Recently I compared the image size of photos taken with my Nikkor 105 mm macro lens and the 18-200 mm DX-zoom at 200mm (both with D90 body and at exactly the same distance from the subject) and found them to be about comparable. I was expecting the 200mm to fill the frame more than the 105mm. There a direct linear relationship between focal length and size of image governed by this formula. My question then is - Why is there no difference in the image size with the two lens? Clearly sensor size is not an issue here as I am using the same body. May be this has to do with how DX lens are rated taking into consideration of the magnification factor of DX format from prime lens which is rated for full frame camera?? Does this make any sense?
Posted by Alan Olander on April 13, 2010 at 8:45 AM
Because of its design, the 18-200mm loses focal length as you focus closer, so at 200mm at infinity it may actually be a 200mm lens, but at close distances it is not 200mm. Nothing to do with DX.
Alan is spot-on with his comment. Another lens from Nikon that exhibits this focal length "breathing" (as some refer to it) is the new 70-200mm f/2.8 VRII. The reason Nikon designs this into the lenses is to improve image quality when close-focusing with the lens. The trade-off is the loss of focal length, which bothers some users. Personally the shortening of the focal length doesn't bother me, but I'm just one shooter and it's totally dependent on one's subject matter and preferred style of working.
Cheers...
Brad
Posted by Mel Ladewig on April 13, 2010 at 3:43 PM
Lenses focus in generally two ways.
Prime lenses generally focus closer by becoming longer focal lengths. The best way to see this is with a bellows camera. If I want to capture a 1:1 image with a 150mm 4X5 lens on 4X5 film, I extend the lens to ~300mm. Is it a 300mm lens now? No, because focal length is based on focus at infinity. This is how extension tubes work.
The other way in which lenses can be focused is internal focus designs and many of these shorten lenses to focus closer than infinity. This situation is usually limited to zoom lenses.
I think what you're seeing is the two lens designs converging on one focal length for a given magnification. If you could continue to focus closer you might see them pass and find the 105 to focus longer than the 70-200mm
Thanks to Alan, Brad and Mel for their clarifications.
If zoom lenses have focal length breathing, then this should be reflected in the EXIF since the camera knows distance of the subject for the autofocus and it would just involve a small correction factor calculation.
Posted by Mel Ladewig on April 15, 2010 at 11:39 AM
It doesn't work that way. Like I said, the focal length is the focal length focused at infinity. It keeps the terminology consistent even if the lens focal mechanism doesn't actually function that way. For instance, there is a large format lens, the Schneider 120HM macro designed for 1:1 macro. It can't even cover the film at infinity and is typically used at 240mm extension for 1:1 or greater. Its still a 120mm lens.
Discussion
5 repliesAlan is spot-on with his comment. Another lens from Nikon that exhibits this focal length "breathing" (as some refer to it) is the new 70-200mm f/2.8 VRII. The reason Nikon designs this into the lenses is to improve image quality when close-focusing with the lens. The trade-off is the loss of focal length, which bothers some users. Personally the shortening of the focal length doesn't bother me, but I'm just one shooter and it's totally dependent on one's subject matter and preferred style of working.
Cheers...
Brad
Prime lenses generally focus closer by becoming longer focal lengths. The best way to see this is with a bellows camera. If I want to capture a 1:1 image with a 150mm 4X5 lens on 4X5 film, I extend the lens to ~300mm. Is it a 300mm lens now? No, because focal length is based on focus at infinity. This is how extension tubes work.
The other way in which lenses can be focused is internal focus designs and many of these shorten lenses to focus closer than infinity. This situation is usually limited to zoom lenses.
I think what you're seeing is the two lens designs converging on one focal length for a given magnification. If you could continue to focus closer you might see them pass and find the 105 to focus longer than the 70-200mm
If zoom lenses have focal length breathing, then this should be reflected in the EXIF since the camera knows distance of the subject for the autofocus and it would just involve a small correction factor calculation.