Focal length question

Thomas Provis

Thomas Provis

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Hi all. Just wondering if you can awnser a question I have. As any photographer knows a lens Has a focal length in which a lens can zoom into a subject. Can anyone explain to me how big a mm is in focal length whem a lens zooms. Please help if this makes sense. Just post a comment asking for more info about this subject.
Thanks
 
Sorry, this does not make any sense :(

Can you rephrase the question?
 
Do you mean the size of the image in frame for any given focal length?
 
A 50mm lens is normally considered the view we see from a given distance. So if you stand at a distance from the subject which is a person, fill the frame with them. Now if you double the focal length the image you see would be from the waist up to the top of there head, that would be with a 100mm lens, double the lens to 200mm and you would see the frame filled from mid chest to the top of there head. A 200mm zoom would be a 4X magnification, a 300mm 6X 400 8X ect...
 
The original post looks almost as if it's a bad google translate, especially the "just post a comment" bit...

As Rowley Birkin QC might say "...Shanghai!"
 
The term "zoom" is usually used with compact cameras, but isnot a very useful term. It's the difference between the long end and the short end. This can give two very different lenses the same "zoom":
  • A 10-50mm lens has a 5x zoom
  • A 100-500mm lens also has a 5x zoom
There are several lens simulators online, the Nikon one is quite good for understanding focal lengths - http://imaging.nikon.com/lineup/lens/simulator/
 
The term "zoom" is usually used with compact cameras

No it is not. The term 'zoom' is usually used for zoom lenses.
 
kendo1 said:
The term "zoom" is usually used with compact cameras

No it is not. The term 'zoom' is usually used for zoom lenses.
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You wouldn't define a 70-200mm lens as a x2.86 zoom, would you? But it's common to refer to a compact as having a x4 or x30 zoom.

The OP isn't clear, and we may be interpreting the question in different ways.
 
You would call it a zoom lens.
You can call a compact whatever you want.
 
An alternate "take" on what the question means could be "how much difference does a millimeter make in the image size as you zoom"? In which case. my answer would be that it depends on the focal length range of the zoom lens. There's no noticeable difference between 1000mm and 1005mm in terms of angle of view and image size; but 5mm makes a big difference if you're comparing 10mm and 15mm. It's a matter of percentage differences when looked at this way.
 
Maybe he is trying to relate magnification of the image to the increase in focal length ……… which is different to the other interpretations above
 
not sure it can be properly answered unless you are referring solely to 1:1 magnification as any zooms focal point is all down to where you actually focus be it 5 feet in front of you or 5000 feet away ona mountain. so how if you take a 200mm lens for example can you judge as to what that translates to?
so no matter what way you look at it a mm is still a mm in terms of distance though in terms of focal length x distance to subject .. hmmm ok so if subject is 1m away ( 1000mm on a 200mm zoom thats a ratio of 5:1 if subject was 10 meters away then a mm is 50:1 ratio on a 200mm zoom
does that sound right .. does in my head?
so then a mm in focal length would be zoom length times distance in meters divided by 1000 ( i think )

so for subject matter 1 meter away on a 200mm then focal length is 1 /50th of a mm
 
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BillN_33 said:
Maybe he is trying to relate magnification of the image to the increase in focal length ……… which is different to the other interpretations above
Click to expand...

Hmm - I thought that was my interpretation of the question...

dean messenger said:
so for subject matter 1 meter away on a 200mm then focal length is 1 /50th of a mm
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Sorry, but that was as clear as mud in the dark to me. A 200mm focal length lens is a 200mm focal length lens no matter where the point of focus is. If you mean that the normally given approximation that runs "image size is difectly proportional to focal length, so an object that gives an image 1 inch high with a 100mm focal length lens will give an image 2 inches high with a 200mm lens" is only true when the object is at infinity, then I can agree. But I doubt if this degree of subtlety was in the OPs mind.
 
but how do you measure that in mm without a reference point. hence using 1 metre focus point as the scale as an example. if your lens is 200mm foal length and you focus on a subject 1metre away then thats 1000mm. so the 200mm focal length in mm is 1/5th of a mm per mm focal length as its 1000 / 200. ( sorry i meant 1/5th not 1/50th.
or in simpler terms based on a 100mm lens. if something is 100mm away in focus from that lens wouldnt that be focal length is 1mm per mm?
though why you would want to know this im not sure.. confused.com!
 
