Focal length in english?

NOJ75

NOJ75

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Jon
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Hi all,

I have been looking at my lenses and can see various markings in mils and mtrs etc etc. Could anyone explain to me about focal length as if they were speaking to a child please. I just want to be absolutely clear in my mind. I.e when to use 50mm or 100mm, what distance I need to be away from the target etc.

Trying to get my head round all this but it sometimes gets a little confusing.

Many thanks :)
 
the focal length of a lens is just the amount of magnification the lens has. So the higher the number, the further the lens will zoom into the subject.

Some lens will have a range say 85mm-200mm which means the lens is telephoto and can be changed to any length between these two numbers.

Different lens will have different minimum focus lengths, which is the closest that an object can be to the end of the lens for the lens to focus on the subject.

How close you need to be to the target depends how how big you want it to be in the photo!
 
Ah right, ok.
So a 50mm lens (for example). If I was souring an apple, the closest the apple could be to the end of my lens is 50mm. Anything under that and the lens would not be able to focus. That about right?

On the other end of the spectrum, if my 80-200mm lens was set to 200mm, again I wouldn't be able to get closer than 200mm as the lens would not focus.

Is that about it in a nutshell or is there more to it?
 
The focal length is the 'size' of the image that the sensor / film sees, so a wide angle at 18mm will show a wide view, a 300mm will be like looking through binoculars to see things closer.

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No thats not right :) im not sure what lawrie meant but for example my 300mm can only focus at minimum 1.4m not 300mm. As for focal length I can't explain simply so ill leave it for someone else!
 
The OP was asking about focal length, I posted the best I could find to show this, having re-read his post, I think he means the minimum focus distance his lens will focus at. That varies with different lens', without knowing what he has, it's anyone's guess.

Jon, can you clarify please
 
lawrie29 said:
Different lens will have different minimum focus Distances, which is the closest that an object can be to the end of the lens for the lens to focus on the subject.

QUOTE]

Is that better??

The minimum focus distance has nothing to do with the MM distance written on the lens.
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NOJ75 said:
Ah right, ok.
So a 50mm lens (for example). If I was souring an apple, the closest the apple could be to the end of my lens is 50mm. Anything under that and the lens would not be able to focus. That about right?

On the other end of the spectrum, if my 80-200mm lens was set to 200mm, again I wouldn't be able to get closer than 200mm as the lens would not focus.

Is that about it in a nutshell or is there more to it?
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The 50mm lens has a focal length of 50mm. That is the magnifiaction it produces.

Your 80-200mm lens magnifies the image much more than the 50mm.



The 300mm end of my bridge camera lens is the equivalent to about 10x magnification, to compare this to how the manufacturers sell this on a compact camera.

But seriously, you've bought a studio set up and have to ask this question?
 
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Focal length - is the distance the light travels from hitting a lens to it's focal point. (the sensor or the back of the lens - can't remember) The longer the focal length - the more 'zoomed in' your image will appear. When looking through a 400mm lens - things will appear closer than looking through a 50mm lens. The body of a xxxmm lens might not actually be xxxmm long though because the lens body has individual glass lenses inside making it behave as if it is longer than it really is.

Minimum Focussing Distance - Is the nearest (to that subject) that a lens can manage to focus. Minimum focus is measured from the focal plane to the subject. Minimum working distance is slightly different as it's measured from the front of the lens to the subject.

If your 50mm lens has a minimum focus distance of say 30cm and your subject was only 20cm away - you would have to move backwards 10cm before your camera could focuson it. A bit like you struggling to read a book with your nose touching the page.
 
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lawrie29 said:
The 50mm lens has a focal length of 50mm. That is the magnifiaction it produces.

Your 80-200mm lens magnifies the image much more than the 50mm.



The 300mm end of my bridge camera lens is the equivalent to about 10x magnification, to compare this to how the manufacturers sell this on a compact camera.

But seriously, you've bought a studio set up and have to ask this question?
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Sorry Lawrie, 50mm is not the magnification the lens produces, you only have to look at macro lenses to see this, you can get for arguments sake a 100mm lens that will produce half and full life size, this is the magnification and has nothing to do with the focal length. The 50mm for a 50mm lens, is the distance between the front lens and the film/ sensor, so a 300 mm lens has 300mm between the front lens and the sensor. look on most film cameras and some digital cameras and there is a mark to show where the film/sensor plan is.

The magnification on bridge and compacts is the difference between the short end and the long end of the zoom, ie 30 - 300mm has 10x magnification a 50-150mm zoom has 3x zoom.
 
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phil8139 said:
The OP was asking about focal length, I posted the best I could find to show this, having re-read his post, I think he means the minimum focus distance his lens will focus at. That varies with different lens', without knowing what he has, it's anyone's guess.

