Hi,
Since somebody mentioned f-stops (focal length/opening) and p-stops (the actual
light that comes through), I decided to do some tests.
I compared my 28/f2.8, 50/f1.8, 50/f1.4 and 90/f2, all preset at f2.8
diaphragm. I used 2 scenes: a painting on newspaper (A4 size (letter for US
readers), newspaper colour, black ink, red and green paint), and a little
cupboard with a built-in halogen spotlight at the top. I could've checked
flare with that light source in the viewfinder, but I forgot :-)
Here's the timings (-x/3 = x/3 of a stop shorter exposure time):
Painting:
28mm f2.8: 1/30th
50mm f1.4: 1/30th-2/3
50mm f1.8: 1/30th-2/3
90mm f2.0: 1/30th
Cupboard:
28mm f2.8: 1/8th-2/3
50mm f1.4: 1/15th-1/3
50mm f1.8: 1/15th
90mm f2.0: 1/8th-2/3
My conclusions:
1) Polarizer:
I first tested the 90mm with a Cokin UV and circular polarizer. It turns out I
lost 1 2/3 stop to the polarizer. This is a good indication that I need to
remove my polarizer when I'm not using it. I didn't know the difference was
this big ! The UV filter was less than 1/3 stop difference.
2) p-stops:
The 50mm f1.4 is 2/3 stops brighter than the 28mm and the 90mm, at the same
diaphragm. So that means the f2.8 indication doesn't mean the same for the
different lenses ! Let's do some conversions:
- If I convert 50mm diaphragm stops to 28mm diaphragm stops, then the 50mm/f1.4
is around a 50mm/f1.1. Not bad, eh ?
- The other way round, the 28mm/f2.8 would be a 28mm/f3.5 in 50mm stops, and my
90mm/f2 would just be a 90mm/f2.5
(I didn't use a calculator, just guesstimated the third-stops.)
3) Flexibility:
If I want to photograph my painting (minimal shutter speed = focal length), I
can do it with the 28mm handheld, but only at f2.8.
I can also do handheld with the 50mm lenses:
- the f1.8 gives me the choice of wide open or 2.8 (just nearly).
- the f1.4 gives me the choice of 3 diaphragms: f1.4, f2.0, f2.8.
The 90mm doesn't allow me to photograph my painting handheld.
I started doing this test, while reading about 50mm lenses on the
ever-interesting MF site : http://medfmt.8k.com/bronlensenvy.html. This is a
very interesting article. One of the main points is that having just one prime
forces you to look at the picture, and to optimize yourself for the
circumstances. Having noticed a certain lens envy in myself on several
occasions, including recently, I need this kind of article sometimes.
Another thing is my plans for a 2-month holiday. Living off a backpack, I
don't want to lug around 3kg of lenses, bodies and the like. This little test
tells me that my 50mm/f1.4 will give more flexibility in low-light situations.
Now, I could've found this out by just checking the lens data, but my little
tests gives the 50mm an extra 2/3 stop advantage.
A couple of things I can think about while preparing for my trip !
Peter.
(why is this little voice shouting that it *needs* the 90mm/f2 within an arm's
reach ?)
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