Lens: full frame vs crop

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Polaroid22

Well-known member
Hello,

I have a question if you shoot with a 50mm on a GH3(2X crop) does the lens behave like a tele lens then? Squesh the background togheter as there is less space between objects(tele effect), or does it just crop the image? And the lens still behaves like a 50mm, but the actual frame has just been cropped 2 times?

Cheers
 
Both: it just crops the image, and this makes it squesh the background together... because you back up and go farther away from your subject, and this changes perspective, which is what causes that compression you're talking about (i.e.: it's not the lens or sensor you use, but how far away from your subject you take the camera).

For FoV, perspective and DoF, think of a 50mm f/1.4 on m43 as a 100mm f/2.8 on FF (for exposure, it's still f/1.4).

More here
 
A 50mm is a 50mm, is a 50mm...

The lens will not change the image it projects onto the sensor; bokeh will remain exactly the same for a given camera to subject distance; all that changes is the field of view.
 
true, but the key here is "for a given camera to subject distance"
that's not constant
if you shoot a portrait (head and shoulders) with a 50mm on FF, you're at about 1.2m from your subject; same portrait with same 50mm on m43, you're at about 2.4m from your subject; now perspective changes, and you get the compression of a 100mm lens on FF shooting that same shot

If you want to shoot that portrait without moving the camera around, you'll put your tripod 2.4m from your subject, and use a 50mm on m43 or a 100mm on FF (and you'll get the same DoF if you set the 50mm at f/1.4 and the 100mm at f/2.8)
 
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Polaroid22 said:
So to wrap things up, my 50mm on my GH3 will behave like a 100mm on a 5D mark III?

Cheers
Click to expand...

I would strongly recommend getting a tool that just gives the important data directly. The 'important' data is 'Field of View' or 'Angle of View'. I have 2 iPhone apps that I use regularly to calculate such questions as ... if a 6 foot tall man is standing 10 feet from my camera, will my 25mm lens on my GH-1 cut off his feet or head...

Alternatively, if I'm taking pictures of surfers out in the ocean, what length lens do I need to have the surfer, on their board, slightly crouched, and have them fill the frame.

The apps also give DoF, etc, for specified subject/camera distances and f-stop...

Using 'crop' factors is sort of like going to a foreign country, and always having convert the 'local' currency into US dollars, then back again... rather than noting it takes 2 euros for a cup of coffee... and 100 euros to stay in a 'nice' hotel... and I only have 102 euros in my pocket... so I'm probably headed for a 60 euro pensione...
 
So, perfect moment to plug my DoF calculator, then :)
It's web-based but it works on smartphones too, just save a bookmark (once you use it, the url contains all the data, so you can bookmark different versions for different cameras, lenses, etc)

Full version:
you give it your crop factor, lens, aperture, and distance to subject, and it will give you picture height and width at focus distance, and DoF

Simplified version (my preferred option):
you give it your crop factor, aperture and desired image height, and it gives you the DoF, and all the lens/distance combinations that will fit the bill

(interesting thing, focal length doesn't affect DoF, if you keep image height constant; the usual belief that longer lenses give you shallower DoF comes from the fact that, once you fall out of the DoF, things get blurry much faster on longer lenses; but the DoF limits are basically the same)
 
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