Sigma Announces 18-35 f/1.8 DC HSM Art for APS-C

[Albi86]

Aperture is just a way to measure the diameter of the iris blade. It's connected to, but it doesn't measure, the real amount of light gathered. This is why T-stops were invented.

T-stops are a measured version of light as opposed to f stops which are theoretical - so they take into account the transmission of light, including effects such as reflections and tinting of glass. However, even T-stops don't take into account the size of the imaging circle or the size of the sensor. That's why a 24-70/2.8 II on FF is more than a worthy rival to this 18-35/1.8 on crop, yet if you mount the same 24-70/2.8 II on crop, it is not.

Exactly. F-stops = T-stops in an ideal lens; in practical terms they are a better measure of DoF than they are of light transmission.

I don't agree with the rest though. Or better, I'm not quite sure what you mean.

 

[indigo9]

Hey guys, read what I wrote. I said f1.8 on crop is brighter than f2.8 on full frame when both have the same shutter speed and ISO, then you start telling me that I'm wrong to say FF is brighter when they're both at f1.8 and the same shutter speed and ISO. I didn't say that, so what gives? I went to great lengths to explain that at the same aperture, shutter speed and ISO, they both expose the same due to the different light gathering of the format being compensated for by the amplifiers being set different. If you can look past that same exposure settings between formats and start to use the higher ISO's with lower noise levels this lower amplification of FF rewards you with, you'll find the true nature of the light gathering of FF lenses on FF sensors.
...

So I was about to chime in that rs is wrong and actually sat down to prove it only to realise that he's right (although not terribly clear).

For the same angle of view:

80mm 1.0x [FF] @ f/2 aperture = 40mm diam pupil
50mm 1.6x [APSC] @ f/2 aperture = 25mm diam pupil

Given that we have the same angle of view, the amount of light falling on the sensor is purely a function of the pupil diameter. f/2 on full frame is not the same as f/2 on crop.

Exposition
In case you're still not convinced, now assume that you have a FF and APSC sensor using the same production process & the same overall resolution -- they will have the same sensitivity per photon. If you take a shot with the same shutter speed, given that more light falls on the full frame sensor you would expect to use less sensor signal amplification [ie a lower ISO setting].

Alternate exposition
Or looking it at another way, if we keep the sensor signal amplification [ISO setting] constant, more light has fallen on each FF sensor pixel [same resolution remember], so you'll have to reduce the shutter speed of the FF to get the same exposure as APSC. Clearly FF f/2 is not the same as APSC f/2.

 

[Albi86]

Hey guys, read what I wrote. I said f1.8 on crop is brighter than f2.8 on full frame when both have the same shutter speed and ISO, then you start telling me that I'm wrong to say FF is brighter when they're both at f1.8 and the same shutter speed and ISO. I didn't say that, so what gives? I went to great lengths to explain that at the same aperture, shutter speed and ISO, they both expose the same due to the different light gathering of the format being compensated for by the amplifiers being set different. If you can look past that same exposure settings between formats and start to use the higher ISO's with lower noise levels this lower amplification of FF rewards you with, you'll find the true nature of the light gathering of FF lenses on FF sensors.
...

80mm 1.0x [FF] @ f/2 aperture = 40mm diam pupil
50mm 1.6x [APSC] @ f/2 aperture = 25mm diam pupil

He is not.

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.

You can't compare different focal lenghts. A 50mm is a 50mm both on crop and FF. However, being the former smaller, you crop the edges to a degree which would be equivalent to the angle of view of a 80mm on FF. This is a quick way to grasp the concept, but it's not exact in a way that you can make calculations.

 

[zim]

and bring on a 24-70 2.8 IS art line for FF I hope it's more 'when' than 'if'

 

[rs]

Hey guys, read what I wrote. I said f1.8 on crop is brighter than f2.8 on full frame when both have the same shutter speed and ISO, then you start telling me that I'm wrong to say FF is brighter when they're both at f1.8 and the same shutter speed and ISO. I didn't say that, so what gives? I went to great lengths to explain that at the same aperture, shutter speed and ISO, they both expose the same due to the different light gathering of the format being compensated for by the amplifiers being set different. If you can look past that same exposure settings between formats and start to use the higher ISO's with lower noise levels this lower amplification of FF rewards you with, you'll find the true nature of the light gathering of FF lenses on FF sensors.
...

