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So a common phrase within photography is this: "You should invest in good lenses before you buy a camera."
This phrase is believed to be true and accurate in all circumstances, but I seem to disagree. I think that it is a little more complicated than first meets the eye.
So a few things need to be clarified: firstly, what are full frame and APS-C sensors? Secondly, why are good lenses bad for APS-C? And thirdly, when would it be good to use an APS-C camera with good lenses?
There are 2 main sizes of sensors in cameras (I am aware that there are lots of sensor sizes I only want to look at 2), APS-C and Full Frame.
Full Frame is a sensor the same size as 35mm film while APS-C has a 1.5 times crop (or 1.6 times if you are Canon and some other companies).
So what is the issue with the 1.5 times crop, or am I just a camera snob with an outlet? Well, yes and no to the snob part. I try to keep a more objective look at photography than some people but that is not to say I am perfect as I only shoot Nikon because I am invested in the gear and I rate the high dynamic range of the sensors they have, but that is a topic for a different month.
The 1.5 times crop makes a higher pixel density when compared to a full frame camera of the same megapixel size and thus creating more detail. On the other hand when it comes down to collecting light the lower pixel density of the full frame sensor makes for the ability of lower ISOs and less noise in your pictures.
At the end of the day its all horses for courses. I have a Nikon D750 (Full Frame) and a Nikon D7100 (APS-C, 1.5 times crop) and they have different uses.
The first thing I would like to highlight is that Full Frame lenses are not designed for APS-C sensors. Likewise, APS-C lenses are not made for Full Frame bodies. When you use a Full Frame lens on an APS-C (or any crop sensor for that matter) two things happen and only one of which is spoken about.
The one which is spoken about is that you need to multiply the focal length by the crop factor, for Nikon APS-C it is a crop factor of 1.5 and for Canon it is 1.6.
This means a 24-70mm f/2.8 lens would be advertised as becoming a 36-105mm f/2.8 lens. Which has a huge lie in it, that everyone (including myself) has fallen for at some point in their lives as photographers.
The lie is that you need to multiply the f-stop as well as the focal length so the 24-70mm f/2.8 becomes a 36-105mm f/4.2. This does not sound like a large difference but it equates to about a stop of light darker. The more important side is that if you buy an f/4 or a f/5.6 lens turns into an f/6 or an f/8.4 which is a huge difference in depth of field.
If you look between the photos in this post, the first is taken with a full frame camera at f/5.6 but the second is taken at f/5.6 on a crop sensor both taken at 300mm. Why does one have far more blur? Well the second photo actually has an f-stop of f/8, which is why a difference in blur would exist.
If you look into the maths (which I am not going into deliberately) you will see that the equation has to stay balanced, and so the f-stop has to get larger.
When would you want to use a full frame lens on a crop body? I can only think of one time, which is wildlife photography. This seems very small minded, it is more when do you need more zoom and less need for a very small depth of field. That is a choice for you to make.