Crop Factor. Something to consider when buying a lens when you are shooting other than FF.

Started Jun 6, 2014 | Discussions thread
Roadrunner123 Contributing Member • Posts: 548
Re: you seem to be thoroughly confused
4

Interesting video from Tony Northrup.  I hope I have this correct, please correct me if I am wrong.  While what he says is true, not everyone buys faster lenses for the dof bokeh, this seems to be his sole consideration.  I want a faster lens to allow me to shoot at lower ISO and faster shutter speed, the dof/bokeh isn't really number one on my thoughts.  Then again I'm not a big portrait shooter.  I suppose it depends on your type of shooting.  Tony completely ignores this.  If manufacturers did what he suggests I would have to do the maths to work out the 'real' speed of the lens to see if it is suitable for me.  To be honest I think his video adds to the confusion.

forpetessake wrote:

dtssounds wrote:

When you shoot full frame with a prime 35mm lens at f1.4, you will get an image with a depth of field relevant to the focal length and aperture.

ok.

You then bring the image into photoshop and make a crop of the image at the size and dimension of an APS sensor. The final image will have a similar angle of view of a 50mm lens.

52.5mm f/2.1 to be precise.

The depth of field, however, will stay the same.

No it won't. After you cropped the image, the circle of confusion changed correspondingly. You do want the same size print, don't you?

Now, when you attach the same lens to an APS sensor camera, the same cropping happens,

Yes.

only this time it happens physically on the sensor itself,

it's no different from the above

thus a 35mm lens will give somewhere around a 50mm angle of view.

52.5mm f/2.1 to be precise.

However, the depth of field is still based on an actual, physical 35mm lens instead of an actual, physical 50mm one.

Again, you are confused. The DOF will be the same as 52.5mm f/2.1 on FF (the same as 35mm/1.4 for 1.5x smaller circle of confusion).

I have to skip the rest, sorry, don't have the patience to parse all that. And the matter is actually very simple. When you want to know what equivalent lenses are for two different formats, i.e. you want to get identical pictures: same DOF, same FOV, same total light, (same noise, same dynamic range on ideal sensor) -- you need to multiply the focal lens and f-stop by the crop factor. And it doesn't matter how you crop. Very simple, really. As the Tony Northrupvideo said: third grade algebra

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