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12-20-2015, 10:56 AM   #1
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Explain: Lens designation compared to 35mm

I may have missed something, so apologies first: when you have a lens say 20-200mm, for a DSLR, some manufacturers may say 'equivalent to 35-270mm in 35mm'. Now my understanding is that in 35mm we can equate to a lens by our own eyesight: 50-55mm is 'normal vision' to us. We can understand a lower mm is wider larger is closer. So how can I figure out DSLR lens designations, because they are not 'true' to us. I'm sure i can figure out something, but why are the lens NOT designated 'true' like a 35mm lens?? So in the above example the manufacturer should designate the lens as 35-270mm not 20-200mm. Would any like to explain this? thanks.

12-20-2015, 11:00 AM   #2
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https://en.wikipedia.org/wiki/Angle_of_view

https://en.wikipedia.org/wiki/Crop_factor

https://en.wikipedia.org/wiki/Normal_lens
12-20-2015, 11:08 AM   #3
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And don't forget https://en.wikipedia.org/wiki/Focal_length , to make it clear that focal length is a physical property of the lens that does not depend on sensor size.

Myself, I started in the DSLR era, and I often find myself wishing that manufacturers didn't give 35mm equivalents so often -- because I keep having to remind myself that on APS-C the 28-35mm range is what's "standard", when really this ought to come naturally to someone who's only ever shot APS-C.
To me, APS-C fields of view *are* what's "true".
12-20-2015, 11:09 AM   #4
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These "equivalent to" explanations are only given for very small sensors, where it is difficult to imagine the FoV of a certain lens. Often one doesn't even know the exact size of these sensors.

12-20-2015, 11:10 AM   #5
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QuoteOriginally posted by fstop18 Quote
I may have missed something, so apologies first: when you have a lens say 20-200mm, for a DSLR, some manufacturers may say 'equivalent to 35-270mm in 35mm'. Now my understanding is that in 35mm we can equate to a lens by our own eyesight: 50-55mm is 'normal vision' to us. We can understand a lower mm is wider larger is closer. So how can I figure out DSLR lens designations, because they are not 'true' to us. I'm sure i can figure out something, but why are the lens NOT designated 'true' like a 35mm lens?? So in the above example the manufacturer should designate the lens as 35-270mm not 20-200mm. Would any like to explain this? thanks.
The first step is to understand that focal length is a property of the lens that doesn't change with sensor size. What changes is the angle of view. This article should help clear things up:

The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

Generally speaking, the focal length labeled on the lens is the true focal length (this applies to all K-mount lenses, at least). So the best thing to do, in my opinion, is to ignore all "equivalencies" and look at the focal length on its own.

On APS-C, a 35mm lens gives a "normal" field of view. On full frame/film, a 50mm lens gives roughly the same field of view.

On APS-C, a 24mm lens is a wide-angle, whereas on full frame, 24mm is closer to ultra-wide since the sensor is larger and sees more of the frame.

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12-20-2015, 11:54 AM   #6
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In a the smallest nutshell I can give you: Since 1913, cameras were made to handle 35mm sprocketed motion picture formatted film, and due to cost, became the defacto popular film standard. The width of the film including the sprockets is 35mm, but the actual image created is 24x36mm.

Although Kodak engineer Steven Sasson created the first digital camera in 1975, it was about the size of Sputnik, and the earliest DSLRs were Kodak cameras in Nikon bodies in the early '90s. So you have a whole legacy of photographers with lenses and manufacturers with decades of bias and experience with the 24x36mm format.

Digital has evolved from this, thus what we call full frame sensors today are 24x36mm in size. With a smaller sensor, like APS-C found on all Pentax DSLRs at present time, the angle of view is essentially cropped on the sensor compared to a larger sensor. So the equivalency notation is to help old-timers or FF shooters like me who can't do the 1.5x math.

NOW if anyone has read this far, I have a peeve that I don't understand. My Pentax 645 (and all other film 645s) are named thus because of the 6 cm. x 4.5 cm size format, which was called the ideal format because unlike 24x36mm, it is more proportionate and a closer aspect ratio to standard 8x10", 11x14", 16x20" prints.

