Beginner Focal lengths what does the mm mean?

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AndyG123

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Basic question here, with a few other little bits. I understand how the focal lengths work, but am clueless as to what the mm means? Such as what is the lowest. Focal. Length you can get? I've seen 12mm ones.
And. Obviously the 300mm brings everything closer I know this isn't classed as a zoom, but if it was an Optical zoom, what equivalent would this be? And how is it calculated?

Thanks.
 
I don't know what the shortest focal length lens is, but certainly 4mm ones exist. BUT....

The mm part on a lens is the focal length. At normal focusing distances, the size of the image is directly proportional to the focal length (means it doubles if the focal length doubles, halves if the focal length halves).

Because a longer focal length gives a bigger image, only part of it will be recorded unless you can magically increase the size of your sensor or film to match it. Hence, a longer focal length will get less in, and the angle of view will decrease.

The normal focal length is usually given as about the diagonal of the format it's being used on. For APS-C sensors, it's about 30mm, for full frame 50mm and for 10"x8" film about 300mm. Hence, your camera matters in determining what magnification you're going to see when you change focal length.

Assuming that you have an APS-C sensor with a 30mm lens, swapping to 300mm would give a 10x larger image; in your terms a 10 times zoom.
 
What Stephen says. But also you ask what is the zoom lens equivalent of a fixed lens 300mm and the answer is 300mm assuming both lenses are for the same camera/sensor/film size.
I suspect you may be confused by the quoted range of a zoom lens which is a measure of the difference between the shortest and longest focal length of the zoom but with a 50-300mm lens the 300 will still be 300 like a fixed length lens.

(Sometimes you may see 300mm equivalent but best to forget about that for now unless you have a specific camera in mind)
 
Thanks for the post, I feel better filled in with the "zoom" part even though I do understand it's not zoom as such. Still a little bit stuck on the mm part? Didn't except it to be a complicated question. But a lot more science seems to be in it.
 
First, the simplified explanation (close, but not accurate :D).

If you focus on a distant object, the focal length of a lens is the distance behind the lens where the sharp image is projected. If you've ever played with a projector (digital or slide) you'll know that the greater the distance between projector and screen, the bigger the image on the screen. That's why the image size depends on the focal length - double the distance (= double the focal length) and double the image size.

If you want to delve deeper, it gets more complex. You'll probably wonder whereabouts on the lens you measure this distance from - and it's not usually the rear element! It depends on the optical design, and I think it wise not to say any more :)
 
Snipped from my link above:

"For example, in a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor."
 
Orangecroc said:
It is the distance from the back of the front lens element to the sensor.
Click to expand...

Or, more exactly "the distance from the rear nodal plane to the sensor (assuming a digital camera) when the lens is focused on infinity". The rear nodal plane may or may not be inside the physical lens; a retrofocus design has the rear nodal plane some distance behind the lens and inside the mirror box (assuming an SLR design). As a simple illustration, the Sigma 10-22mm lens doesn't have the back of the front element only 10mm away from the sensor.
 
Al1944 said:
Snipped from my link above:

"For example, in a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor."
Click to expand...
I've seen this, but a 300mm lens isn't 300mm away from the sensor?
 
In post 10, I mentioned retrofocus designs which place the lens further away than the focal length implies. At the other end of the focal length scale as it were are telephoto designs. These let the designer produce a lens that is shorter than the focal length implies - the rear nodal plane may be outside and in front of the physical lens. I said in my first reply that my definition was simple but not quite accurate - now you pretty well have the full picture. Except for an explanation of front and rear nodal planes, principal planes and a few other things :D

Note that all telephoto lenses are long focal lengths for the format that they cover, but not all long focal lengths are telephoto. The distinction is (lamentably) often ignored, but it is important in some cases. Telephoto designs are harder to use with a view camera, for example. And you don't need to understand the previous sentence :)
 
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Thanks guys, I know this isn't going to help me be a better photographer but I do like knowing what little things mean, always have even as a child haha.
 
Stephen is right in post #10 (and strictly speaking there are errors in both links posted) but the exact definition of focal length and how it is measured is not at all important. What matters is the field of view (or angle of view) that the lens produces, and that's defined by a) the focal length, and b) the sensor format. One without the other is meaningless. Eg, a 50mm lens has the same field of view on a full-frame camera, as a 31mm lens on a (Canon) crop-format camera, which is the same as a 25mm lerns on M4/3 format.

The difference is easily calculated from the crop-factor, 1.6x for a Canon cropper and 2x for M4/3, but that's a different question.
 
Short answer is that the mm 'specification' of a lens, these days means probably very little at all, it's all 'equated' or 'equivalent', ie semi make believe distances, rather than real ones you could actually measure with a ruler, tape-measure or micrometer!

