Cameras these days come in all shapes and sizes and if there is one thing that makes me cringe is the 35mm equivalence argument. First off one thing to get out of the way is the fact that if you have both crop and full frame sensors then this is a valid concern as you might want to match focal legths when changing cameras for the same subject. This would apply for instance to a wedding photographer that has a full frame as a primary body and a crop sensor as a secondary. Now for the rest of us...
Here is some sensor sizes compared:
Sensor sizes. license:CC
If you are new to the field of photography you are most likely wondering why every website and store is converting a crop sensor specific lens to a 35mm equivalent. The real answer is marketing. There is no such thing as 35mm standard. In the days of film the most popular film format was 35mm. When the first digital cameras came out the sensors where all APS-C size (crop sensor). Back then this was the only format available and therefore the equivalent of a 35mm was a reference for people switching from film. A new standard emerged, namely APS-C. When technology finally was able to produce 35mm equivalent sensors (full frame) suddenly people started to get conscious about the crop factor. Why? Because of two reasons. Companies wanted to sell their more expensive full frame cameras. By telling the consumer that full frame was the standard they are actually telling you subconsciously that your smaller sensor is not up to the standard they expect. Second, the older generation of photographers using interchangeable lenses were used to 35mm film. The obvious weakness to this argument is that with the introduction of APS-C in the first digital cameras these companies created a generation of photographers accustomed to the crop sensor that never held a film camera in their lives.
So all of a sudden a snob attitude is created with people using full frame cameras looking down on APS-C sensor cameras and calling them amateurs. If you read my previous blog you'll remember that I said that when a photographer knows what he is doing then having the better equipment will produce the better photograph. Well, digital photography has evolved to a level that 5 years ago some of the APS-C cameras produced today would put to shame some full frame cameras. This is not an evolution of 20 years of technology but just of meer 5!
Besides the marketing argument, the biggest fallacy people commit is the fact that they have no idea how this affects them. If you always had an APS-C camera and your lens says 18-55mm then converting that to full frame makes absolutely no sense. Why? Because unless you are familiar with how that looks when you look through the viewfinder of a full frame camera then you are none the wiser. In fact I believe that some people will be disappointed when they realise that their long lenses won't have as much reach on a full frame sensor as it had on their crop. Then again you are gaining in the wide angle so there is a trade-off again.
Putting the argument of focal length conversion aside, the second sin people commit in my point of view is the conversion of aperture. This is as mind boggling as it comes. Some photographers claim that when you have a 50mm 1.8 lens in the crop sensor this is equivalent to 75mm f2.8. Now that we know that the focal length of the lens can be converted in cases where you want to achieve the same frame for the picture when you switch camera bodies how does this fit in? Well it doesn't. The lens itself is always an f1.8 there is no changing that. People are confusing depth of field and bokeh (background blurring) with compression. Here is the thing. If you take a picture with a full frame camera at 50mm and crop the image to a 75mm equivalent then compare that to the same picture at 50mm with a crop sensor body you will have the same blurry background. There will be no difference. The problem is that in most tests people change the focal length of of the lens in a full frame body to match what it would look like in the crop sensor. The key is in the word CROP. They are compressing instead of cropping. Finally, the most convincing argument for me is this: Your f3.5 lens is f3.5 because of the blades closing to a particular diameter no matter which sensor you put it on. The f number is a product of your lens not your sensor. Your sensor cannot alter the amount of light entering the lens only the lens can. Therefore your lens is what it says it is for aperture.
Writing all this down might seem confusing to some but the bottom line is that if you own one type of camera body or the other the conversion is pointless. The second point is that if you do have to multiply for practical purposes then doing so with the aperture is not right. Don't get bullied by sales people about equivalency. What the lens claims on its body is 100% correct and unless you switch to different sensor bodies nobody can tell you your 50 is not a 50.