80mm 1.0x [FF] @ f/2 aperture = 40mm diam pupil
50mm 1.6x [APSC] @ f/2 aperture = 25mm diam pupil

He is not.

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.

You can't compare different focal lenghts. A 50mm is a 50mm both on crop and FF. However, being the former smaller, you crop the edges to a degree which would be equivalent to the angle of view of a 80mm on FF. This is a quick way to grasp the concept, but it's not exact in a way that you can make calculations.

Thanks indigo9, nice explanation. Albi86 - a 1.6x crop camera crops the FoV, so to get the same framing, you have to use a different focal length. This 18-35 crop lens is a rival to a 24-70 on FF, not a 16-35. 80mm on FF does frame the same as 50mm on crop - and if they both have the same aperture, the 80mm lens has to have a bigger entrance pupil.

And using indigo9's simple (yet still misunderstood) explanation, 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.

Neuro - do you want to chime in at this point? People respect your opinion

And as I said before though, this is a very interesting lens, and its a great option for crop sensor users - while it lets in slightly less light and has a narrower zoom range than the 24-70/2.8, its a great alternative to moving to a more expensive body, and the internal zoom is a great feature for a normal lens.

 

[Albi86]

...

You are mixing two concepts that are unrelated.

If we talk about framing, then you're right. The smaller effective aperture is the reason why DoF is bigger on crop at a given aperture and angle of view (note that I didn't say focal length).

However, framing has nothing to do with light gathering. f-stops are a function of focal length, not angle of view. A 50mm is a 50mm on every camera. It's the angle of view that changes in relation to sensor size, not the focal length.

Another quote from Wikipedia:

A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil is larger than that of the 100 mm lens, but given the same light transmission efficiency, both will produce the same illuminance at the focal plane when imaging a scene of a given luminance.

 

[Sella174]

I think the wheels come off because we are using "full-frame" lenses on "crop-frame" cameras, and insist on comparing everything to the "35mm" (aka "full-frame") format.

 

[Albi86]

I think the wheels come off because we are using "full-frame" lenses on "crop-frame" cameras, and insist on comparing everything to the "35mm" (aka "full-frame") format.

This can be done with no problems. The important thing is not to confuse focal length with angle of view. We can say that a 50mm gives you the same framing ( = angle of view) of a 80mm on FF; however, other parameters related specifically to focal length and not to angle of view are not affected.

 

[rs]

...

You are mixing two concepts that are unrelated.

If we talk about framing, then you're right. The smaller effective aperture is the reason why DoF is bigger on crop at a given aperture and angle of view (note that I didn't say focal length).

However, framing has nothing to do with light gathering. f-stops are a function of focal length, not angle of view. A 50mm is a 50mm on every camera. It's the angle of view that changes in relation to sensor size, not the focal length.

Another quote from Wikipedia:

A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil is larger than that of the 100 mm lens, but given the same light transmission efficiency, both will produce the same illuminance at the focal plane when imaging a scene of a given luminance.

But to compare a crop lens, crop sensor combo to a FF lens, FF sensor combo, there's no point in comparing two with different framing - otherwise you'd be arguing this 18-35 crop lens is a direct equivalent of a 16-35 FF lens on FF.

To exaggerate, is a 100mm f5.6 large format lens with its huge image circle the same as a 100mm f5.6 lens and its tiny image circle on a compact? Is it wrong to compare lenses which give the same framing? Surely from a photographers point of view, they're two very different lenses?

Wikipedia doesn't take into account imaging circle in that equation you're quoting. The aperture of a lens is a bit like working out the speed of water in a hose pipe. The imaging circle is a bit like the diameter of the pipe. Widen the pipe and keep the speed the same, you get more coming through.

