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Author Topic: Checking Real Focal Length - 70-300l  (Read 1638 times)

Jens_T

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Checking Real Focal Length - 70-300l
« on: September 03, 2013, 04:01:44 PM »
Hello,

today I bought the EF70-300L lense to replace my old Tamron 70-300VC. When playing around I noticed that the 300mm on the Canon seem to be a good bit wider than on my Tamron.

Is there a website that actually checks the "nominal" focal length? I remember Lensrentals sometimes comments on the the real focal length but they don't list the details.

Or is there a simple way to calculate it without too much hassle?

Thanks in advance
jens

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Checking Real Focal Length - 70-300l
« on: September 03, 2013, 04:01:44 PM »

AlanF

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Re: Checking Real Focal Length - 70-300l
« Reply #1 on: September 03, 2013, 06:16:30 PM »
You can calculate it using the simple lens formulae to a good approximation using a tape measure.  Measure the height of an object, h, at a distance v away from your lens, where v is much greater than the focal length, f, say 15 metres for a 300mm lens. Calculate the height of the image, i, from the number of pixels it spans and the size of the pixels (which you can calculate yourself or read in websites). Then, f = vi/h. For an extra couple of % of accuracy you should use u = vi/h, where 1/v + 1/u = 1/f.
« Last Edit: September 03, 2013, 06:26:05 PM by AlanF »
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neuroanatomist

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Re: Checking Real Focal Length - 70-300l
« Reply #2 on: September 03, 2013, 07:29:50 PM »
Two issues at play here, rounding and breathing.

The specified focal length is usually rounded slightly, usually not by much. I don't know the exact values, but for example the 70-300L might be 71.9 - 296.8 mm.  This usually isn't a big enough difference to notice.

Breathing is the change in apparent focal length with focusing distance (distance to subject).  The actual (and specified) focal length(s) are with the lens focused at infinity. As you focus closer, the 'apparent' FL gets shorter.  For the 70-300L, that decrease in apparent FL is greater than many lenses, due to the design. Many zoom lenses lose a significant amount of their sharpness when they are focused near or at the minimum focus distance (MFD).  The 70-300L does not, because Canon used a floating focus system in the lens.  However, if you look at the specs for the lens, you see that it has the same MFD and maximum magnification value of the 70-200/2.8L IS II.  That means that while the 70-300L is 300mm at the infinity focus point, by the MFD it is giving you the FoV of a 200 mm lens.

Did you try comparing the lenses in question with both of them focused at infinity?
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ahab1372

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Re: Checking Real Focal Length - 70-300l
« Reply #3 on: September 03, 2013, 10:18:27 PM »
I noticed the other day that the 5D3 records an EXIF tag "35mm equivalent focal length" (or similar) with a value of 291 when completely zoomed out. I'll have to test if that value changes with the focus distance

neuroanatomist

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Re: Checking Real Focal Length - 70-300l
« Reply #4 on: September 03, 2013, 10:34:31 PM »
I noticed the other day that the 5D3 records an EXIF tag "35mm equivalent focal length" (or similar) with a value of 291 when completely zoomed out. I'll have to test if that value changes with the focus distance

Interesting!

I've found a couple of 70-300mm patents from Canon, one with 72.48-290.86mm and another with 72.50-290.90mm (neither are the current 70-300L, since both start at ~f/4.6 and end at ~f/5.9 (but you can see how the rounding works in their favor!).
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Jens_T

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Re: Checking Real Focal Length - 70-300l
« Reply #5 on: September 04, 2013, 01:06:05 AM »
Two issues at play here, rounding and breathing.

Breathing is the change in apparent focal length with focusing distance (distance to subject).  The actual (and specified) focal length(s) are with the lens focused at infinity. As you focus closer, the 'apparent' FL gets shorter. 
Did you try comparing the lenses in question with both of them focused at infinity?

Thank you Neuroanatomist - now I remember reading about breathing and suspect this is the answer. I was playing around with the lenses in my flat to check how well AF performs in bad light and noticed the focal length difference. Will compare the lenses outside at infinity.

BR
Jens

Jens_T

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Re: Checking Real Focal Length - 70-300l
« Reply #6 on: September 04, 2013, 01:10:52 AM »
You can calculate it using the simple lens formulae to a good approximation using a tape measure.  Measure the height of an object, h, at a distance v away from your lens, where v is much greater than the focal length, f, say 15 metres for a 300mm lens. Calculate the height of the image, i, from the number of pixels it spans and the size of the pixels (which you can calculate yourself or read in websites). Then, f = vi/h. For an extra couple of % of accuracy you should use u = vi/h, where 1/v + 1/u = 1/f.

Thanks Alan, I might try that if focus breathing isn't the answer.
What variable is u?
f = distance * image height / object height

BR
Jens

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Re: Checking Real Focal Length - 70-300l
« Reply #6 on: September 04, 2013, 01:10:52 AM »

LetTheRightLensIn

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Re: Checking Real Focal Length - 70-300l
« Reply #7 on: September 04, 2013, 01:46:21 AM »
Hello,

today I bought the EF70-300L lense to replace my old Tamron 70-300VC. When playing around I noticed that the 300mm on the Canon seem to be a good bit wider than on my Tamron.

Is there a website that actually checks the "nominal" focal length? I remember Lensrentals sometimes comments on the the real focal length but they don't list the details.

Or is there a simple way to calculate it without too much hassle?

Thanks in advance
jens

The 70-300L has a TON of focus breathing and the Tamron has VERY little. At MFD the 70-300L turns 300mm into only 200mm! At 25' it is still only about 282mm. Even at 50' is still doesn't seem to yet reach 300mm. Maybe on the moon it does  ;D. Or maybe never. Even shooting across the length of a soccer field, the long way, the old simga 120-300mm seemed to deliver about 280mm if what you call the 300 2.8 IS delivering at 300mm at that distance. (the sigma also seemed to be about f/3.0 if you call the 300 2.8 IS f/2.8) and the difference seemed to be about the same shooting much closer in too so I think the old sigma 120-300mm was simply really a 120-280mm f/3.0 lens  ;D.


At 25' I had to set it to read EXIF 72mm to get same FOV of the 70-200 f/4 IS at 70mm.
« Last Edit: September 04, 2013, 01:49:31 AM by LetTheRightLensIn »

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Re: Checking Real Focal Length - 70-300l
« Reply #7 on: September 04, 2013, 01:46:21 AM »