650mm f2 @ $1000. Too good to be true?

@henryqiu:

what this 3x mode does is that turns your 1.6x crop camera into a 4.8x crop camera, because it only uses the central third of your sensor to create the whole image

therefore, 135mm f/2 on full frame is equivalent to 648mm f/9.6 on 4.8x crop camera in terms of FoV and DoF (and yes, compression)
 
NormanBates said:
@henryqiu:

what this 3x mode does is that turns your 1.6x crop camera into a 4.8x crop camera, because it only uses the central third of your sensor to create the whole image

therefore, 135mm f/2 on full frame is equivalent to 648mm f/9.6 on 4.8x crop camera in terms of FoV and DoF (and yes, compression)
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I don't understand. When I use my 400mm f5.6l to film a area with aperture 5.6 and shutter speed 1/50, the camera set my ISO as 6400. When I use my 17-70 (aperture:f4, shutter speed 1/50, 3x digital zoom, focal length 70mm), I get ISO 3200. So I thought the camera should be treating it as a 336mm f4 instead of a 336mm f19.2.

Actually my ultimate question is whether I should get the 135mm f2 to film some performance. In the past, I struggled to film the performance using my 400mm f5.6l as the aperture is too small and I do not have cash to buy a 400mm f2.8. With a 3x digital zoom, it will become 405mm f2 which in theory is perfect for low light situation. Will the DOF be too shallow that only a small part will be in focus only? Thanks all for replying to this thread.
 
Fung Wei Liang said:
I don't understand. When I use my 400mm f5.6l to film a area with aperture 5.6 and shutter speed 1/50, the camera set my ISO as 6400. When I use my 17-70 (aperture:f4, shutter speed 1/50, 3x digital zoom, focal length 70mm), I get ISO 3200. So I thought the camera should be treating it as a 336mm f4 instead of a 336mm f19.2.

Actually my ultimate question is whether I should get the 135mm f2 to film some performance. In the past, I struggled to film the performance using my 400mm f5.6l as the aperture is too small and I do not have cash to buy a 400mm f2.8. With a 3x digital zoom, it will become 405mm f2 which in theory is perfect for low light situation. Will the DOF be too shallow that only a small part will be in focus only? Thanks all for replying to this thread.
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That the f5.6 lens required iso 6400, and the f4 (actually f4.5) lens required iso 3200 is correct.

FStop's are not linear, f4 is one stop faster than f5.6.

A 135/2 is a perfect lens for shooting performances. When doing DOF calc, don't use aps-c or the D-Reb in the calc, use a 2/3 sensor instead, as that's about the size in crop mode.

To need 400mm to show a performance means it's quite far away. A 135/2 in 3x crop mode will have approx 14ft of DOF @ 100ft at "normal viewing" distances. If you expect this to be viewed larger than a typical size then don't forget that apparent DOF will be reduced.
 
thisisapocalypse said:
A 135mm is always a 135mm lens, always, and will retain the characteristics of a 135 design, it won't have the compression of a 650mm nor the look of a 650mm f/2.
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Don't think you're right about that. The DoF won't be the same as a 650mm but "compression" is based on distance from the subject and not the lens. If one backs up the same distance as one would a 650mm lens and uses crop mode, one should get the same level of depth compression with a 135mm in crop as one would with a 650mm normally.
 
Fung Wei Liang said:
I don't understand. When I use my 400mm f5.6l to film a area with aperture 5.6 and shutter speed 1/50, the camera set my ISO as 6400. When I use my 17-70 (aperture:f4, shutter speed 1/50, 3x digital zoom, focal length 70mm), I get ISO 3200. So I thought the camera should be treating it as a 336mm f4 instead of a 336mm f19.2.
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that is perfectly OK

what I said is: the 135mm f/2 will become a 650mm in terms of FoV and compression, with f/2 in terms of light gathering, and a f/9.6 in terms of DOF (i.e. no, you won't have problems with just the subject being in focus, in fact it is as if you were shooting with a camera with a smaller sensor and thus perpetually deep DoF)

apart from that deeper DoF (which may be good for you) the only issue is sharpness, because you're stretching your bayer sensor and your lens a bit too thin; if you're happy with the sharpness in the 3x crop mode with your current lenses, you'll probably be happy with the 135mm

in any case, I understand this is can be quite confusing, so you may want to consider renting or borrowing the lens before buying it...
 
