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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?

 
RPCrowe
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Feb 08, 2011 12:27 as a reply to  @ post 11802011 |  #16

Early in my Navy career...

Early in my Navy career we referred to focal lengths in inches when we wanted to generalize and in mm when we wanted to be specific.

Accurate measurements of focal lengths were very important, especially when doing photo reconnaissance wherein measurements were made from the photograph.

However, regarding today's lenses, specifically zooms. The listed focal length of a zoom lens is only accurate when the lens is focused at infinity...


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Skaperen
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Feb 08, 2011 18:28 |  #17

Roy Mathers wrote in post #11801252 (external link)
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).

Even then it might be 1.509 x 3.521 from one mill, and 1.488 x 3.496 from another, but 1.487 x 3.495 on Tuesdays. Or it could vary that much from one piece to the next.

What I'm wanting to understand here is the scale of quality control that actually exists, in a numeric sense, as well as the designed targets. If a lens is supposed to be 100.000 on the nose, plus or minus a certain percentage, that's one thing. If it is supposed to be 102.00 on the nose, plus or minus a certain percentage, but marketed as 100.0, that's another. But I'm looking for this in terms of "false marketing or such". I just want to get a sense of scale about what we are dealing with.

In the extreme wide angles, where numbers are getting very small, I see virtually no fractional wavelengths. Certainly not in the Canon or Nikon lines. But I have seen such lenses in other contexts where things did have a fractional wavelength that implied a certain greater level of precision.


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Skaperen
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Feb 08, 2011 18:34 |  #18

KhanhD wrote in post #11801574 (external link)
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.

There's a good reason to know. You'd need to have a specially matched pair if there is much difference between units. I know in electronics and radios there are cases where matched pairs or matched sets are essential. Matched resistors, even if they are 10% inaccurate, need to be within 0.05% of each other in some kinds of circuits. Radio transmitter tubes were frequently sold in matched pairs.

Canon could run tests on their L lenses to actually check the true specs, and pick out ones that match others and sell them in matched pairs for triple the price. Would you buy that for 3D shooting?


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Skaperen
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Feb 08, 2011 18:49 |  #19

Roy Mathers wrote in post #11801137 (external link)
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.

It's kind of academic interest. I want to know, when I do calculations, what level of accuracy is the norm.

Yes, it is easier to say 30 fps. But when I generate a synthesized slate clock by computer, I need to know that it is 30000/1001. So 30 fps was the "rate class" as opposed to the specific rate. Now ATSC, the digital standard here, supports the exact rates of 24, 30, and 60, as well as the "NTSC compatible" rates of 23.976[...], 29.97[...], and 59.94[...]. So just saying "30" isn't as certain as it once was (but is still far more often really 29.97 or so). The video engineers have to know these numbers.

So I presume lens engineers know if their design is 100.0000 on the nose, or is 102.3333 just because that was the easiest or best way to tweak the design into having less CA and/or less BD. It's still a "100mm class" lens that I'm sure most of us could not see any difference with (though it might show in critical 3D or photometry shots).


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KhanhD
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Feb 08, 2011 20:18 |  #20

Skaperen wrote in post #11804513 (external link)
Canon could run tests on their L lenses to actually check the true specs, and pick out ones that match others and sell them in matched pairs for triple the price. Would you buy that for 3D shooting?

Of course not. I prefer Nikkors for shooting video. :p


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macroimage
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Feb 08, 2011 23:59 |  #21

I remember reading a while back that marked focal lengths are ±5% when focused at infinity. Some lenses change focal length a lot with focus distance, some only a little.

I haven't seen noticeable sample to sample variation in focal length in the cases where I have tried multiple samples of the same lens. It stands to reason that there must be some variance but it may be quite small given how precise lens elements must be made.


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smacafee
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Feb 09, 2011 01:48 |  #22

Canon describes their crop sensors as multiplying the focal length by 1.6x but it's actually closer to 1.62x

It seems Canon doesn't care too much about giving us particularly accurate or precise numbers.




  
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macroimage
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Feb 09, 2011 03:47 |  #23

smacafee wrote in post #11806671 (external link)
Canon describes their crop sensors as multiplying the focal length by 1.6x but it's actually closer to 1.62x

The 1.6x factor varies from model to model:
300D: 1.586x
350D: 1.622x
400D: 1.622x
450D: 1.622x
500D: 1.614x
550D: 1.614x
600D: 1.614x
650D: 1.614x
700D: 1.614x
750D: 1.614x
760D: 1.614x

1000D: 1.622x
1100D: 1.622x
1200D: 1.614x
100D: 1.614x

DCS3: 1.648x (advertised as 1.7x and 5:4 ratio)
D2000: 1.579x
D30: 1.586x
D60: 1.586x
10D: 1.586x
20D: 1.600x
20Da: 1.600x
30D: 1.600x
40D: 1.622x
50D: 1.614x
60D: 1.614x
60Da: 1.614x
70D: 1.600x
7D: 1.614x
7DmkII: 1.607x

EOS M: 1.614x
EOS M2: 1.614x
EOS M3: 1.614x

The 300D really does make a slightly wider picture than a 350D if you look carefully.

