Lens Correction

March 15, 2007

Write this down – I was wrong!

I said that when a camera lens is manufactured “for digital cameras” that it meant two things:

1. It would cause severe vignetting if used on a 35mm film camera (or a digital with a 35mm sensor)

2. That the 1.5x multiplier typically employed to understand how much real estate you’re going to get into the frame when shooting digital didn’t apply. That is to say, if the lens says 50mm on the side you’d typically think “on my digital that’s 75,” but not with “for digital” lenses. With those (so I said), if it says 50 you get 50.

And I was wrong! Partly. The first item is correct: you do get vignetting if you use a “for digital” lens on a film camera. But the focal length printed on the side of the lens is still being reported in exactly the same way as it is on other lenses. The 50mm “for digital” is still giving you an effective 75mm. The two things above are independent of each other.

So why do lens makers make “for digital” lenses, anyway? You can’t even use them on film cameras! Turns out it’s to keep the cost and weight down. Because most digitals use smaller sensors, the lens doesn’t have to be so huge to cover it.

So why don’t they start printing the effective focal length on the sides of lenses made “for digital” cameras? Tough call. I think the industry position is that it would cause confusion (ha!). They want to employ the same metric to all lenses, and since the focal length is really a property of the lens and is completely independent of the size of the sensor, they stick with the old way of reporting it. Makes sense if you think about it.

Anyway, to recalculate my current lens lineup… My new Sigma 18-50mm f/2.8 “for digital” lens is giving me an effective 27-75mm.



No comments yet

  1. I can now sleep at night. I was actually thinking quite a bit about your post as I was researching lenses. I couldn’t find anything to support your previous statement. Glad to have that cleared up;)

  2. I think it’s somewhat deceiving of the lens maufacturers not to state that the conversion is still required. They go on about how the lens is designed to fit digital camera sensors and should not be used on film cameras or digitals with full frame (35mm) sensors. It leads one to believe you are getting, for example, a true 18-50mm lens since conversion factors are not mentioned in the features, specs or FAQs.

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