Sigma 18-35 f/1.8 Really Just 28-55 f/2.8?

Just want to make sure I'm not falling into the traps of marketing, but is that really the case for these lenses when it comes to crop sensors? Was just having a conversation with an older photographer (swears by his 5D mark 1) and he was just breaking down a few things to me before the fire works started.

What he means is that the lens will give about the same FOV, DOF and low light capability on 1.6X that a hypothetical 28-55/2.8 lens would give if used on a similar tech level FF body. That is a correct assessment.

Note that the low light part assumes the FF body is about 1 stop cleaner, which is reasonable if we compare a 5D2 or 5D3 to a current 1.6X body, but it might not be true for his older 5D.

But for DOF and FOV the statement is correct.

Of course, this comparison is more meaningful if you own both formats. If you only have a 1.6X body then what hypothetical lens it compares to on a format you do not have is kind of academic. Maybe a bit of interest is to know that such a fast zoom is not giving the smaller format an advantage in DOF or low light. The fast f/1.8 is actually offsetting the other differences noted between the formats in use.

Yes and no.

From my limited understanding of the technical aspect of lens and sensors: the DOF of f1.8 on a crop is comparable to f2.8 on FF and the angle of view of 18-35 is comparable to 28-55 on ff. But f1.8 is still allowing more light to reach the sensor. So the lens still have a 1 1/3 stop advantage over an f2.8 lens.

(I'm sure I'll be corrected if I'm misunderstanding the technical stuff)

gremlin75 wrote in post #16991825
Yes and no.

From my limited understanding of the technical aspect of lens and sensors: the DOF of f1.8 on a crop is comparable to f2.8 on FF and the angle of view of 18-35 is comparable to 28-55 on ff. But f1.8 is still allowing more light to reach the sensor. So the lens still have a 1 1/3 stop advantage over an f2.8 lens.

(I'm sure I'll be corrected if I'm misunderstanding the technical stuff)

You are correct, but usually a FF sensor camera will also have an advantage in noise at high ISO. So even the light gathering can be offset. But this is more situational depending on the camera. A 5D Mark 2 might have a one stop advantage over a 7D for example and so the aperture difference would be offset by using one stop higher ISO with the 5D2.

But the older 5D classic might have no noise advantage over a 7D, so the light gathering difference would not be compensated.

The FOV and DOF differences are not situational like that.

gremlin75 wrote in post #16991825
Yes and no.

From my limited understanding of the technical aspect of lens and sensors: the DOF of f1.8 on a crop is comparable to f2.8 on FF and the angle of view of 18-35 is comparable to 28-55 on ff. But f1.8 is still allowing more light to reach the sensor. So the lens still have a 1 1/3 stop advantage over an f2.8 lens.

(I'm sure I'll be corrected if I'm misunderstanding the technical stuff)

not necessarily.

think of it as bigger rain drops. The larger raindrops are falling into a cup (crop sensor), where smaller rain drops are falling into a bowl (full frame sensor). Which collects more rain?

In this particular case, it's slightly more in favor of the crop sensor, but that's not the whole story either. High quality optics refracting on a bigger surface will have more resolving power and color tonality in general.

JeffreyG wrote in post #16991805
What he means is that the lens will give about the same FOV, DOF and low light capability on 1.6X that a hypothetical 28-55/2.8 lens would give if used on a similar tech level FF body. That is a correct assessment.

Note that the low light part assumes the FF body is about 1 stop cleaner, which is reasonable if we compare a 5D2 or 5D3 to a current 1.6X body, but it might not be true for his older 5D.

But for DOF and FOV the statement is correct.

Of course, this comparison is more meaningful if you own both formats. If you only have a 1.6X body then what hypothetical lens it compares to on a format you do not have is kind of academic. Maybe a bit of interest is to know that such a fast zoom is not giving the smaller format an advantage in DOF or low light. The fast f/1.8 is actually offsetting the other differences noted between the formats in use.

That's wrong, the rest before that is right. f/1.8 stays f/1.8 in terms of letting thru light.. But the FoV and DoF you're right.

This is the main reason I'm switching back to full frame... DoF is of great importance to me, since I like nicer bokeh and thinner DoF...

Somedude18 wrote in post #16991906
That's wrong, the rest before that is right. f/1.8 stays f/1.8 in terms of letting thru light.. But the FoV and DoF you're right.

This is the main reason I'm switching back to full frame... DoF is of great importance to me, since I like nicer bokeh and thinner DoF...

Read my explanation in post 4. Sometimes it is true, sometimes it isn't.

JeffreyG wrote in post #16991912
Read my explanation in post 4. Sometimes it is true, sometimes it isn't.

Sorry, it wrong. A specious argument.

The noise performance has no bearing at the light coming in. High ISO performance should not be lumped in with aperture to lead at arguments 'the lens is 2 stops better in the 5DII vs the 5D'.

What happened to shutter speed? f2.8 is f2.8 as far as the lens is concerned. The one stop better shutter speed to be afforded at high ISO is a different thing altogether. Is 1/500 in the 5D 1/1000 in the 5DIII?

Basically the sigma has now provided a tool that gives crop owners a final image very closely matching what f2.8 zooms have been providing the ff body.

Assume the following. Person A has an aps-c body with a sigma 18-35 f1.8 lens. Person B has a 24-70L f2.8 on a FF. They both shoot the same subject material with the same framing with the same shutter speed.

