EFS vs EF lenses interms of Aperture

[fpcadiente]

Hi! Sorry but maybe most of you might think this is a dumb question, I just really want to under so I'll post away.

Recently Sigma has annouce it's latest Art lens, the 18-35mm F1.8 DC USM. It is design for APS-C cameras. It was mention that "The lens will offer the depth-of-field equivalent of a constant F2.7 on full-frame".

What does that mean? Does that mean the amount of light gather at F1.8 on an APS-C is the same as F2.7 on full frame?
If I shoot same object and nearly exact time, same shutter speed and ISO, will I be able to get nearly the same exposure?

e.g.
Canon EOS 60D + 18-35 F1.8
Aperture F1.8

Shutter Speed 1/100

ISO 400

Canon EOS 6D + 24-70 F2.8

Aperture F2.8

Shutter Speed 1/100

ISO 400

thanks

[diverhank]

I believe this has to do with depth of field only and not exposure.    If you use the same lens and aperture on a crop and full frame camera to take pictures of the same subject with identical framing (meaning you have to get closer  with a full frame)...the depth of field taken by the full frame will be shallower by around 1.3 stop (more blurred background).

[Skirball]

---

@diverhank wrote:

I believe this has to do with depth of field only and not exposure.    If you use the same lens and aperture on a crop and full frame camera to take pictures of the same subject with identical framing (meaning you have to get closer  with a full frame)...the depth of field taken by the full frame will be shallower by around 1.3 stop (more blurred background).

---

This is correct - no change to exposure, only depth of field.

For the record I think it's useless marketing fluff.  I understand pointing out the equivilent focal length on a FF (sort of), but the DoF is a bit much.  Anyone who knows enough about cameras to know what kind of DoF to expect at a certain aperture, at a certain focal length, knows how switching between FF and crop is going to affect it.

[cicopo]

As above I think the claim relates to the DOF more than settings but you can use this tool to dial in different settings on different bodies to see the effect.

http://dofmaster.com/dofjs.html

[hsbn]

With any lens, on a bigger sensor you'll get a shallower DOF. Put a F2.8 lens on a full frame (just for example), you get creamy nice out of focus background. To get the same effect the APS-C, you'll need F1.8 lens. But it won't affect your exposure at all. The same thing why Pont and shoot camera that has F2 lens and you don't have the smooth bokeh.

[fpcadiente]

thank you very much for all your reply. Now I kinda understand 🙂

[ebiggs1]

There is and always has been a misconception of the focal length of lenses when attached to different bodies.

A 50mm lens is a 50mm lens whether it is on a 1.6 crop sensor body like a Rebel, it is 50mm on a 1D series 1.3 crop sensor and it is still 50mm on a FF such as a 5D series.

The angel of acceptance is what is different. The smaller sensor makes the subject "appear" larger because it has less coverage area from the lens. This also causes the DOF to appear less but actually it is the exact same.

A lens can not change it's physical properities only the sensor can effect what it sees.

[ScottyP]

Although, for entirely different reasons, one can usually get by with higher ISO settings on a FF camera, so in a sense FF also gives you a couple of stops of good IQ shootings at higher ISO's.

[ebiggs1]

In a broad general statement, yes, but it is actually pixel size that counts not sensor size.