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.

 

[Mantanuska]

Given that we have the same angle of view, the amount of light falling on the sensor is purely a function of the pupil diameter. f/2 on full frame is not the same as f/2 on crop.

Exposition
In case you're still not convinced, now assume that you have a FF and APSC sensor using the same production process & the same overall resolution -- they will have the same sensitivity per photon. If you take a shot with the same shutter speed, given that more light falls on the full frame sensor you would expect to use less sensor signal amplification [ie a lower ISO setting].

More light falls on the FF sensor but it is spread out over a bigger area. actual intensity (or in this case it helps to think about it as density) of the light is the same.

The only reason why you are able to use a higher ISO on FF is because the pixels are larger on FF which means better signal to noise.

 

[Albi86]

...

You are mixing two concepts that are unrelated.

If we talk about framing, then you're right. The smaller effective aperture is the reason why DoF is bigger on crop at a given aperture and angle of view (note that I didn't say focal length).

However, framing has nothing to do with light gathering. f-stops are a function of focal length, not angle of view. A 50mm is a 50mm on every camera. It's the angle of view that changes in relation to sensor size, not the focal length.

Another quote from Wikipedia:

A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil is larger than that of the 100 mm lens, but given the same light transmission efficiency, both will produce the same illuminance at the focal plane when imaging a scene of a given luminance.

But to compare a crop lens, crop sensor combo to a FF lens, FF sensor combo, there's no point in comparing two with different framing - otherwise you'd be arguing this 18-35 crop lens is a direct equivalent of a 16-35 FF lens on FF.

You can compare what you want, as long as you it correctly. The way you do it is the way you can compare angles of view. This is fine as long as you don't make the mistake to consider angle of view = focal length and to put this value into a f/stop calculation.

To exaggerate, is a 100mm f5.6 large format lens with its huge image circle the same as a 100mm f5.6 lens and its tiny image circle on a compact? Is it wrong to compare lenses which give the same framing? Surely from a photographers point of view, they're two very different lenses?

Focal length is what it is. You can mount an old Zeiss for Hasselblad lens on your Canon camera and the focal length will not change. It would give you the same framing of a 100mm Canon lens.

Wikipedia doesn't take into account imaging circle in that equation you're quoting. The aperture of a lens is a bit like working out the speed of water in a hose pipe. The imaging circle is a bit like the diameter of the pipe. Widen the pipe and keep the speed the same, you get more coming through.

More flow, but not more pressure. Same with light: you get more light coming in in total, but the amount of light / surface of the sensor would be the same. Any extra light that comes in will not affect exposure.

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.

Same as above. A smaller sensor is actually a smaller mouth to feed. This is why you can have compact cameras with 1" sensor and f/1.8 lenses.

 

[rs]

You can compare what you want, as long as you it correctly. The way you do it is the way you can compare angles of view. This is fine as long as you don't make the mistake to consider angle of view = focal length and to put this value into a f/stop calculation.

Angle of view is a combination of focal length and sensor size. If we're talking about two different sized sensors, to get the same angle of view, we have to adjust the focal length to suit.

Focal length is what it is. You can mount an old Zeiss for Hasselblad lens on your Canon camera and the focal length will not change. It would give you the same framing of a 100mm Canon lens.

You're looking at it the wrong way here - while the 100mm Zeiss frames the same as the 100mm Canon on the Canon SLR, it frames very different from how it would on the Hasselbald. On the Canon, you've cropped out a fair old proportion of the image circle, resulting in much of the light being cropped out and a narrower FoV.

More flow, but not more pressure. Same with light: you get more light coming in in total, but the amount of light / surface of the sensor would be the same. Any extra light that comes in will not affect exposure.

That's my exact point - the intensity of the light at any area of the sensor isn't greater, its just there's a bigger area of it, so in total more light is captured by the system.

Same as above. A smaller sensor is actually a smaller mouth to feed. This is why you can have compact cameras with 1" sensor and f/1.8 lenses.

