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Archive 2017 · 100mm f/2.8 non-L macro

  
 
beanpkk
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p.1 #1 · p.1 #1 · 100mm f/2.8 non-L macro


Just for kicks I made an inexact attempt at calculating how much focus breathing this lena actually does. From my calculations, it looks like it's only a "100mm" lens when focused at infinity. At 10 feet or so the lens is up to 120mm, and at MFD the lens changes to a whopping 300mm focal length. The focal length increase isn't linear, of course...changing much faster as focusing approaches MFD than at the infinity end.

I have never seen official numbers for this, and I'm sure mine aren't perfect but this does explain why this lens doesn't do well at focus stacking when up close to the subject.

Comments welcome as always.

keith




Feb 19, 2017 at 10:13 AM
jcolwell
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p.1 #2 · p.1 #2 · 100mm f/2.8 non-L macro


beanpkk wrote:
Just for kicks I made an inexact attempt at calculating how much focus breathing this lena actually does. From my calculations, it looks like it's only a "100mm" lens when focused at infinity. At 10 feet or so the lens is up to 120mm, and at MFD the lens changes to a whopping 300mm focal length. The focal length increase isn't linear, of course...changing much faster as focusing approaches MFD than at the infinity end.


You might want to check your calculations. With "focus breathing", focal length decreases as you get closer to the subject.



Feb 19, 2017 at 11:43 AM
Don Clary
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p.1 #3 · p.1 #3 · 100mm f/2.8 non-L macro


Yes. I tried to calculate focal length of 100-400 II, when set at 400mm and closest focus distance. Not sure if I was accurate, but focal length appeared to be 190mm.


Feb 19, 2017 at 11:52 AM
EB-1
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p.1 #4 · p.1 #4 · 100mm f/2.8 non-L macro


beanpkk wrote:
Just for kicks I made an inexact attempt at calculating how much focus breathing this lena actually does. From my calculations, it looks like it's only a "100mm" lens when focused at infinity. At 10 feet or so the lens is up to 120mm, and at MFD the lens changes to a whopping 300mm focal length. The focal length increase isn't linear, of course...changing much faster as focusing approaches MFD than at the infinity end.

I have never seen official numbers for this, and I'm sure mine aren't perfect but this does explain why this lens doesn't do well at focus
...Show more

I've owned nearly a dozen macros over the past 35 years and they all have shorter focal lengths at 1:1 or 1:2. The older helical focus macro lenses tend to lose only a small amount whereas the more modern 1:1 internal focus type tend to lose a lot more.

EBH



Feb 19, 2017 at 12:12 PM
beanpkk
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p.1 #5 · p.1 #5 · 100mm f/2.8 non-L macro


Thanks for the comments. All I can do is give you my method and you can tell me where I went wrong:

Set the camera and lens up at MFD, and measure the width of the image at the subject. Then measure the distance from subject to sensor, and calculate the angle of view. Angle of view leads directly to focal length given a constant format, 35mm in this case. Then repeat the procedure farther away. Here are my numbers:

With the camera sensor about a foot from the subject (MFD +/-), and going by the viewfinder in a 5D, the width of the image at the target was 36mm. I calculate that to be an angle of view of about 6.4 degrees. At 10 feet, image width was 2'10" for an angle of 17.7 degrees. That calculates to about a 120mm focal length. I didn't try it at Infinity but I assume it would come out 100mm for that. At MFD, 6.4 degrees comes out to 300mm.

Here's the site that gave me the focal length: http://tawbaware.com/maxlyons/calc.htm

According to that site, a FL of 100mm should give a horizontal angle of view of about 20 degrees, right in line with my 17+ at less than infinity. To get an angle of 6.9 degrees requires a FL of 300mm. B&H lists the angle of view of the 100mm non-L at 24 degrees (diagonal) which lines up with my 20+/- degrees horizontal.

If you put the 100mm lens on a camera and just move the focus ring, you can see the image get dramatically bigger as you focus closer.

What am I doing wrong?

Thanks,
keith



Feb 19, 2017 at 04:20 PM
charlyw
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p.1 #6 · p.1 #6 · 100mm f/2.8 non-L macro


You may want to check your calculations but for the old 100mm f/2.8 USM lens I know for a fact that focal length at MFD is about 70mm. For the newest Sigma 150mm f/2.8 DG OS HSM lens it's 95mm that remain from that 150mm at infinity focus.

You can find a spreadsheet with the correct calculations at http://www.jeffree.co.uk/pages/macro-lens-calcs.html



Feb 22, 2017 at 04:21 AM
charlyw
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p.1 #7 · p.1 #7 · 100mm f/2.8 non-L macro


beanpkk wrote:
What am I doing wrong?


I now checked closer what was going on and you were using calculations which are only applicable at infinity focus, they do not apply to close focusing distances - there the calculations I posted are correct.



Feb 22, 2017 at 05:37 AM
beanpkk
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p.1 #8 · p.1 #8 · 100mm f/2.8 non-L macro


charlyw: thank you for the reference. I will definitely take a look at it.

keith



Feb 22, 2017 at 05:32 PM





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