Ok, it's been 38 years since my last optics class, and 13 years since I had to deal with optics in any way whatsoever. So I'm going to forget everything I thought I knew, and ask questions. Is there a difference in depth of field with various imager sizes? Does a 50mm lens on a film SLR with an imaging size of 24 x 36 mm have the same depth of field as its equivalent (about 35 mm) on a digital SLR, with a smaller imaging size? Or is it that the 50 mm lens has the same depth of field for the imaging size, and it's just that the digital SLR crops out the edges? What happens when you get down to the really small imagers? My wife's Sony has an 8-24 mm zoom, which is an SLR equivalent of about 35-105 mm. I don't care about pixel count. I'm just looking for an explanation of why an 8-24 zoom on a tiny imager would have more depth of field than a 35-105 zoom on a bigger imager.
I have always thought the depth of field varies in accordance with the focal length used, the shorter/wider the lense the shallower the depth of field. The conversion rules from film (24x36mm) to digital (CCD size) or vice versus remains a fuzzy lesson to say the least. My take on it is the conversion applies only to equivalent angle of views and not the magnification power, but I could be all wet on this. I believe using an 35mm on a digital body will only use a middle portion from the lense image and the far edges dropped off as you mentioned.
Sounds like a glass design issue. Depth of field has to do with the aperture setting, and the way the glass is ground, right?
As I said, I'm going to forget my 1967 physics class. I believe that, for any given imaging size (say 35mm's 24 x 36), with equal lens length (say 50mm), and equal aperture (say f11), the depth of field is roughly the same. Expensive lenses will have more depth of field than cheap lenses but that is a minor factor in the equation (I think). I'm trying to figure out if my wife's $300 camera with a lens length of 8mm (equivalent DSLR is about 24 mm) has more depth of field than my DSLR at 24mm. I used to think that, at the same aperture, the depth of field would be roughly equivalent. I've been told it's not, and I'm trying to figure out why that would be true.
Ok, I found an answer. Yes, my wife's point and shoot has much better depth of field. That's because depth of field is determined by the actual lens length, not the equivalent lens length. An 8 mm lens is going to have more depth of field than a 24 mm lens on a DSLR, which is going to have more depth of field than a 35 mm lens on a SLR. These are roughly equivalent as to field of view. Case closed?
Yes, Lenny. I thought it would relate to field of view (kind of the equivalent lens length), but, no, an 8 mm lens just has terrific depth of field. Now, it is hard to spread that across a 35 mm imaging plane, but pretty easy for a small imager. It's not really intuitive, is it?
There's a very detailed article on the topic here: Warning: highly technical http://www.photo.net/learn/optics/dofdigital/
Well, when, I look back to when I used to shoot almost exclusively with prime lenses, I would always take the 50mm one over say a 85mm or 135mm lense when I wanted the better depth of field. So something must have been clicking in the old noggin.
As my true professional photographers say, "A wide angle is the photographer's best friend." I work with three of them regularly, and they all prefer prime lenses. My photo tasks are not so demanding, and I can't justify the expense of primes for hobby purposes. I've also read that some older Nikon primes will damage the electronics in the D70 and D100. One problem with the Nikons is that I just can't squeeze them into some of the spaces I would like to. So I'm looking for something small, with adjustable aperture, for those spots. Or else it's mirrors and Photoshop.
Well, when, I look back to when I used to shoot almost exclusively with prime lenses, I would always take the 50mm one over say a 85mm or 135mm lense when I wanted the better depth of field. So something must have been clicking in the old noggin.
The old Kodak X15, which was a plastic bodied camera with a piece of flat plastic for the lens, had the best depth of field ever! Focus from 1" to infinity, no distortion!
