cowpunk52
Veteran
That lens is only 1.9 at the widest focal length. It ramps down to 2.8 as you zoom in.
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That lens is only 1.9 at the widest focal length. It ramps down to 2.8 as you zoom in.
Yeah, lots of zoom lenses do that. Not unusual.
combatentropy
Veteran
Long I pined for a large sensor with a fixed zoom. Then, about a year ago, after poring over lenses and doing math, I found out that the depth of field would be the same. Here's the breakdown:
The HPX-250's lens opens to f/1.6. With its 1/3-size chips, though, that's the depth of field of about f/5.6 in S35. And that's at its widest angle. The lens ramps, to f/3.2, which is like f/16. Hence the deep focus we all know in 1/3-chip cameras.
An S35 lens with the same zoom range and depth of field would be an 18-400mm that ramps from f/5.6 to f/16. I'm guessing it would be about the same size and weight. But the depth of field you want takes f/2.8 to f/4, right? An 18-400mm lens that opens to f/2.8 would weigh approximately 100 lb and be four feet long, I'm guessing. I have no example. I've never seen such a lens.
"Yeah, but the HPX-250's lens is 22x. I'll settle for 10x," you might say. f/2.8 10x S35 lenses are rare, but there's an Angenieux like that. It weighs 24 lb. and is a foot and a half long.
"I'll take 5x," you say. Okay, RED makes one, and it's 10 lb., 11 in.
"That's ridiculous. They're doing it wrong. I've seen 10x SLR lenses at reasonable sizes." Yes, and they ramp from f/3.5 to f/6.3. And they're variofocal, which I'm guessing is helping reduce size and weight. Parfocal, f/2.8 lenses exist for SLRs, but their zoom range is 3x. So, I think that's what you're looking at, a 3x zoom.
I think you'll run into the same problem if you go with a 2/3 chip. The needed focal lengths are shorter, like 7-70, so the lens can be smaller than S35. But the needed f-stop for the depth of field you want is correspondingly bigger --- exactly corresponding. You'll want a zoom that opens to f/1.2, to equal f/2.8 in S35. Most zooms for 2/3 open only to f/2. That sounds good, but f/2 in 2/3 is equivalent in depth of field to f/5.6 in S35. I have a feeling that an f/1.2, 7-70mm zoom lens for 2/3-size chips would be just as big as that Angenieux f/2.8 24-290mm (about 20 lb., and a foot and a half long).
It seems that another way to calculate depth of field is simply to measure the size and weight of the lens.
EDIT: After googling, I found that my f-stop equivalences are probably wrong. For example, f/1.2 in 2/3 would be like f/3.2 in S35. At this point, based on what I've read, it seems that "stop factor" is exactly the same as "crop factor," and I was going by stop factor = 1/2 crop factor. I'm still not sure exactly what it is, but my point is that f/2.8, for example, is not always great shallow focus. It depends on sensor size.
The HPX-250's lens opens to f/1.6. With its 1/3-size chips, though, that's the depth of field of about f/5.6 in S35. And that's at its widest angle. The lens ramps, to f/3.2, which is like f/16. Hence the deep focus we all know in 1/3-chip cameras.
An S35 lens with the same zoom range and depth of field would be an 18-400mm that ramps from f/5.6 to f/16. I'm guessing it would be about the same size and weight. But the depth of field you want takes f/2.8 to f/4, right? An 18-400mm lens that opens to f/2.8 would weigh approximately 100 lb and be four feet long, I'm guessing. I have no example. I've never seen such a lens.
"Yeah, but the HPX-250's lens is 22x. I'll settle for 10x," you might say. f/2.8 10x S35 lenses are rare, but there's an Angenieux like that. It weighs 24 lb. and is a foot and a half long.
"I'll take 5x," you say. Okay, RED makes one, and it's 10 lb., 11 in.
"That's ridiculous. They're doing it wrong. I've seen 10x SLR lenses at reasonable sizes." Yes, and they ramp from f/3.5 to f/6.3. And they're variofocal, which I'm guessing is helping reduce size and weight. Parfocal, f/2.8 lenses exist for SLRs, but their zoom range is 3x. So, I think that's what you're looking at, a 3x zoom.
