Focus breathing of Nikon 18-200mm

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Focus breathing of Nikon 18-200mm

Postby jeremy1302 » Fri Apr 16, 2010 12:02 pm

Ok, I did this quick test to satisfy my own curiosity, and thought I'd share it with you guys.

I knew that at 200mm the lens would have a different effective focal length at different focal distances, but I wanted to see the extent of the effect.

I set my D5000 to f36, ISO 6400 and the lens to 200mm. The aperture was to get a maximum depth of field and the high ISO was to be able to shoot at f36 handheld. Yes, i did this handheld, but remained in exactly the same position each time, like I said, it was only a quick test.

This first image was with the lens at its shortest focussing distance, of just under 0.5m:


This next one focussed to 1m:


This one focussed to 3m:


And finally, this one focussed to infinity:


I've then tried to roughly overlay a grid of the infinity focussed size on top of the minimum focal distance image:


I know that the box isnt central, but thats because I was handholding.

From some rough calculations that could probably be wrong, I made the focal length of this closely focussed image to be just over 105mm, assuming that the lens is a true 200mm when focussed at infinity.

I think that this is actually a significant reduction! People buy an 18-200mm when actually it can behave like an 18-105mm if your style of photography means using close focussing distances.

Do I care? No, normally when I use 200mm, I am focussing at distant objects and so get a true 200mm. In fact, even when focussing at 3m the effect is nearly trivial.

Hope that my rambling makes sense, and you can see what Ive done!


Postby jeremy1302 » Fri Apr 16, 2010 12:06 pm

Ps on a proof read, I can see that the yellow box is possibly not quite lined up correctly, but if anything it needs to be tighter and so the lens is actually less than at 105mm.

Also please note that all my focal lengths are the true focal lengths, and not full frame equivalents

Postby beren23 » Fri Apr 16, 2010 12:55 pm

Yes, you illustrate the problem quite well with your shots.

This is yet another compromise of a lens such as this (it's not unique to Nikon's super-zoom, yet I cannot tell you how bad it is for each brand, I guess there'll be no much difference between them though). Surely engineers make this compromise knowing that, most likely, one will use the 200mm focal length to shoot distant objects where, focused at infinity, you get a true 200mm anyway(as you well said), or for portrait shots where you'll care more of getting a shallow DOF than of the effecive focal length of your lens in that momment.

Postby Bernie » Fri Apr 16, 2010 3:30 pm

Maximum Reproduction Ratio: 0.22x
Minimum Focus Distance: 1.6ft.(0.5m)

Simple lens mathematics

F = D / ( 1/M + M + 2 )

D = focusing distance (distance between subject and sensor)
F = focal length
M = magnification

Let's use this with what we know from the 18-200 ...
M = 0.22
D = 500 mm

F = 500 mm / ( 1/0.22 + 0.22 + 2 )
F = 74mm
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Postby jeremy1302 » Fri Apr 16, 2010 5:58 pm

Great, thanks for adding that Ernie, so actually worse than I estimated!

I really dont want to put anybody off getting one of these lenses in these tests, they really are a great bit of glass. Just look at how much the situation improves moving from 0.5m to 1m to see how little this is likely to affect you in real life. I dont think many people use this lens as a macro lens :wink:

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