Nikon 105mm f2.8 macro lens isn't f2.8!!!

I'm really into close up photography, so I thought the time was right for me to get a proper macro lens, rather than live with my 18-55 kit lens. I tried out the Nikon 105mm f/2.8 macro lens in a shop, and loved it... for the most part. However, I had 2 main problems with it:

1) The lens seemed to have trouble autofocusing sometimes. It would zoom to infinity, zoom back, and then just give up, even when there was ample light. I know you should manual focus in macro shots, but I'm not sure I have enough skill to do so.

2 (and this is the big one)) When focusing at a 1:1 reproduction ratio, the lens wasn't actually at f/2.8, actually more like f/4.8. If I focused on something 3 metres away, it was at 2.8, but it seems as though if it is marketed as an f/2.8 lens, then I'd better damn well get f/2.8!!!

Is there anything I can do to help stop these problems? And if there isn't, would someone mind telling me just why this happens? Thanks ever so much for the help.

~TamiyaGuy

 

the manual has a graph explaining this. it's normal unfortunately.
I was disappointed as well when I got my 60mm 2.8 and 105mm 2.8;

 

Why the hell would you want 2.8 in 1:1 ??! Ah... Probably DOF is too large... HMMMMM

 

A lens that is f/2.8 at infinity and f/4.8 at 1:1 is good. The Nikon achieves that by shortening the focal length. Not only is it not f/2.8, but it isn't 105 mm. Same with the 60 mm and current 200 mm - they shorten their focal length as they focus closer, thus decreasing the amount of change in the effective f-number.

Think about it. Just suppose that the focal length was fixed. The image distance (conjugate) at 1:1 is twice the focal length, so if the aperture (entrance pupil) diameter and focal length are the same, the f-number doubles*. Therefore a regular f/2.8 lens would have an effective aperture of f/5.6 at 1:1. That's normal.

*The effective f-number is based on the image distance divided by the entrance pupil ("aperture") diameter. It also depends on the 'pupil magnification'.

Best,
Helen

 

Interesting, My Canon maintains f2.8 at the minimum focus distance (1:1).

Not sure why you would ever want such a narrow DOF tho.

My focus will hunt sometimes, but it's fine most of the time in adequate light.

 

How do you know that the effective aperture is f/2.8?

Best,
Helen

 

well, other than looking at the aperture on the camera, I guess I don't.

How does one tell the effective aperture?

 

Thanks for all the info, guys. So basically, the lens shortens the closer you get? That would explain it. To be honest, Helen, some of that went way over my head, but I get the gist of it. I've also looked a bit more into it, and I've heard that basically, you either have that, or you get the lens protruding while it's focusing. Thanks!

 

" How does one tell the effective aperture?"

Take a picture of something like a grey card or a white piece of paper at f/2.8 from at least ten times the focal length (ie one metre) to get a benchmark, then keeping the same shutter speed and aperture take a picture of it at 1:1, making sure that the illumination is unchanged. Take two or three more pictures at 1:1, at progressively longer shutter speeds but keeping f/2.8 all the time. Compare the pixel values of the target. The shutter speed used for the 1:1 image that has the same pixel values for the target as the benchmark image should show you how far from f/2.8 you are at 1:1.

Best,
Helen

 

Have Mercy. Thanks Helen.

 

Here's another way of explaining it.

F-number is focal length divided by the diameter of the aperture (in fact the entrance pupil, but let's ignore that for now). A 105 mm f/2.8 lens has an aperture of 105/2.8 = 37.5 mm diameter.

As you focus more closely, a simple lens would have to move further away from the sensor/film. At 1:1 a simple lens would be twice as far from the sensor/film as it would be when focused at infinity. At infinity the simple lens would be one focal length from the sensor/film. At 1:1 the simple lens will be two focal lengths from the sensor/film - ie 210 mm in this case.

The f-number is now 210/37.5 = 5.6.

Because your Micro-Nikkor is cleverly designed, it achieves f/4.8 at 1:1 instead of f/5.6.

Best,
Helen

 

Helen is right on the money on this, as usual. I don't know if you have a Mac, but if you do, you can use the EXIF View program to look at your effective aperture... it shows up when I use my 105 VR.

As far as the crappy focusing goes with this lens, yes, it is quite true... the thing hunts like a golden retriever...

I have no idea why Nikon did such a bad job on this lens in the autofocus department... it is horrible on my D40 and D80, although it works just fine on my D300 when shooting up close in macro.

On the other hand, I virtually never autofocus when shooting macro anyway...

On the bright side, the lens is EXCEEDINGLY sharp.