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Understanding focal length 
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The title doesn't quite say it all, because I have a pretty good idea of many of the facts and implications of focal length, but I'll put my case forward and anywhere that you can clarify a point or two would be welcome to me and probably others.

I've been having a read of this- http://www.cambridgeincolour.com/tutori ... lenses.htm and it still leaves me with a few puzzlers.

I have clarified that the focal length is the distance between the lens and the point at which the light rays cross. My only problem continues to be- where exactly is this in the set up?

One site I found from Google made me think that the lens aperture was sited at or near this point. The first diagram on the above site only shows the light rays converging on the sensor which I know can't be right. Then the second diagram- like most that I have found- doesn't relate the focal point to a camera and lens. Can someone please help clear this up for me?

I KNOW for example that a standard lens for 35mm is 50mm- the distance from opposite corners of a 35mm frame and that this is 1.6 times the standard for my EOS300D because the sensor is smaller than a 35mm frame.

I know that my 300mm lens when mounted on my DSLR is the equivalent of a 480mm focal length on my 35mm SLR.

I have only just learned this evening that the zoom figure listed on compact digital cameras is the difference between the shortest and longest focal length of a zoom lens i.e. 75mm to 300mm = x4 zoom. My Dad has 18x zoom on his Fuji Finepix camera (one of those SLR lookalikes), but I now know that this could mean 5mm to 90mm or 10mm to 180mm or 20mm to 360mm. You can only relate it to another product by knowing how large the sensor is.

But, how do you work out how much magnification you are getting with a camera lens for a given focal length. Obviously, on my SLR, a 50mm lens is standard, but it has a small telephoto effect on my DSLR. Is there a formula based on focal length and sensor/film frame size?

I've had a quick Google around tonight and have not quite found satisfactory answers- many pages are either too simple or appear far too complex.

Thanks.

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Thu May 14, 2009 10:07 pm
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To further muddy the waters, DLSR manufacturers are making what are known as “short focus” lenses - which are designed for non full-frame sensors. The Canon EF-S lenses are such devices. These give you the focus length as stated - so a 10mm short focus lens will give you the 10mm.

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Thu May 14, 2009 10:19 pm
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So, what you are saying is that the focal length will be less, but they are telling the user that it is 10mm so that a direct comparison can be made to a 35mm SLR with a (normal) 10mm lens?

And the hits just keep on coming :lol:

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Thu May 14, 2009 10:23 pm
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The focal length is, as you say, the distance between the lens and the point of convergence of light which is coming from infinity. So, if you were focused on an object at infinity with a 50mm lens, then the lens would be exactly 50mm from the sensor. Light rays from objects which are closer than infinity would need a greater distance to converge, which is why the front element of a lens would move further away from the camera body as you focused on closer objects.

It's completely arbitrary that we use the 35mm system as the basis to work out what a standard, wide or telephoto lens it. 50mm is seen as 'normal' because when a photo taken with one is blown up to print size (e.g. about 5" x 7") then it looks 'normal', as you get a similar angle of view as with the human eye. However on 645 medium format cameras, for example, you need to use an 80mm lens to achieve a similar effect, as a 50mm lens is wide-angle. There is no direct way to convert a focal length into a magnification factor unless you are talking about the image size on the sensor or film itself; as soon as you make a print from a negative, project a slide, or view a digital file on screen, you are adding an extra magnification factor.

To summarise, the focal point of a lens will have to be on the recording medium for pictures to be in sharp focus, and when focused on infinity this will be the focal length from the lens, regardless of what format of film or size of sensor you are using. The apparent 'magnification' of APS-C is simply due to the cropping effect of a smaller sensor.

I hope this helps.

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Fri May 15, 2009 6:06 am
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Nick, I think that I understand what you are saying in as much as I have come to realise that we are talking about the angle of the view, rather than magnification. So a 7x5 image from a 35mm frame with a 50mm lens is around the same as the way the image would be seen with a human eye. Fair enough. If the focal length is increased, the viewpoint is narrowed and if it is shortened, the viewepoint is wider than the human eye. Sorted.

