Crop Factor sensors - Lenses - Apertures ??

[kazalou]

Most Digital Camera manufactures out there selling cameras with sensors smaller then Full Frame
Like my Pentax K7 which uses an APS-C Sensor which I owned combine with a Tamron 90mm F/2.8 prime lense is in fact the equivalent of a (1.5 x 90mm) = 135mm. But most company never mention that the Aperture F/2.8 is also affected ?
are we not suppose to multiply the Crop Factor 1.5x by F/2.8 which result approximately to a F/4.0 .

And if this make sense then all these Bridge Cameras like for example a Sony RX10 using a 1" inch sensor which represent a Crop Factor of 2.7x if multiply this by the F/2.8 represent an Aperture close to 7.6 which would explain the lack of DOF compare to a Full Frame camera using an F/2.8 aperture.

So are they right to say: The Sony RX10 featuring a 24-200mm equivalent (8.3x optical zoom) lens
with a continuous F2.8 maximum aperture

Am I completely wrong on this subject, please as I need to understand better the implication of using smaller sensors...
Kazalou

[Rorschach]

You've got most of it. Relative to the sensor, it's not misleading to specify a maximum aperture of f/xyz.

Focal lengths and apertures are constant regardless of the sensor size e.g. a 300mm /f2.8 lens on my 70D is still 300mm at f/2.8 on my friend's 5D Mark III despite my camera having an APS-C sensor while my friend's is full-frame. It's just that the crop factor gives the appearance that the focal length on my 70D is ~480mm whilst the reduced amount of light getting to with a smaller sensor gives an aperture equivalent of f/4.5.

It's absolutely fine to specify the f/2.8 aperture, you just have to bear in mind that you will not get the same performance in terms of depth of field or the amount of light coming in when you compare it to a camera with a larger sensor. To get the equivalent light and DoF as an f/2.8 lens on a full-frame sensor with an RX10 would need an aperture of ~f/1.0 so as you've rightly worked out, one of the implications with a smaller sensor is that it's more difficult to get a shallow depth of field.

[kazalou]

This next link is a Video presented by Tony Northrup which to me has made a clear demonstration of what I am trying to understand and like we always say a Picture is worth a thousand words.
Please let me know if you think it make sense ...

https://www.youtube.com/watch?v=f5zN6NV ... 2K2p9tnwYQ

kazalou..

[Rorschach]

It definitely makes sense but to be honest, I expected Tony Northtup to explain it in a digestible fashion anyway.