What defined focal length of lenses?

Luckless · Feb 28, 2019

Humans like 'nice' numbers, so while looking at common focal lengths on the market today it is probably worth considering how many grew out of designs that may have originally been designed to imperial scales and lengths that are 'nice' in that system.

For example, if we go digging around the history of some 70mm designs then I wouldn't be remotely surprised to find a 2 3/4" lens, which is 69.85mm, and that readily becomes 70mm after being 'nicefied' by a marketing department.

M-88 · Feb 28, 2019

Sirius Glass said:
Are you expecting adults to act rationally??

Guilty as charged, I keep forgetting that.

Luckless said:
Humans like 'nice' numbers, so while looking at common focal lengths on the market today it is probably worth considering how many grew out of designs that may have originally been designed to imperial scales and lengths that are 'nice' in that system.

For example, if we go digging around the history of some 70mm designs then I wouldn't be remotely surprised to find a 2 3/4" lens, which is 69.85mm, and that readily becomes 70mm after being 'nicefied' by a marketing department.

And yet I would still be surprised if that

Theo Sulphate said:
I read somewhere that Minolta made it's f/1.2 lens in the 58mm focal length because -- at that time -- they couldn't design f/1.2 with a smaller diameter objective lens and a 50mm focal length. I don't know if that had anything to do with the characteristics of the SRT's mount.

Probably same goes for Olympus which had 55 mm f/1.2 before they developed 50 mm f/1.2 eight years later or so.

AgX · Feb 28, 2019

Theo Sulphate said:
I read somewhere that Minolta made it's f/1.2 lens in the 58mm focal length because -- at that time -- they couldn't design f/1.2 with a smaller diameter objective lens and a 50mm focal length. I don't know if that had anything to do with the characteristics of the SRT's mount.

Wouldn't the exit lens need even be bigger? To the contrary I assume in that design the total lenght too big, respectively the back focus distance too small for the mirror movement.

This also is the reason why at compact cameras you find 40/38mm Tessars whereas at SLRs the 50mm is the standard lens fot high speed lenses..

Ian Grant · Feb 28, 2019

It also has to be remembered that Focal Lengths are sometimes nominal, so a 150mm lens might actually be 152mm or 148mm, although more typically it's a 1mm or less either way.

The typical range of Focal Length lenses for 35mm cameras pre-dates companies like Topcon, we'd need to look at Leica, Zeiss Ikon (Contax), Ihagee (Exacta), KW (early Prakiflex/Praktica and Praktina) and pre and post WWII German lens design from a wide variety of companies.

Remember that the first full professional system camera was the Praktina, although the pre-WWII Exacta was earlier but lacked motor drives and a bulk film back.

Ian

M-88 · Feb 28, 2019

Ian Grant said:
It also has to be remembered that Focal Lengths are sometimes nominal, so a 150mm lens might actually be 152mm or 148mm, although more typically it's a 1mm or less either way.

The typical range of Focal Length lenses for 35mm cameras pre-dates companies like Topcon, we'd need to look at Leica, Zeiss Ikon (Contax), Ihagee (Exacta), KW (early Prakiflex/Praktica and Praktina) and pre and post WWII German lens design from a wide variety of companies.

Remember that the first full professional system camera was the Praktina, although the pre-WWII Exacta was earlier but lacked motor drives and a bulk film back.

Ian

Of course! That is why I have doubts about 35 mm gear engineers using Imperial unit system. That would have been more logical for medium format and large format cameras. And let's not forget that Nikon and Canon copied German 39 mm thread (and not just the thread) in their rangefinders, while Topcon adopted whimsical lens mount of Exakta and Pentax took M42 thread. I would consider Exakta to be a system camera, because were there even any motor drives before the second world war?

Ian Grant · Feb 28, 2019

M-88 said:
Of course! That is why I have doubts about 35 mm gear engineers using Imperial unit system. That would have been more logical for medium format and large format cameras. And let's not forget that Nikon and Canon copied German 39 mm thread (and not just the thread) in their rangefinders, while Topcon adopted whimsical lens mount of Exakta and Pentax took M42 thread. I would consider Exakta to be a system camera, because were there even any motor drives before the second world war?