Jon, can you clarify please
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Basically yes. Depending on what lens you have, what distance should you be from the target. For example, if I am using 70mm, what distance should I be? Is there a set distance or is it what ever I want to be?
And to clarify, is 70mm the closest I can be if set to that on the lens?
 
lawrie29 said:
But seriously, you've bought a studio set up and have to ask this question?
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The reason I bought it whilst still being very amateur is because a friend is moving abroad and offered me a complete set up at a price I couldn't refuse.
 
NOJ75 said:
Basically yes. Depending on what lens you have, what distance should you be from the target. For example, if I am using 70mm, what distance should I be? Is there a set distance or is it what ever I want to be?
And to clarify, is 70mm the closest I can be if set to that on the lens?
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Check the specifications on the lens, sometimes it's written on the side of the lens (minimum focussing distance may be abbrevited as "min" or "MFD" and will usually have a figure in metres).

Minimum focussing distance will vary between lenses and may be different for two models of the same focal length, depending on differences in their design.

As long as the distance between the focal plane of the camera and the subject is equal or greater than the minimim focussing distance, you can be as far away as you like.
 
Finally, just to be clear. I fully understand what 70-300mm (for example) zoom is. I know that you can zoom into the target s lot more the higher you go.
I was simply trying to understand what focal length is and how that and distance to the target had an impact. How important it is and the easiest way to understand it.

Many thanks to everyone for having an interest and replying. I think I'm starting to get the idea :)

(sorry about the multiple posts to answer these. I'm on my iPhone and can't see a way to multi quote)
 
Focal length put simply is the distance from the objective lens to the film/sensor plane, when focused on a subject at infinity.

When you focus closer than infinity the focal length changes.

You should use the lens that gives the best perspective, focusing range and angle of view for whatever you decide to shoot.
 
NOJ75 said:
Finally, just to be clear. I fully understand what 70-300mm (for example) zoom is. I know that you can zoom into the target s lot more the higher you go.
I was simply trying to understand what focal length is and how that and distance to the target had an impact. How important it is and the easiest way to understand it.

Many thanks to everyone for having an interest and replying. I think I'm starting to get the idea :)

(sorry about the multiple posts to answer these. I'm on my iPhone and can't see a way to multi quote)
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"I fully understand what 70-300mm (for example) zoom is." Then you understand focal length, because that's what the focal length range of that lens is. I think you're confusing it with focusing distance which is completely different.

Technically speaking, the focal length is the distance from the rear nodal point of the lens to the sensor, when focused on infinity. That's not very helpful, so converted into more meaningful terms, it defines the field of view illustrated in Phil's images above.

However, those numbers must be considered in conjunction with the size of the sensor, and it is the combined effects of focal length and sensor size that control how much you get in the picture.

If you want a shorthand for what focal length is, then 'magnification' is pretty much it, but measured at infinity focus.
 
NOJ75, forgive me if I'm speaking out of turn here but it appears that you're looking for a link between focal length, minimum focal distance and distance to "target/subject".

Quite simply there is no link.......

Minimum focal distance is the minimum distance you can be from your target/subject, this will vary from lens to lens, my old Sigma 170-500mm had a minimum focal distance of 3m so anything I wanted to shoot had to be at least 3m from where I was standing. My new Canon 100-400mm has a minimum focal distance of 1.6m. I think my 15-85mm is about 30cm.

Distance to target/subect..... Well provided it's further away than the minimum focal length then it completely open. Say you're taking a landscape shot and there's a tree 50m away it may not completely fill the from top to bottom but by zooming in a bit you could make this happen.

There's no optimum distance to target/subject, this will depend entirely on what the target is and how you want to compose the shot.
 
While not disagreeing with any of the explanations of focal length in terms of the distance from the focal plane etc, surely the easiest way of explaining it for the original poster is that the higher the focal length the narrower the field of view that the lens will produce.

There is no particular distance that the subject has to be from the lens, as long as it is between the minimum focussing distance and infinity (providing you want it sharp). Have a look at the diagram on this page for an idea of how focal length alters the field of view.
 
Cast your mind back to when you used to burn holes in pieces of paper with a magnifying glass. You focussed the sun to a point (or nearly so) on the paper and it caught fire. The distance between the magnifying glass and the paper is the focal length. It's the distance from the lens to where infinity is brought to focus. It has nothing to do with how close an object can be to the lens and still be focussed. That is entirely down to the lens manufacturer, and is why real-world prime lenses have many elements.
 
Thank you all very much for your detailed replies and links to very useful resources. I understand focal length now. In fact, once explained properly, something I thought was really complicated is in fact rather straight forward.

Many thanks to all of you that have made an input to this thread. Much appreciated. :)
 
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