Crop Factors

You walk into a shop and

the sales staff start telling you that because the crop factor of 1.5 the 200mm lens you are about to purchase is the same as a 300mm lens sounds to good to be true and it is. You will in fact see an area of the same as a 300mm lens but not zoomed into the area with as much detail ill try to show this in more detail in a moment. At the end of the day regardless of the crop factor a 300mm lens will give better results than a 200mm lens you cant get away from that fact, the crop factor crops an area to the same size as a full frame masking out external detail.

Crop factor is based on the old style 35mm film camera this is the industry standard reference point and is often referred to as full frame

If you compare the size of the film in a normal SLR (film is 35mm) to the image sensor in most DSLRs you’ll find that the size of the DSLRs sensor is generally smaller (unless you get what’s called a ‘full frame’ DSLR). Until recently full frame sensors were far and few between but more models supporting full frame sensors are becoming more affordable and are not limited to only the professional photographer.

If you take a photo with a smaller sensor and the same lens it will only show a smaller area of the scene.

To illustrate this I’ve show how different cameras with different image sizes will see an image.

Black – Full Frame
 Yellow– 1.3x Crop Factor
– Red 1.5x Crop Factor 
Green – 1.6x Crop Factor

When you enlarge images to the same size from different sensors the ones with the smaller sensors will be enlarged more – making it seem bigger.

As a result – when you fit a lens to a camera with a smaller sensor the lens is often said to have a larger equivalent lens size. But remember the truth is you are loosing detail through your lens compared with a full frame sensor

The table below that shows the equivalent lens sizes for different crop factors. The column on the left is the lens focal length on a full frame camera.

Full Frame	1.3 x	1.5 x	1.6 x	2.0 x
12mm	15.6	18	19.2	24
24mm	31.2	36	38.4	48
50mm	65	75	80	100
70mm	91	105	112	140
100mm	130	150	160	200
200mm	260	300	320	400
300mm	390	450	480	600
400mm	520	600	640	800
600mm	780	900	960	1200
800mm	1040	1200	1280	1600