« on: April 24, 2013, 06:13:55 PM »
For willful obfuscation, you take the biscuit (and the competition has been intense).
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While you should be comparing pictures/scenes shot with both and then presented on comparable mediums (screen or paper).Nope, comparing shutter speeds of 70/2.8 to shutter speeds of 35/1.8 is not helping at all.I didn't compare it specifically with 70mm, I compared it with a 24-70mm, because that was the comparison others made that started the debate.
Otherwise the comparison is meaningless.
Just the opposite. The end result is about the same, because it is mostly limited by purely optical considerations. No matter what technical solutions are employed between the subject and the final image on paper or on monitor.You seem to think that all that matters is the amount of light falling on a square micrometer of the sensor.If you read the thread, you'll understand that my objective was to dispel the notion that f/1.8 on APS-C is in effect the same as f/2.8 on full frame. That notion is actually an unhelpful
and circuitous way of saying that APS-C sensors are, because of their typically smaller pixels, usually noisier than full-frame sensors - simple as that.Nope. They aren't noisier "because of their typically smaller pixels". Given the same size of the lens' entrance pupil (i.e. the same amount of light captured by the lens from an area on the subject), the same angle of view and enough of pixel well depth, they all will provide about the same amount of noise on the final medium.
They are noisier because the entrance pupils of their smaller lenses are generally smaller than on the lenses that provide the same angle of view for larger sensor formats. There are physical limitations in making large entrance pupils for smaller focal lengths, as pupils bigger than f/0.5 are impossible even theoretically.
Nope, comparing shutter speeds of 70/2.8 to shutter speeds of 35/1.8 is not helping at all.
You seem to think that all that matters is the amount of light falling on a square micrometer of the sensor.
Yes, all nice. But i am not worried about the need of such a lens. More about the possibility to even build it! (For a price someone can pay)
A 135mm f/1.8 lens has the same size physical aperture as a 200mm f/2.8 lens. I'm pretty sure this lens would be cheaper than Sigma's $1,300 70-200mm f/2.8 OS, seeing how it's a much simpler design. I'd also guess that it'd be cheaper than Canon's $1,000 135mm f/2 L, because that tends to be how Sigma rolls. I'd personally guess somewhere in the $800 range.
Going back to the 35mm equivalent discussion, consider this:
On APS-C (like the 7D) compared to full frame (like the 5D Mark III)
The sensor is 1.6 x 1.6 times smaller.
35mm equivalent aperture - Multiply F-Number by (1.6 ) . (an f stop is a base 2 log, so even though we have 1.6x1.6 times as much light we take the square root, which is 1.6 to multiply the F number by. (example 2.8 x 1.6 = 4.48, 4.0 x 1.6 = 6.4, 1.8 x 1.6 = 2.88))
35mm equivalent focal length - Multiply by 1.6
35mm equivalent ISO or light sensitivity - Multiply by (1.6 x 1.6) (bet you haven't heard of that, but if you do the math the an APS-C sensor amplifies the signal 1.6x1.6 times more at a given PIXEL than the a full frame camera, so even if both say ISO 800, ISO 800 on the an APS-C it is multiplying the light from each individual pixel the same as ISO 2000 full frame, assuming they had identical resolution, if resolution differs)
OK, now I know you're trolling. You failed to quote the part of my post that answers this:So you agree that a larger enlargement makes noise more visible? If so, it seems like we're agreeing on this, but arguing about semantics.The reasons why FF sensors typically produce less noisy images in practice than APS-C sensors are that a) the (usually) bigger individual pixels produce a better signal-to-noise ratio, and b) the native image requires less enlargement when printing.
From my point of view, amplification and magnification amount to one and the same thing. Take two different sized sensors with the same number of MP (1D X and 7D for example), then the smaller sensor clearly has smaller pixels. Each pixel receives less light, so in order to give the same electrical signal to create a calibrated ISO 100, it has to amplify to a greater level the smaller signal created from the smaller number of photons collected at its smaller pixel. However, take two sensors of different size with pixels of the same density (eg 1Ds mk III and 30D, or D800 and D7000), and on a pixel level, each pixel is the same size, so it collects the same number of photons, creates the same strength electrical signal and requires the same amount of amplification. But it is magnified less from that larger sensor (each pixel, together with its noise is a less significant part of the whole image), so it's noise has a lower effect on the whole image. Not all FF and crop comparisons fall into one of these two convenient categories, so it's usually a combination of amplification and magnification differences between the two. But one thing is for sure - take the whole image, and the bigger sensor will be less noisy. And all things being equal (same generation technology etc), you'll find the noise ratio is directly in line with the area ratio.
