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05-17-2014, 04:52 PM   #1
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50 mm vs 55 mm lens focal length

Most of the major lens manufacturers have produced both 50 and 55 mm standard lenses, including specifically Pentax and Canon. Pentax has made 55 mm lenses in DA (1.4), K (1.8 and 2.0), and Takumar (1.9, 2.0, and 2.2), but not in F, FA, M, or A as far as I can find out.

Wondering if anyone can explain why a lens lineup would include 50 mm as well as 55 mm? Are the 5 extra mm particularly significant for certain applications, or perhaps some other reason. On cropped sensor, where a 55 mm shows up as 82.5 mm, the slightly more narrow focal length seems to me to be helpful in making the 55 mm a very nice portraiture lens. However, most of the 55s seem to have been produced during film days, the widely acclaimed Pentax DA 55 1.4 notwithstanding.

05-17-2014, 05:21 PM   #2
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The difference really isn't significant- perhaps it was due to manufacturing limitations that there were similar 55's and a 50's back in the old days.

The reason the DA* 55mm is that focal length is to directly emulate a 85mm portrait lens on film.

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05-17-2014, 05:26 PM   #3
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Good question. I think the focal length is so close as to make no difference but in theory for APS-C 55mm is the best for portraits. Why 55mm for film I don't know.
I do know that the Takumar 55mm (I have a SMC f 1.8) is a great lens for across frame sharpness so it may have more to do with construction of the lens being distinct rather than the focal length itself.
05-17-2014, 05:39 PM   #4
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The difference of 5mm is really not a big deal. The difference in lens design can be fairly significant. The DA *55 is a very nice lens. Obviously the 50mm f1.2 and Voigtlander 50mm f1.4 also are sharp lenses with great out of focus rendering. Anyway, I would choose between these lenses not on the basis of focal length, but on the basis of the other qualities they have or don't have.

05-17-2014, 06:12 PM   #5
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1.) Possibly because a major camera maker would not want to be missing a FL another camera maker offered. Although a macro (micro) Nikon had a very popular lens in the 55mm. Likely the other important camera companies had also.
2.) And prior to retrofocus design it was probably easier to design a 55 or 58mm lens.
3.) I also think the difference in 50 to 55mm is not trivial in the view one has--at least I find it so using a 55mm and 50 mm (on Nikon film camera).
05-17-2014, 06:24 PM   #6
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I know I read somewhere (might have been on here somewhere) that 55mm was the easiest focal length to get good quality at for early lenses, and so a lot of the oldest fast lenses are 55mm (or 58 for some manufacturers) instead of 50.
05-17-2014, 09:04 PM - 1 Like   #7
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QuoteOriginally posted by Rondec Quote
The difference of 5mm is really not a big deal. The difference in lens design can be fairly significant. The DA *55 is a very nice lens. Obviously the 50mm f1.2 and Voigtlander 50mm f1.4 also are sharp lenses with great out of focus rendering. Anyway, I would choose between these lenses not on the basis of focal length, but on the basis of the other qualities they have or don't have.
The focal length of the Voigtlander 58mm f1.4 is 58mm. (There is a 50mm f1.8.) Not 50mm, not 55 mm, ... but 58 mm. In fact Voigtlander has a couple of unusual focal lengths: 58mm, 125 mm.

Pentax has also some unusual focal lengths: 31 mm, 43mm, 77mm.
05-17-2014, 09:48 PM   #8
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QuoteOriginally posted by narual Quote
I know I read somewhere (might have been on here somewhere) that 55mm was the easiest focal length to get good quality at for early lenses, and so a lot of the oldest fast lenses are 55mm (or 58 for some manufacturers) instead of 50.

not really, we had 50mm f1.5's back in the 30's


This is a good general read of lens designs,
History of fast 35mm film optic lenses

I think the current 55mm lens length we are left with today is more a function of marketing then principle, ie. this Nikkors, Nikkor 55mm f/1.2 Standard Lenses - Version History - Part III were extremely popular once SLR's had gained traction.

05-17-2014, 10:53 PM   #9
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In the Pentax world of the late 1960’s and early 1970’s 55mm was more of a slower “consumer” budget lens and 50mm was faster and more of a “professional” lens.

When I bought my first Pentax SLR kit in 1975, I could not afford the K50/1.2 or K50/1.4 and settled on the cheaper K55/1.8.

Of course all three lenses are superb, but that was the main difference between the 50mm and 55mm back then, price and speed.

Phil.
05-18-2014, 07:12 AM   #10
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The difference is extreme. So if you happened to have six or eight 50mm lenses, you could still get another five 55mm lenses and no one would be inclined to add the two groups together.
05-18-2014, 11:05 AM   #11
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i think it has something to do with what was considered a normal lens over the years. I think first cameras would use 60, 58, 55mm then came the 50, 47, 45, 43 and 40mm. From there on, 37, 36, 35mm, etc. were considered wide angle lenses. So ranging from 35-60mm you can have wide normal to long normal lenses. but disregard the 3mm or less difference because even a lens called "50mm f/1.4" could actually be 52.5mm or 48mm if measured precisely. Optical formula is another matter of course. A 58mm f/1.4 could be very different form a 60mm f/2 or 55 f/1.8 but not because of focal length difference.
05-18-2014, 01:32 PM   #12
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QuoteOriginally posted by carrrlangas Quote
i think it has something to do with what was considered a normal lens over the years. I think first cameras would use 60, 58, 55mm then came the 50, 47, 45, 43 and 40mm. From there on, 37, 36, 35mm, etc. were considered wide angle lenses. So ranging from 35-60mm you can have wide normal to long normal lenses. but disregard the 3mm or less difference because even a lens called "50mm f/1.4" could actually be 52.5mm or 48mm if measured precisely. Optical formula is another matter of course. A 58mm f/1.4 could be very different form a 60mm f/2 or 55 f/1.8 but not because of focal length difference.
Not only that, but I believe lenses can vary quite a bit in focal length depending on focus setting as well, so that the actual focal length will differ depending on you focusing on infinity or minimal focus distance.
05-18-2014, 03:05 PM   #13
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The "rated" focal length of a lens is measured from the infinity setting (you need some standard reference point). The focal length is measured from the optical (not physical) center of the lens to the focal point.