Nikon copied the Contax, so used the bayonet mout. Probably the only SLRs using Imperial parts were the Wrayflex, of course the Corfield Periflex and some US mae cameras would have been Imperial as well.

I agree Exacta's were system cameras jut not a full professional system like the Praktina's.

Ian

M-88 · Feb 28, 2019

Ian Grant said:
Nikon copied the Contax, so used the bayonet mout. Probably the only SLRs using Imperial parts were the Wrayflex, of course the Corfield Periflex and some US mae cameras would have been Imperial as well.

I agree Exacta's were system cameras jut not a full professional system like the Praktina's.

Ian

Oh yes, they adopted the lens mount and overall body design from Contax, but swapped the complex shutter with simpler Leica type. Thanks for refreshing my memory! As for Praktina, I looked up its motor drive and film back. Looks quite intimidating.

Dennis-B · Feb 28, 2019

At the time, a lot of these lenses were designed and manufactured before modern CAD/CAM came into existence. It took huge sums of money to hold the necessary tolerance. Even the drawings, when taken down to the .1mm +/- tolerances, couldn't be manufactured to the same dimensions.

Before the 20th century, the Swiss were able to mill two metallic surfaces which could not be separated, just surface tension. However, these tolerances couldn't be held over volumes of work.

In manufacturing, there's the adage that you measure it with a micrometer, mark it with a grease pencil, and cut it with an axe.

Alan Gales · Feb 28, 2019

Dennis-B said:
In manufacturing, there's the adage that you measure it with a micrometer, mark it with a grease pencil, and cut it with an axe.

Now, that's funny!

Mick Fagan · Feb 28, 2019

Dennis-B said:
At the time, a lot of these lenses were designed and manufactured before modern CAD/CAM came into existence. It took huge sums of money to hold the necessary tolerance. Even the drawings, when taken down to the .1mm +/- tolerances, couldn't be manufactured to the same dimensions.

Before the 20th century, the Swiss were able to mill two metallic surfaces which could not be separated, just surface tension. However, these tolerances couldn't be held over volumes of work.

In manufacturing, there's the adage that you measure it with a micrometer, mark it with a grease pencil, and cut it with an axe.

I think you are talking about "gauge blocks"?

If so, they were invented by a Swedish person, Carl Edvard Johansson, sometimes referred to as, the world's Master of Measurement.

He was the inventor of the set of precise pieces of perfectly flat, hardened steel known to this day as gauge blocks, slip gauges, or, to his honour and in his memory, as Johansson gauges, or quite simply Jo blocks.

He got the idea while on a train journey home from Germany to the Mauser factory, where they had a very high respect for ultraprecise measurement. He found their scheme wanting. According to legend, he was pondering the idea of making improvements to the forthcoming Swedish operation while on a long and other wise tedious rail journey home.

This was in 1896.

Source: Exactly, 2018 by Simon Winchester, pages 167-168.

Mick.

M-88 · Mar 1, 2019

Mick Fagan said:
I think you are talking about "gauge blocks"?

If so, they were invented by a Swedish person, Carl Edvard Johansson, sometimes referred to as, the world's Master of Measurement.

He was the inventor of the set of precise pieces of perfectly flat, hardened steel known to this day as gauge blocks, slip gauges, or, to his honour and in his memory, as Johansson gauges, or quite simply Jo blocks.

He got the idea while on a train journey home from Germany to the Mauser factory, where they had a very high respect for ultraprecise measurement. He found their scheme wanting. According to legend, he was pondering the idea of making improvements to the forthcoming Swedish operation while on a long and other wise tedious rail journey home.

This was in 1896.

Source: Exactly, 2018 by Simon Winchester, pages 167-168.

Mick.

I tried to break one of those as a kid. I can confirm they are quite rugged and durable!

lxdude · Mar 1, 2019

Chan Tran said:
Well first of all they chose the normal focal length. Why 50mm and not 43mm I don't know.

