The new Sigma 18-200 lens I got for Christmas is advertised as being specifically designed for digital cameras. That got me wondering if it was subject to the 1.5 multiplication factor that I apply to my other lenses to calculate effective focal length.
I’ve written about this before when I first got my DSLR, comparing my film Nikon N60 with the Nikon D50. Digital SLRs have a smaller image sensor than 35mm film, so the image is cropped, which appears to increase the telephoto of lenses designed for film. The focal length of the lens doesn’t change. However, the effective focal length is extended due to cropping. For Nikons, this is about 1.5X. For Canons, I think it’s about 1.6X. Higher end DSLRs have full-frame sensors that match 35mm film cameras.
First, I did a bit of research to see what this “designed for digital” really means. According to Sigma’s website…
DC (DC Lens)
These are special lenses designed so that the image circle matches the smaller size of the image sensor of most digital SLR cameras. Their specialized design gives these lenses the ideal properties for digital cameras, the compact and lightweight construction is an added bonus ! including compact and lightweight construction.
So, that would lead me to believe that the multiplication factor doesn’t come into play. To test this, I set up a little experiment. I set up my tripod with my older Sigma 70-210 lens set at 200mm and snapped a shot. I then took a picture of the same scene with the new 18-200 lens set at 200mm. The results tell the story below…
The image on the left was taken with the 70-200 non-digital lens. You can clearly see increased magnification. The image on the right was taken with the digital 18-200 lens. There is clearly a wider field of view. You can click on the images for a larger view.
I don’t think I’m going to miss the additional magnification. The sharpness and versatility of the lens far outweighs a little bit of magnification. And if I need that additional zoom, I’ve still got my 70-210.
Out of curiousity, I tried the new 18-200 with my Nikon N60 film camera while I was out shooting yesterday. There was a pronounced barrel vignette, which I guess is due to the smaller image circle. I’ll have to see what really happens when I develop the film. I also wonder what would happen if I used this camera with a full-frame digital. I’m guessing I wouldn’t be happy with the results.
[tags]sigma 18-200, photography, lens comparison, sigma 70-210, Nikon, camera[/tags]
2 thoughts on “More on the new lens”
The magnification factor does come into play and there is no way to avoid it. The reason your 18-200 has a smaller image at the 200 point is because that 18-200 does not really go to 200! Zoom lenses tend to be a little optimistic at both ends and are allowed leeway in their labeling. An 18-200 is likely to be a little longer at the wide end and a little shorter at the long end — but when competing with Nikon’s 18-200, a 20-180 might not be as competitive. Also, the fstop may not be exactly a 3.5 to 6.3 but an f/4 to f/8 — but since we are taking readings thru the lens and not a separate meter we can’t tell but if you used a separe meter you could — and it isnt just Sigma playing this game — they all do it. For many years, the Leica 50mm Summicron was actually a 52mm. Also, the Nikon factor is actually 1.52x and Canon has two factors on different models: 1.6x and 1.3x. T
David, Thanks for the clarifications. Even so, I’m curious to see how this lens behaves with my 35mm SLR, given the barreling effect.
Are you saying that Nikon’s 18-200 is a true 18-200, or is it optimistic at both ends, too?