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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 04 Feb 2007 (Sunday) 23:01
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Student Needs Understanding of Variable Aperature Lens

 
nitekatt2006
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Feb 04, 2007 23:01 |  #1

Hello shooters. I know a student photographer who is basically starting out from the absolute beginning and doesn't quite understand why her new Canon 28-135 mm 3.5-5.6 IS zoom doesn't stay at a constant aperature. On the wide end, we know that the larger aperature of 3.5 is the constant, but when extending to 135mm, the largest f stop is 5.6. in other words, she is having trouble understanding why she can't shoot at 3.5-4.0 at 135mm. She is ordering the new Sigma 70-200 f2.8 which of course is a constant aperature. She may order a 1.4 extender as well

How can I explain why lenses with variable aperatures perform the way they do and you have no option to choose the f stop scale on the entire range of the lens focal length. One thing is, variable aperature lenses are much less expensive than f2.8 constant aperature lenses. She is starting to understand that on a higher quality f2.8 zoom, any shutter speed can be selected. Canon makes many variable aperature lenses and I am certainly getting sharp images from my own 28-135mm IS

Can someone give an explanation on how to better explain how and why variable aperature lenses work.

Thanks, for any help, as i am not so technically an expert, I just try to visualize the shot I try for and get it with whatever lens i have with in reason

katt :cool:


Canon 10D and 30D. Tokina 80-200mm Pro 2.8 zoom, Sigma 50mm 2.8 macro, Sigma 10-20mm f4-5.6, Canon 28-135mm f3.5-5.6 IS, 2 AB 800

  
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liza
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Feb 04, 2007 23:05 |  #2
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This should give you the information you need:

http://www.uscoles.com​/fstop.htm (external link)



Elizabeth
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islandtime
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Feb 04, 2007 23:18 |  #3

On any zoom (28-135, etc) lens that has a variable aperture (3.5-5.6, etc) the smaller number is the widest aperture at the wide end (28mm) and the larger number is the aperture at the zoom end (135mm). On a lens like the 70-200 the aperture is constant for each end.

She is starting to understand that on a higher quality f2.8 zoom, any shutter speed can be selected.

Any shutter speed can be selected for any lens whether it be a constant 2.8 or a variable 3.5-5.6.

Try these virtual cameras to get a feel for how shutter vs aperture works.

http://www.camerasinte​ractive.com/virtual_ca​mera/main.htm (external link)

http://www.photonhead.​com/simcam/ (external link)

Both of the sites mentioned above are great reading for a new photographer.


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Mark_Cohran
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Feb 04, 2007 23:31 |  #4

This thread should help:

https://photography-on-the.net/forum/showthre​ad.php?t=98636

Mark


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nitekatt2006
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Feb 04, 2007 23:57 |  #5

Thanks to everyone for their input. i think what the student needs to grasp is since the 28-135 has a variable aperature, she can't understand why she can't zoom at 135 and shoot at f3.5 and has to shoot at f5.6 and of course the meter will change as the light entering the lens is different at the zoom, right? . She thought the variance of 3.5-5.6 should have given her that option. All the posts and links you recomended, i shall pass on to her.

Thanks katt


Canon 10D and 30D. Tokina 80-200mm Pro 2.8 zoom, Sigma 50mm 2.8 macro, Sigma 10-20mm f4-5.6, Canon 28-135mm f3.5-5.6 IS, 2 AB 800

  
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Lightstream
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Feb 05, 2007 00:37 |  #6

The f/3.5-5.6 figure is quoted for maximum aperture (throughout the zoom range). She's thinking of useable aperture range (ie. what the lens can stop down to).

So.. the 28-135 is

28mm - f/3.5 to f/22
135mm - f/5.6 to f/36

http://www.canon.com …f_28~135_35~56i​s_usm.html (external link)




  
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Pixel9ine
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Feb 05, 2007 00:52 as a reply to  @ Lightstream's post |  #7

The point to remember is, the max. aperture isn't physically changing as you zoom the lens. The fact that it's "variable" is due to the aperture being a function of the focal length, hence, as the focal length changes so will the relative aperture size (although the relation is not linear).


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Student Needs Understanding of Variable Aperature Lens
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