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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 08 Feb 2011 (Tuesday) 09:10
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How accurate are focal lengths?

 
Skaperen
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Feb 08, 2011 09:10 |  #1

If a lens spec stats 100mm, just how accurate is that? Two aspects of this question apply. One is the design, where the actual value might be, for example 101.2mm and it is just marketed as 100. The other aspect is unit to unit variation. I'd expect the latter to be quite small, more so for the better lenses.

It's kind of like people saying 30 frames per second for NTSC video, which it's closer to 29.9700299700299y(...) and is exactly 30000/1001 ... within the precision of the oscillator used to drive the video (which is usually very accurate ... within 0.00028% for TV broadcast under the old analog standards in USA ).

Things like electrical components, resistors and capacitors, tend to have a few percent variation. A 1000 ohm resistor may really be 950 ohms. But with lenses, I'd think if there is that much variation in glass, different elements would not match well together.


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gasrocks
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Feb 08, 2011 09:40 |  #2

How big is a 2X4? Numbers given are nominal they will say. Some 500mm lenses are really 478mm, etc. Especially shows up when comparing images taken by 2 different zooms with overlapping ranges.


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Roy ­ Mathers
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Feb 08, 2011 09:45 |  #3

Does it matter how accurate the focal length is, or is your question purely academic and out of interest. It seems to me that, for instance, saying 30 frames per second for NTSC video is easier that saying 29.9700299700299 frames per second. Similary, it's easier to call a lens 100mm than, say, 102.36.




  
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KhanhD
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Feb 08, 2011 09:58 |  #4

gasrocks wrote in post #11801098 (external link)
How big is a 2X4? Numbers given are nominal they will say.

Lumber is denoted by its rough cut size before final milling.

A 2x4 is 2x4" when its cut from the log, then milled to 1.5x3.5" finished size.
A 2x6 is 2x6" when its cut from teh log, then milled to 1.5x5.5" finished size.
A 4x4 is 4x4" when its cut from the log, then milled to 3.5x3.5" finished size.


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gasrocks
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Feb 08, 2011 10:00 |  #5

6 mil plastic is 4.5 mil thick.


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Roy ­ Mathers
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Feb 08, 2011 10:01 |  #6

Skaperen's point would presumably be that, with timber, you would know that a 2 x 4 would actually be 1.5 x 3.5 finished but, with a lens, you would have no such guarantee. (The same applies to gasrocks' plastic example).




  
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KhanhD
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Feb 08, 2011 10:23 |  #7

gasrocks wrote in post #11801247 (external link)
6 mil plastic is 4.5 mil thick.

Because a "mil" is not the same as a "millimeter"

EDIT: Furthermore, 6 mil plastic is nowhere near 4.5mm thick.


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Roy ­ Mathers
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Feb 08, 2011 10:27 |  #8

I'm not sure that that affects the principle.




  
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KhanhD
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Feb 08, 2011 10:33 |  #9

It kinda does. Its like saying "Well..24 inches is actually 2 feet." Theyre different units of measurement.


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Pasukun
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Feb 08, 2011 10:40 |  #10

Unless specified focal length and actual focal length is greatly out of order (ex: more than 15%), I wouldn't mind so much about the difference.
It is my assumption but I will bet just about every lens is off by some.
There is no way they can make it all exactly 50mm, 100mm or 200mm, even within the same copies.
But there is no point in labeling them as 53mm, 98mm or 193mm, as far as I can see.


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Feb 08, 2011 10:45 |  #11

Looking at the textbook for lens design, I can see that existing designs are always slightly off the target in terms of FL. As I do my own lens design research, I can tell you why: FL is just one of many targets when you numerically optimize a lens; the optimization is designed to get you close to your targets, but you can never get it exact.

From that textbook, here are target and real FLs for different lenses (they are not necessarily by Canon):

55mm f1.2: FL=55.54mm,
90mm f2.5: FL=89.81mm,
20mm f1.4: FL=20.60mm,
35mm f1.2: FL=36.00mm,
200mm f2.8: FL=197mm,
300mm f2.0: FL=296mm.


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KhanhD
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Feb 08, 2011 10:51 |  #12

Didnt DPReview have various 50mms that all had different effective Focal Lenghts, even within the same type?

IMO That would be a problem for 3D shooting where you need 2 setups, set up exactly the same.


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SkipD
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Feb 08, 2011 10:53 |  #13

Another thing that photographers need to understand is that - even with the rounding of numbers that takes place - all advertised focal lengths stated for lenses are with the lens focused at "infinity". When the lenses are focused closer than infinity, two competing "100mm" lenses could provide very different fields of view.

From what I've read, zoom lenses seem to be particularly prone to not matching up the field of view to that of competitors' lenses when focused close in.


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Roy ­ Mathers
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Feb 08, 2011 11:57 |  #14

I must ask - how much does it matter?




  
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KhanhD
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Feb 08, 2011 12:02 |  #15

Roy Mathers wrote in post #11801969 (external link)
I must ask - how much does it matter?

For 99.9% of people..it doesn't. For those pairing SLRs to shoot HD/3D, it does.


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How accurate are focal lengths?
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