Question about variable aperture

[kirbinster]

I know some lenses don't have a fixed aperture such as the Nikon 70-300VR which is f/4.5 at the short end and f/5.6 at the long end. What I don't understand is why my Nikon 105mm AF-S f/2.8 does not offer constant aperture as you would expect from a fixed aperture lens. When you focus far out you get f/2.8 but when you focus close in the widest you get is f/4.8. I've never seen another lens do this.

[RichardTaylor]

Does the actual aperture change or does it appear to change?

If it does appear to change the maths behind it is here. (It is usual bellows/extension tube stuff).
PDF file.
http://scubageek.com/articles/tubes.pdf


If it really changes then it appears it is the way it is designed.
Nikon 105mm f/2.8G AF-S VR Review by Thom Hogan

[kirbinster]

I'm not sure what you mean about does it appear to change or does it change? When I look at the top display on my D300 it shows in aperture priority that while I have it set at f/2.8 while I focus on something close the f/2.8 on the display changes to f/4.8.

[RichardTaylor]

If the indicated aperture changes, then it is the design.

On my Tokina 100mm F2.8 macro the indicated aperture does not change, however the amount of light coming in through the lens drops off as it is extended, and the lens does get a lot longer..

[autofocus]

Originally Posted by kirbinster

I'm not sure what you mean about does it appear to change or does it change? When I look at the top display on my D300 it shows in aperture priority that while I have it set at f/2.8 while I focus on something close the f/2.8 on the display changes to f/4.8.

Kirb, does it happen on one of your other bodies?

[RLucas]

Originally Posted by kirbinster

I'm not sure what you mean about does it appear to change or does it change? When I look at the top display on my D300 it shows in aperture priority that while I have it set at f/2.8 while I focus on something close the f/2.8 on the display changes to f/4.8.

Kirb, I have seen an article about this somewhere before. I can't find it now, but has to do with the way the lens focuses in macro mode. It will stop down at close focus to maintain a managable dof. If I can find the link, I will post it. But that was it in a nutshell. I don't think the physical aperture changes, but it is measured from the light falloff.

Edit: Here it is..

I have news for you: all of the fixed focal length Micro-Nikkors aren't actually fixed focal length. In order to keep from being enormously long when focused at 1:1 magnification, Nikon (as well as many other macro makers) plays with the optical formula in order to keep from having an ever telescoping lens barrel. In macro work, you wouldn't want that, anyway, as a lens barrel that telescoped significantly to get to 1:1 would reduce working distance and potentially start hitting things in your scene at close working distances. Thus, at 1:1, this lens becomes about f/4.8 and does not extend even a millimeter. The aperture loss is actually a bit less dramatic at lower magnifications and the non-extension is very much welcome for macro use.

Nikon 105mm f/2.8G AF-S VR Review by Thom Hogan

lol! I was way off!

[Nicole]

And a bit more info here too: http://photo.net/nikon-camera-forum/00Bzl1

[sk66]

It has to do with the focus mechanism/design...It changes the "entrance pupil" which is the "effective aperture". I would suspect your lens has a lens/lens group between the aperture and the camera body which moves to achieve close up focus which is changing the entrance pupil diameter...
I would also call that "misrepresentation"...My constant aperture lenses all keep a constant aperture regardless, including my 150mm f/2.8 macro.

[kirbinster]

Oh so this is a "feature" of the famous Nikon 105mm micro then?

Yes it does it on all three bodies.

[ravncat]

its a design compromise - and not a bad one. I would expect that (in a world full of light) one would want to stop down for depth of field anyway when doing macro work.

The aperture change is from an effective focal length change to preserve working distance. It`s nice that the effective aperture shows in the nikon bodies - as with some other macro lenses, tamron tokina and canon all have macro`s that don`t show the effective aperture change - but hold the lens up to a uniform light source and you can see it dim by focusing closer.