Lens Magnification Test

[Commander]

Neo, just did a very simple test with two lenses, the 70-180mm micro and the 18-200mm VRII.

These parameters are constant :

Distance between subject and sensor plane = approx 750mm
ISO 200 ; f11 for all 3 shotsShot #1 : Nikkor 70-180mm @ 180mm
Shot #2 : Nikkor 18-200mm @ 170mm
Shot #3 : Nikkor 18-200mm @ 200mm

Interestingly, I cannot set the 18-200mm @ 180mm no matter how I turn the zoom ring. :thinking:

Anyway, I hope this sheds some light on your magnification mystery. Let me know if you want any more comparisons by moving other parameters.

[Sky Blue]

how come 200mm still wider than 180mm?

[Common Rose]

yalor... that's what i'm puzzled with... the magnification at the 200mm end is actually smaller than 180mm of another lens when shooting at close range...

I suspect that one of the reason is due to the fact that the physical length of the 18-200mm is short, so at close range, the distance from the subject to the first lens of the lens (:hmmm:) is longer and much significant as compared to that of the 180mm.

[Sky Blue]

I think it got some thing to do with the focal length calculation... but to me, this is not 200mm, and at wide side, really 18mm?

[Commander]

Hmm... most of you guys have lenses covering the various focal lengths. Go try it out lor...

Fix camera location with the camera body on the tripod. Then change the lenses accordingly. Fix a subject at a constant distance from the camera. Then shoot with various lenses at the same focal length e.g. 28-80mm, 18-70mm, 50mm prime = all at 50mm focal length. You will find that the angle and magnification are all different for each lens... :thinking:

[Common Rose]

well, if differ by a bit, it's no big deal... but the difference in this comparison test is really BIG. :what:

[Commander]

Maybe the 70-180mm's construction as a macro lens has something to do with it? If I were to focus it at infinity, the field of view should be the same as a lens at 180mm? The closer I go to the object, the macro lens construction kicks in, and it functions more like a 1:1.3 macro lens at 180mm. :thinking:

Anyway, u guys should try it and see for yourself, if you have various lenses at home.

[Rustic]

Maybe the 70-180mm's construction as a macro lens has something to do with it? If I were to focus it at infinity, the field of view should be the same as a lens at 180mm? The closer I go to the object, the macro lens construction kicks in, and it functions more like a 1:1.3 macro lens at 180mm. :thinking:

That is what I have been trying to tell Neo. :P

It is the lens construction. The marking is good for infinity focus, but not for near subjects.

Add: For the Sigma 70-300. Once u push the switch, the rear part will be fixed. It acted as an extension tube for macro purpose. So, the optical formula for inf and near is definitely different.

[Common Mime]

I think the test should be taken place from a pretty fat for the telephoto test. In near minimum focusing distant, the built in "macro" feature (the flower sign) will do somthing to the magnification rate. Do you think a T-90 can produce a bigger image at 750mm (from lens to subject) or the 200mm lens?

I think T-90 give a bigger image...

[Rustic]

http://www.kenrockwell.com/nikon/18200.htm

Actual Focal Lengths

Some folks are worried that they got cheated and 200mm isn't really 200mm. I pulled out my 80 - 400 VR and 80 - 200 AFS and compared. At 500 feet (200 meters) the 200 mm setting of my 18 - 200 VR gave exactly the same field of view as my 80 - 400 VR and 80 - 200 AFS at their 200 mm settings. For the long distance shots for which you need 200 mm you have an honest 200 mm.

Those who are worried misunderstand how internal focusing (IF) works. Internal focusing allows this 200 mm lens to focus down to 1.5 feet at 200 mm, which is closer than any other 200mm lens! Not even Nikon's 200mm macro lens focuses as close as the 18 - 200 VR.

IF works by twiddling many internal factors to allow focusing without the whole lens having to move in and out. Standard lens design equations tell us that even if you extend a 200 mm lens by an extra 8" (200 mm) to focus more closely that it only focuses as close as 32" (800 mm). Do you really want your 18 - 200 to poke out an extra 8" as you focus, and then only focus to three feet? Of course not. This would be in addition to the extension you get when zooming to 200 mm.

IF works by deliberately shortening the focal length as you focus closer. Shortening the focal length internally has the same effect as moving the lens out to focus. Focal length is 200 mm at infinity where you need it and shortens as you focus closer.

IF lens are supposed to appear to have shorter focal lengths when compared to traditional lenses at close distances. Don't drive yourself crazy comparing since every IF design is a little different. My 24 - 85 AFS IF lens matches 18 - 200 VR at close distances, while the 18 - 200 VR at 6 feet and 200 mm matches my 80 - 200 AFS set to 130 mm.

So what? The 18 - 200 VR focuses far closer and and gets tighter close ups than any other 200 mm lens, except of course for the $1,000 Nikon 200mm macro. The effective focal length has to shorten as you focus closer for the lens to work. When doing the nerd comparisons above you'll notice that the lenses to which you're comparing the 18 - 200 VR can't focus anywhere near as close or get as much magnification as the 18 - 200 VR does, regardless of comparative focal lengths.

The only loss you have with the IF system of the 18 - 200 VR at 200 mm is if you're shooting at the restricted minimum focus distances of the older lenses, which were eight feet for the 80 - 400 VR and five feet for the 80 - 200 AFS. At these distances you'll match the 200 mm setting of the 18 - 200 VR at about the the 135 mm - 120 mm settings of the older lenses. So what? Just walk one step closer with the 18 - 200 VR and you'll get a tighter image than the older lenses which can't focus that close.

Even at 25 feet this is irrelevant. My 18 - 200 mm at 200mm gives the same image as my 80 - 200 AFS does set to 190 mm.

[yanyewkay]

long and informative post. in short.. it means that the intended focal lengths are measured when focussed to infinity.

At a distant object eg:200 in the report above. it is sufficiently far enough. at the same 'focal length' you try to focus it to something nearer, the image size would be different.