Photoclubalpha

David,
In your review of the Samyang 12mm f/2, you say that "It is equivalent to an 18mm on full frame".
Surely this is incorrect? It will be equivalent to 8mm on Full Frame if my understanding of these lens descriptions is correct.
However, I have to admit that I find these conversions confusing.

Art,

You're going backwards! APS-C has a smaller sensor, so any lens used on it has a narrower FOV than one used on a full frame camera. Conversion factor is (lens focal lenth) * 1.5, so a 12mm is the same FOV on an APS-C camera as an 18 would be on a FF camera.

With APS-C, you're telephotos go a lot farther, but your wides are not as wide. If you have access to both an APS-C and a FF camera (can even be old film Maxxums) just look thru both with a wide lens and you'll see the difference.

Hope this helps,

Dusty

Thanks, Dusty, for your response but I think you have it wrong. To make sure, I did as you suggested and looked at how my lenses work on the two sensor sizes.

When I put my Sony 10-18 zoom on my A6000 , then put it on my A7, the field of view on the A7 is wider than on the A6000.
To get the same FOV as I get at 12mm on the A6000, I need to set it to 18mm on the A7.

The confusion arises because the lens definition is not related to the sensor size; they are all based on the same optical definition, if that is the correct way to label it.
So your last para was correct; all lenses are longer on APS-C than on FF. So the Samyang is shorter or wider on FF than on APS-C.

artHarris wrote:To get the same FOV as I get at 12mm on the A6000, I need to set it to 18mm on the A7.

Based on your very own experiment, it sounds like 12mm on aps-c is equivalent to 18mm on FF. Or am I missing something?

I'll put it another way-
If I set the 10-18 zoom to 12mm, the focal length of the Samyang (which is an APC-S lens - correction!), on the A6000 the FOV is smaller than on the A7R.
My "18mm" comment is the setting which makes the FOV with this zoom on the A7 to same as 12 mm on the A6000 - the FOV with a given focal length is always smaller, i.e. the lens is longer, on APC-S cameras than on FF cameras. After all, the sensor is smaller. All these lenses are defined on the same basis, even if APC-S lenses are smaller.

Or, put the other way, a given lens will be wider on FF than on APC-S because more of the lens's FOV is used (not talking about vignetting with APC-S lenses, which complicates the issue - unusually, the Sony 10-18 fills the FF sensor at most focal lengths).

I think this is the easiest way to think about it. Unless I have made some inexplicable mistake.

Both your experiment and everything you just stated affirms that 12mm on aps-c is equivalent to 18mm on FF. The fov is the same in both setups therefore one is equivalent to the other. If you want to capture the same fov on an aps-c camera that you would have if you used a ff coupled with an 18mm lens then you would have to use a 12mm lens on the aps-c. That is what the equivalent statement is saying. This also fits what you just stated in the previous post. I'm not sure where the confusion is at. Maybe I'm just missing something obvious.

I see my point of confusion:
When a FF lens is used on an APC-S camera, only the centre of its FOV is used, so it acts as a wider angled lens. The A7 allows the camera to capture all the lens's FOV, by deactivating the APC-S framing, which is switched on by default. With a lens like the Sony 10-18, this allows the lens to be wider than would be expected from its 'definition'.
When a "normal' APC-S lens is used on a FF camera, the FOV is limited by the lens, so, on a FF camera, with the APC-S 'framing' activated, the image captured is of the same FOV but it is enlarged to fill the sensor, so it appears that the lens is a longer lens.
I have this APC-S function switched off on my A7, so that I can use whatever a lens captures. Hence my confusion.

Back at my desk after a week away, and I should have noted that this 18mm 'achievement' comes by the use of the digital zoom facility, something which in most contexts is not advised. Much better to do the zooming on the computer.
Unlike the longer focal length obtained using FF lenses on an APC-S camera, this longer focal length is not real, but digital.

I await the angument restarting!