NormanBates said:
that is perfectly OK

what I said is: the 135mm f/2 will become a 650mm in terms of FoV and compression, with f/2 in terms of light gathering, and a f/9.6 in terms of DOF (i.e. no, you won't have problems with just the subject being in focus, in fact it is as if you were shooting with a camera with a smaller sensor and thus perpetually deep DoF)

apart from that deeper DoF (which may be good for you) the only issue is sharpness, because you're stretching your bayer sensor and your lens a bit too thin; if you're happy with the sharpness in the 3x crop mode with your current lenses, you'll probably be happy with the 135mm

in any case, I understand this is can be quite confusing, so you may want to consider renting or borrowing the lens before buying it...
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Hi thanks for enlightening me. Just two more question before I make the purchase, will the same idea works if I buy a 50mm f1.8 and turning into a "240mm f1.8". Will it be able all actions on stage?
 
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a 50mm f/1.8 will become the full-frame equivalent of a 240mm for FoV, with f/1.8 in terms of light intake and f/8.6 in terms of DoF
and, again, you'll have some minor sharpness issues which you may or may not care about

if you're not going to rent the 135mm f/2 before you buy it, going first for a cheaper lens like this one is a good idea
if you're not taking stills and can live without autofocus, there's a vintage nikkor 50mm f/1.8 that's just as cheap and supposedly much better
 
henryqiu said:
"I have 90mm wide angle lenses designed for..." <-- If the same language is spoken (i.e., in 35mm film terms), this 90mm has to be equivalent to the focal length of, say, a 24-105mm lens zoomed to 90mm. So, 90mm lenses are not wide angle lenses, which is common sense, even though you may get a wide shot with them with a much larger format camera.
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The definition of a telephoto lens is "a lens in which the physical length of the lens is shorter than the focal length." Lenses designed for smaller sensors can have a smaller design, and lenses designed for larger lenses will be larger. So the point at which a lens is defined as telephoto or wide-angle is different. Focal length alone is not enough to determine whether a lens is wide-angle or telephoto.

He is not talking about lens design in a wierd way, he is talking in factual, absolute terms. However there are so many variables and misconceptions that doing so can cause some confusion with people who don't fully understand all the science,

henryqiu said:
"A 135/2 has the DOF of a 135/2." <-- This makes no sense, because DoF is not an attribute of a lens
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Yes it is. DOF in an artifact produced entirely by the lens before the image is transferred onto an imaging plane. Sensor size is introduced after the DOF has already been determined, though can be added to the equation afterwards to manipulate the results. For example, with the same lens, if you use a smaller sized sensor, you will have to move further away to get the same framing. By increasing the distance-to-subject, you also increase the DOF.

Lenses determine the DOF. Sensors utilise the determined DOF in different ways depending on their size.
 
that's just 720p and encoded to vimeo, so it looks terribly soft but maybe it's not the 3x crop mode's fault... the user is happy with the results

whether image quality in 3x crop mode is good enough or not is a relative issue: some users think it is, some say it's too soft; you'll have to judge yourself
I see you already have the camera and a some lenses, so try to repeat your test with a more suitable subject (read: not a picture; just go out and point the camera at some buildings)
 
Fung Wei Liang said:
This is an example how 3x digital crop can be used. http://vimeo.com/24660572 My eyes are not trained, but can anyone tell me whether the quality degradation is significant or not significant in 3x crop mode?
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The 3x zoom looks fine, the 10x is definitely very soft.

Just keep in mind that the further you are away from your subject, atmospheric factors come into play. Even at night in the dark, thermals will cause a wavy look if your shooting across lots of people generating heat.
 
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