Similarly speaking, a 1.6x crop of FF is no different from a 1.6x crop sensor. The exposure doesn't differ with different sized sensors. The noise does. Bigger area to capture light equals more light captured. That equals lower noise. Or you can turn it around by bumping up the ISO and create the same noise with greater sensitivity. That is what makes an f2.8 lens on FF equal to an f1.75 lens on 1.6x crop.
A FF sensor behind an f2.8 FF lens gathers 2.56x as much light as an f2.8 lens does on a 1.6x crop sensor due to the sensors 2.56x bigger surface area. If you only capture a fraction of all that FF f2.8 light by cropping it, well, the obvious happens from the light gathering point of view. The reason why using an f1.8 lens wide open on crop gives a brighter image than f2.8 on FF (when both are at the same ISO and shutter speed) is the amplification of the crop cameras sensor is 2.56x greater, at the expense of noise at any given ISO rating.
...surely you can see the 1.6x crop gives just over a stop less light gathering than a full frame sensor - making f1.8 on crop equal in light capturing terms to an aperture 1.6x smaller on FF - f2.88.
...to compare the 18-35/1.8 to the 24-70/2.8 on FF we'd need to set them both to a focal length to give an equal field of view. So for the sake of this example, lets use the long end of the Sigma's zoom - set the 18-35/1.8 to 35mm, which is equivalent of 56mm on the 24-70. We get the following:
56mm 1.0x [FF] @ f/2.8 aperture = 20mm diam pupil
35mm 1.6x [APSC] @ f/1.8 aperture = 19.4mm diam pupil
So, vaguely less light gathering from the new Sigma lens, as well as a vaguely wider DoF.
Or think about it like this - imagine a photo taken with a FF lens and a FF sensor. Now you take that same photo and you crop out just the centre 40% - you've taken away 60% of the image - which is also 60% of the light that passed through that FF lens. You're left with only 40% of the light. That's what crop does. You need a faster lens on crop to make it capture the same amount of light in that smaller area.
...the 24-70 on FF goes wider, longer, is capable of a vaguely narrower DoF and capturing vaguely more light.
My argument is simply the total quantity of light a system can capture is more than just aperture - it is a combination of aperture and sensor size.
Print a whole FF image at 1m wide, the noise hides quite well. Now take a crop of that same image and print that crop at 1m wide, do you think the noise will be no less apparent?There is some truth in this part of your statement but it has nothing whatsoever to do with the dimensions of the sensor.So you're saying pretty much all FF sensors are better than pretty much all crop sensors because of something other than their bigger size? I'm confused by your logic now
If I use masking tape around the edges of my full size sensor to leave only an APS-C sized area, what does it do to the quality of the image on the exposed area? Nothing. What does it do to the exposure value? Nothing.
You'd need to shoot the cropped image at a lower ISO to show an equivalent amount of noise in the final print.
The reasons why FF sensors typically produce less noisy images in practice than APS-C sensors are that a) the (usually) bigger individual pixels produce a better signal-to-noise ratio, and b) the native image requires less enlargement when printing.
There is some truth in this part of your statement but it has nothing whatsoever to do with the dimensions of the sensor.So you're saying pretty much all FF sensors are better than pretty much all crop sensors because of something other than their bigger size? I'm confused by your logic now
ps - it seems like you're slowing coming around to understanding my argument. Using that metabones analogy, you got the bit about how both the combined effect of aperture and image circle is equal to the total amount of light coming through the system. You now seem to get the other end of the argument - FF allows you to work at higher ISO's than crop. If you could just accept this missing part of the puzzle about it being the larger size of the sensor which allows you to work at these higher ISO's, I think you'll have it. Or are you going to start arguing about some other random part of the rationale?
What about using the FF system at a higher ISO? As the FF sensors larger area allows it to capture 2.56x more light, you can use an ISO 2.56x higher (just over a stop), without suffering from any more noise than the crop sensor.
