Steve
captured D3 / 24f1.4 AFS
“How do I figure out how fast a lens I need to buy for the depth of field I think I want?” Dang, that’s one helluva great question that came in this morning! Back in the day when you bought a camera body it came with a 50f1.4, wasn’t an option. Then they started to offer other slower 50mm lenses to lower the overall price until finally, you could buy the body only. Now a days, you can buy “kits” but fast lenses aren’t part of those kits typically. Determining what a shallow depth of field just isn’t as easy as it use to be when you were forced into it. But shooting with a narrow DoF is a powerful tool and like the writer asked, how does one figure that out prior to buying?
Waco
captured D5 / 105f1.4 AFS
Being old, I shot with the 50f1.4 from the start and still do. These days my camera bag has the 24f1.4, 50f1.4 and 105f1.4 and I own those lenses to shoot at just, f1.4. To me, it doesn’t make much sense to spend the big buck for f1.4 if you’re just going to close down the lens so I shoot these at f1.4. Now how does one determine which focal length, f1.4 lens to BUY before doling out the big bucks is a helluva a question though! The obvious answer is to rent and test but this obvious answer has some flaws. Speaking personally, when I get a new fast lens, I shoot with it for a month straight, pounding it to make sure I like it. I did that with the 85f1.4 AFS when it came out and while a spectacularly sharp lens, I found that focal length just didn’t fit my style of photography. You could look at other photographer’s photographs but not knowing the back story, that might not help. The photo of Steve above, how close is that whiskey bottle to the film plane, how far is it from Steve? Those numbers are important to understanding the DoF of the 24f1.4. You could borrow a fast lens from someone on a workshop with you but that’s even less time to spend with a lens. You could shoot with a focal length you like and then in Photoshop screw around with DoF with a filter and while it sounds doable, applying that to the field I can tell you first hand doesn’t go very smoothly.
So, I have to admit I’m stumped by this question. I don’t have an answer in how you learn which fast lens to buy prior to spending the money. There are a lot of old, 50f1.4 lenses in drawers from the days when you had to get it with the body and that’s a shame as it’s a lens and focal length that can be an effective tool. And that’s what you’re after with your hard earned money, an effective tool.