Sigma 30mm A vs 35mm A?

Sdiver2489 wrote in post #17381403
Just an FYI...the 30mm siggy is about 1/3 a stop faster than the Canon F2 IS. I just don't want you to get your hopes up of a huge difference in shutter speed between the two. The 35mm sigma is brighter (5/6th a stop faster).

I'm confused. I thought both Sigmas are f/1.4? Isn't that a full stop faster than f/2?

F1.4 is a theoretical rating...some lenses actually hit it...some are a bit off. Some are a lot off (siggy 30 F1.4).

Take a look at DXOmark lens reviews and pull up the stats on each and look at the light transmission rating. This is effectively the F-stop.

Sdiver2489 wrote in post #17381412
F1.4 is a theoretical rating...some lenses actually hit it...some are a bit off. Some are a lot off (siggy 30 F1.4).

Take a look at DXOmark lens reviews and pull up the stats on each and look at the light transmission rating. This is effectively the F-stop.

Thanks, didn't know that. That is quite disappointing to hear.

But light transmission is a T-stop and is different from an F-stop. Light transmission has little to do with depth of field.
I own the 30mm A as well as a couple Canon f/1.8 lenses (50mm and 85mm). In terms of IQ and color rendering the 30mm takes the cake (in my opinion at least, I love it), but I honestly can't say if the light transmission is any brighter than the Canons.

By the way, if you can live with some vignetting the Sigma does mount to a full frame camera and it does really well on my 5d. The 6d will ask more of the lens though, as its resolution is quite a bit higher.

Sdiver2489 wrote in post #17381412
F1.4 is a theoretical rating...some lenses actually hit it...some are a bit off. Some are a lot off (siggy 30 F1.4).

Take a look at DXOmark lens reviews and pull up the stats on each and look at the light transmission rating. This is effectively the F-stop.

I don't think it works that way.

As I understand it, the opening of the aperture -the physical size - is described by the F-stop.
It's the focal length / aperture size. So F/2 on an 85mm lens means the aperture has a diameter of 42mm. This is important for the depth of field.
The F-stop isn't theoretical at all. You can calculate this if you know the maximum aperture size and the focal length.

The transmission, which determines how much light actually hits the sensor, is the T-stop.
The aperture has its role here too, but the coatings and light fall-off play a part as well. If you install a 3-stop ND filter the T-stop changes.

To return to the Sigma, DXO reports it has a T-stop of T/1.8.

As I understand it, the opening of the aperture -the physical size - is described by the F-stop.
It's the focal length / aperture size. So F/2 on an 85mm lens means the aperture has a diameter of 42mm. This is important for the depth of field.
The F-stop isn't theoretical at all. You can calculate this if you know the maximum aperture size and the focal length.

The transmission, which determines how much light actually hits the sensor, is the T-stop.
The aperture has its role here too, but the coatings and light fall-off play a part as well. If you install a 3-stop ND filter the T-stop changes.

To return to the Sigma, DXO reports it has a T-stop of T/1.8.

How do you think it works? To quote dxomark:


DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination. T-stop, a measurement widely used in the industry, especially the motion picture industry, is a measurement of the effective quantity of light transmitted by the lens at a certain f-stop. Thus, T-stop takes into account every reflection or absorption due to the lens. It can be considered as an effective (versus theoretical) way of measuring the aperture.

Sdiver2489 wrote in post #17381727
How do you think it works? To quote dxomark:

DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination. T-stop, a measurement widely used in the industry, especially the motion picture industry, is a measurement of the effective quantity of light transmitted by the lens at a certain f-stop. Thus, T-stop takes into account every reflection or absorption due to the lens. It can be considered as an effective (versus theoretical) way of measuring the aperture.

You posted DXO's definition of T stop yet your last sentence in your last post said T stop was effectively the F stop and that is what I think he wasn't sure about. I'm not 100% sure either but for DOF I think it's physical aperture size.

FEChariot wrote in post #17381815
You posted DXO's definition of T stop yet your last sentence in your last post said T stop was effectively the F stop and that is what I think he wasn't sure about. I'm not 100% sure either but for DOF I think it's physical aperture size.