A couple of bits of terminology for you; a 'lens' strictly is the one bit of shaped glass that magnifies or shrinks an image. The focal length of that lens is the distance from the focal plane, it needs to be placed to project a 'focused' image on that plane.....BUT what we tend to call a 'lens' is actually an abreviation of 'COMPOUND Lens', ie an arangement of individual lenses into one assembly... NOW, if they are all held rigidly in place and don't move, the phsical distance that they focus, could be measured from any of the individual lens elements, and it all starts to get a bit confoundling, hence, the stated mm focal length isn't usually a real world dimension, but the 'equivalent' magnification you get from the compound arrangement, compared to a simple single element arrangement. Make sense?

Oh-Kay... so now make a 'zoom' lens, this is one where elements inside a compund lens are shifted forwars and back, to vary the 'eqivilent' focal length.... now the same 'compund lens' is the same pysical lenth from the camera to the front element, and possibly even the rear element to sensor... but it has an 'equivilent' focal length at its short setting to say a simple single lens of 18m and an equivalent focal length at its longest setting to a simple single lens of 55mm.. the body of the lens hasn't changed, there's probably no dimension you can measure between the cameras sensor and any bit of the lens that is either 18mm or 55mm, to all extents and purposes its a almost imaginary dimension, an 'equvilece'

RIGHT.... 'The cop factor'.... the focal length is a guide to the angle of view of the lens.... the longer the focal length the more 'telephoto' it is, the more 'zoom' it has. Shorter it is, the wider angle of view it has.

OK.. grab a torch ad turn the lights off, shine torch on the wall, you get a circle of light. Move closer to the wall, circle gets smaller (and brighter) move away, circle gets bigger (but dimmer) Now change the size of the wall....... The circle of light wont change size, unless you move closer or further away, but f you have a big wall, you'll only get a circle of light on a small bot of it. If you only have one brick... torch will probably light it a up and fall off the edges behind it,,, and with a lens projecting a similar circle of 'image', not onto a brick or a wall, but a bit of film or 'sensor' that is probably more useful to us, we want to fill our frame with picture, so how big a circle do we need to be able to cover our sensor? OR how much of the image circle do we let fall off the edge... or 'crop' from the circle.

THIS is what's known as the 'crop factor'... if you have a big sensor behind a lens, then it needs a wider angle of view to cover it, and that begs a longer (equivalent!) focal length lens..Hence, the 'crop-factor' is a quick convenience to get another 'equivalence' this time for the field of view you get from a lens when you put a different sized sensor behind it.

Oh-Kay... the 'normal' angle of view, ie one that is neither telephoto, or wide angle, is usually accepted to be provided by a lens that has an angle of view of aprox 45 deg. Wider angles of view are err. 'wide angle' smaller angles of view are telephoto.

The term'Full-Frame' is derived from 35mm film cameras that have a negative 'frame' that is 24x36mm. Full Frame Digital cameras have a 'sensor' instead of a film trap, but its approximately the same size.

If you do a bit of Pythagoras, you find that the diagonal of that oblong is 50mm and a 50mm (equivilant!) focal length lens, on a 24x36mm negative or sensor gives you that 46 degree angle of view. (Pedantry begs I tell you I am not being strictly accurate though... if you do the maths, the diagonal of a 24x36mm frame is 43.25mm.... and that would give a 52deg angle of view, ... B-U-T.. it's close enough!)

Right... I have a rather antique 120 roll-film or 'Medium format' camera. This uses large roll film, and has a film trap that puts a 6x9cm image in the negative. If you do the same Pythagoras on that to find the length of the diagonal, you find that it's 10.8cm.. and the 'normal' angle lens that camera is fitted with is marked as 105mm.. which again isn't exactly the same, but is still close enough and provides 'close' to that 45 deg angle of view. BUT that lens is still a 105mm focal length lens... and if I put that on a 35mm film camera, it would STILL be a 105mm lens, only now the middle of the mage it projected would be falling on the film, the rest would be falling off the edge.

And I would get a negative that is aproximately 1/6th the area, and showing me only 'part' of the scene that the lens might project onto the big film, and that negatve would have a 'cropped' angle of view of only 23deg or so... BUT when that negative is 'enlarged' to make a print, or the dig-data taken from a sensor transcribed onto a screen, that 'smaller' bit of the image projected by the lens, 'caught' by the film or sensor, has to be enlarged more to the same viewing scale, so using a smaller senor or film format it has the 'equivalent' effect to using a much 'longer' or more telephoto lens.

And this is important, because a lens has a focal length, and that is a fixed attribute of that lens, but the angle of view it delvers depends on the size of sensor behind it.