I think what Thomas was trying to ask (and I couldn't answer hence why he asked on here) is how exactly is the focal length of a lens worked out? ie what makes a 50mm lens 50mm?
He was asking me why his 18-105 is not physically 18mm long when zoomed out and 105mm long when zoomed in. The physics of things like that are beyond me so I'm hoping someone can answer for the both of us:)
Hope that makes a little more sense
 
sbowler579 said:
A 50mm lens is normally considered the view we see from a given distance. So if you stand at a distance from the subject which is a person, fill the frame with them. Now if you double the focal length the image you see would be from the waist up to the top of there head, that would be with a 100mm lens, double the lens to 200mm and you would see the frame filled from mid chest to the top of there head. A 200mm zoom would be a 4X magnification, a 300mm 6X 400 8X ect...
Click to expand...
I might be thick or out of date, but surely that depends on film/sensor size? I always saw 50mm as being a standard focal length for 35mm film, or full frame DSLR. Not for an APS-C or smaller sensor.
 
True should have mentioned the crop factor usually 1.5 or 1.6 but the zoom ratios are the same.
 
XA2inmypocket said:
I might be thick or out of date, but surely that depends on film/sensor size? I always saw 50mm as being a standard focal length for 35mm film, or full frame DSLR. Not for an APS-C or smaller sensor.
Click to expand...

sbowler579 said:
True should have mentioned the crop factor usually 1.5 or 1.6 but the zoom ratios are the same.
Click to expand...
Just No!
Focal length is a physical measurement, and 'focal length equivalent' is a kludge that should be ignored because it eventually creates as many questions as it tries to answer in it's simplicity.

JulesP said:
A little google throws this up
http://www.dpreview.com/glossary/optical/focal-length
Makes sense now to me - hope it helps T:)
Click to expand...

The above is all you need to know, and the answer to the OP's question.
 
Phil V said:
Just No!
Focal length is a physical measurement, and 'focal length equivalent' is a kludge that should be ignored because it eventually creates as many questions as it tries to answer in it's simplicity.
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Agreed.

when I use medium or large format, I don't apply a crop factor. It's better to just know what a 'standard' focal length is and work it out from there.


Steve.
 
I'm not going to confuse him about crop factors - he's 13 and just starting out. He's keen to learn and asks quite a lot of good questions.
I never think about the crop factor on my lenses anyway (mostly use FX lenses on DX body)
 
As I understand it you times by the crop factor?! So at 200mm with a 1.6 crop camera its 200/50=4 then 4x1.6=6.4. So that's a zoom of 6.4x. That's why full frames are so much better for macro. PLEASE CORRECT ME IF IM WRONG.
 
JulesP said:
I'm not going to confuse him about crop factors - he's 13 and just starting out. He's keen to learn and asks quite a lot of good questions.
I never think about the crop factor on my lenses anyway (mostly use FX lenses on DX body)
Click to expand...

Thing is if you mention these things he'll think whats this all about then and look it up. That's how I lurnt a lot anyway.
 
Ive read a few of the replies and a few of them ake no sense because theyre all numbers and so confusing. anyway. it seems like noone knows what im om about apart from my dad and he doesnt know either. thanks for the help guys. appreciate it. but in the end it turned into an argument about zooms on a DSLR and a Compact. Thanks anyway guys. :):jaffa:
 
If you have a photograph that's 20"x16" and want to put it in a frame that's 10"x8", you have to cut off 10" from one side and 8" from the other; that's a "crop factor" of 2, and you're really physically cropping. It's the same with lenses. The lens might prduce an image that will cover a full 35mm frame, but if you have a camera that has a smaller sensor, the camera will only record part of the image - it crops it. Naturally, lens makers make lenses with smaller coverage for smaller cameras. Crop factor merely means that the sensor is smaller.
 
Thomas Provis said:
Ive read a few of the replies and a few of them ake no sense because theyre all numbers and so confusing.
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Numbers should make things clearer since they imply precision. That said, I didn't understand some of the posts in this thread and I've been a photographer for about 60 years :)
 
Thomas Provis said:
i just dont understand. Thats all. its all very confusing for a 13yr old
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Hi Thomas, I thought you said earlier that you had understood an answer.

Anyway, here's the crux, I'm not sure any of us really understand your question, but if you ring me on the number on my website, I'll hopefully be able to sort it for you.
 
This thread has given me a headache....................:)
 
Hi Thomas,

I've sat and read your question a few times over and I think I understand what you mean so I'll do my best to write it out succinctly.

You're asking how big a mm relates to in zoom terms to which I think your trying to correlate zoom and how much you can fit into a given mm at a given point in the range. For example 1m fits in 1 mm and 0.10m fits in 10mm of zoom. Is that what your trying to ask?

That stab is probably as confusing as the first post but I couldn't think of a better way of putting it.

James
 
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