Chris Kenny
09-17-2006, 04:32 AM
This issue has come up a lot in various threads here, which makes sense, since it's something a lot of people who have primarily used 1/3" CCD cameras haven't had to deal with much, but will certainly need to understand if they buy a RED.
I think it's worth clarifying a couple of things, because there's widespread confusion and even supposedly reliable sources give what amounts to misleading information.
A wider lens, in general, can not be used to achieve the same primary subject size (same field of view in the plane of the subject) as a longer lens, but with deeper DOF. If you toss on a wider lens and then move the camera up so your field of view is the same as it was before, you end up with basically the same DOF you had before.
To toss some actual numbers in here from a DOF calculator... if I'm at f/5.6 with s35, focused 5' away with a 38mm lens, I've got an field of view (FOV) about 34 deg wide, and 1'6" of DOF. Now, if I switch to a 100mm lens, but keep the FOV the same (which requires that I move back to 13'2"), when I check my DOF, surprise, it's still 1'6". It doesn't get any shallower just because I'm using a longer lens.
This might lead people to believe lens length doesn't matter, and that DOF is determined entirely by your FOV relative to your format size, but be careful, because it's actually a little more complex than that. Lenses of different lengths have different hyperfocal distances, and if you're comparing a bunch of lenses, there really will be DOF differences as you approach the hyperfocal distence of one or another of them.
To return to the numbers above, say a 38mm lens has a hyperfocal distance of 33'5" (your numbers may differ if you assume a differently sized circle of confusion). A 100mm lens has a hyperfocal distance of 231'. Now, as we saw above, with close focus (5' for the 38mm), DOF at a given FOV was the same for both of these lenses. But what happens if we focus at, say, 20', which is almost 2/3 of the way to the 38mm's hyperfocal distance, but not even 1/10 of the way to the 100mm's?
With the 38mm, we get 37'1" of DOF. With the 100mm (now we have to back off to 52'8" to keep the same FOV), we only get 25'2").
Also, while wider lenses don't (except as noted above) really give you deeper DOF, they can often create the appearance of such. Wider lenses, as everyone knows, exaggerate perspective. This means that if we set up two shots with the same main subject size, anything in the background will be smaller in the shot with the wider lens. This means the background will have more detail per unit of area, which will typically make it appear to be sharper.
Playing around with a dSLR and a zoom lens is a great way to get a feel for this stuff. Also, if you have a Palm, go download pCAM (http://www.davideubank.com/) right now, it's the best $15 you'll ever spend.
I think it's worth clarifying a couple of things, because there's widespread confusion and even supposedly reliable sources give what amounts to misleading information.
A wider lens, in general, can not be used to achieve the same primary subject size (same field of view in the plane of the subject) as a longer lens, but with deeper DOF. If you toss on a wider lens and then move the camera up so your field of view is the same as it was before, you end up with basically the same DOF you had before.
To toss some actual numbers in here from a DOF calculator... if I'm at f/5.6 with s35, focused 5' away with a 38mm lens, I've got an field of view (FOV) about 34 deg wide, and 1'6" of DOF. Now, if I switch to a 100mm lens, but keep the FOV the same (which requires that I move back to 13'2"), when I check my DOF, surprise, it's still 1'6". It doesn't get any shallower just because I'm using a longer lens.
This might lead people to believe lens length doesn't matter, and that DOF is determined entirely by your FOV relative to your format size, but be careful, because it's actually a little more complex than that. Lenses of different lengths have different hyperfocal distances, and if you're comparing a bunch of lenses, there really will be DOF differences as you approach the hyperfocal distence of one or another of them.
To return to the numbers above, say a 38mm lens has a hyperfocal distance of 33'5" (your numbers may differ if you assume a differently sized circle of confusion). A 100mm lens has a hyperfocal distance of 231'. Now, as we saw above, with close focus (5' for the 38mm), DOF at a given FOV was the same for both of these lenses. But what happens if we focus at, say, 20', which is almost 2/3 of the way to the 38mm's hyperfocal distance, but not even 1/10 of the way to the 100mm's?
With the 38mm, we get 37'1" of DOF. With the 100mm (now we have to back off to 52'8" to keep the same FOV), we only get 25'2").
Also, while wider lenses don't (except as noted above) really give you deeper DOF, they can often create the appearance of such. Wider lenses, as everyone knows, exaggerate perspective. This means that if we set up two shots with the same main subject size, anything in the background will be smaller in the shot with the wider lens. This means the background will have more detail per unit of area, which will typically make it appear to be sharper.
Playing around with a dSLR and a zoom lens is a great way to get a feel for this stuff. Also, if you have a Palm, go download pCAM (http://www.davideubank.com/) right now, it's the best $15 you'll ever spend.