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

TamiyaGuy · Mar 22, 2008

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

SCHNOOBS · Mar 22, 2008

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;

Val · Mar 22, 2008

Why the hell would you want 2.8 in 1:1 ??!

Ah... Probably DOF is too large... HMMMMM

Helen B · Mar 22, 2008

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

TCimages · Mar 22, 2008

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.

Helen B · Mar 22, 2008

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

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

Best,
Helen

TCimages · Mar 22, 2008

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

How does one tell the effective aperture?

TamiyaGuy · Mar 22, 2008

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!

Helen B · Mar 22, 2008

" 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

TCimages · Mar 22, 2008

Have Mercy. Thanks Helen.

Helen B · Mar 22, 2008

TamiyaGuy said:
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!

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

sabbath999 · Mar 22, 2008

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.

Garbz · Mar 22, 2008

Sabbath what you said is self contradictory. The lens isn't doing a bad job at autofocus. The camera is. The only time this is the lens' fault is if the lens is not sharp at f/2.8. And I'm sure we can both agree that this lens is pretty damn sharp wide open for a change.

One thing to realise is that given when you're doing really tight macro photography the depth of field is tiny. So the slightest change and you've lost all contrast. Now a clever autofocus system may be able to figure out which way it moved, but given just how out of focus something appears wide at f/5.6 1:1 when you move even a couple of mm, even a complex phase detection system may not know if it's moving the lens in the right direction which is why the focus hunts. I've never once had the lens actually MISS the focus point when searching. Only when it loses focus sometimes it runs the wrong way trying to find it again.

sabbath999 · Mar 23, 2008

Garbz said:
Sabbath what you said is self contradictory. The lens isn't doing a bad job at autofocus. The camera is. The only time this is the lens' fault is if the lens is not sharp at f/2.8. And I'm sure we can both agree that this lens is pretty damn sharp wide open for a change.

One thing to realise is that given when you're doing really tight macro photography the depth of field is tiny. So the slightest change and you've lost all contrast. Now a clever autofocus system may be able to figure out which way it moved, but given just how out of focus something appears wide at f/5.6 1:1 when you move even a couple of mm, even a complex phase detection system may not know if it's moving the lens in the right direction which is why the focus hunts. I've never once had the lens actually MISS the focus point when searching. Only when it loses focus sometimes it runs the wrong way trying to find it again.

Certainly, if it works on one camera and not the other two then the problem is in the software somewhere... whether that software has been corrected to compensate for an error in the lens on the D300 (like how they fixed the defects in the lens on the Hubble space telescope by redesigning the software on the camera) or whether the D40's and D80's ship with defective software, I don't know.

When I say this lens hunts, I am not saying it "misses focus" then hunts. When it hunts, it just goes nuts. I have actually had the thing wrack in and out four, five and even six times, never finding a single focus spot (background, foreground or whatever), when I was well outside the minimum focus distance. It's not like a normal hunt lens which will wrack out, and wrack back in and find something, it will go all the way out, all the way in, all the way back out again, all the way back in again, etc until either you let off the button or it finally finds something.

It only does this on macro stuff though, not on stuff a meter or two away... I have never had a problem with further away stuff at all, just when you are say 3:1 or closer.

My guess is that Nikon fixed whatever was wrong with the camera's CPU or added compensation for the error built into the 105's CPU in the D300. It isn't perfect, it will still OCCASIONALLY hunt, but all lenses will do that.

BTW this is why I recommend to all to buy the one YOU have instead of the 105 VR version... Yours doesn't have this autofocus defect, and VR doesn't work very well at macro distances anyway.

Both are equally sharp, and the AF 105 f/2.8 is a bunch cheaper (pre-owned) than the inferior VR version I have.