So WHY is the Pentax 645Z named such with a 43.8 x 32.8mm size sensor? That is like a medium format crop sensor or what APS-C is to FF. It messes with my 35mm, 75mm, and 200mm Pentax 645 A primes. From a marketing and cost perspective, I get it. But call it what it is, not what it appears to be, but isn't. The Pentax 4433?
12-20-2015, 12:08 PM   #7
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To fstop18: All the comments above are correct.

However, I really hate talking about "35mm equivalents". You are correct that 50mm was considered "normal" on a 35mm camera. Given that all current Pentax dslrs use what is referred to as an APS-C sized sensor, which is approximately 16x24mm, the angle of view for a given lens is narrower.

You just need to learn a couple of new reference points. On a 35mm camera, and on the upcoming Pentax full-frame dslr, 50mm is normal. Less than 50mm is wide angle and greater than 50mm is telephoto.

On an APS-C camera, approximately 33mm focal length is normal. That is, it will give the equivalent angle of view as a 50mm will on 35mm. Less than 33 is wide, more than 33 is tele. That's all you really need to know about "equivalents".

There is a little more to it than that, particularly when referring to depth of field, but as far as focal length is concerned "wide < 33mm (normal) < telephoto".
12-20-2015, 12:27 PM   #8
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QuoteOriginally posted by noblepa Quote
To fstop18: All the comments above are correct.

However, I really hate talking about "35mm equivalents". You are correct that 50mm was considered "normal" on a 35mm camera. Given that all current Pentax dslrs use what is referred to as an APS-C sized sensor, which is approximately 16x24mm, the angle of view for a given lens is narrower.

You just need to learn a couple of new reference points. On a 35mm camera, and on the upcoming Pentax full-frame dslr, 50mm is normal. Less than 50mm is wide angle and greater than 50mm is telephoto.

On an APS-C camera, approximately 33mm focal length is normal. That is, it will give the equivalent angle of view as a 50mm will on 35mm. Less than 33 is wide, more than 33 is tele. That's all you really need to know about "equivalents".

There is a little more to it than that, particularly when referring to depth of field, but as far as focal length is concerned "wide < 33mm (normal) < telephoto".
And I concur here too. Technically 58mm was the closest to the human eye field of view, but due to cost of the design, 50mm became the practical approximation with 35mm film or FF. Thus on an APS-C sensor, 33mm is the approximate equivalent to 50mm and 39mm the APS-C version of a 58mm FF.

Because I shot with a 58mm f/1.2 prime for many years on a FSLR, I find the DA 40mm f/2.6 Limited very natural for me, while others find it awkwardly long and prefer 31mm.

But all this seems to be very subjective because there are articles and documentation that have 43mm pegged as the true target for a single human eye field of view and 22mm for both eyes (35mm equivalent=FF). Although I'm a prime fan, this is a great argument for an advantage of zooms.

12-20-2015, 12:43 PM   #9
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So.. simply put.. not all cameras have the same size sensor ("film"). This means if you put the same lens on them, they might not all capture the same frame. A lens with a certain focal length will project a certain image. But the angle of view of the actual recorded photo can be bigger or smaller than that image projection. So a certain lens can appear to have a different angle of view when you put it on, for example, Pentax Q camera, Pentax K-5, and Pentax 645Z. Because these cameras have a different sized sensor, which can capture less of the projected image (like the Q, which is very small) or a bigger sensor which captures more of the projected image (like 645Z, very big sensor).

Equivalent focal length was merely something that manufacturers told people who already know what a 50mm lens or a 200mm lens looks like on the old 24x36mm film SLR cameras. These people could be surprised if they put the same 50mm lens from their old Pentax K-1000 onto a new K-5, and the recorded image would be different.

