Very new here, and very new to the photography world. But it's surely becoming more than a hobby to me. I shoot with the A7S2 and I just bought my first manual vintage lens. It's a bit of a weird one, and pretty rare, but I've done my research and I was expecting great results. The lens is a Vivitar 135mm f2.8 Close Focussing which goes up to a 1:2 magnification. It comes in a Minolta MD mount and I've adapted it to the E-mount with a dumb adapter. I tried it out and I'm impressed. It handles beautifully and it's super sharp. The only thing I noticed while using it is that I was expecting it to let in more light than it actually does. So I did a little experiment. I set up my Sony FE 24-240 F3.5-6.3 on the same tripod, same lighting conditions, I set it to approximately 135mm and shot to expose properly at f6.3 and f11. After that I slapped on the Vivitar, same shutter speed, same ISO, same aperture values, and guess what... the Vivitar was not only much darker but also much narrower.
Time for examples, straight out of the camera, I only combined the two in one image:
Here you can see the Vivitar on the left, the Sony on the right, this is at f6.3. I also have one where I repositioned the tripod so the angle of the Sony would resemble the Vivitar, and this time at f11, the other way around with the Sony left and Vivitar right:
So I know it's an old lens, and the difference in angle probably had something to do with the Close Focussing feature. But the loss of light I can't explain, and it's no small difference. Light entering the camera through different lenses with same apertures should be the same right? Any thoughts on what may be the cause of this?