Why does 200mm on my Sigma does not go as far as my Nikkor's?

[Bobalongwhale]

Hello

Although I would call myself a keen amateur, I have to admit I do no know much about the maths behind focal length.

I have just bought a Sigma DC 18-200 lens and I notice that at 200mm it does not zoom as close as with my Nikkor 55-200. My camera is a Nikon D300s.

Can someone explain to me why that is?

Thanks

[vandergus]

My guess is a phenomenon called "focus breathing". When lens manufacturer's specify the focal length of a lens, it is always at infinity focus. As you focus at closer distances, the focal length may change so at the nearest focusing distances the lens may have an actual focal length of 175mm, or something like that. It's a compromise that is often made in lenses with large zoom ranges (like your 18-200mm) and/or lenses that are internally focused. I know that a lot of Nikonians were upset with the latest 70-200mm f2.8 redesign because it is significantly less than 200mm at the closest focusing distances. Your 55-200mm probably exhibits less focus breathing because it has a smaller zoom range and has a less complex design.

[OsmosisStudios]

Originally Posted by vandergus

My guess is a phenomenon called "focus breathing". When lens manufacturer's specify the focal length of a lens, it is always at infinity focus. As you focus at closer distances, the focal length may change so at the nearest focusing distances the lens may have an actual focal length of 175mm, or something like that. It's a compromise that is often made in lenses with large zoom ranges (like your 18-200mm) and/or lenses that are internally focused. I know that a lot of Nikonians were upset with the latest 70-200mm f2.8 redesign because it is significantly less than 200mm at the closest focusing distances. Your 55-200mm probably exhibits less focus breathing because it has a smaller zoom range and has a less complex design.

This. Most superzooms breathe rather heavily.

[Aegea]

Indeed. I tested a Sigma 150-500 and found it had an effective maximum FL of 430mm at 20 feet.

[gturner]

Either that or the dude in Sigma factory has been trying to convert 6 inches into mm to impress a colleague

[ravncat]

The nikon 18-200 gets down to 135 or so at very close distances,

as stated above, the focal lengths are written for focus at infinity.

[TheEagleSpirit]

I love my Nikkor f-2.8 70-200 VRII lense.

[OsmosisStudios]

Originally Posted by TheEagleSpirit

I love my Nikkor f-2.8 70-200 VRII lense.

Thanks but thats only slightly relevevant: the 70-200 f/2.8VRII has some significant breathing issues - so much so that many pros are using x1.4 or x2 teleconverters on them to get the focal length they want and need