Happy Moon landing anniversary!
The short of it is that a crop sensor is smaller and to get the true/equivalent focal length the listed focal length needs to be multiplied by 1.5 (Sony, Minolta, Nikon, Pentax) or 1.6 (Canon) or 2 (Olympus). a 50mm becomes a 75mm/80mm/100mm. The same lens on a Full Frame would stay the same. If you want more details then continue reading if not then check out the pictures for examples.
The Strawberry Moon captured at 300mm with a Full Frame and with a Crop sensor. Same lens, same location, both 24mp Sony sensors, taken minutes apart. ISO 400 f8 1/1600th sec shutter.
I am writing this on the night of the first Moon landing in 1969. Considering the subject that I used I felt that it was a fitting date to finish writing the post.
Back in the days of film this was not really an issue. If you had a DSLR then you generally had the same size film as the other brands, 35mm. A full frame (FF) sensor is the same size 36x24mm. Crop sensors vary a little bit from APS-H @ 28.7x19mm to APS-C 23.6x15.7mm or 22.2x14.8mm to 17.3x13mm and need to multiply the focal length by 1.3, 1.5, 1.6, or 2 respectively to get the equivalent length.
I have only until recently had APS-C cameras. I have noticed that I don’t have as much reach with my FF as I used to. The reason is because the 300mm lens is actually 300mm not acting like a 450mm lens.
Through the pictures above there is a noticeable difference.
It can be an advantage or not depending on the type of photography that you plan on doing and budget. If it is a hobby and you want to take pictures of things far away a crop sensor might be the better option. A 300mm kit lens becomes a 450mm or 480mm or 600mm depending on the manufacturer. Getting a lens with the same reach for a FF camera can easily get expensive. For example a lens from Sigma or Tamron that reaches 400mm starts around $800 dollars. A zoom lens that gets to 300mm can be gotten for less than half that price, some manufactures even have them as part of kits.
Here is the Moon at 200mm, iso 400, f8, 1/1600th second shutter same lens different cameras. The a77 is a APS-C sensor and is really 300mm. The 3rd image is from the FF at 300mm to show that they are relatively the same size on the sensor. They are different lens, all the other settings are the same though.
~Scott