Yep sorry, factor of 4 out but the ratio (and hence the increased counts) is the same. Changing the magnification (ie focal length) changes the counts per pixel (both star and sky background counts) but makes no different to the counts in the aperture, either the star counts or the sky background counts. The only difference is if you spread the light over more pixelsthan you need to, you increase the camera noise contribution. In practise this is only the thermal noise, not the read noise as you can compensate for this (in CCD cameras) by binning pixels. Thermal noise is very low in modern CCD, particularly at typical exposure times used for photometry so the net effect is that changing focal length does not improve photometry SNR, only increasing the aperture does this
Robin