I think you'll run into the same problem if you go with a 2/3 chip. The needed focal lengths are shorter, like 7-70, so the lens can be smaller than S35. But the needed f-stop for the depth of field you want is correspondingly bigger --- exactly corresponding. You'll want a zoom that opens to f/1.2, to equal f/2.8 in S35. Most zooms for 2/3 open only to f/2. That sounds good, but f/2 in 2/3 is equivalent in depth of field to f/5.6 in S35. I have a feeling that an f/1.2, 7-70mm zoom lens for 2/3-size chips would be just as big as that Angenieux f/2.8 24-290mm (about 20 lb., and a foot and a half long).
It seems that another way to calculate depth of field is simply to measure the size and weight of the lens.
EDIT: After googling, I found that my f-stop equivalences are probably wrong. For example, f/1.2 in 2/3 would be like f/3.2 in S35. At this point, based on what I've read, it seems that "stop factor" is exactly the same as "crop factor," and I was going by stop factor = 1/2 crop factor. I'm still not sure exactly what it is, but my point is that f/2.8, for example, is not always great shallow focus. It depends on sensor size.
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The Scarlet 2/3's zoom was an 8x, 28-224mm (35mm equivalent), constant T2.4. It wasn't that big, physically. Obviously the DoF wasn't nearly as tight as any S35. The weakness in the Scarlet 2/3 was with the zoom, ironically. I don't think they could get it to work.
However, such a zoom would be good enough for TV news, documentaries, etc. Even films.
However, such a zoom would be good enough for TV news, documentaries, etc. Even films.
Scarlet 2/3 Fixed's zoom was f/2.4 and T2.6. I suspect that it wasn't a true mechanical parfocal but a varifocal with focus electronically compensated for during zooming. For whatever reason, it failed to hold focus, so Red scrapped it.
combatentropy
Veteran
For depth of field, what is the 35mm equivalent of F2.4?
From what I've read, a format's crop factor (for focal length) is also its "stop factor" (for depth of field). The crop factor for 2/3 is 4: a 7mm lens in 2/3 equals a 28mm lens in FF35. So, F2.4 in 2/3 equals F9.6 in FF35, for depth of field.
This is based on three independent articles I read when googling "equivalent depth of field." I myself have never conducted a scientific test. I've seen a post by someone here that has, but I don't know where it is. If this is true, that crop factor also equals stop factor, then my last post was wrong, as far as the f-stop equivalences. I was using stop factor = 1/2 crop factor.
From what I've read, a format's crop factor (for focal length) is also its "stop factor" (for depth of field). The crop factor for 2/3 is 4: a 7mm lens in 2/3 equals a 28mm lens in FF35. So, F2.4 in 2/3 equals F9.6 in FF35, for depth of field.
This is based on three independent articles I read when googling "equivalent depth of field." I myself have never conducted a scientific test. I've seen a post by someone here that has, but I don't know where it is. If this is true, that crop factor also equals stop factor, then my last post was wrong, as far as the f-stop equivalences. I was using stop factor = 1/2 crop factor.
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dustylense
Veteran
I would still buy the fixed scarlet 2/3's. Too bad they failed at it.
Samuel H
Major Contributor
it is true
was this article among the ones you found on google? (I guess not, but just curious, SEO-wise) http://www.similaar.com/foto/doftest/doftest.html
But 7 mm is pretty wide-angle for a 2/3-inch sensor, so deep DOF might be expected. A "normal" lens is approximately equal to the diameter of the format (for 2/3-inch it would be about 11 mm, and for FF35, about 45 mm). If you're shooting a closeup, you would use a longer lens, which would minimize distortion and, at the same, DOF.
combatentropy
Veteran
Right, and an 11mm lens at F2.4 would be like a 45mm lens at F9.6.
And a 18mm lens on 2/3 at F2.4 would be like an 72mm lens at F9.6 on FF35.
And a 25mm lens on 2/3 at F2.4 would be like a 100mm lens at F9.6 on FF35.
So the Blackmagic Cinema camera seems to have everything I want except the ND filters and my lord the price is right.... Really exciting stuff. The thread is here:
http://www.dvxuser.com/V6/showthread.php?280186-BlackMagic-Cinema-camera!!!!-2-5k-for-3k