Why 50mm is not a 50mm

denisbergeron · August 22, 2012

A lot of people always say that : a 50mm is a 50mm what ever the camera format you got...

Sorry, but not for me.

You can said that someone can take a 50mm from a medium or large format and put it on a dslr and take the same picture that the one using the 50mm from the dslr cie.

But, than again, when you take a single 50mm lens and put it on different format you can't get the same picture.

Think about it, if your enough lucky to have a 50mm f3.5 that can work on a 6x7 (cm) format sensor.

You will get a wide lens with a shallow DOF.

You get the same lens, on a 35mm sensor, you got a normal lens with some shallownest DOF

You get the same lens on a small camera with a P&S style sensor, you got a real telephoto lens without any DOF.. something like 600mm with f8 on a 35mm but with the f3.5 opening.

So, when you change format, with a single lens, the more you shrink the sensor, the more telephoto you get, the less DOF you get and more light for the multiplication ration you got.

Sorry, but a 50mm is a 50mm only in number when you change the platform.

keith reeder · August 22, 2012

Doesn't matter - 50mm is 50mm, and that's true regardless of the body a lens is on.

The Field Of View changes, by the focal length does not.

HenryUK · August 22, 2012

It always has a 50 mm focal length. That is a physical property that does not change whichever camera it is used on.

What changes is the angle of view. This is simple geometry; the bigger the sensor or film the wider the field of view.

Henry

carbon_dragon · August 22, 2012

You can look at it that way, but I think of my 35mm as a 50mm when on my APS camera. True it's really a 50 with only

the center portion visible, but I like just to do the multipliers because field of view makes more sense to me.

graham_line · August 22, 2012

The original premise only works if the user equates "50mm" with "normal," which isn't the case across different formats. 50mm is a physical dimension, not a functional description.

nathangardner · August 22, 2012

But the characteristics of that lens and the perspective of the shot will all be identical no matter the format. The field of view may not be the same, but if you crop a wide photo from a 50mm on a medium format camera to the same field of view as a 50mm on 35mm format, the photos will be identical. Bokeh, perspective, everything will be the same. If you put a 300hp engine in two different cars it may perform differently in each, but it's still the same engine and still has 300hp;all of it's objective characteristics are identical, it just happens to have a different affect on cars of different specs. You don't say that a 300hp engine "acts like" a 480hp engine in a two door car, it's still a 300hp engine.

nathangardner · August 22, 2012

Also, this has nothing to do with Canon EOS...

JDMvW · August 22, 2012

As the following from Wikipedia shows, the focal length is not determined by anything to do with the size of the image used by the person operating the camera

In general, the focal length or EFL is the value that describes the ability of the optical system to focus light, and is the value used to calculate the magnification of the system. The other parameters are used in determining where an image will be formed for a given object position. For the case of a lens of thickness d in air, and surfaces with radii of curvature R1 and R2, the effective focal length f is given by

</blockquote>

frank_skomial · August 22, 2012

.. and there is a matter of "focus breathing" that possibly changes focal length of lens when at very close focus. Some few lenses, mostly zooms, suffer from this defect.

E.g, for example Nikkor 70-200/2,8 VR II lens is only 134mm short when set at 200mm zoom and focused at the closest distance.

Read more from Thom Hogan, on his review of the lens and the problem description. <a href="http://www.bythom.com">www.bythom.com</a>

<a href="http://www.bythom.com/nikkor-70-200-VR-II-lens.htm">http://www.bythom.com/nikkor-70-200-VR-II-lens.htm</a>

keith reeder · August 22, 2012

Dammit, I meant angle of view, not field of view..!

mendel_leisk · August 22, 2012

Maybe turn it around, say:

A normal lens on full frame is 50mm. A normal lens (say) on 1.6 crop is 32~mm.

keith reeder · August 22, 2012

It you're talking the about the same "normal lens" on each camera, that's not true either, Mendel - it's still only 50mm.

To say otherwise has no more meaning than to suggest that a lens' focal length changes if you move closer to, or further from, the subject.

fmueller · August 22, 2012

It you're talking the about the same "normal lens" on each camera, that's not true either

</blockquote>

It can not be the same lens, because a 50mm lens is a 50mm lens, not a 32mm lens. ;-)

keith reeder · August 22, 2012

Indeed, Frank..!

pjdilip · August 22, 2012

I had raised a similar question recently...but the consensus was that nothing changes if the same lens is used on different formats, except that the sensor or film registers a different portion of the image.

However, I hazard one more issue: does the design for a larger image circle lens make changes in the image projected on the sensor? Maybe design adjustments have to be made to bring a larger field of view to focus on to the plane of the sensor. If that's true then actually a 50mm large format lens may not be giving the same image as a 50mm small format lens in the cropped area...

Just a thought for the experts!t

scott_ferris · August 22, 2012

Dilip,

The lens design is different to make a larger, or smaller, image circle, but the projection of the center portion is identical whatever size your capture medium.

