UWA lenses Full frame vs Crop

[GPT]

As we all know the focal length of a lens doesn't change whether it is fitted to a crop sensor camera or a full frame camera (a 50mm is still a 50mm), what changes is the angle of view. So on a crop sensor camera (assume 1.5x) a 50mm lens gives an angle of view equivalent to 75mm lens fitted to a full frame camera.

As we also know, focal length affects the perspective in a photograph, longer focal lengths compressing perspective and shorter exaggerating it.

My question is specifically around ultra-wide lenses where the effect is most obvious.

I have a Nikon DX camera and the 10-24mm lens. At 10mm this gives the same angle of view as a FX camera with a lens at 15mm. My questions is does anyone own both systems and lenses capable of those (or similar) focal lengths. I would love to see a side by side comparison of the same scene from both the DX and FX systems to compare the affect focal length has on two pictures with the same field of view.

Most people talk of lenses on crop cameras and their 35mm equivalence (in terms of field of view) but no-one seems to mention the effect of the change in focal length.

 

[StewartR]

As we also know, focal length affects the perspective in a photograph, longer focal lengths compressing perspective and shorter exaggerating it.

Not quite.

Focal length does not affect the perspective. What DOES affect the perspective is the relative placement of subjects at different distances. It tends to be the case that one chooses different subject placements with different lenses, but that's a compositional choice, not a property of the lens.

Thought experiment: Take a photo with a 500mm lens. Without moving the camera, switch to a 10mm lens and take a photo. Then zoom in to the second photo until it's the same size as the first photo. The result is that the two photos will look the same. The perspective hasn't changed.

So what is your question, exactly?

 

[StewartR]

I would love to see a side by side comparison of the same scene from both the DX and FX systems to compare the affect focal length has on two pictures with the same field of view.

It has no effect.

 

[GPT]

Interesting.

So using a 50mm on a 1.5x crop camera to take a portrait from 12 feet away would give exactly the same result (forget DOF for a minute) as using a 75mm on a full frame camera from 12 feet away.

The difference comes about when using a 50mm on both cameras but standing further away with the crop camera to give the same FOV.

 

[Nod]

I thought I had a set of photos to demonstrate just these points (12mm on crop and 18mm on FF as well as 12mm on both) but I'm afraid I can't find them. If I come across them, I'll try to remember to post them.

 

[StewartR]

So using a 50mm on a 1.5x crop camera to take a portrait from 12 feet away would give exactly the same result (forget DOF for a minute) as using a 75mm on a full frame camera from 12 feet away.

The difference comes about when using a 50mm on both cameras but standing further away with the crop camera to give the same FOV.

Exactly so. If you use a 50mm on both cameras, and you want the subject to be the same size in the frame with both cameras, you'll want to stand 50% further away from your subject with the DX camera than you would with the FX camera. But although you'll be 50% further away from the subject, you won't be 50% further away from the background, so the background will appear larger in your image relative to the subject.

 

[woof woof]

I sold my APS-C camera but I have a 5D (FF) and a G1 (MFT, x2 crop.) As the G1 is x2 the math is easier

I've just taken these two shots out of my window. 5D, 50mm f5. G1 25mm f2.8.

They're pretty much the same picture IMVHO and you'd have to look pretty closely to see any real difference, which would be mostly down to the lenses anyway.

Here they are...





PS. Looks like my finger slipped so the 5D shot is actually at f4.5.

Just for fun, this is the G1 fitted with a 50mm lens at f2.8. I had to move back maybe 4 ft or so to get the dragon and car more or less the same size in the frame.

 

[GPT]

Thanks all (especially the sample shots), this is all very interesting to me and adds to my understanding of photography. Still much to learn.

Cheers.