Again, it looks like you've nearly got it here - it does need that faster aperture to make it equivalent.

 

[Albi86]

Angle of view is a combination of focal length and sensor size. If we're talking about two different sized sensors, to get the same angle of view, we have to adjust the focal length to suit.

Exactly. And you have to decide if we're talking about focal length or angle of view. You can't freely interchange the two concepts, because they are two different things. You can compare one to the other, but within limits.

Focal length is a property of the lens, and the lens alone. Whatever is a function of focal length, is not affected by the sensor in your camera, which - as you said - instead will affect the angle of view.

A Nikon 1 10mm f/2.8 lens will have an equivalent focal length (read: angle of view) of a 27mm lens on FF, but it will be nevertheless a 10mm lens. Assuming transmission is the same, the exposure would be the same for this lens and a 10mm F/2.8 lens on FF. The Nikon 1 lens can be smaller because the sensor is smaller. The total amount of light gathered by a FF will be more, but the amount of light hitting the sensor / surface of the sensor would be the same. You could mount the FF lens on the Nikon 1 and it would be the same - because the extra light will fall off the sensor. That's why exposure is not affected.

If you now transpose this concept to this lens, you will understand that exposure-wise a f/1.8 lens is a f/1.8 lens disregarding of the system. This, again, is because f-stops depend on focal length and focal length is a parameter of the lens and not of the camera.

 

[insanitybeard]

Debate regarding the light gathering ability of this lens and equivalent DOF vs FF aside, I must admit this is an interesting lens. This is coming from somebody who up until this point has only considered Canon glass. Maybe it will give Canon a push to put out some more high quality and fast dedicated crop lenses of it's own. The implications of using this lens on a crop camera cannot be ignored, if it is good optically.

 

[AprilForever]

What makes this lens epic is that there is no equivalent FF lens. In very low light, this can focus when f2.8 cannot!!!

 

[Albi86]

What makes this lens epic is that there is no equivalent FF lens. In very low light, this can focus when f2.8 cannot!!!

+1

It's epic that it takes crop shooters where they've never been before.

 

[Lee Jay]

What makes this lens epic is that there is no equivalent FF lens. In very low light, this can focus when f2.8 cannot!!!

No cameras I know of have f/1.8 AF sensors. f/2.8 sensors "see" at f/2.8 regardless of the actual f-stop of the lens. An f/1.4 lens is no brighter to f/2.8 AF sensors than an f/2.8 lens is.

 

[indigo9]

Exactly. And you have to decide if we're talking about focal length or angle of view. You can't freely interchange the two concepts, because they are two different things. You can compare one to the other, but within limits.

Sensor dimensions taken from: http://en.wikipedia.org/wiki/Full-frame_digital_SLR
FoV/Focal Length formulas: http://paulbourke.net/miscellaneous/lens/

Calculations:
(Using an 81mm lens here to be more accurate, 36/22.2 = 1.62 crop factor)

APSC sensor: 22.2mm width, focal length 50mm = 0.4369 radians = 25.0 degrees
Full Frame sensor: 36mm width, focal length 81mm = 0.4373 radians = 25.1 degrees

Focal length and Field of View are the same thing. The only reason they might not match in practice is because manufacturers don't quite tell the truth and the advertised focal length is slightly different from what they say (if it's 52mm, they're going to advertise it as a 50mm anyway)

A Nikon 1 10mm f/2.8 lens will have an equivalent focal length (read: angle of view) of a 27mm lens on FF, but it will be nevertheless a 10mm lens. Assuming transmission is the same, the exposure would be the same for this lens and a 10mm F/2.8 lens on FF.

A Nikon 1 10mm f/2.8 lens will have a pupil diameter of 3.57mm.

A 27mm f2.8 lens will have a pupil diameter of 9.64mm.

There are 2.7^2=7.3 times as many photons are coming through that 27mm lens as through the 10mm lens at the same f stop. Since the same proportion of those photons falling on the sensor is the same [same aspect ratios fitting in a circular aperture], the full frame sensor is receiving 7.3 times as many photons at the same f-stop.

