Focal Length/Sensor Size

[londonsnapper]

Hi all,

What's a standard focal length on a APS-C and Micro Four Thirds sensor? I know that it's around 30mm and 25mm respectively, but isn't that a completely different focal length that will change the depth perception rather than just compensate for the restricted field of view?

 

[HoppyUK]

Hi all,

What's a standard focal length on a APS-C and Micro Four Thirds sensor? I know that it's around 30mm and 25mm respectively, but isn't that a completely different focal length that will change the depth perception rather than just compensate for the restricted field of view?

To get equivalent field of view, multiply by crop factor. Eg, 50mm lens on full frame, same as 33mm on APS-C 1.5x, and 25mm on Four Thirds 2x.

If you also want to restore equivalent depth of field, multiply f/number by crop factor also. Eg, f/4 on FF is f/2.7 on APS-C and f/2 on 4/3rds.

 

[gad-westy]

You are completely right and it's where a lot of people seem to confuse this issue (you hear phrases like a 50mm becomes a 75mm on a crop camera when of course it doesn't). The focal length determines perspective (or depth perception if I've read your post right) and of course it doesn't change no matter what size sensor the lens is pointed at. The only thing that changes is 'field of view', i.e. what appears in shot or maybe more accurately what is chopped out.

 

[HoppyUK]

You are completely right and it's where a lot of people seem to confuse this issue (you hear phrases like a 50mm becomes a 75mm on a crop camera when of course it doesn't). The focal length determines perspective (or depth perception if I've read your post right) and of course it doesn't change no matter what size sensor the lens is pointed at. The only thing that changes is 'field of view', i.e. what appears in shot or maybe more accurately what is chopped out.

Focal length does not affect perspective. Distance to subject changes perspective.

 

[londonsnapper]

You are completely right and it's where a lot of people seem to confuse this issue (you hear phrases like a 50mm becomes a 75mm on a crop camera when of course it doesn't). The focal length determines perspective (or depth perception if I've read your post right) and of course it doesn't change no matter what size sensor the lens is pointed at. The only thing that changes is 'field of view', i.e. what appears in shot or maybe more accurately what is chopped out.

Thanks. That's what I thought. So, does this mean that you can't achieve anywhere near the same classic 50mm 'standard' look on a cropped sensor?

 

[londonsnapper]

Focal length does not affect perspective. Distance to subject changes perspective.

But doesn't a telephoto focal length compress the perspective while a wider focal length elongates it? Surely that must mean that focal length effects perspective?

 

[HoppyUK]

Thanks. That's what I thought. So, does this mean that you can't achieve anywhere near the same classic 50mm 'standard' look on a cropped sensor?

If you want to replicate the look of a classic 50mm lens on full frame, then just multiply both focal length and f/number by the crop factor. Images will be identical in terms of framing, depth of field, and perspective.

But doesn't a telephoto focal length compress the perspective while a wider focal length elongates it? Surely that must mean that focal length effects perspective?

No, you get compressed perspective with a telephoto because they're generally used at greater distance. Easy test, shoot a distant subject with a long lens and a short focal length, then enlarge the shorter focal length image so it matches the framing of the long lens. Perspective will be identical.

 

[londonsnapper]

If you want to replicate the look of a classic 50mm lens on full frame, then just multiply both focal length and f/number by the crop factor. Images will be identical in terms of framing, depth of field, and perspective.

No, you get compressed perspective with a telephoto because they're generally used at greater distance. Easy test, shoot a distant subject with a long lens and a short focal length, then enlarge the shorter focal length image so it matches the framing of the long lens. Perspective will be identical.

So, is it true to say that a telephoto lens makes things seem flatter, or foreshortened, simply because it offers a narrower field of view?

 

[HoppyUK]

So, is it true to say that a telephoto lens makes things seem flatter, or foreshortened, simply because it offers a narrower field of view?

No. Perspective is a function of distance, nothing else. Try the test I suggested above.

 

[Nod]

Hoppy, as a Canon man (you, not me, god forbid!!!), I'm surprised you used 1.5x as the APS-C crop factor (Nikon's sensor size) rather than 1.6x (Canon's)!

As a general rule, a "standard" lens is generally reckoned to be the diagonal dimension of the recording medium. That makes the standard FL for 35mm 43.2666mm, although it's more usual to regard a 50mm as a standard lens on 35mm (and hence FF digital) bodies. If my rough arithmetic is approximately correct, that makes the standard length (well, closest commonly available prime) for a 1.5x crop body 30mm. That's the maths but it's generally safe to ignore that and to use whatever focal length you like.

 

[woof woof]

Just out of interest I pointed cameras and lenses at the house over the field with a Chinese dragon in the foreground, trying to keep it the same size in the frame but with the focus on the house. For the 135mm shot I had to move back, maybe 6 feet or so. The 5D+50mm and G1+28mm look essentially the same with only sight differences due to not getting the framing exactly right etc.

