Is 120mm really 120mm on the 24-120mm f/4?

I went to do some more chess shots this morning and stumbled upon something very disconcerting. I was shooting with my 24-120mm f/4G and decided I wanted more depth of field, so I grabbed my 85mm f/1.8G. I had been shooting at 120mm on the zoom, so imagine my shock when I put the prime on and had almost the identical image framed!! That's 35mm of additional focal length that doesn't appear to be there. I figured I must have forgotten that I moved the tripod, so I put the zoom back on and I'll be damned if 120mm on the zoom isn't almost exactly like the view from the 85mm. I zoomed out to 85mm and sure enough it was wider than the prime.

Nikkor 85mm f/1.8G



Nikkor 24-120mm f/4G at 120mm



Nikkor 24-120mm f/4G at 85mm



Nikkor 24-85mm f/3.5-4.5G at 85mm



Sigma 105mm f/2.8 Macro




Now, I've read a lot of reviews for this lens, both before and since buying it, and nothing prepared me for what I'm seeing here. I suspect that some of it may have to do with me using the far end of the zoom against a rather close subject (funny thing is, the zoom focuses at this distance and max aperture at 120mm but the 85mm will not), but still, I'm not really thrilled that I'm getting closer with my 105mm than the zoom at 120mm.

Is there some optical property that I'm forgetting that will cause this, or do I need to continue this investigation with a wider field of view? The two zooms seem to be in lock step, so is it possible that the primes tend to magnify more when they approach minimum focusing distance? That's the only thing I can think of.

What you're describing sounds and looks to me like focus breathing.

Informative Link: http://www.bobatkins.com/photography/technical/focus_breathing_focal_length_changes.html
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Horoscope Fish said:

Isn't that focus breathing?

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Click to expand...

Ah-yup!

I've dealt with focus breathing before, but never to the extreme of losing 35mm. Interesting, particularly given that both zooms are approximately the same at 85mm (i.e. they "breathe the same").

Probably similar to my Nikon 70-200mm @ 200mm will have similar FOV with my 135mm prime at minimum focusing distance.

Marked focal length only applies to focus at infinity. Many lenses change at short focus distances. If you repeat your tests at longer outdoor distances, you will get different results.

How does it do when dealing with subjects further away?

WM

This is interesting...

Thanks, gents. When I get home from errands this afternoon I'll try something similar outdoors at distances, but as I wake up (LOL) I realize that you're correct regarding the focus breathing and that at normal subject distances we'll fall in line.

It was still cool to see you document focus breathing. It may help new shooters understand what is going on with their gear.

Noticed that before though on a different accord. Suppose standards have slopped since the good old days, but you don't really hear everyone complaining so...

Hmm. By chance I found this thread after discussing this with my dad earlier today. He had found that he had a 'gap' between to zooms that both went to 70mm but from different ends. He thought that it had to do with the FX/DX crop, because one of the lenses were an older lens not minded explicitly for DX. I hold that this could not be the case, as this is often refuted right here on the forum

Now that I have come home, I have tried a few lenses myself, all 55mm

18-55, DX
18-55 @ 55 by jonas_sandager, on Flickr

55-200, DX (sorry for the bad focus, the cup was just a bit closer that closest focus distance)
55-200 @ 55 by jonas_sandager, on Flickr

55mm macro, FX
55 macro by jonas_sandager, on Flickr

All were at f/5.6 and 1/15 second. My stand is very wobly, so the framing is not exactly the same. But close enough to see the big difference in field of view.

As you can see, the 18-55 is much closer to the 55 macro in it's field of view than the 55-200 which clearly outrule the FX/DX explanation. Focus breathing seems like a much likelier candidate. It's dark outside now, but I will try to remember to take some tests at infinity tomorrow.

BackdoorHippie said:

Thanks, gents. When I get home from errands this afternoon I'll try something similar outdoors at distances, but as I wake up (LOL) I realize that you're correct regarding the focus breathing and that at normal subject distances we'll fall in line.

Click to expand...

Jake, did you ever retest this lens at different distances?

@nidding: take also a so-called "lens breathing" into account...

The focal length is determined with the standard that the lens is focused at infinity.

DraganDL said:

@nidding: take also a so-called "lens breathing" into account...

Click to expand...

Yep. That's why I want to test at infinity focus tomorrow

Very interesting, never heard of this before, will have to read up and investigate!

From what I have read, the problem is the IF (internal focusing) mechanism. For maintaining constant focal length the barrel has to increase and decrease its length while focusing. In case of IF one group of lenses moves back and forth, and this changes the focal length.

Breathing (lens) - Wikipedia, the free encyclopedia
Focus Breathing (Focal length variation with focus distance)- Bob Atkins Photography