The fact that one sensor is larger or smaller is completely irrelevant -- both sensors are covering the same proportion of the image circle (because we're talking about 35mm equivalent focal lengths here -- if you want to argue proportion of image circle covered then you need to do the comparisons using the same focal lengths -- see below)

The Nikon 1 lens can be smaller because the sensor is smaller. The total amount of light gathered by a FF will be more, but the amount of light hitting the sensor / surface of the sensor would be the same.

See above -- the amount of light (ie photons) hitting the sensor is most definitely not the same. The physical aperture size and the field of view are the only 2 things that control how many photons go through the lens.

You could mount the FF lens on the Nikon 1 and it would be the same - because the extra light will fall off the sensor.

Huh? A Nikon 1 sensor is 2.72x2.72 = 7.44 times smaller -- 87% of photons that would be hitting a FF sensor are being ignored. Assuming sensors are equivalent technology, at the same shutter speed the simple fact is that the Nikon 1 will have to amplify the signal coming off the sensor 7 times more than the full frame camera to get a proper exposure. How is this possibly equivalent?

That's why exposure is not affected.

More photons hitting the sensor = more electrons being excited = less amplification necessary to properly expose an image = less amplification of background sensor noise.

Note that I deliberately haven't mentioned ISO settings at all -- ISO settings are just calibrated labels to measure how much sensor signal amplification the camera needs to perform to properly expose a scene to a certain level, they are not an inherent property of the sensor itself. Go to dxomark and compare the SNR on a Canon 5D2 and a 60D -- the 60D is far noisier despite having a newer sensor, because ISO 1600 on a 60D actually means a lot more signal amplification than ISO 1600 on a 5D2. The ISO number itself is irrelevant, what is relevant is how much noise is amplified along with the useful signal.

If you now transpose this concept to this lens, you will understand that exposure-wise a f/1.8 lens is a f/1.8 lens disregarding of the system. This, again, is because f-stops depend on focal length and focal length is a parameter of the lens and not of the camera.

Yes, a f/1.8 lens is a f/1.8 lens regardless of the system. A f/1.8 lens on a full frame will perform better than a f/1.8 lens on crop with equivalent focal lengths though, and that's what the original question was.

Imagine that tomorrow I introduce a new camera system for amateur astronomers called "Super 100x35mm", sensor dimensions 360mm x 240mm, same ~4000x2800px resolution as a crop system. I stick 35mm-equivalent 50mm focal length lens on it, which for this system would be a 500m f/2.8. The pupil diameter of this lens is 178mm -- this pupil is 10 times wider than a full frame lens. According to the logic above, the fact that this pupil & sensor has 262 times more light gathering capability than a crop 32mm f/2.8 lens is irrelevant; "exposure-wise" all f/2.8 lenses are equivalent? Does this really sound sensible to you?

If that's the case, why are astronomy telescopes so wide?

• It's not to get a narrow FOV -- you could get http://pan-starrs.ifa.hawaii.edu/public/home.html FOV using a 650mm lens on a 35mm system.
• It's not resolution, you could just move the focal plane farther back to fit in more sensors -- eg 36cm could give you 1.7gigapixels with off the shelf components http://ambivalentengineer.blogspot.com.au/2012/08/argus-is.html
• It's because a wider aperture gives you more light = better low light performance

The original question was whether a crop camera with a given lens at f/2.8 performs the same as an equivalent focal length f/2.8 full frame. The answer is no, for equivalent focal lengths at the same f-stop the full frame sensor receives a lot more light and so gives less noise. In light of that, rs is right -- the lens might be nice for someone like me who only owns a crop camera but if you care terribly about low light performance then you should be on full frame.


Native sensor ISO? Let's not open *that* can of worms

 

[michi]

Lots of high tech talk I am not privy to understand or care to understand. The lens however seems great. Impressed with Sigma. Unfortunately I have decided to go and stay with full frame. If anything remotely similar was available for full frame, I would be all over it.

 

[AdamJ]

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.