Fixed focal length macro versus zoom lens

[MatthewA]

I was having a play comparing a 180mm dedicated macro lens versus the 18-200mm zoom to compare field of view close up.

The zoom lens will focus down to 0.5m, but these shots were at about 0.7m. All shots were taken using the camera mounted on a tripod so that the distance to the object remained the same. Maybe many of you already know about this effect, but I have to say I was VERY surprised to see the difference between the macro and zoom lens when used close up :eek:

I wonder if all zoom lenses show similar results compared to fixed focal length lenses or is it just ultra-zooms :thinking:

No. 1 180mm Sigma approx 0.7m

http://www.talkphotography.co.uk/gallery/data/500/DSC_1504_800.jpg


No. 2 Same as No. 1 but with 18-200mm Nikon zoom at 200mm

http://www.talkphotography.co.uk/gallery/data/500/DSC_1506_800.jpg


For comparison, here's the 2 lenses focussed in the distance - the subject is not great, just a shot through the window, but you can see the field of view is now pretty similar.

No. 3 Sigma 180mm

http://www.talkphotography.co.uk/gallery/data/500/DSC_1508_800.jpg


No. 4 Same as No.3 but with 200mm zoom

http://www.talkphotography.co.uk/gallery/data/500/DSC_1507_800.jpg

[Canon Bob]

It's common to relate angle of view to focal length but it's not quite so simple. The focal length of the lens is always measured at infinity focus (and, in reality, doesn't change). The angle of view is actually a function of the reproduction ratio and reduces as the magnification increases. It's quite normal that the angle of view of a close focussing lens will be about half of it's AOV when focussed to infinity. The same thing is happening with your zoom but the change is exponential and has an ever more rapidly increasing effect as the magnification increases (ie, closer focussing distance). A 180mm macro lens has probably got the equivalent AOV of a 100mm lens focussed at infinity.

Bob

PS...your sundial will tell the wrong time unless you get it levelled.

[MatthewA]

Thanks for the explanation Bob. I knew the AOV varied somewhat as focus gets closer but I hadn't appreciated it changed to such an extent for non-macro lenses, so I'm even moe impressed with macro lenses now (and they're pretty good at infinity too!)

I wondered why I was always late for lunch, now I know it was the sundial running slow :D (It's the ants that have moved it, little blighters)

[Canon Bob]

Matthew,

I can bore you further with some equations if needed but I suspect that you were only after a quick summary/explanation.

Bob

[MatthewA]

Yes thanks Bob, just a quick explanation is enough... although just to clarify, the effect would be the same for a non macro fixed focal length lens as it is for the zoom lens, yes?

Now you've got me thinking, would the use of extension tubes affect this (assume lens focussed at infinity) or is that a silly question? Hmmm, I think I need to read up a bit on it after all, have you any suitable links that don't need a PhD in maths?

Thanks
Matt

[Canon Bob]

... the effect would be the same for a non macro fixed focal length lens as it is for the zoom lens, yes?

Now you've got me thinking, would the use of extension tubes affect this (assume lens focussed at infinity) or is that a silly question? Hmmm, I think I need to read up a bit on it after all, have you any suitable links that don't need a PhD in maths?

Thanks
Matt

Matt.....the effect is certainly there for any focal length (and zoom). The closer your point of focus then the larger the magnification and the narrower the AOV. The effect is exponential so the close focussing macro's really highlight the effect.

Tubes will increase the potential magnification...depends how closely you focus but the same effect will be seen.

I'll have a dig for some non-mathematical reading....maybe not today so I'll PM you if I find it.

Bob