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if you were focused on an object at infinity with a 50mm lens, then the lens would be exactly 50mm from the sensor.


This is the part that I don't get. Surely if the focal point is the point at which the light rays converge, then all you would get on the sensor/film is a spot where all of the light rays meet at the same point.

I was always of the understanding that images are projected onto film upside down. This fits with what I have learned yesterday and so I have produced this little diagram.

Clicky

This shows my understanding of focal point, and I am guessing that A is the back of the lens (meaning the entire lens, not an element) but I am really not sure.

B is the focal point. That I do understand, but I also think that if the sensor is here, then all you will get is a spot of light where all of the rays converge.

The line that I have put at C is where I would have thought the sensor is positioned (relatively). This explains to me why images are upside down. But this brings me to a third factor and one I didn't consider last night- The distance of the focal point from the film/sensor, because of course you can buy rings to extend this distance to enlarge images on the film/sensor.

So from my point of view, there are three factors in determining how much magnification you get- focal length, sensor size and distance of the focal point from the sensor. You may have to factor in the production of the finished article, but all I am trying to consider at the moment is how can you work out magnification based on these factors.

I'm not saying that it is important- I know that a longer focal length means higher magnification- I'm just trying to rationalise it so that I can know exactly what I'm on about AND, I can surprise compact camera users (and idiots in electrical stores) by asking them how much magnification they really get and then explaining it to them. :)

Now you have managed to bring focus into it and I am possibly confusing myself even more by thinking of zoom lenses, but when I focus with a zoom lens it stays focussed no matter what focal length I change to, so I cannot rationalise what you are saying with what I am trying to understand at the moment.

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Fri May 15, 2009 6:52 am
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nickminers wrote:
To summarise, …
Thank you, Professor Miners.

Mark

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Fri May 15, 2009 6:52 am
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I've just tried (and failed miserably) to understand the general Wikipedia entry on focal length.

I would like to understand the principals, but I'm finding it hard. I have now realised that focal length is all about angles, rather than magnification, but I am still left wondering how you can compare a digital compact camera with a x3 zoom lense to a DSLR with a 18mm to 75mm lens (for example).

In my ignorance, I had thought that- like binoculars- x3 (or x18) was the amount of magnification, but now I know what it really means, I am no closer to being able to compare my father's camera's zoom performance to my DSLR (just out of interest).

I am still not quite there with the understanding of the relative locations of focal point and sensor and whether the focal point is within a lens assembly or behind it- based on my understanding of the images being projected upside down- But, come to think of it, if I look through a lens off camera, the image is upside down- suggesting to me that the focal point is somewhere inside the lens assembly.

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Fri May 15, 2009 7:05 am
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A 50mm focal length lens on a film camera is equivalent to "life-size" reproduction, so a 500mm lens on the same camera is the equivalent of 10x magnification.

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Fri May 15, 2009 7:28 am
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trigen_killer wrote:
I've just tried (and failed miserably) to understand the general Wikipedia entry on focal length.

I would like to understand the principals, but I'm finding it hard. I have now realised that focal length is all about angles, rather than magnification, but I am still left wondering how you can compare a digital compact camera with a x3 zoom lense to a DSLR with a 18mm to 75mm lens (for example).

In my ignorance, I had thought that- like binoculars- x3 (or x18) was the amount of magnification, but now I know what it really means, I am no closer to being able to compare my father's camera's zoom performance to my DSLR (just out of interest).

I am still not quite there with the understanding of the relative locations of focal point and sensor and whether the focal point is within a lens assembly or behind it- based on my understanding of the images being projected upside down- But, come to think of it, if I look through a lens off camera, the image is upside down- suggesting to me that the focal point is somewhere inside the lens assembly.