Up to this point, person B's FF results might be cleaner than person A's APS-C results at the same ISO levels depending on which bodies we are talking about (5D vs 70D? 7D vs 5D3? 50D vs 5D2?). The FF would also have a thinner DOF because they would have changed the focal length to 55mm to get that framing as 35mm on the crop.

Now person A can shoot at f1.8 at 35mm, reducing their DOF, and possibly lowering the ISO due to the faster aperture. They now have a result that would more closely match the FF results, at a lower ISO, narrowing the ISO gap from the FF or even bettering it, again depending on the bodies we are talking about. This result could never be obtained in the past unless person A would shoot with a fast 35mm prime.

Sharpness is a different issue altogether, but it appears that the Sigma resolves detail very well on a crop, so that difference might be negligible. Distortion at the wide end might be a consideration too (18mm on a crop vs 28mm on a ff).

gremlin75 wrote in post #16991825
the DOF of f1.8 on a crop is comparable to f2.8 on FF and the angle of view of 18-35 is comparable to 28-55 on ff. But f1.8 is still allowing more light to reach the sensor. So the lens still have a 1 1/3 stop advantage over an f2.8 lens.

Pretty much this.

This is an advantage in situations where you don't want really shallow DOF but do want f1.8 speed. Like trying to shoot a table of people at an event: using f1.8 on a crop body will give you a deeper DOF allowing you to keep more people in focus while still letting in f1.8 light to keep your ISO down or your shutter speed up.

Try not to over think it I would suggest you rent the Sigma for a weekend and after you fall in love, buy one for yourself and don't worry about the crop factor stuffs

Charlie wrote in post #16991903
not necessarily.

think of it as bigger rain drops. The larger raindrops are falling into a cup (crop sensor), where smaller rain drops are falling into a bowl (full frame sensor). Which collects more rain?

In this particular case, it's slightly more in favor of the crop sensor, but that's not the whole story either. High quality optics refracting on a bigger surface will have more resolving power and color tonality in general.

That's the best illustration of it I've seen so far. But we need to keep in mind that the light coming thru the lens reaches more than one sensor in the camera. I.e. the extra light (or "larger rain drops") from the f/1.8 lens can help the AF performance in low light, which is often an important aspect of "low light capability".

MakisM1 wrote in post #16991953
Sorry, it wrong. A specious argument.

The noise performance has no bearing at the light coming in. High ISO performance should not be lumped in with aperture to lead at arguments 'the lens is 2 stops better in the 5DII vs the 5D'.

I get your point, but it isn't specious. Whenever I think about these kinds of questions between formats, the first thing I think of is "What would allow each format to take the same picture?"

In my mind, noise associated with high ISO is part of this 'same picture' question. I mean, there are really small format cameras like the Panasonic LX line that have f/2.8 zoom lenses, and nobody would think that these have the 'same low light' capability as a 5D Mark III equipped with a f/2.8 zoom.

We know the 5D3 at say, f/2.8 and ISO 3200 is going to be different in output from a Panasonix LX3 at f/2.8, the latter is simply not as capable in low light.

The same comparison holds for the narrower difference between 1.6X and FF so long as the development time of the cameras are close, as I stated.

When the angle of view is held constant between different formats (which we are talking about here) then you need to use the same shutter speed to get the same subject freezing or handholding. So of course we would hold shutter speed constant across formats in this comparison.

You guys are all so intelligent haha, I hope one day I will be able to go in depth about the topic like this, so from what I'm reading I think to a certain degree what the guy was saying was correct, is that why the lens is APC-S only? Lens like the Tokina 11-16 that can only shoot at 16 on full frame is that lens still shooting 2.8 on full frame? Or is that an entirely different situation?

KeenanRIVALS wrote in post #16992101
You guys are all so intelligent haha, I hope one day I will be able to go in depth about the topic like this, so from what I'm reading I think to a certain degree what the guy was saying was correct, is that why the lens is APC-S only? Lens like the Tokina 11-16 that can only shoot at 16 on full frame is that lens still shooting 2.8 on full frame? Or is that an entirely different situation?

Lens specs are provided with no regard to what sensor type you plan to use with that lens (FL or f-number, ex. 10mm EFS = 10mm EF). The only thing that matters on a lens designation, for Canon, is EFS vs EF, and denotes a different mount ring. Canon is the only company to make EFS lenses. All other APS-C 3rd party lenses are EF mount, and can mount to FF or APS-H if you don't mind the heavy vignetting.

TeamSpeed wrote in post #16992116
Lens specs are provided with no regard to what sensor type you plan to use with that lens (FL or f-number, ex. 10mm EFS = 10mm EF). The only thing that matters on a lens designation, for Canon, is EFS vs EF, and denotes a different mount ring. Canon is the only company to make EFS lenses. All other APS-C 3rd party lenses are EF mount, and can mount to FF or APS-H if you don't mind the heavy vignetting.

I think ultimately I'm phrasing my question wrong, I understand crop and the 1.6 crop factor. But if I mount my 18-35 on a 5DIII would the aperture still be 1.8? Or will it be 2.8 (that's what the guy was saying)

I was defending that despite vignetting it would still be a f stop of 1.8, he was saying it wouldn't be, it would be an f stop of 2.8, Sigma is just calling it 1.8 because they know it will be on a crop sensor. He compared it to TV's and 240hz which are really 120hz or something for that nature.