I think it is around 50mm because of the aspect ratio, 50% wider than it is high. 135 has a little bit of a panoramic quality to it as a result, and I think the longer focal length diminished that compared to the wider angle 43mm.

AgX · Mar 1, 2019

lxdude said:
I think it is around 50mm because of the aspect ratio, 50% wider than it is high. 135 has a little bit of a panoramic quality to it as a result,...

Aspect Ratio is not related to Focal Lenght.

Dennis-B · Mar 1, 2019

Mick Fagan said:
I think you are talking about "gauge blocks"?

If so, they were invented by a Swedish person, Carl Edvard Johansson, sometimes referred to as, the world's Master of Measurement.

He was the inventor of the set of precise pieces of perfectly flat, hardened steel known to this day as gauge blocks, slip gauges, or, to his honour and in his memory, as Johansson gauges, or quite simply Jo blocks.

He got the idea while on a train journey home from Germany to the Mauser factory, where they had a very high respect for ultraprecise measurement. He found their scheme wanting. According to legend, he was pondering the idea of making improvements to the forthcoming Swedish operation while on a long and other wise tedious rail journey home.

This was in 1896.

Source: Exactly, 2018 by Simon Winchester, pages 167-168.

Mick.

You are correct. I was working from memory, and conflated Swiss and Swedish. Thanks.

Sirius Glass · Mar 1, 2019

What drives some focal lengths to be come more common? Preferences: I have used 58mm, 50mm, 43mm, 35mm, 28mm, 24mm and 21mm over the years. Using 50mm as normal I found that 35mm was too close to 50mm and prefer 28mm as a wide angle lens. I like the 24mm as wider but would rather use a 21mm for my very wide angle lens. Others have similar experiences. Some prefer 35mm over 28mm.

Dennis-B · Mar 1, 2019

Sirius Glass said:
What drives some focal lengths to be come more common? Preferences: I have used 58mm, 50mm, 43mm, 35mm, 28mm, 24mm and 21mm over the years. Using 50mm as normal I found that 35mm was too close to 50mm and prefer 28mm as a wide angle lens. I like the 24mm as wider but would rather use a 21mm for my very wide angle lens. Others have similar experiences. Some prefer 35mm over 28mm.

"Why" is as diverse as the entire 35mm community.

A lot depends on how you learned photography, and what was/is pleasing to your eyes. Did you like the vistas of the 21mm, or the 24mm? As for me, I prefer the 24mm, since that's the widest that I can use without having to worry about distortion from off-lens axis shots; but, that's me. Wide angles at 24mm also provide a bit of wiggle room for framing. I do like 28mm, but it's like a 35mm after using a 24mm lens.

I've often given thoughts to a 35mm as a "walk around" lens, but even using a zoom at that length, I always thought the photos were a bit flat for my liking. I'm not a real fan of the 50's, but they generally have faster speed, and when I need wide open, f/1.4 is really handy.

And, of course, a lot depends on what you're trying to accomplish. Do you need compression or expansion? Is the 135mm telephoto working, or do you need 105, or 200?

It seems that the selection is merely "horses for courses", and what you eye calls for you to use to get the image you want.

Luckless · Mar 1, 2019

It would be interesting to see a formal experiment on what people choose if we could control their education and external influences.

- How many people choose a given lens as their 'one and only go-to lens', not because they actually have any real reason to prefer that over any other, but because "that's what the expert/book/random internet post" said was 'the best'?

I kind of want to go and buy 3 lenses of the same model to repackage in new casings, mark them differently, and then offer to loan them to different photographers while claiming they're each a slight variant on a new design coming to market in a few years... Might be interesting to see how many come back saying "They're all the same lens".

AgX · Mar 1, 2019

There are by now manufacturers doing so...

Joel_L · Mar 1, 2019

I think 45-50mm is considered "normal" in 35mm format because it most closely resembles what you see with your eyes.