Looking through a lens with the naked eye is misleading, as your eye has its own built in lens which confuses matters! To get an object in focus, every point on the object needs to be mapped to a DIFFERENT focal point on the film or sensor (that's why it's called 'focal' length). If the object is at infinity, then the lens aperture will be exactly the same distance from the film plane or sensor as the focal length. If the focal point were inside the lens, you'd never get a sharp image.

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Last edited by nickminers on Fri May 15, 2009 7:38 am, edited 3 times in total.



Fri May 15, 2009 7:35 am
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timark_uk wrote:
nickminers wrote:
To summarise, …
Thank you, Professor Miners.

Mark

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Fri May 15, 2009 7:35 am
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OK, Nick. I get what you are saying now. The diagram that I (and other places) have used shows only the light rays from the outside edges and not from the rest of the lens. So when you factor in all of the other rays, you get an image.

I am really starting to get this (and already I am wishing I'd never troubled myself to worry about it :) )

So, taking into account something I mentioned earlier-

If I take my 100-300mm lens, adjust the zoom to 150mm, focus on a subject, then take the lens off, fit an extension ring, replace the lens and point it at the same subject, the lens will now be an additional 25mm (for example) from the camera and the focal point falls short of the sensor by 25mm.

Now I adjust the lens focus to bring the focal point back onto the sensor.

So, the magnification is increased because the focal length has been increased to 175mm, even though the lens setting has not been changed.

So, I am getting there and I am assuming that dogbert10 is right as you haven't corrected him, but nowhere that I have visited puts it in such simple terms, making me think that such things are either beneath those that really know or that no-one really knows or cares. :lol:


Stop Press. One final thing. From all that we have discussed, there is still one thing to consider. When the focal length of a zoom lens is adjusted, this will move the focal point further away from or nearer to the sensor. As the image does not go out of focus, I am assuming that the lens automatically compensates for the change in focal length in order to keep the focal point correctly focussed on the sensor. Otherwise, it would be necessary to adjust the focal length and then adjust the focus as two separate operations.

Did the earliest zoom lenses require the user to refocus after adjusting focal length?

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Fri May 15, 2009 8:07 am
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Extension tubes are more for macro photography than for extra magnification; they don't actually change the focal length of the lens, but they do move the focal range so that though you can't focus on infinity, you CAN focus much closer up. The focal length is defined as the distance from the lens when focused on infinity, so adding extension tubes (which don't contain any glass) doesn't change this number, but since you've moved the lens further from the body, the focus dial will no longer be correct.

To actually change the focal length you need to use a teleconverter, which is like an extension tube but contains extra glass to effectively multiply the focal length by 1.4x or 2x. This has the side-effect of reducing the aperture (which is expressed as a fraction of the focal length), so a 200mm f/4 lens with a 2x converter would become 280mm f/5.6 with a 1.4x converter, and 400mm f.8 with a 2x converter.

As you rightly say, with zoom lenses, changing the focal length alters the focal point. Such lenses contain several lens elements, so even though you still sometimes have to refocus after you've zoomed, it's not by as much as the difference in focal length would suggest.

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Fri May 15, 2009 11:37 am
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Oh, do you ever wish that you'd never bothered to think about something? :lol:

I would say that- leaving magnification out of it - I am prepared to accept all that I now know about focal length and get on with my life. Thanks, Nick and others for the input. It's been a blast.

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Fri May 15, 2009 12:32 pm
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paulzolo wrote:
The Canon EF-S lenses are such devices. These give you the focus length as stated - so a 10mm short focus lens will give you the 10mm.
My understanding is that, certainly with the Canon EF lenses, the focal length indicated is still given in "35mm, Fullframe format", even though this will never be obviously be used on such a camera, purely for comparison with other, EF, lenses. (I know it's not the most reliable of sources, but see this table on wiki).

Edd

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Mon May 18, 2009 6:20 pm
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