More depth of field and perspective fun.

First, trying to match framing with different focal lengths.

28mm @ f/5.6


50mm @f/5.6


85mm @ f/5.6


105mm @ f/5.6


200mm f/4 @ f/5.6


What if we try to minimize depth of field? These are all shot wide open. Some of these lenses (esp. the 50mm) really do better stopped down a little. Although the 50 is still sharp in the center if you nail the focus.

28mm f/2.8 @ f/2.8


50mm f/1.4 @ f/1.4


85 f/1.8 @ f/1.8


105mm f/2.8 @ f/2.8


200mm f/4 @ f/4


Either the 85 or the 200 win at depth of field isolation, at least for this subject. Technically you can figure out the theoretical aperture opening of each lens (focal length over f/stop), which is supposed to also relate to how out of focus the background gets, but this does not account for moving to preserve framing, only the relative depth of field if we were shooting without moving.

Lens: 28mm f/2.8 Lens opening: 10mm
Lens: 50mm f/1.4 Lens opening: 35.7mm
Lens: 85mm f/1.8 Lens opening: 47.2mm
Lens: 105mm f/2.8 Lens opening: 37.5mm
Lens: 200mm f/4 Lens opening: 50mm

I'd say the empirical data agrees, the 50mm and 105mm have similar sized blur circles wide open, and ditto for the 85mm and 200mm, despite the 85mm being around 10 feet from the subject where the 200mm was 20+ feet from the subject. Here are some crops. I don't think I can say for sure if there is a difference. It doesn't help the background has changed, and the character of the out of focus backgrounds might change ones impression, my focus and exposure, etc. I think the background of the 200mm might be slightly more out of focus?

85mm f/1.8 cropped:


200mm f/4 cropped:


And...

50mm f/1.4 cropped:


105mm f/2.8 cropped:


I guess the 50mm f/1.4 won't win any bokeh beauty contests wide open. The 200mm makes all the rest look bad wide open!

For fun, here are some of the fastest Nikon lenses, which should have some of the greatest depth of field isolation (not counting the super telephotos...)

Lens: 50mm f/1.2 Lens opening: 41.6mm
Lens: 85mm f/1.4 Lens opening: 60.7mm
Lens: 105mm f/1.8 Lens opening: 58.3mm
Lens: 135mm f/2 Lens opening: 67.5mm
Lens: 180mm f/2.8 Lens opening: 64.3mm
Lens: 200mm f/2 Lens opening: 100mm (!)

Of course, to really control depth of field you need a tilt-shift lens or a large format camera...

Moral of this story: depth of field and perspective are kinda complicated. The basic ideas are certainly easy: long lens, faster f/stop, closer subject mean less depth of field, but when you start to change multiple things I think it is hard to predict the results. Also, it is fun to take pictures of stuff.