60mm f/2.8 Macro- Why the inconsistant maximum aperature?

[andrew_pike1]

I just got the AF Micro-Nikkor 60mm f/2.8D lens for my birthday on

the 7th and took it out to the arboretum for some fun.

 

One thing that surprised me was that when I looked through the

viewfinder of my F100, I found that as the focus went in and out,

the maximum aperature got smaller and larger. The closer it focused,

the smaller it got. I was under the impression that only zooms

suffer from this, not single focal length lenses.

 

Why is this happening?

[vivek iyer]

As the magnification is increased, the effective aperture goes down.

This is clearly indicated in 60mm f/2.8. Nothing unusual.

 

Vivek.

[jbq]

This is because of bellows factor: as you focus closer, the lens gets further away from the film, and the numerical aperture becomes a bad approximation for the effective aperture. The 60/2.8 reports the effective aperture.

[todd peach seattle, washi]

(deleted a pair of antagonistic responses - moderator)

 

Vivek succinctly nailed it, it has always been so with macro lenses. The newer 'gee-whiz' cameras report this more or less accurately. With older cameras, the photographer had to be aware of the 'lens extension' factor, particularly when dealing with flash exposures.

[andrew_pike1]

I see. So, it rather is like in a zoom lens. The farther the front of the lens moves from the aperature blades, the amount of light hitting the aperature blades decreases, thus lowering the effective aperature. Because macro lenses have such greater focusing ranges than other fixed-focal length lenses, the effect is much more pronounced.

 

As with my Nikkormat EL I used a 55mm f/3.5, the effect was less apparent, so I was suprised at first when I saw the aperature go down as I focused.

 

Thanks a lot.

[wogears]

Hi!

 

That's neat! Auto bellows factor indication. Do you happen to know if my D70 will do this? (Don't have a Nikkor macro, only the cheapie Vivitar. Will try that tonight, but I assume the "D" factor is important?)

 

Les

[jbq]

"So, it rather is like in a zoom lens [...]"

 

Not quite. Actually, in a "simple" lens model the entire lens moves away from the film, including the aperture. From behind the lens, the light radiates out of the aperture, and when focusing close the aperture (the source of light) gets further away from the film.

[jemini_joseph]

Same question I've posted a day before!!!

[jemini_joseph]

Sorry forgot to add the link <br>

<a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=009S3Y">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=009S3Y</a>

[todd peach seattle, washi]

Les -

 

The 'true aperture' trick is mostly a function of the lens, and how it communicates electronically with an AF camera. I would guess that the Vivitar doesn't do this trick, but perhaps.....