The problem is that all of this "equivalent" stuff can fool new photographers into thinking that buying a 200mm lens from another film or sensor format will give them a different angle from the 200mm they have on their camera. But it won't. On your camera, all 200mm lenses will appear the same. Or 24mm or 50mm or whatever. Doesn't matter if the lens was made in 1950s or if it was made for 67 format. Just note that if you use a lens made for a smaller format (like for the Q), then the recorded image will have black edges; as the lens' projected image will not cover the whole sensor. Whole thing is a big silly: equivalent means that the lens on a certain camera will have an angle of view similar to a different lens mounted on a different camera.


Anyway, if you are only taking photos with one camera just ignore all of the "equivalent" stuff. You will only have to learn about this if you use cameras of different sensor size (or film size). For now, just look at the actual focal length of the lens and remember what it looks like on your camera. All lenses with a given focal length will give (approximately) the same angle of view on your camera.

QuoteOriginally posted by fstop18 Quote
So in the above example the manufacturer should designate the lens as 35-270mm not 20-200mm.
No, because the lens only appears to be equivalent to that second number when mounted on certain cameras. Just look at the true focal length, which is a property specific to the lens and has nothing to do with the camera it is mounted on, and remember its look on your camera. The only time you will look at equivalences is when you will mount those lenses on cameras with different sensor size. Then you might notice that 35mm is wide angle on one camera, but not wide angle on another; even though the lens itself doesn't change, its just the camera recording more or less of its projected image.

Last edited by Na Horuk; 12-20-2015 at 12:49 PM.
12-20-2015, 01:49 PM   #10
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Other people have given great responses but I will give my take on it too.

What most of the time you are reading is about FIELD OF VIEW... NOT FOCAL LENGTH.

A 28mm lens is 28mm regardless of the format of the camera or the sensor size.

To illustrate my point ...imagine if you had a chair sitting looking directly out a window.

In example 1 your window is 2 foot by 2 foot square.

In example 2 your window is 3 foot by 3 foot square.

In example 3 your window is 4 foot by 4 foot square and so on and so forth.

Assuming your chair never moved... which ones of the windows do you see more of the world through and which ones to you see less of the world through? Obviously you see more of the world with a bigger window...that just means more in terms of horizontal and vertical views.

Your window is your sensor size. In that experiment you however have never moved your chair. Your distance to the window has never changed, hence your 'focal length' has also never changed.

All the different sensor sizes... that is all that is happening...

Now, moving on a little bit farther.... 35mm 'film' or 'sensors' have been considered the basic standard for basically arbitrary reasons. Let's just say they randomly picked a 2.5 foot by 2.5 foot window and called it 'standard'. They were just popular I guess so that 35mm sensor is considered the 'standard' window size.

What happens though is what if you have a 2 foot by 2 foot window but you want to see the same stuff you see through a 2.5 foot by 2.5 foot "standard" window... basically to compensate you have to move closer to the window, which would equate to a shorter focal length.

Sensor size is the actual size of the window. Focal length is how far away from the window you really are.
12-20-2015, 08:48 PM   #11
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As far as the 645Z sensor goes, other manufacturers decided on the sensor size. My guess is that the current "medium format" size has to do with how expensive it would be to make a larger sensor. Larger sensors exist but are much more expensive.
12-20-2015, 09:01 PM   #12
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OK, my take on "normal" lenses, repeated from a post elsewhere here many months ago. The old standard for normal focal length lenses, dating back to the 1800's, was the diagonal length of the the image size (=diagonal length of a negative, chrome, or sensor). By this criterion, a normal lens on 35mm would be about 43mm, not the 50mm that became standard (much less 55 or 58mm). However, one of the photo magazines (Popular or Modern Photography, forget which) sometime I think in the 1970's, decided to look carefully at the perspective used by French painters for urban scenes and compare this to lenses for 35mm cameras. I forget which artist they selected, but they took a painting of a French town, found the exact spot the painter must have been (based on how the buildings were rendered, how they overlapped each other, etc.), then tried to reproduce exactly what the painter recorded on the canvas. Turned out to be a 105mm lens, that is, the artist focused upon and recorded a much narrower FOV than what the human eye can see, which is actually much wider than what is covered by a lens whose focal length equals the diagonal of the the recorded image.
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