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

[GaryJ]

I've decided to offer some help to Sigma with an FAQ set for potentially confused customers.

+1

[rs]

I've decided to offer some help to Sigma with an FAQ set for potentially confused customers.

+1

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 10000 on crop, or a 24-70/2.8 at 56mm f2.8 1/100th, ISO 25600 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)

Regardless of how you understand this difference between full frame and crop, and either agree or disagree with me, this Sigma lens really ups the game for crop users. From the specs point of view (and samples images), it looks great.

[AdamJ]

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.

I confess this the first time I've heard of a metabones speed booster but I get the idea and, yes, I'm with you so far.

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.

No, you've broken the sequence of logic from your metabones analogy. You can only say that the amount of light hitting the FF sensor is more because the FF sensor is bigger. The amount of light hitting each pixel is exactly the same, regardless of sensor size.

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).

.

This is a compound error from the previous incorrect statement. Sensor size is not a factor in this.

[rs]

This is a compound error from the previous incorrect statement. Sensor size is not a factor in this.

My original argument which has been hotly debated was merely that as nice as this Sigma 18-35/1.8 is on a crop camera, it's not quite as nice* as a 24-70/2.8 II is on a full frame camera.  I was comparing two complete systems, sensor and all. I know that a 24-70/2.8 is no match for this Sigma when they're both mounted on a crop body.

*by nice, I mean 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. The entrance pupil size when both systems have an equivalent FoV is a simple way of quantifying that.

But if you disagree with me and think that an f1.8 lens on 1.6x crop captures more light than a lens with an equal field of view at f2.8 on full frame, then fair enough. I've tried explaining this concept in many different ways, and you still don't get it. So I give up.

[Wildfire]

But if you disagree with me and think that an f1.8 lens on 1.6x crop captures more light than a lens with an equal field of view at f2.8 on full frame, then fair enough. I've tried explaining this concept in many different ways, and you still don't get it. So I give up.

Why do you care how much light the lenses capture? That's irrelevant. What I care about is the final exposure.

With an f/1.8 lens, the final exposure will be brighter than with an f/2.8 lens no matter what (even if the f/2.8 lens is "capturing" more light, it doesn't "capture" enough to be brighter than the f/1.8 lens in-camera). This is true regardless of whether you use one lens on crop and one on FF, or both on FF, or both on crop.

My original argument which has been hotly debated was merely that as nice as this Sigma 18-35/1.8 is on a crop camera, it's not quite as nice* as a 24-70/2.8 II is on a full frame camera.

That is your subjective opinion. The fact of the matter is that the lenses will have their different uses and there will be situations where each lens is better than the other. Because the Sigma is an f/1.8 lens, it may be quite possible that the 24-70 does not create a bright enough exposure for a photographer in a low-light situation, so the Sigma will be much "nicer" to use in that situation.

[rs]

Why do you care how much light the lenses capture? That's irrelevant. What I care about is the final exposure.

Because the Sigma is an f/1.8 lens, it may be quite possible that the 24-70 does not create a bright enough exposure for a photographer in a low-light situation, so the Sigma will be much "nicer" to use in that situation.

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.

[AdamJ]

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.

You're trolling, surely.

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.

There is some truth in this part of your statement but it has nothing whatsoever to do with the dimensions of the sensor.

[rs]

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?

[dirtcastle]

What about depth of field? Is the depth of field always in a 1:1 relationship with the f-stop? In other words, will this lens get equivalent depths of field of an f/2.8 lens on a full-frame camera?

[LOALTD]

I've decided to offer some help to Sigma with an FAQ set for potentially confused customers.

+2

So much mis-information on this, nice work!

[AdamJ]

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

Bingo!

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.

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.

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?

No, I'm content to stick with facts rather than come round to your argument.

[rs]

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

Bingo!

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.

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?

You'd need to shoot the cropped image at a lower ISO to show an equivalent amount of noise in the final print.

[AdamJ]

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

Bingo!

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.

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?

You'd need to shoot the cropped image at a lower ISO to show an equivalent amount of noise in the final print.

OK, now I know you're trolling. You failed to quote the part of my post that answers this:

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.

[jemping]

Not sure if anyone posted this already, but here is the website that has the sample images for this lens.

http://lcap.tistory.com/entry/Sigma-ART-18-35mm-f18-Preview

[rs]

OK, now I know you're trolling. You failed to quote the part of my post that answers this:

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.

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.

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.