A 50 is a 50 is a 50.

g dan mitchell · August 22, 2012

Saying that a 50mm lens is not always a 50mm lens if you put it on different cameras is roughly like saying that 60mph is not always 60mph if you compare a larger car to a small car.

50mm is the focal length of the lens. It is the same focal length whether you put it on a cropped sensor camera or a full frame camera or a four/thirds camera or a p&s camera or a MF camera or your head. ;-)

The effect of using the 50mm focal length is not the same on all cameras, and the format size is largely responsible for the differing effect. Another analogy. 50 pounds is 50 pounds, but it is "heavier" to a 5 year old than to a NFL lineman.

Dan

fmueller · August 22, 2012

The underlying problem here is that for some reason lost in the mist of history, focal length was chosen as the dominant specification when describing photographic lenses. We don't know who made that decision, but it is pretty clear that the person who made it was likely a physicist, and certainly a geek well out of touch with the material world.

Focal length is the distance between a lens' focal point and its optical center. Where both of these points lie is impossible for a photographer to say when holding a lens in hand. That means focal length is not a tangible measure. If you have an unknown lens and somebody asks you what the focal length is, you can not take out a ruler and measure it. Furthermore, if you know that a lens has a focal length of 50mm, you know nothing meaningful about that lens.

Focal length only takes on meaning with experience. Especially old school photographers who started out using prime lenses after a while know exactly how a 50mm lens behaves on a camera using 35mm film, and they call it a normal lens. Only because of experience, they also know how lenses with shorter focal length behave, and they call those wide angle lenses, while lenses with longer focal length are called tele lenses.

Film shooters moving to medium format found out soon enough that a 80mm medium format lens behaves much the same as a 50mm lens on 35mm film. That's why in the medium format world a lens of about 80mm focal length is considered a normal lens. Similar correlations could be made for large format and other film sizes. People using other formats than the standard 35mm film were comparatively few in the film days, and there were virtually no photographers without 35mm experience. Only in the digital days with a multitude of sensor sizes has all of this become an issue, because as I said before, if you know a lens has a focal length of 50mm, you know nothing about how this lens will behave. If the sensor size is smaller than 35mm film, the lens will be a tele lens, but how strong of a tele lens you won't know unless you know the sensor size. Even if you do know the sensor size, you pretty much need experience with that format and other lenses in that system to appreciate how your 50mm lens will behave in that environment.

The most sensible thing might be to specify angle of view, because the angle of view is always roughly the same for a normal lens regardless of format. As the name suggests, a wide angle lens has a wider angle of view, and accordingly that of a tele lens in narrower. Also, from basic geometry class most people probably have a feel for a 180 degree angle, 90 degree angle, and 45 degree angle as reference points. A practical downside is that there are three angles of view, horizontal, vertical and diagonal. If Nikon decided to use one, Canon the other, and the rest of the world the third, that would be a mess. More important might be that currently few people have a good feeling for angles of view, because they have never been in wide use as a specification. On the other hand, lots of folks still have experience with film, and know what focal lengths mean in that environment. That's why lenses are often specified by stating the 'full frame equivalent focal length'. That specification has nothing to do with the actual focal length of the lens, but '50mm full frame equivalent' simply means when used on the camera it was designed for, the lens acts like a normal lens; that is like a 50mm lens on a 35mm film camera. The shorter it gets, the more wide angle the lens will be, and the longer the more tele. If you have experience shooting film, that means a lot to you. If not, you might be none the wiser. That's all there is about it. The actual number means nothing, but then the focal length of a lens has never had any tangible meaning.

johnw63 · August 22, 2012

It's a good variable to know, when computing the field of view.

50mm on 35mm camera = 39.6 degrees horizontal field of view.

sarah_fox · August 23, 2012

Frank, actually if you know just a tiny bit about optics (the basic formulas pertaining to meniscus lenses being quite simple), focal length tells you quite a lot about what a lens will do. I can't think of a single factor more important to the description of a lens than focal length, the next most important being maximum aperture. But I grant you that a lens could be described by angle of view, provided one is set on a single format. (Describing an EF lens might be difficult, though, as it would have two different specifications, depending on whether it is used on a FF or crop camera.)

Denis, as others have said, a 50 is a 50 is a 50, at least as regards DoF issues. Not all formats are the same or utilize a 50 the same way, but that's another matter.

I think all things pertaining to DoF will become immediately clear and intuitive to you if you read/digest/understand Harold Merklinger's excellent articles on DoF and the object field method of characterizing DoF and blur. See here:

<a href="http://jimdoty.com/learn/dof/dof_merk/dof_merk.html">http://jimdoty.com/learn/dof/dof_merk/dof_merk.html</a>

While Merklinger's methods may make a few oversimplifications (e.g. his focusing to infinity, which I would argue places too much emphasis on physical size and too little emphasis on visual angle -- read the articles, and you may agree), the object field method of describing DoF provided a great conceptual leap forward for me in my understanding of DoF. I think it may be just as meaningful to others who want to invest a few hours of study and thought.

