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rhythmaningI use two lenses with my DSLR: an 18-50mm zoom and a 55-200mm zoom, both Sigma. Before I went on holiday in June, I bought one lens to cover the same range – another Sigma, 18-200mm.
When I got home, I wanted to compare the new lens quality to my old lenses. I set it to 200mm and took a picture; then did the same with my 55-200mm.
The quality wasn’t an issue; but the two pictures had quite different fields of view – despite both being “200” mm. I estimated that at its full extent, the new lens was equivalent to c. 130mm on the older lens.
I wasn’t happy about this – having just forked out a couple of hundred quid, I had failed to duplicate the full range. I took the lens back to the shop, where we duplicated the test, and the salesman agreed that the field of view was significantly less than I had expected. He explained that this was because the 10-200mm lens was not a zoom but a “versatile” lens. This sounded like saleman’s flannel – surely 200mm should be 200mm? (He was good enough to refund my money, and I would recommend them for good service and prices.)
My brother was visiting this weekend, and he has a Nikon 18-200mm zoom, so we repeated the experiment – with exactly the same results. The image from the Nikon lens had a much wider field of view, equivalent to about 130mm on the Sigma.
Here’s the image from my original lens, set at 200mm:
Here’s that from the Nikon 18-200mm, set to 200mm:
Here are the two overlaid, the Sigma image resized (not a perfect fit):
And here are the outlines of the two, for comparison – the Nikon 18-200mm in red, the Sigma 55-200 in blue:
The metadata recorded by the camera says that both lenses were shooting at 200mm.
Clearly, 200mm is not always 200mm. I don’t know which lens is truer to 200mm: I don’t have access to a fixed 200mm lens.
Does anyone have an explanation for the difference between the lenses?
Any views appreciated!
(crossposted)
When I got home, I wanted to compare the new lens quality to my old lenses. I set it to 200mm and took a picture; then did the same with my 55-200mm.
The quality wasn’t an issue; but the two pictures had quite different fields of view – despite both being “200” mm. I estimated that at its full extent, the new lens was equivalent to c. 130mm on the older lens.
I wasn’t happy about this – having just forked out a couple of hundred quid, I had failed to duplicate the full range. I took the lens back to the shop, where we duplicated the test, and the salesman agreed that the field of view was significantly less than I had expected. He explained that this was because the 10-200mm lens was not a zoom but a “versatile” lens. This sounded like saleman’s flannel – surely 200mm should be 200mm? (He was good enough to refund my money, and I would recommend them for good service and prices.)
My brother was visiting this weekend, and he has a Nikon 18-200mm zoom, so we repeated the experiment – with exactly the same results. The image from the Nikon lens had a much wider field of view, equivalent to about 130mm on the Sigma.
Here’s the image from my original lens, set at 200mm:
Here’s that from the Nikon 18-200mm, set to 200mm:
Here are the two overlaid, the Sigma image resized (not a perfect fit):
And here are the outlines of the two, for comparison – the Nikon 18-200mm in red, the Sigma 55-200 in blue:
The metadata recorded by the camera says that both lenses were shooting at 200mm.
Clearly, 200mm is not always 200mm. I don’t know which lens is truer to 200mm: I don’t have access to a fixed 200mm lens.
Does anyone have an explanation for the difference between the lenses?
Any views appreciated!
(crossposted)
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no subject
Date: 2011-08-06 09:22 am (UTC)If you have an APS-C sensor in your camera picking up the light, then the focal length is effectively 1.5-1.6x larger than a full frame sensor. So if one lens had an actual focal length of 200mm with an effective focal length of 320mm, and the other an effective focal length of 200mm, that would fit what you're seeing. i.e. one lens was designed to match the size of the sensor in your camera, the other was designed for a larger sensor.
There's more about it on wikipedia, as always.
http://en.wikipedia.org/wiki/Crop_factor
http://en.wikipedia.org/wiki/Image_sensor_format
no subject
Date: 2011-08-11 10:48 am (UTC)The consensus view seems to be that it is a result of the lens qualities - both would be 200mm if I had focused on infinity, rather than the bookcase (at 3m).
I shall experiment next time I get access to the lens...
no subject
Date: 2011-08-11 10:51 am (UTC)