Why is my f2.8 lens a f.4?

Joined
Jan 13, 2009
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Josh, no. The Nikkor Micro lenses are designed to accurately communicate the value of the actual f-number to the camera as they focus closer. Other close-focusing Nikkors (such as the 35mm f/2) do not. But when the 60mm says f/3, 4 or 5, it actually is getting less light onto the sensor, and thus needs to use the higher f-number to calculate the correct exposure. With non-micro lenses, the difference between the stated f-number and the effective f-number is not great enough to effect the exposure. It can change up to a third of a stop with out a noticeable difference.

What doesn't change is the actual diameter of the lens opening.
Im confused lol, so the blades dont change the opening during macro focusing?? what about during normal behavior, my lense is still F3 for almost everything except 10 feet and infinity..
 
Joined
May 4, 2008
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Stafford, UK
Maybe Stupid Question?

I knew that current Micro Nikkors (60/105/200) change F stop when focusing close (similar to attaching ext tube), ok all clear.

Does thw same thing happen to other makes, in particular to much vaunted Zeiss 100mm f2 Macro Planar.

Many thanks,
 
Joined
Jun 4, 2007
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Rockville, MD
The reason the f/number of the lens changes as you get down to 1:1 is because the effective focal length of the lens is actually shortening with the games they play on these lenses. The 105mm VR is not truly 105mm at 1:1. It's something shorter. So apply Jerry's formula (F-stop # = (lens focal length)/(diameter of aperture)) and you get something less than f/2.8 since the focal length shortened.

I think my 70-300VR does something similar because whenever I'm focusing pretty closely at 300mm I have to end up adding a lot of positive exposure compensation. Rather than really being an f/5.6, I think it's actually more like an f/8 here because I'm always having to add about +1.0EV. I guess they only throw the appropriate close focus effective aperture corrections onto the Micro lenses and correctly assume that most of the time on lenses like the 70-300vr you're not going to be shooting like that.
 
Joined
Jan 13, 2009
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Lol Steve im still confused on a few things. I understand the aperature changes as i get closer, doesnt bother me really or affect me.. just the one comment chris made confused me..

About the lense not closing down during macro(i assume he means aperature blades) but the camera tricking it.. that would mean it would have DOF of 2.8 but light of F4.8 at 1:1

I know how the lense works, at least for me.. Face portraits can get very shallow at even F4, so i dont want 2.8 that much really for face shots.. at F6.3 parts of ears and hair arnt in focus either.. its amazing how this lense works on FX..

Im not asking anymore questions lol about this subject.dont want to waste anyone elses time. I dont understand the physics of it, i just understand what the lense does (light wize) and how it affects my light..

I love the 105VR
 

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