Originally posted by Rondec Yes. But the question is whether you convert just the focal length or the aperture too. I think you should convert the aperture, so the DA *55 is not exactly the same on a K-3 as the DFA *85 f1.4 would be on a K-1, even though angle of view will be similar. I prefer the full frame rendering of the 85, even though I like the DA *55 quite well too.
Yes, the angle of view is cropped on APS-C, but if you compare a 61 megapixel full frame (A7r IV) to a K3, you don't get any extra length at all to your lenses. So, this cropped angle of view doesn't mean much. If you shoot with your hypothetical 450mm lens on APS-C and 700 mm on full frame with the same pixel density, you have the ability to crop down the full frame to the APS-C image, just not the reverse.
You convert neither focal length or aperture. They are fixed. They cannot be converted as they are measurable physical properties. It is all about angle of view.
Nobody claims that you cannot get APS style images from an FF camera. However, if you crop a FF image to APS size you are not comparing APS to FF anymore but APS to APS. One of them a waste of money, size and pixels.
To illustrate how subjective and situation dependent the so called objective equivalence is can be illustrated by an example:
Say, you are an astro photographer taking images of the milky way with your APS 1.4 lens. Then you figure out that you can actually rather use FF and an F:2 lens, which is much cheaper, according to equivalence. To your surprise you find that either the shutter speed gets one stop longer giving streaks and comparable to the use an F.2 lens for APS, or you have to boost ISO one stop loosing the noise advantage you presumable bought the FF camera for in the first place. There's simply no equivalence of free lunch. If you want that FF advantage over APS for this situation you have to use an F:1.4 on FF as well. What parametres you find important when comparing formats and lenses is purely subjective....
Last edited by Pål Jensen; 07-08-2021 at 07:54 AM.