FEChariot...Those are DxOmarks words not mine. They said it is an effective way of measuring aperture. This is my understanding of T-stop from my reading into it in the past.

Not all lenses meet their calculated F-stop in terms of actual light gathering ability. This is why I was pointing out to the OP that while the Canon 35 F2 IS meets its F2 rating...the Sigma 30mm F1.4 is a F1.8 in terms of effective light gathering (T-stop).

Oh and I agree that T-stop should not be used in terms of light gathering...but for the OP's issue regarding F2.0 not being fast enough getting a 1/3 faster shutter speed is probably not going to be significant.

Sdiver2489 wrote in post #17381902
FEChariot...Those are DxOmarks words not mine. They said it is an effective way of measuring aperture. This is my understanding of T-stop from my reading into it in the past.

Not all lenses meet their calculated F-stop in terms of actual light gathering ability. This is why I was pointing out to the OP that while the Canon 35 F2 IS meets its F2 rating...the Sigma 30mm F1.4 is a F1.8 in terms of effective light gathering (T-stop).

Oh and I agree that T-stop should not be used in terms of light gathering...but for the OP's issue regarding F2.0 not being fast enough getting a 1/3 faster shutter speed is probably not going to be significant.

They didn't say that. F is a measure of how big the opening is. T is a measure of how much light makes it through. They are related but not equivalent.

If you put a ND filter on the lens the size of the opening (F) doesn't change, the amount of light (T) does change. Different glass transmits light differently.

travisvwright wrote in post #17381939

Sdiver2489 wrote in post #17381938
FEChariot...Those are DxOmarks words not mine. They said it is an effective way of measuring aperture. This is my understanding of T-stop from my reading into it in the past.

They didn't say that. F is a measure of how big the opening is. T is a measure of how much light makes it through. They are related but not equivalent.

If you put a ND filter on the lens the size of the opening (F) doesn't change, the amount of light (T) does change. Different glass transmits light differently.

Yes they did:

And we are talking light gathering abilities here because the OP wants faster shutter speeds. If the light doesn't make it to the sensor then the theoretical aperture most people care about doesn't matter.

Sdiver2489 wrote in post #17381727
How do you think it works? To quote dxomark:

DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination. T-stop, a measurement widely used in the industry, especially the motion picture industry, is a measurement of the effective quantity of light transmitted by the lens at a certain f-stop. Thus, T-stop takes into account every reflection or absorption due to the lens. It can be considered as an effective (versus theoretical) way of measuring the aperture.

I see what you are saying here but I think you are taking it out of context. They are describing the quantity of light here based on the aperture which is not the whole story as it leaves out the whole how DOF is effected by the aperture.

Sdiver2489 wrote in post #17382008
Yes they did:

And we are talking light gathering abilities here because the OP wants faster shutter speeds. If the light doesn't make it to the sensor then the theoretical aperture most people care about doesn't matter.

Well we started off saying it was light gathering, but your statement below changed the discussion slightly:

Sdiver2489 wrote in post #17381412
This is effectively the F-stop.

Maybe, "This is effectively the F-stop with respect to light gathering properties of the lens." would be a more accurate statement.

FEChariot wrote in post #17382191
I see what you are saying here but I think you are taking it out of context. They are describing the quantity of light here based on the aperture which is not the whole story as it leaves out the whole how DOF is effected by the aperture.

Well we started off saying it was light gathering, but your statement below changed the discussion slightly:

Maybe, "This is effectively the F-stop with respect to light gathering properties of the lens." would be a more accurate statement.

I haven't been arguing from a DOF point of view...my comments are strictly from a light gathering perspective to hell the OP pick a lens that will help give him the fastest shutter speed he is looking for. In that regard, just because a lens is rated F1.4 doesn't mean it gathers rate like every other F1.4 lens. Hopefully we're on the same page now

Sdiver2489 wrote in post #17382437
I haven't been arguing from a DOF point of view...my comments are strictly from a light gathering perspective to hell the OP pick a lens that will help give him the fastest shutter speed he is looking for. In that regard, just because a lens is rated F1.4 doesn't mean it gathers rate like every other F1.4 lens. Hopefully we're on the same page now

No I am with you on that, but you don't need to bring fire and brimstone into the discussion.