Arklatexian · Mar 2, 2019

Luckless said:
Humans like 'nice' numbers, so while looking at common focal lengths on the market today it is probably worth considering how many grew out of designs that may have originally been designed to imperial scales and lengths that are 'nice' in that system.

For example, if we go digging around the history of some 70mm designs then I wouldn't be remotely surprised to find a 2 3/4" lens, which is 69.85mm, and that readily becomes 70mm after being 'nicefied' by a marketing department.

Could it be that the next lens coming down the assembly line would actually be 70.25 mm? These lenses are "manufactured" items to be sold to a "mass" market. If you want closer tolerances, you will pay far more than most are willing to pay. What is the old expression: "close enough for government work". I should add except for NASA here in the USA. (maybe).........Regards!

Arklatexian · Mar 2, 2019

Joel_L said:
I think 45-50mm is considered "normal" in 35mm format because it most closely resembles what you see with your eyes.

+1

Luckless · Mar 3, 2019

Arklatexian said:
Could it be that the next lens coming down the assembly line would actually be 70.25 mm? These lenses are "manufactured" items to be sold to a "mass" market. If you want closer tolerances, you will pay far more than most are willing to pay. What is the old expression: "close enough for government work". I should add except for NASA here in the USA. (maybe).........Regards!

I've often wondered how consistent different models of lenses are to different examples within a given line, and whether or not there have been any trends in production tolerances over the years. Are they getting more consistent, or does computer automation allow manufacturers to run looser tolerances for individual glass elements and batch stuff together into functional lenses more easily?

Bill Burk · Mar 3, 2019

Here’s an illustration from Petersen’s Pentax System.

They’re all nicely spaced. In the range of 85 to 105 I think some emphasis was given to the greatest aperture they could get for a given (49 mm) filter size, or if they could have a very bright lens hang the filter size restriction (haaa why 67mm filter for their earlier 35mm f/2).

Once you get to the hundreds thery jump by hundreds.

Dennis-B · Mar 3, 2019

Luckless said:
I've often wondered how consistent different models of lenses are to different examples within a given line, and whether or not there have been any trends in production tolerances over the years. Are they getting more consistent, or does computer automation allow manufacturers to run looser tolerances for individual glass elements and batch stuff together into functional lenses more easily?

One of the things I learned in my career, was that manufacturers sought tighter tolerances. If the high/low limits grow, it becomes more likely that a part with a low variation may eventually not work with another part which has a high variation.

On the Science Channel, there's a series titled "How It's Made". Now, you're not going to become an expert on any of the subjects they show, unless you already have it. However, one of the things they highlight is the constant computerized quality checking now done in a lot of products. In many instances products not meeting tolerances will be sent back for rework automatically, without human intervention. One of the programs showed how camera lenses were made, and the process is remarkable.

I worked for a major auto manufacturer for 38 years in finance and IT, and our bosses wanted us to be familiar with all phases of manufacturing, not necessarily to work there, but how to recognize changes in engineering and manufacturing. In our plants, the torque guns used to drive screws and nuts send torque settings for every screw, nut and bolt installed, to file servers so the vehicle has a complete history of manufacturing. Now, most of us can't imagine why a door panel screw torque history, but someone needs it, either to improve the process or design.

M-88 · Mar 9, 2019

Interesting excerpt which explains why there were 55 and 58 mm lenses:

"
This lens, the first normal lens for the Nikon F is a four(4)-group, six(6)-element Gaussian type with an additional weak concave element at the front, for a so-called retrofocus type lens of seven(7)-element construction. (See Fig. 1.)

As you know, for an SLR camera lens, the back focus must be sufficient that the lens does not interfere with the quick-return mirror, which makes the lens design harder the larger the aperture and the shorter the focal length. For this reason, at that time the normal lens for an SLR usually had a focal length slightly longer, at 55mm or 58mm.
The challenge for lens designers at that time was to bite the bullet, and increase the number of elements in the lens, to achieve a 50mm focal length."

Taken from here:
https://imaging.nikon.com/history/story/0002/index.htm

What defined focal length of lenses?

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