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04-09-2013, 03:17 PM   #1
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pancake lenses

Hello!

I own DA70mm pancake lens, it is fantastic little lens. The fact that it is pancake designed lens makes it even more special. Thinking about pancake lenses, it occurred to my mind that da70mm is the longest lens in pancake design. So I have a question - a bit silly one, but I do not understand the physics and lens design at all, so forgive my ignorance. Is it theoretically possible to make, let's say, 135mm pancake lens? It would be great to have 100mm or 135mm pancake lens with f3.5 aperture.

Marko

04-09-2013, 03:47 PM   #2
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QuoteOriginally posted by hegedus Quote
Hello!

I own DA70mm pancake lens, it is fantastic little lens. The fact that it is pancake designed lens makes it even more special. Thinking about pancake lenses, it occurred to my mind that da70mm is the longest lens in pancake design. So I have a question - a bit silly one, but I do not understand the physics and lens design at all, so forgive my ignorance. Is it theoretically possible to make, let's say, 135mm pancake lens? It would be great to have 100mm or 135mm pancake lens with f3.5 aperture.

Marko
Take a look at the Pentax M 135mm F3.5. That's about as small as they'll go.

The DFA 100mm WR macro also employs a pancake design, but at that focal length it's not much of a pancake any more

Adam
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04-09-2013, 04:28 PM   #3
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Adam i'm a bit lost with what you mean with 100mm macro and the pancake design?


You could call some of the lenses for large formatt pancake lenses and they can easily range from 100mm to 180mm.
from what i know the register distance is the limiting factor in this

It's very easy to find for example 40mm pancake lenses for 135format camera's but if you look at for example mirorless the pancake lenses are all around 20mm.
04-09-2013, 04:59 PM   #4
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QuoteOriginally posted by Anvh Quote
Adam i'm a bit lost with what you mean with 100mm macro and the pancake design?
The outer barrel uses the same finish, and the diameter has been made as small as possible (i.e. compare it to the FA version!).


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04-10-2013, 02:53 AM   #5
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QuoteOriginally posted by Adam Quote
The outer barrel uses the same finish, and the diameter has been made as small as possible (i.e. compare it to the FA version!).
Ah you mean the limited finish

The 100mm WR certainly is nicely made, still have the non WR version though
04-10-2013, 03:33 AM   #6
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For the OP, if you consider focal length, a simple lens needs to be placed at its focal length away from the image receiver(sensor, or film) to focus at infinity and move forward I.e. away from the sensor to focus closer.

What makes a pancake lens possible, at the 40-70 mm range is that the Pentax system regestry distance is 46.46mm this is the distance from the focusing plane to the lens mount. What makes pancake lenses possible is simple optical designs with minimum elements thick allow a very compact overall lens body simply because they are at or close to the system regestry distance.

You can male longer lenses shorter physically than their focal length, and in fact, that is the true meaning of telephoto. But there are practical limits in lens design that make true pancake designs possible for tepephotos
04-10-2013, 04:30 AM   #7
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Thanks guys, that cleared things up for me a little bit. One can only dream that Pentax can find a way to make 100mm pancake lens, but obviously there are some limits that can not be avoided.
04-10-2013, 04:46 AM   #8
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Technically it is possible - but it requires some pretty unusual lens geometry and exotic glass types to make a pancake lens that is significantly longer than the mount flange - Diffractive optics allow for lenses to be considerably shorter than they ordinarily would be if they were designed with standard refractive optics*. Diffractive lenses have been used in lenses made by Canon - these lenses do have drawbacks similar to mirror lenses, bokeh has circular rings in it that bear a striking resemblance to the rings in a fresnel lens from a lighthouse. Also lenses with diffractive optics have reduced image sharpness, offering performance somewhere between a standard refractive lens and a mirror lens.

*For instance the Canon EF 400mm f/2.8L IS is 317mm long and weighs in at 3Kg- the Canon EF 400mm f/4 DO IS has a length of 233mm and weighs 1,930Kg - not much bigger than the Canon EF 300mm f/2.8L IS

04-10-2013, 07:55 AM   #10
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Morning coffee project, I made a diagram of the -M 1:2.8 40mm

And copied the diagram trying to get used to the SMC Pentax -M Zoom 1:2.8~4 40~80mm on the K-01
This lens becomes a fixed 80mm "close focus" when the end is pulled out. On a tripod pointing down vertically this copy creeps off focus,
as the push pull macro nose is quite heavy.
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04-10-2013, 08:51 AM   #11
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QuoteOriginally posted by Lowell Goudge Quote
What makes a pancake lens possible, at the 40-70 mm range is that the Pentax system regestry distance is 46.46mm
Don't forget that there are also K-mount pancakes down to 20mm focal length,
like the M20/4, DA 21/3.2, or Voigtlaender Color-Skopar 20/3.5.
04-10-2013, 12:21 PM   #12
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QuoteOriginally posted by lytrytyr Quote
Don't forget that there are also K-mount pancakes down to 20mm focal length,
like the M20/4, DA 21/3.2, or Voigtlaender Color-Skopar 20/3.5.
The M20/4 is, I agree a small lens but it is NOT a pancake in the true sense of the M40 as a comparison.
04-10-2013, 03:09 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
The M20/4 is, I agree a small lens but it is NOT a pancake in the true sense of the M40 as a comparison.
It's about the same length as the Color-Skopar, and that's considered a pancake:

Voigtlander Color Skopar 20mm f/3.5 SL II Aspherical (Canon EOS) - Lab Test / Review

"it is a ultra-wide angle pancake lens"
04-10-2013, 05:35 PM   #14
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The 40mm (especially XS) and the 70mm are so flat, they really raise the standard. On other systems much thicker lenses are called "compact" and even pancakes. Its all relative, I guess. Pentax certainly spoils us in that regard. But I think that is the reason why this thread was started, to clear up whether "pancake" lenses are just coincidentally compact, or if there is a special design property (and whether it can be used for other focal lengths). So we got an answer, which is that the camera mount and flange distance allow for certain designs to be made very compact, but there is a limit to it, and Pentax has probably either reached it or is very near to reaching it.
04-10-2013, 06:19 PM   #15
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QuoteOriginally posted by lytrytyr Quote
It's about the same length as the Color-Skopar, and that's considered a pancake:

Voigtlander Color Skopar 20mm f/3.5 SL II Aspherical (Canon EOS) - Lab Test / Review

"it is a ultra-wide angle pancake lens"
But it is really not. The pancake lenses are the absolute minimum in thickness for controls and nothing more. These lenses as I said are small but boot real pancake
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