ISO 10,000 on a typical APS-C sensor gives the same noise as ISO 25,600 on a typical FF. If you do choose to make use of the higher ISO's made available to you, the final exposure is the same, and the f1.8 crop system offers no low light advantage over an f2.8 full frame system.
Think of the metabones speed booster. Imagine now that they created one which made a FF lens create an imaging circle to match the APS-C crop sensor found in a Canon camera - a 1.6x telecompressor. That way you could mount, say, a 24-70/2.8 on crop with the metabones adapter, and get an identical FoV that the lens achieves on FF.
The 24-70/2.8 would be turned into a 15-44/1.75, right?
Now, we all know f1.75 is faster than f2.8. No-one is disputing that. If you mount this lens on the crop sensor camera with the telecompressor, it will allow for more than a stop faster shutter speeds at equal ISO's. It is an f1.75 lens, and no-one can argue with that. But where does this metabones get this extra speed from? Its not magic - its just it compresses the larger image circle into a smaller one - that extra light from that larger FF image circle is now condensed down into a smaller, more intense imaging circle, and is then received by a smaller sensor. However, in total its only the same amount of light/photons coming in through the lens which hits the sensor. The FF sensor and the APS-C sensor with a telecompressor both receive an identical number of photons, but the APS-C sensor has brighter light presented to it - more light per area - in other words its just over a stop brighter.
ISO's are rated to make exposure calculations work. What one camera does to achieve ISO 1600 isn't the same as another camera at ISO 1600 - especially with different size formats. The larger sensor as a whole has more photons hitting it at a particular aperture, so it needs to amplify the resulting electrical signal less for any given ISO. And even some cameras with the same sensor have to do different amplification, such as the Sony NEX 7 and Sony SLT A77.
If you can't see that, haven't you ever wondered why FF sensors are typically just over a stop better than crop sensors when it comes to noise? This faster aperture of the Sigma simply allows the noisier sensor to work at lower ISO's to finally fight back. Use a 18-35/1.8 at 35mm f1.8 1/100th of a sec ISO 1000 on crop, or a 24-70/2.8 at 56mm f2.8 1/100th, ISO 1600 on FF and you'll find its the same framing, depth of field, exposure and noise. (well, it would be if the Sigma was slightly brighter at f1.75)..
There has been a lot of nonsense on this thread. It's pointless contributing now because the ones spreading the nonsense have entrenched themselves so deeply that they have no prospect of coming out of this with any dignity or credibility.
Let's not say these people have no dignity...that's personal and uncalled for. They're simply sharing their opinion.
No. A FF sensor behind an f2.8 FF lens gathers 2.56x as much light as an f2.8 lens does on a 1.6x crop sensor due to the sensors 2.56x bigger surface area. If you only capture a fraction of all that FF f2.8 light by cropping it, well, the obvious happens from the light gathering point of view. The reason why using an f1.8 lens wide open on crop gives a brighter image than f2.8 on FF (when both are at the same ISO and shutter speed) is the amplification of the crop cameras sensor is 2.56x greater, at the expense of noise at any given ISO rating. In other words ISO 10,000 on crop is pretty much equal to ISO 25,600 on FF in terms of noise. So feel free to shoot smaller apertures on FF and use higher ISO's to get the same light gathering and noise.This is great for crop shooters. However, before everyone gets excited over the f/1.8 bit, you have to remember that f/1.8 on a crop sensor is nothing like f/1.8 on a FF sensor. This lens will give the same angle of view, image noise for given exposure parameters (*1), depth of field at a given AOV and subject distance (*2), etc. etc. etc. as a 28-50mm f/2.8 full frame lens.
In other words, if the lenses and sensors are perfect, this lens on a crop sensor would give identical results to a 28-50mm f/2.8 on a FF sensor.
You mean identical DOF. f1.8 will still give more light on APS-C than f/2.8 on full frame.
The different amplification levels is a bit like how the Sony NEX 7 and the Sony SLT A77 both share an identical sensor, and give the same exposure with the same shutter speed, aperture and ISO, yet the SLT camera has a semi translucent mirror permanently in front of the sensor, acting like a neutral density filter you can't get rid of. Sony just cranked up the amplifier on the A77 a bit more to make it all seem good - at the expense of noise.
So f1.8 on a 1.6x crop is equivalent to f2.88 on FF in terms of both depth of field and light gathering.