Do film-era lenses become longer on a crop-sensor DSLR?
Asked 2/19/2012
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I’m using two older Tokina lenses from a Minolta film SLR on a Sony APS-C DSLR: a 28–80mm and a 100–300mm. I’ve been told to multiply the focal length by 1.5 because of the smaller sensor, so they would be equivalent to about 42–120mm and 150–450mm. Is that actually correct? If so, what’s the catch?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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That is partly true. You won't actually get a lens with a longer focal length, but you get a lens with the same field of view as a lens with a longer focal length. The actual focal length remains the same, but as you are using a smaller part of the image circle you get a larger magnification, similar to what a longer focal length would give.
This way of multiplying the focal length to get an equivalent focal length is only applied to DSLRs, although the difference in how the field of view corresponds to the focal length has always existed, for example between medium format cameras and small format SLRs. It's only when there was a lot of consumers moving from SLRs to DSLRs that there was a need for an overly simplified way to compare how lenses work on different formats.
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
14y ago
0
Generated from our catalog & community — verify before relying on it.
The lenses do not actually change focal length. A 100–300mm lens remains a 100–300mm lens.
What changes on an APS-C Sony DSLR is the field of view: the smaller sensor captures a smaller central portion of the image circle, so the view looks narrower. That gives the same framing you’d get from about a 1.5× longer lens on a 35mm film or full-frame camera.
So your lenses behave like this in terms of angle of view:
- 28–80mm ≈ 42–120mm equivalent
- 100–300mm ≈ 150–450mm equivalent
The “catch” is that you are not getting extra optical magnification or a physically longer lens—you’re just using less of the image projected by the lens. A new Sony 100–300mm would still be a 100–300mm lens; on the same APS-C camera it would give the same field of view as your old one.
In short: the crop factor changes equivalent field of view, not the actual focal length.
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