If you are in the market for a camera and if you have been searching forums you would have come across the long discussion on Sensor Sizes the references to APS-C, Full Frame crop factor etc. If you don't know what I am talking about you can head over to forums at dpreview you will find what I am talking about :)
Since this is a question I am asked often by people who thinking about purchasing a DSLR I thought I will simplify my life a bit by talking about crop factor, but first a brief history. Sometimes the answers to the present lie in the past. Back in the day there were many sizes of film but it was the 35 mm format which was portable along with Leica cameras and SLR's which made photography popular really popular.
Since the film size was 36×24 mm and all SLR's which accepted this film sizes made lenses corresponding to this size. All was well in the world of photography till digital came along. With digital we got DSLR's with sensors which were smaller than the 35 mm format . There are some advantages to it. Such cameras are smaller than the full frame DSLR. Now if you look at the sensor sizes diagram below you will see the various smaller sensor sizes
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| Various Sensor Sizes - Image Credit: Wikipedia |
Now when the sensor became smaller the camera manufactures figured that they could make lenses which were smaller than the full frame sensor. The reason was simple the light which came from a full frame sensor covered a full frame sensor. Now that the sensor was smaller there was light information which was not getting captured -- a smaller lens solved the problem so the light information fit exactly on the sensor.
This is where things start getting a bit confusing. All our reference points so far have been 35 mm equivalent. So a 50 mm on a FF (Full Frame) is normal lens any thing above this is telephoto lens and anything less than this is wide-angle lens. However, when the lens was made smaller it made a direct impact on focal length. Depending on how such smaller the sensor was from 35 mm format know as Crop Factor (CF) a different focal length was a normal lens. So if the crop factor was 1.5 then a 35 mm lens would approximately give a field of view of a 50 mm on a FF. (35*1.5 = 52.5 mm) So if you had a crop body in your hand and your friend had FF body then both of you would almost the same field of view. Now if both of you had a the same focal length then both of you will have a different field of view something like what you see below.
Now if your friend who had a FF camera wanted to get the same field of view as you then she would have to step in closer. The moment you step in closer you impact depth of field(DOF). DOF is factor of distance from subject, the closer you step in the shallower the DOF.
Now if you put a FF lens on a crop body it has impact on DOF as not the lens no longer acts as the focal length it is suppose to. If you have a point and shoot you would have the same scenario. If you had a point and shoot and your friend had a FF camera and if you two stood at the same place then your friend would have the shallow DOF compared to you to get the same field of view with the same focal length In other words you can never achieve the shallow of FF on a crop body of point and shoot.
As I have already stated the FF lens gives you a different field of view when put on a crop body, it acts like a magnifier. Now the only place this starts to make a difference is when you start to purchase lenses. If in the long rung you decide to purchase a FF camera then purchasing FF glass is better, However, if you intend to stay on the crop sensor only then purchasing crop lenses is better. One advantage going with FF apart from DOF is that there are distance markers on the lens which help in case you are manually focusing. Crop body on the hand are smaller and much more portable. At the end of the day this is just technical stuff. What matters most is your vision and what you want to do. If are a portrait photographer then a FF or medium format may make more sense. If you are a street photographer then something smaller and more discreet is better, even range finders would be good in such a case like the one from Fuji Film or Leica.
Here are some links you might want to read up on to understand more on what happens due to crop factor. Happy clicking
http://trevorsowersphotography.blogspot.in/2011/03/distance-to-subject-and-its-impact-on.html
http://www.photozone.de/focal-length-and-perspective#
