Crop factor and max aperture question

Should be kind of obvious, but just to confirm: a 85mm lens will act like a 135mm on a crop body.

What about max aperture? If I have 50mm f/2.8, will it behave like 80mm f/2.8 or 80mm f/3.5?

uOpt wrote in post #11641781
Should be kind of obvious, but just to confirm: a 85mm lens will act like a 135mm on a crop body.

What about max aperture? If I have 50mm f/2.8, will it behave like 80mm f/2.8 or 80mm f/3.5?

The aperture is a physical opening represented as a ratio of the focal length. The focal length is not actually changing (it's still 50mm) and neither is the aperture (it's still 50mm/2.8 or 17.85mm in diameter). You will still be delivering f2.8 light to the sensor no matter the crop factor.

However, when comparing equivalence, the lens will give similar DOF and FOV as an 80mm f4.5 lens on FF when used at the same distance to subject. So in a sense, you do have an aperture crop factor.

tkbslc wrote in post #11641806
The aperture is a physical opening represented as a ratio of the focal length. The focal length is not actually changing (it's still 50mm) and neither is the aperture (it's still 50mm/2.8 or 17.85mm in diameter).

However, when comparing equivalence, the lens will give similar results as an 80mm f4.5 lens on FF when used at the same distance to subject. So in a sense, you do have an aperture crop factor.

Only from a DoF point of view, exposure remains unchanged.

themadman wrote in post #11641824
Only from a DoF point of view, exposure remains unchanged.

Yeah, I edited my original post to hopefully make that more clear.

Hmmm, I thought that the "apparent aperture" would go down with the square, so from 2.8 to 3.5? 2.8 * sqrt(1.6) = 3.5?

uOpt wrote in post #11641834
Hmmm, I thought that the "apparent aperture" would go down with the square, so from 2.8 to 3.5? 2.8 * sqrt(1.6) = 3.5?

The aperture size is what's important here. As I mentioned, a 50mm f2.8 has a 17.85mm aperture. That's 80/4.5. You pretty much just multiply both the aperture and focal length by the crop factor for FOV and DOF equivalence. You are getting 80mm equivalent focal length, but your aperture is still only 50/2.8.

uOpt wrote in post #11641781
Should be kind of obvious, but just to confirm: a 85mm lens will act like a 135mm on a crop body.

This is incorrect. An 85mm lens will always be and act like an 85mm lens regardless of the camera it is mounted to.

An 85mm lens mounted on an APC-C format camera will provide the same field (angle) of view as a 136mm lens mounted on a 35mm film format camera. However, the characteristics of the lens itself does not change in any way when you mount it on different format (format refers to the physical size of the film frame or digital sensor in a camera) bodies.

uOpt wrote in post #11641781
What about max aperture? If I have 50mm f/2.8, will it behave like 80mm f/2.8 or 80mm f/3.5?

The aperture settings of any particular lens will be identical regardless of the camera it is mounted to. If you have a 50mm lens mounted to a 7D and set it to f/2.8, you will have the same exposure intensity as if you put the lens (or any lens, for that matter) on a 5D and set the aperture to f/2.8 (as long as the shutter speeds and ISO settings are the same, of course).

Please click on this link to an article describing "crop factor" issues. I think it will help you understand the facts.