how to read the numbers on lenses?


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gunawan

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Nov 14, 2008
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justicebaogong.blogspot.com
#1
Hi guys,

Hope some senior members can guide this noob here:

I've spent like almost a hundred hours reading the internet on dslrs until eyes blurr. Still totally confused with the various confusing terms, esp those terms regarding focusing, depth, length, circle confusion, etc.. :confused:

I know 18mm - 55mm means this lens can zoom close at 18mm and far at 55mm and the zoom aspect is 55 / 18 to get the number of x zoom.

1) but how far is 18mm in real life distance and how far is 55mm in real life distance?

2) So is it true that the smaller the 1st number, the wider the lens? ie 10mm/50mm is wider than 18mm/50mm?

3) Then how far can 10mm/50mm zoom as compared to 50mm/ 250mm ? do we base on the number of times zoom of do we base on the 2nd number on the lens? :think:

4) if we use prime lens ie 50mm f1.4, does it mean this lens only got 1 fixed clear area at 50mm? meaning we have to walk front and back to get the ideal sharpness? can autofocus if slightly behind or in front of focus area?

:confused::confused:would appreciate some foto guru advice? many tks:sweat:
 

ortega

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Nov 2, 2004
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#2
1) but how far is 18mm in real life distance and how far is 55mm in real life distance?
not how far that is important but the angle of view

2) So is it true that the smaller the 1st number, the wider the lens? ie 10mm/50mm is wider than 18mm/50mm?
yes

3) Then how far can 10mm/50mm zoom as compared to 50mm/ 250mm ? do we base on the number of times zoom of do we base on the 2nd number on the lens? :think:
base on the angle of view

4) if we use prime lens ie 50mm f1.4, does it mean this lens only got 1 fixed clear area at 50mm? meaning we have to walk front and back to get the ideal sharpness? can autofocus if slightly behind or in front of focus area?
prime lens mean cannot zoom, can focus but cannot zoom
 

night86mare

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Aug 25, 2006
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www.pbase.com
#3
1) but how far is 18mm in real life distance and how far is 55mm in real life distance?

2) So is it true that the smaller the 1st number, the wider the lens? ie 10mm/50mm is wider than 18mm/50mm?

3) Then how far can 10mm/50mm zoom as compared to 50mm/ 250mm ? do we base on the number of times zoom of do we base on the 2nd number on the lens? :think:

4) if we use prime lens ie 50mm f1.4, does it mean this lens only got 1 fixed clear area at 50mm? meaning we have to walk front and back to get the ideal sharpness? can autofocus if slightly behind or in front of focus area?

:confused::confused:would appreciate some foto guru advice? many tks:sweat:
1) there is no "how far" or "how near". i hope i do not confuse you further by throwing in this term called "crop factor", i.e. 18mm should be multipled by this crop factor to translate this into a comparable universal scale - which is in 35mm film terms. in any case, xx mm refers to a perspective, i.e. what you will get what you look through the lens, and some would tell you that 50mm (after crop factor, meaning ~35mm BEFORE crop) will give you closest to what the human eye sees when it concentrates on a certain area of vision.

2) yes. but there is no 10-50 mm lens today. not yet anyways.

3) how far? with a longer FOCAL LENGTH (xx mm) you have a longer reach with your lens, the perspective is more compressed, among other things. as for how far, it depends on what you are shooting, doesn't it?

4) no. it just means that you have a fixed perspective. the focal length comes about from the lens topic in physics, it is all very theoretical and if you have not learnt it don't bother, really. you can focus from near to far, but you cannot zoom in and out, because 50mm is a PRIME lens, or rather fixed focal length lens. as for how near, it depends on which 50mm lens you are talking about.


read these

link
link

and feel free to clarify if you still don't understand
 

Blur Shadow

Senior Member
Sep 17, 2005
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#4
1. Refer to the link below to have a rough take on how wide or how far each focal length is. Take note that the website uses a Nikon DX format camera, so the crop factor is 1.5.
http://megaz.arbuz.com/2008/01/08/lens-zoom-on-dx-15-crop-factor-cameras

2. Yes. The smaller the number, the wider it is.

3. Well, let's put it this way. The larger the number, the "further" it can reach or see. Perspective will be "compressed", but if you're unsure of what that means, then you can simply leave it out for now.

4. A prime lens means it has a fixed focal length. You are not able to zoom in or out. You can, however, focus.
 

Rashkae

Senior Member
Nov 28, 2005
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#5
I suggest you read the newbie's guide sticky in the Newbie's forum to understand photographic terms.
 

Rashkae

Senior Member
Nov 28, 2005
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#6
3) Then how far can 10mm/50mm zoom as compared to 50mm/ 250mm ? do we base on the number of times zoom of do we base on the 2nd number on the lens? :think:
Oh... And as an FYI... Both of these would be 5x zoom. ;)
 

tjhan

New Member
Feb 11, 2007
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#7
Ok, do not bother with howmany TIMES zoom. This is not a microscope, where it is straightforward. 10 x zoom does not mean the image becomes 10 x as big! It's a stupid trick manufacturers use to con people into buying compact cameras.

Now since you appear to be very confused, you can just go by rule of thumbs to make it simpler. The smaller the number (mm), the wider the wides. The larger the number the long you can reach but narrower view.

Ok your eyes are prime lenses. They cannot zoom. They can focus at will though. Your eyes are bloody good prime lenses too. Unless your eyes can zoom, then I will call you ROBOCOP or TERMINATOR.
 

dision

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Mar 25, 2008
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#8
FYI 50mm prime lens on 35mm sensor will give about the same viewing perspective as ur eyes naturally looking forward. Hence its call standard lens.
 

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lsisaxon

Senior Member
Nov 29, 2004
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#9
FYI 50mm prime lens on 35mm sensor will give about the same viewing perspective as ur eyes naturally looking forward. Hence its call standard lens.
It's called standard lens because it comes standard with the older bodies. The correct term for a lens with normal perspective is Normal lens.
 

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limcc

New Member
Apr 10, 2004
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#10
1) but how far is 18mm in real life distance and how far is 55mm in real life distance?
I think the wikipedia on Focal Length should be able answer this question.

or this site which explain in a more layman term: it is the distance from the lens to the film, when focused on a subject at infinity.



4) if we use prime lens ie 50mm f1.4, does it mean this lens only got 1 fixed clear area at 50mm? meaning we have to walk front and back to get the ideal sharpness? can autofocus if slightly behind or in front of focus area?
the angle of view of a prime lens should be fixed. but in real world, it may vary depends on the film/sensor size(crop sensor) and even selectable for some camera(D3?). maybe in this case, focal length = Angle of view will be easier for you to understand? the angle of view for the 50mm when used on most camera will have a fixed angle of view, every thing else remains the same when compare to zoom lens.
 

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