You asked what was the 'shortest' focal length lens you an get.... well, amongst my cameras the shorted (equivalent!) focal length lens have is about 4.5mm. My 'crop' or APS-C sensor or 'DX' format Digital SLR, has that smaller sensor, aprox 18x24mm, about half the size of a 'full-frame' sensor. This 4.5mm lens is a rather specialist lens called a 'fish-eye' and provides an incredibly wide angle of view. It actually delivers a full 180 degree field of view on both horizontal and vertical axis of the frame, and delivers a circular image, 'wasting' some of the wide-side of the frame to get it all in. If I were to put that lens onto an FX format or 'full-frame' camera, it would still be a 4.5mm lens, and it would still deliver a round image circle onto the sensor, and that circle would still be aprox 18mm in diameter.... there would just be more 'wasted' sensor not seeing any image around that circle.

If I were to put that lens onto a Micro-Four-Thirds format camera, that has a sensor approximately 13x17mm.. now the 18mm diameter circle is going to start falling off the edge, certainly on the shorter vertical side and 'cropping' the image, and reducing the angle of view. If I fitted that lens in front of a smaller still, 'micro-sensor' in a bridge or compact camera, that is maybe 6x8mm, so the image circle would cover the whole sensor, and I would get a greater angle of view still, and more effective 'zoom' from a higher 'crop-factor' of equivalence.

I actually have a vry small micro-sensor action cam, that has a 4.5mm focal length the same as the fish-eye, but with a sensor so small, that focal length gives the 'equivalent' mild wide angle of view of aprox 35mm on a full-frame camera.

SO.. to all extents and purposes the specified focal mm length of a lens or the specified mm focal length 'range' of a zoom lens, is pretty meaningless, its the focal length in combination with sensor size that matters.to what we are usually really interested in which is the Angle of View. Andit is almost always NOT an actual physical dimension of the actual lens, but an 'equivalence' compared to a simple single element model.

It is used, pretty much for convenience and comparison. On my DX sensor Digtal SLR the 'normal' (45deg-ish) angle of view is provided by a (compound) lens that is 'equivalent' to a (single element) lens of 30mm.. anything shorter than that gives me a 'wider' field of view, anything longer a more 'telephoto' field of view... shorter the focal length, more wide I get longer the focal length the ore telephoto... for the same sensor size!!!

Back to how short can they go? Well, 4.5mm is 'about' the shortest I have encountered. That is about as short as you can get for a digtal SLR. And is still a mild wide on even a micro-sensor used in an action cam or camera-phone, and with an 18mm diameter image circle, it would be rather pointless using it on a larger FX or Full-Frame sensor camera, where the image it projects wouldn't cover any more of the sensor so give any more 'wide' angle, and a lens maybe 8 or 9mm focal length would probably be delivering the same equivalent 180 degree field of view and round image on the sensor.

How WIDE can you go.. well, that is a slightly different question, but is answered by the same 'circular frame' full round fish-eye lenses, that can capture a full 180 degrees on horizontal and vertical axis... any wider and the camera would be trying to take a photo of itself! (And with a 180/full roumd, it's already perilously close and you have to be very careful to keep feet and the camera strap out of the field of view!)

In years past, it was 'rare' to find a lens that had a angle of view much wider than 'about' 90 degrees, which is delivered on a 'crop' sensor camera by an 18mm focal length lens, or around 28mm for full-frame or 35mm-film. For film there were occasionally lenses around the 22-24mmrange, but that was generally about as short/wide as they got, and you were heading into the realms of fish and the image circle falling off the film at about 12mm. Now, for Digital, and with smaller 'crop' sensor cameras begging shorter lenses still to get such wide angles of view, you can get down to around 10mm, where the Angle of View is around maybe 120 degrees, but still delivering an image circle that is covering the whole sensor and delivering a more conventional oblong picture. I have one of the widest angle 'Ultra-Wide-Angle' lenses for DX/Crop sensor digital camera, an 8-16mm, that claims to have a little more than 120 deg angle of view on both axis, but still delivers a rectangular image over the whole sensor. though it IS starting to get a tad distorted at the edges, particularly for closer subjects.. which is to start raising more questions...

But, the salient bit is, to a certain degree the quoted focal length of a lens is pretty irrelevant, what usually is of importance is the Angle of View, and you don't get that 'just' from the focal length but the focal length and the sensor size in combination..

Clear as mud? More questions raised than answered? Good! Welcome to the wonderful world of cameras! IT'S ALL like this, and its almost ALL a moving feast, where the 'rules' change depending on circumstance!
 
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Mike, there's a lot in your post that I think is more likely to confuse than enlighten. You've used some terms in a way I haven't come across before, and seem to be confusing the hideous term "crop factor" (which has lead to more confusion than anything else in recent years to judge from forum posts from newcomers) with image circle. I certainly don't recognise your explanation of focal length from anything I've read in optics textbooks. I think you're overly fond of "equivalents" and find them in too many places...
 
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