10:35 PM

Originally Posted by 713alan

If Pentax can make some lenses as pancake design that are shorter than their conventional lenses, why can't they make shorter versions of their other conventional lenses?

In the good old times, lenses used to be at least as long as their focal length when focused to infinity and much longer when focused closer. But the lens designers noted that by including a negative element behind a long focal lens, the optical path was shortened, therefore allowing to make shorter lenses, which were called telefocus lenses.

That's why today a 500mm lens is not 500mm long. Unfortunately, there is a limit to the power of the negative element, because an element too strong would create a lot of aberrations. That's why it is not yet possible to make pancake lenses with long focal length.

Cheers!