Originally posted by Class A - It is not the same aperture. It is just the same f-ratio, leading to different aperture diameters for different focal lengths.
- If you change the focal length from 60mm to 220mm then the respective f-ratio changes from f/2.8 to f/10, not to f/16.
- The cropped image has the same image quality that you'd get from using a 220mm lens on the larger sensor, stopped down to f/10.
As I said in the beginning what matters for image quality is the total amount of light (number of photons) captured, not the exposure.
Exposure tells you about the amount of light for a unit area. It is a good measure for the light coming in, but not at all for the light recorded. Let's assume the same f/2.8 equivalent exposure for two sensors. One has the size 1mm x 1mm and the other has the size 24mm x 36mm. The latter collects 864 times more photons than the smaller one. The difference corresponds 9.75 stops. Let's assume that the same part of a scene is projected on both sensors (which is the only way to do an "apples to apples" comparison). Are you telling me that you don't mind which image you get; the one that uses say 1000 photons or the one that uses 864,000 photons, just because you can argue that both got the same "exposure"?
The above is an argument why looking at f-ratios that correspond to "exposure" is not helpful when talking about image quality. Another argument is that the widest f-stop for a lens is simply its focal length divided by its aperture diameter. If you calculate FF-equivalent parameters, you need to multiply the focal length by the crop factor. In order to get the same DOF, the FF-equivalent lens needs to maintain the same aperture diameter, hence the widest f-stop of the FF-equivalent lens will be crop_factor*original_focal_length/aperture_diameter which is the same as crop_factor*original_widest_f-stop. End of story. Exposure (in the sense of the amount of light on a unit area) doesn't play into it at all.
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
You can see from the statement above that you first point is complete balderdash relative aperture and F-ratio are the same thing so saying one is different yet the other the same is contradicting yourself.
I fail to understand how you can think that putting the same lens with the same registration distance can suddenly change its aperture it is just ludicrous F-stop is a fixed number based on lens physical length and diameter of entrance pupil.
I've proved it in the images above if you wish I can add the teleconverter image and you'll see the shutter speed drop and the entrance pupil shrinks.
Let me say again
F-stop is the ratio of the lens's focal length to the diameter of the entrance pupil It is a
dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
I've highlighted the important piece in this discussion,
The only thing that alters in a crop sensor is the perceived focal length due to the reduced angle of view, DoF , exposure etc all remain the same as the lens would deliver on any format camera
You could crop the identical image of the Q above from the K5 image below it and they would be identical (if lower resolution) your suggesting that once I've cropped the image the effective F-stop changes to f10 ? !!!!! Juts complete madness IMO
As for Image quality (how you think that's related to sensor size I don't know) it is a factor DR, resolution micro lens accuracy, noise floor , lens quality well depth etc etc nothing related with sensor size .
Again I think your confused by the observation larger sensors tend to deliver higher IQ at the same settings, This is because they tend to have better heat dissipation (all else being equal resolution etc) leading to lower noise floor.