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Do you also have to convert the speed to 35mm format?
22.8.2017
Translation: machine translated
The same focal length looks different on different sensor sizes, which is why it is sometimes converted to 35 mm format. Do you also have to do this with the aperture and speed?
The starting point for the idea of converting the light intensity is as follows: If you photograph the same image section with the same aperture using different sensor sizes, the photo will still not look quite the same: the depth of field is different. The unfocussed background is more blurred on the large sensor.
Tony Northrup shows this very nicely in his explanatory video, here is a screenshot of it. You have to pay attention to the window frames in the background:
You can now also offset the f-number with the crop factor in order to achieve a comparable depth of field. Example: APS-C: 75 mm and f/4 corresponds to an image in full format with 112.5 mm focal length and f/6.
So far, so good. But you have to be extremely careful not to draw the wrong conclusions. In particular, you must not confuse the converted values with the real ones.
Full frame camera, 35 mm, f/4.APS-C, same focal length and aperture (35 mm, f/4).Detail of the shot with full-frame camera. The degree of blurring in the background is the same as in the shot with an APS-C camera.It is also not the case that a lens with a speed of f/4 on a full-frame sensor would then only have a speed of f/6. The real light intensity remains at f/4, it does not change. It is a fixed property of a lens, which is why it is not a marketing gimmick to write this property on the lens.
As is well known, the aperture not only says something about the depth of field, but also how long the exposure can be under certain conditions - and this is the main point when talking about "light intensity".
How strong the exposure is depends on three factors: Exposure time, aperture and ISO sensitivity. If a particular photo is correctly exposed at 1/60th of a second, f/2 and 100 ISO, then that's always the case, regardless of the sensor size and the lens used. If I start to convert the aperture, the exposure is no longer correct.
There are now people who claim that you also have to convert the ISO value, i.e. multiply the crop factor by the square. 1.5 squared is 2.25. In the example above: APS-C: f/2 and 100 ISO corresponds to f/3 and 225 ISO in full format.
This double conversion means that the exposure is correct again. But the whole calculation only relates to the comparability of images, not lenses. A lens with a speed of f/6 (if that even exists) does not have a speed of f/4 on APS-C. It is still f/6.
Conclusion
If you want to use a full-frame lens on an APS-C camera, you must observe the following:
It's best to forget all the rest, it only causes unnecessary confusion.
David Lee
Senior Editor
David.Lee@digitecgalaxus.chMy interest in IT and writing landed me in tech journalism early on (2000). I want to know how we can use technology without being used. Outside of the office, I’m a keen musician who makes up for lacking talent with excessive enthusiasm.
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