5D+50mm.

G1+28mm.

G1+55mm.

G1+135mm.

 

[gad-westy]

Alan, the first two shots with the G1, the 28mm and 55mm, Was the camera in the same position for both?

 

[woof woof]

Yes, well... near enough... I just sat on the corner of the bed and shot out of the window. The shots aren't cropped, they're full image shots, so there can only be a matter of inches in it as a result swaying back and trying to get the dragon the same size in the frame. I only got up and moved for the 135mm shot.

 

[gad-westy]

Cheers. That's interesting because there is clearly a lot of 'telephoto compression' between the two but similar framing which sort of goes against what Hoppy was saying earlier but the comparison between 5d 50mm and G1 28mm demonstrates exactly what Hoppy was saying. I'm feeling a bit enlightened by this thread but I think I'll do my own experiment to verify.

 

[Nod]

No, you get compressed perspective with a telephoto because they're generally used at greater distance. Easy test, shoot a distant subject with a long lens and a short focal length **, then enlarge the shorter focal length image so it matches the framing of the long lens. Perspective will be identical.

I'm guessing that this is the post from Hoppy you're referring to?

What Hoppy seems to have left out is "from the same viewpoint" where I have added **

As woof woof said, he moved back for the 135mm shot to keep the dragon the same size in the frame; it's that change in position that has changed the perspective. Even swaying back and forth can involve a foot or 2 of movement - enough to change perspective.

 

[HoppyUK]

Cheers. That's interesting because there is clearly a lot of 'telephoto compression' between the two but similar framing which sort of goes against what Hoppy was saying earlier but the comparison between 5d 50mm and G1 28mm demonstrates exactly what Hoppy was saying. I'm feeling a bit enlightened by this thread but I think I'll do my own experiment to verify.

Yes do your own tests, very easy.

You need something with a clear near subject and background, maybe a parked car shooting down the street. Start with the telephoto lens and frame it up so the car is a certain size, maybe with the front filling exactly half the frame. Then without moving, swap the lens to something wider and take the same shot, and enlarge the wider shot to match the framing. Perspective will be identical.

Now fit the wider lens, and move forward so the car fills the frame exactly like the first shot, and you'll see a dramatic change in perspective against the background.

The point is, and it's not even contentious, is that distance changes perspective and not focal length. But it's also true to say that, almost always, longer lenses deliver compressed perspective, and wide-angles more exagerated perspective, because almost by definition you shoot closer with wide-angles and from further away with telephotos. It's just important to know that the perspective changes are not an inherant characteristic of the focal length.

 

[woof woof]

I'm guessing that this is the post from Hoppy you're referring to?

What Hoppy seems to have left out is "from the same viewpoint" where I have added **

As woof woof said, he moved back for the 135mm shot to keep the dragon the same size in the frame; it's that change in position that has changed the perspective. Even swaying back and forth can involve a foot or 2 of movement - enough to change perspective.

Well, no, not quite...

I only moved back about six foot or so to get the dragon to be about the same size in the frame and that six feet isn't going to make much difference to the size of the house in the frame. The tele "compression" and the "look" that you get with longer lenses just can not be duplicated by cropping a wider angle shot if the composition isn't friendly, like here with the dragon and the house, when there's something in the foreground.

Here's an example with a friendly composition. Here I took two shots from within feet of each other, one wide angle and one short tele. As you can see the whole image shots look quite different but the crops look very similar. In this case cropping a wide shot creates the same image as the long shot but if there's something in the frame relatively close to the camera there's no way you can create an identical shot by cropping, as you can see with the G1 28, 55 and 135mm shots. You'd have to selectively increase the size of some things in the frame relative to others.

Wide shot...

85mm shot...

Crop, wide and then 85mm...

Here's the same thing at Luminous Landscape. The piece is about DoF but ignore that and just look at the size of the puppet and the tower, it's the same thing as with my dragon and house...

http://www.luminous-landscape.com/tutorials/dof2.shtml

With shots like this you can't make 28mm look like 135mm, not just by cropping and not without some serious magic in Photoshop.

As has been said, the best way to see this is to try it for yourself

 

[londonsnapper]

Thanks everyone for the info and experiments. so is it true to say the following:

'Telephoto lenses seemingly compress the sense of distance, as they offer a much narrower field of view'.

 

[HoppyUK]

Thanks everyone for the info and experiments. so is it true to say the following:

'Telephoto lenses seemingly compress the sense of distance, as they offer a much narrower field of view'.

Not quite.

"Perspective is the relative size of objects at different distances as they are rendered in the image. It is the camera-to-subject distance that defines perspective. Longer focal length lenses tend to deliver compressed perspective, because they are generally used to photograph more distant subjects. By the same token, wide-angles tend to show exagerated perspective, because they are often used closer to the subject."