After having learned the object field method, I no longer need DoF tables, nor are they particularly useful when I have them. Lens markings are equally irrelevant. I can mentally figure DoF in a more meaningful way for any lens and any aperture in terms that are relevant to what I am trying to achieve. Equally importantly, I can also estimate the magnitude of blur that is needed for a given object in a photo and mentally estimate the aperture needed to achieve it. I don't even need to get very numerical about it -- no paper, no pencils, no calculator, just eyeballing the scene.

Anyway, after digesting Merklinger's articles, you'll see that a 50 is very definitely a 50.

As far as the difference in 50's for different formats with different image circles, I would say that a 50mm wide angle for large format would be a more difficult lens to design and implement. Aspects of the design needed to broaden the image circle may rob the image of a bit of sharpness in terms of lines/mm -- which is OK, because less resolution (in terms of lines/mm) is needed to achieve a detailed image. Unfortunately this is not something I understand well, nor have I read a cogent discussion of the issue. That said, two 50mm lenses of different format will render a scene exactly the same way, ignoring differences of sharpness, color, contrast, bokeh, etc.

fmueller · August 23, 2012

Frank, actually if you know just a tiny bit about optics (the basic formulas pertaining to meniscus lenses being quite simple), focal length tells you quite a lot about what a lens will do.

</blockquote>

That might well be where I fall short! When I comes to maths problems, my wife is in charge ;-)

Describing an EF lens might be difficult, though, as it would have two different specifications, depending on whether it is used on a FF or crop camera.

</blockquote>

Good point. I hadn't thought of that.

Not all formats are the same or utilize a 50 the same way, but that's another matter.

</blockquote>

Agreed

That said, two 50mm lenses of different format will render a scene exactly the same way.

</blockquote>

That seems to contradict your previous point. A 50mm on 35mm film will render a scene like a normal lens. A 50mm on medium formal will render a scene like a wide angle lens. In fact the same 50/1.8 on a 5D will render a scene like a normal lens, while it will render the scene like a short tele on a 7D. To fill the frame of different formats with the same scene using a 50mm lens, you need to change the camera to subject distance, giving you a different perspective.

I guess my main point is that for the newcomer the numerical value of the focal length has no practical use. It is only through experience with lenses of certain focal length that the value gains any meaning. If you know how one 50mm lens behaves on 35mm film, you know how all lenses of 50mm or thereabouts will behave in the same format. But that tells you nothing about how 50mm lenses will behave in a different format. On the other hand, if somebody tells you that a 31mm lens on APS-C will behave like a 50mm on film, then a person who has used 50mm lenses on film will gain a lot from this information. However, for somebody without film experience, the information that the lens has an actual focal length of 31mm is as useless as the information that it will behave like a 50mm on film.

sarah_fox · August 23, 2012

That seems to contradict your previous point.

</blockquote>

Sorry for the murkiness. What I was trying to say was that two 50mm lenses DESIGNED with image circles to suit different formats will render a scene the same way on any given format. For instance, two 50mm lenses, one designed for the 35mm format and the other for medium format (and having a larger image circle), will both render a scene the same way if used on a 35mm camera. (Note here: I don't know whether it's possible to adapt a 50mm medium format lens of any sort to a 35mm camera, but I'm assuming it is, for the sake of argument.)

I guess my main point is that for the newcomer the numerical value of the focal length has no practical use. It is only through experience with lenses of certain focal length that the value gains any meaning.

</blockquote>

True.

If you know how one 50mm lens behaves on 35mm film, you know how all lenses of 50mm or thereabouts will behave in the same format. But that tells you nothing about how 50mm lenses will behave in a different format.

</blockquote>

Well, I suppose you have to start out knowing for SOME format how focal length relates to angle of view -- e.g. what constitutes wide, normal, tele. Then armed with that knowledge and a crop factor, it's possible to port this knowledge/experience to a different format. However, if you don't have that starting point, then yes, as you say, it would all be meaningless.

To me, it makes the most sense simply to learn the focal lengths that go with one's choice of format. Then learn how to apply a crop factor if dabbling in a new format. I really think focal length would be no less confusing to a person than angles of view. I believe many people could not accurately draw a 30 degree angle, and I suspect very few could intuitively draw a 10 deg angle without dividing up a quadrant into thirds and then thirds again. And even then, it's another matter to look out over a scene and say, "I need a lens with a 20 deg angle of view to frame that in." (I would have to mentally mark the scene into fractions of quadrants, and I'm quite good with math!) I'm more likely to know what focal length of lens I need from experience. I've just become accustomed to applying the 1.6 crop factor when moving between my full frame and crop cameras.

fmueller · August 23, 2012

Sarah - I think we are on the same page now :-)

mark_medin · August 23, 2012

And in 35mm cine (half-frame to us still film photogs), the 50mm (or 2") lens was considered the "normal", showing normal has different meanings for different uses.

IIRC, focal length "breathing" became an issue on primes when IF lenses came about.

igord · August 25, 2012

50 is not 50 only under the water ;)

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