Crop confused

[horse.collier]

Came across this and am a bit confused about 'crop' being used in this context.

Does it mean cropping the size by removing unwanted area around your image?

Is it to do with ratio or saving the image in a lower format

Any help welcome, but please no blinding with science.

"A 20mm f/2 and a 100mm f/2 and a 96MP full frame camera. Then by cropping I'll have:

20mm f/2 at 96MP

40mm f/4 eq. at 24MP

80mm f/8 eq. at 6MP

100mm f/2 at 96MP

200mm f/4 eq. at 24MP

400mm f/8 eq. at 6MP"

 

[swanseamale47]

Well I'm confused, mind I am tired, but that doesnt make sense to me.

 

[woof woof]

I don't know about the maths but I'm going to comment anyway but about perspective.

I think cropping is a good thing as long as you end up with enough pm's to do whatever you want to do but another thing to consider is perspective. For example if you stand in one spot and take a series of pictures and then crop them to create the effect of different focal lengths all is well and dandy and your perspective will remain the same but if you would normally take, for example, a 20mm shot from point X and a 200mm shot from point Y your perspectives will look different and can not be reproduce by cropping... unless you move, for example you stand at the point at which you'd take the 200mm shot, take the shot with the 20mm and then crop it to 200mm FoV and you'll get that 200mm position perspective from the 20mm lens.

Sorry if all that is irrelevant but I do think that it's easy to forget the position and distance we shoot from and the resulting perspective.

 

[Musicman]

Does it mean cropping the size by removing unwanted area around your image?

Essentially, yes, that is what they are on about.

If you take a full frame, 96 megapixel image using 20mm f/2 lens, then if you were to crop it to half the original size, it would have the same angle of view as if you had used a 40mm lens.

If you print both images out onto the same size of paper, then the amount of blur in out of focus areas of the cropped version will appear as if you had taken the image at an aperture of f/4 with that 40mm lens.

Then that scales doubling the amount of crop each time and is then repeated for a different focal length.

Of course your crop to 6 megapixels is not going to be able to sustain printing onto larger sizes of paper as well as the 96 megapixel version, because you will start to see the individual pixels if you look closely at the print. At that point, you only have 1/16 of the information that you started with.

 

[horse.collier]

Right. Am with you. Will try it on some pics I have taken myself. Seeing it done practically will help. May thanks.

 

[horse.collier]

I don't know about the maths but I'm going to comment anyway but about perspective.

I think cropping is a good thing as long as you end up with enough pm's to do whatever you want to do but another thing to consider is perspective. For example if you stand in one spot and take a series of pictures and then crop them to create the effect of different focal lengths all is well and dandy and your perspective will remain the same but if you would normally take, for example, a 20mm shot from point X and a 200mm shot from point Y your perspectives will look different and can not be reproduce by cropping... unless you move, for example you stand at the point at which you'd take the 200mm shot, take the shot with the 20mm and then crop it to 200mm FoV and you'll get that 200mm position perspective from the 20mm lens.

Sorry if all that is irrelevant but I do think that it's easy to forget the position and distance we shoot from and the resulting perspective.

Bit confusing but I get the gist. Will try the last part myself, hopefully the proof is in the eating

 

[woof woof]

Bit confusing but I get the gist. Will try the last part myself, hopefully the proof is in the eating

I just thought perspective was worth mentioning as I think sometimes people get confused about it or maybe even just forget about the effect it has.

Hope you get it all sorted in your mind. Good luck with it and good luck experimenting.

 

[sk66]

Came across this and am a bit confused about 'crop' being used in this context.

Does it mean cropping the size by removing unwanted area around your image?

Is it to do with ratio or saving the image in a lower format

Any help welcome, but please no blinding with science.

"A 20mm f/2 and a 100mm f/2 and a 96MP full frame camera. Then by cropping I'll have:

20mm f/2 at 96MP

40mm f/4 eq. at 24MP

80mm f/8 eq. at 6MP

100mm f/2 at 96MP

200mm f/4 eq. at 24MP

400mm f/8 eq. at 6MP"

Yes, kind of.
96 MP is 12000 x 8000 px at 20mm f/2. Cropped to 1/4 area (50% H x W) for 40mm FOV would be 6000 x 4000; 24MP (and so-on).

If you then take the cropped image and enlarge it to the same output size that is 4x the enlargement, which results in 1/4 the light density (2 stops, f/4). And if you took the picture from 2x the distance for the same composition/comparable image, you also get more DOF (f/4 equiv).

So, if you are comparing equivalent output images (same physical size/compositions) the effective aperture is also two stops less in terms of image noise (light/light density/data) and depth of field, as noted.
It does not matter if you do the cropping by changing the actual lens (to longer/slower), adding 2x teleconverters, or cropping in post... the effect is the same.

If, however, you are not comparing equivalent output images, then the f-ratio equivalence doesn't correlate. E.g. 40mm f/4 is less DOF than a 20mm f/2 from the same distance (different compositions).