There is no easy answer if you have to choose between these two lenses, the Zeiss 21mm F2.8 and the Zeiss 21mm F4.5. Of course there is no wrong answer either.
I used the two lenses for a total of three rolls of film at the Caribana Festival in Toronto. The pictures were shot at F5.6 with both lenses and the film was the same type. The camera I used was the Voigtlander Bessa R4A since it has framelines for 21mm.
Of the two lenses I preferred to use the C - Biogon 4,5/21mm ZM. It was a breeze to focus, and just so small. I couldn't get over how small this lens was. Now I was at an event during the middle of the day so there was plenty of light for either lens... the fact that the lens is slower wasn't an issue on this day.
As far as image quality goes, either lens would make great enlargements, although I've read the lens reviews and most favour the slower C - Biogon 4.5 for critical sharpness, however no one would find the faster lens wanting.
I really just have one test for judging camera equipment..... was it enjoyable to use or not. Both these lenses were great to use but the big difference for me was just the size. I loved having the smaller lens on the camera. It made the whole camera system seem smaller.
So the question is, would you need the speed of F2.8 yourself or would the slower, smaller package work just fine.
I opened these new lenses just for this review, used it over a couple hour period and am now selling them as store demos. If you're interested you can find them listed on my website forTraditional Photo Supplies.