With short focal lengths the manufacturers are able to make smaller and lighter lenses. Consider f-number (N) = focal length (f) divided by the diamter (D) of entrance pupil or effective aperture ( N = f / D).

So for a 58mm f/1.4 lens you would need lenses that is around 41.4mm in diameter. For a 50mm f/1.4 only around 35.7 mm. That's mostly why my Minolta 58mm f/1.4 Rokkor has a 55mm filter ring vs 49mm on my 50mm f/1.4 Minolta MD lens.

The shift to the 45-50mm range for "normal" lenses could also have been due to changes in what is considered as the "normal" focal lengths for a format. The current definition is +/-20% of the diagonal of the image format. Thus for Full Frame (135 film) diagonal 36 x 24 = 43.3mm (+/-20%) = 34.6 to 51.9 mm. 58mm would be more like 25%
05-18-2014, 08:46 PM   #14
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This has I think to do with convention and to space available for the mirror box in an SLR.


The focal length is the distance between the rear element and the plane of focus. It was usual to not employ a retrofocus design if possible, (a retrofocus design is a design that keeps the rear element away from the camera body by using extra elements) because with 50mm and longer lenses there was more than enough space to stop the mirror hitting the lens as it lifted,


The distance needed for this varied in different camera manufacturers between 42mm and 46mm. Miranda and Canon had small mirror boxes, Pentax has slightly longer, Nikon had the largest. Thats why if you put a pentax lens on a Nikon body it wont focus to infinity. But any lens from any manufacturer will focus to infinity on a canon body, and in fact beyond.


Now lets consider pentax.


By convention, the 50mm would be a good focal length photographically, however lenses at that focal length bring the rear element so close to the mirror box that conflicts can happen. The 50mm f1.4 for example is unusable on some bodies due to the mirror striking it. The 55mm f1.8 has no conflicts with the mirror on any camera that I know of.


I have heard that the 50mm f1.4 did in fact hit the mirror in at least one pentax model.


There is no significant difference between 50mm 55mm or indeed 60mm, so if a 50mm is a bit too close to the mirror box it makes good business sense to use 55mm in your design rather than the almost identical 50mm focal length.


With the 50mm f1.4 Pentax chose it seems to live with the mirror conflict possibility at this shorter focal length, rather than go the more expensive route of retrofocus design with extra elements.
05-19-2014, 06:11 AM   #15
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QuoteOriginally posted by Imageman Quote
This has I think to do with convention and to space available for the mirror box in an SLR.


The focal length is the distance between the rear element and the plane of focus. It was usual to not employ a retrofocus design if possible, (a retrofocus design is a design that keeps the rear element away from the camera body by using extra elements) because with 50mm and longer lenses there was more than enough space to stop the mirror hitting the lens as it lifted,
Focal length is measured from the principal plane (H') of the lens (often called the optical center) to the primary focal point or plane. A single thin lens could be placed at H' to achieve the same focal length but the light would focus to a point.

Volume 1, Chapter 33. The Human Eye as an Optical System

Consider this diagram - H prime (H') is before the front element.


QuoteOriginally posted by Imageman Quote

Now lets consider pentax.


By convention, the 50mm would be a good focal length photographically, however lenses at that focal length bring the rear element so close to the mirror box that conflicts can happen. The 50mm f1.4 for example is unusable on some bodies due to the mirror striking it. The 55mm f1.8 has no conflicts with the mirror on any camera that I know of.

I have heard that the 50mm f1.4 did in fact hit the mirror in at least one pentax model.

There is no significant difference between 50mm 55mm or indeed 60mm, so if a 50mm is a bit too close to the mirror box it makes good business sense to use 55mm in your design rather than the almost identical 50mm focal length.

With the 50mm f1.4 Pentax chose it seems to live with the mirror conflict possibility at this shorter focal length, rather than go the more expensive route of retrofocus design with extra elements.
The registration distance for Pentax bayonet (K) and screw (M42) mount is 45.46mm.The registration distance is measured from the base of the flange on the lens (or top of the mount on the camera body). That would mean the rear element on a 50mm need only be 4.54mm away from the body or 4.54mm into the lens body.

If focal length is always measured from the rear element then there would have been no need for anything on a 50mm lens to protrude past the flange other than the aperture linkages. It would also mean that telephoto lenses would have a tube the equal to the focal length less 45.46mm. It would mean zoom lenses would always have floating rear-element that travels the range of the zoom less 45.46mm (not all zooms have floating rear elements). It would mean wide-angle lenses rear-elements would have to protrude into the camera body by 45.46mm less the focal length of the lens. Even with retrofocus designs if focal length is measured from the rear element then the K-mount would be limited to say no wider than 30mm.
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