The 1.3x cameras vary even more:
DCS1: 1.304x
D6000: 1.304x
1D: 1.254x
1DmkII: 1.254x
1DmkIIn: 1.254x
1DmkIII: 1.281x
1DmkIV: 1.290x

Even the 5D wasn't quite "full frame"
5D: 1.006x
5DmkII, 5DmkIII: 1.000x
5DS, 5DS R: 1.000x
6D: 1.006x
1Ds, 1DsMkII, 1DsMkIII: 1.000x.
1Dx: 1.000x


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SkipD
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Feb 09, 2011 05:20 |  #24

smacafee wrote in post #11806671 (external link)
Canon describes their crop sensors as multiplying the focal length by 1.6x but it's actually closer to 1.62x

It seems Canon doesn't care too much about giving us particularly accurate or precise numbers.

The "crop factor" is a number that can be used to compare the field of view change when using the same lens on different format cameras. There is no multiplying of focal lengths involved, though. The focal length of a lens does not change when it is used on different format cameras.

Different DSLR cameras have different size sensors and all are being compared against the conventional 24mm by 36mm size of the classic 35mm film camera format. That's why the the "crop factor" numbers for various models are different as shown in the previous post.


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smacafee
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Feb 09, 2011 13:47 |  #25

SkipD wrote in post #11807073 (external link)
The "crop factor" is a number that can be used to compare the field of view change when using the same lens on different format cameras. There is no multiplying of focal lengths involved, though. The focal length of a lens does not change when it is used on different format cameras.

Different DSLR cameras have different size sensors and all are being compared against the conventional 24mm by 36mm size of the classic 35mm film camera format. That's why the the "crop factor" numbers for various models are different as shown in the previous post.

yes, thank you I understand that.




  
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JHutch
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Feb 09, 2011 13:57 |  #26

macroimage wrote in post #11806905 (external link)
The 1.6x factor varies from model to model:
300D: 1.586x
350D: 1.622x
400D: 1.622x
450D: 1.622x
500D: 1.614x
550D: 1.614x
600D: 1.614x

1000D: 1.622x
1100D: 1.622x

DCS3: 1.648x (advertised as 1.7x and 5:4 ratio)
D2000: 1.579x
D30: 1.586x
D60: 1.586x
10D: 1.586x
20D: 1.600x
20Da:1.600x
30D: 1.600x
40D: 1.622x
50D: 1.614x
60D: 1.614x
7D: 1.614x

The 300D really does make a slightly wider picture than a 350D if you look carefully.

The 1.3x cameras vary even more:
DCS1: 1.304x
D6000: 1.304x
1D: 1.254x
1DmkII: 1.254x
1DmkIIn: 1.254x
1DmkIII: 1.281x
1DmkIV: 1.290x

Even the 5D wasn't quite "full frame"
5D: 1.006x
5DmkII and all 1Ds are 1.000x.

Do you happen to have the correct factor for a d700?


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macroimage
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Feb 09, 2011 20:54 |  #27

JHutch wrote in post #11809715 (external link)
Do you happen to have the correct factor for a d700?

I don't think that is even a Canon. :)

The D700 appears to be 1.001x based on its 36x23.9mm sensor based on ratio of its diagonal to the 24x36mm standard. It doesn't have a perfect 3:2 aspect ratio either.

24x36 is a bit approximate anyway as film gates could vary a tenth or two of a mm anyway.


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Skaperen
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Feb 10, 2011 06:53 |  #28

macroimage wrote in post #11806349 (external link)
I remember reading a while back that marked focal lengths are ±5% when focused at infinity. Some lenses change focal length a lot with focus distance, some only a little.

I haven't seen noticeable sample to sample variation in focal length in the cases where I have tried multiple samples of the same lens. It stands to reason that there must be some variance but it may be quite small given how precise lens elements must be made.

If focus is done by moving all elements together, then there is no change of the lens' focal length. Classic lens design does this. But that also results in a narrowing of the angle of view because that is based on the lens' actual distance, not its focal length. We just usually try to match those up to bring the subject in focus.

There's also a reduction of effective aperture. What matters is how far the aperture is from the film/sensor although.

The internal focus lenses can change the focal length of the lens system. To the extent they reduce the variation of angle of view or reduce the variation of aperture, that should be a good thing.

I'm curious how much of that 5% variance is between manufactured units of the same model, or marketed FL vs. engineered FL vs. manufactured FL. Is the variation in grinding glass, or mounting the glass in tubes, the greatest source of variation of the whole lens system?


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Joaaso
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Feb 10, 2011 08:17 |  #29

from what I can remember, the sigma 50mm is noticeably wider than the canon 50/1.4, so its quite obvious that there is some variation or unprecise specs there, but I have no idea about the actual numbers...


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Jannie
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Feb 10, 2011 10:08 |  #30

Approximately, they vary from one to the other on the same camera, my 24L was wider than my 24-70L when set at 24mm.


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