query about perspective, magnification and small sensors

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Hi i am considering getting a dslr for portraiture photography and most likely a camera with aps-C sized sensor. Now i understand that this smaller than full frame sensor creates a cropping factor or magnification factor of 1.6. And there for in order to get the same field of view that you would with a 50mm lens you now need to use a 30mm or 35mm lens. however i am still a little unclear about perspective because it is my understanding that despite there being a 1.6 magnification when using any lens the lens's perspective has not actually changed so a 50mm lens still gives the same perspective and when shooting portraits you use a 50mm lens for its true to life persective not the field of view. So i do infact still want to use a 50mm lens for portraits? is this correct? thanks,
 
As I understand it, a 50mm lens on a 1.6x crop body will be similar to an 85mm on a full frame body!
 
Perspective is related to the distance between the camera, subject, foreground and background. As stated, to maintain the perspective of the scene shot, a 1.6 crop sensor will need a lens of 32mm to replicate that taken using a full frame body with a 50mm lens mounted.

The final part of the equation is the aperture setting needed. To maintain the same level of subject isolation, the aperture value used on a 1.6 crop sensor body will need to be 1.6x less than the full frame body.
This is the downside of small sensors if you're trying to match a full frame shot exactly....you'll need faster, more expensive lenses.

HTH

Bob
 
Hi BW, and welcome to TP!

I was going to just say what the short answer to your question is, but then I realised I wasn't quite sure! So here's the not-so-short thinking-out-loud version. Hope it makes sense.

Firstly, perspective isn't really about the lens. It's about the relationship between the different objects in your image, and how far they are away from the camera.

If you want to take a head-and-shoulders portrait using a 10mm ultra-wide-angle lens, you have to be very very close to the subject in order to frame it properly. The lens will be only a few centimetres from their nose. At this distance, other features of their face (eyes, ears, chin etc.) will be a few centimetres further away, but in proportional terms that's a lot - maybe two or three times further away. That's why you get such an exaggerated perspective, with the nose looking so large.

If you want to take a head-and-shoulders portrait with a 600mm super-telephoto lens, you need to be a long long way away from the subject in order to frame it properly - maybe 30 meters or so. At this distance, the other features of their face (eyes, ears, chin etc.) will still be those same few centimetres further away, but in proportional terms that's very little - maybe 0.1% further away. That's why you get such an flattened perspective.

But the difference between these two cases isn't the lens - it's the distance to the subject. If you had the subject 30 metres away and took the photo with the 10mm lens, they'd be very very small in the middle of the picture. But if you enlarged it a lot you'd get essentially the same effect as with the 600mm lens, because their nose would only be 0.1% closer to the camera rather than half the distance.

OK so far?

So ... here's the short answer. (And it's not the one I was expecting!) If you use a 50mm lens on a full-frame camera for a portrait, then you should use a 30mm lens on a camera with an APS-C sensor. Why? Because the field of view is the same, and therefore the distance to the subject is the same in order to achieve the same composition, and it's the distance to the subject which determines whether or not the perspective is flattering.

(For what it's worth, I think a slightly longer lens is better anyway. I'd prefer 85-135mm on a full-frame camera, and therefore 50-80mm on a 1.6x crop sensor. But that's just a matter of taste.)


If anybody else is reading this, please feel free to comment. As you can see, I'm not 100% sure of the ground on which I'm standing. (But don't just tell me I'm wrong - tell me why I'm wrong!)



EDIT: Canon Bob hadn't posted when I started my reply ... I was distracted by something in the middle of writing it!
 
I thought one of the primary reasons for using an 85-110mm lens for portraiture was that the way the lens works flatters the subject as opposed to a shorter or longer focal length that can be less flatering. In which case you should still use an 85mm even on a crop body.

I could however be talking total tosh.
 
a1ex2001 said:
I thought one of the primary reasons for using an 85-110mm lens for portraiture was that the way the lens works flatters the subject as opposed to a shorter or longer focal length that can be less flatering. In which case you should still use an 85mm even on a crop body.

I could however be talking total tosh.
Click to expand...
I think you could be talking total tosh. ;)

What is it about "the way the lens works" that flatters the subject, if it's not the distance to the subject?
 
StewartR said:
....If anybody else is reading this, please feel free to comment. As you can see, I'm not 100% sure of the ground on which I'm standing.....
Click to expand...

Your appraisal matches my understanding exactly but I think aperture needs to come into the equation if we're talking about replicating the same result....DOF and bokeh play a large part in portaiture.

Bob
 
StewartR said:
I think you could be talking total tosh. ;)

What is it about "the way the lens works" that flatters the subject, if it's not the distance to the subject?
Click to expand...

You could turn that on it's head and say the distance is a result of the focal length :)

I think the correct answer is to stand 1.6x further away and then the results would be the same. If you're now thinking what about DoF well don't forget the CoC is also different by 1.6x so it balances out.
 
StewartR said:
I think you could be talking total tosh. ;)

What is it about "the way the lens works" that flatters the subject, if it's not the distance to the subject?
Click to expand...

I know it sounds a bit odd but I'm sure I read it in a magazine, something to do with people and features looking nicer in that focal length. I'm sure it said that wide angle could exagerate features like pointy noses of plumpness where as the 85mm to 110mm range made people look slimmer and features less exagerated. Again could be total tosh.

This has to be one of the worst things to come out of affordable DSLR's the crop sensor debate must take up too or three threads a day and every time I think I've got it nailed someone comes up with a slightly different question.
 
The OP was right in his assumption - while perspective is to a large degree influenced by distance to subject, the lens used will also alter the elements in a scene.

Shooting a wider angle focal length on a crop body will not give you the same image as it would using the equivalent focal length on a FF body. The difference is small but noticeable.

For portraits for example, a 35mm lens on a crop body will not give as flattering a photo as 50mm lens on a FF body. With both the above cameras, distance to subject would be identical(ish) to retain the same field of view.

The 35 lens however would accentuate the distance between foreground and background elements in the scene - the nose would be slightly larger in ratio to the ears for example. This is how wideangle lenses work.
 
WeddingHack said:
For portraits for example, a 35mm lens on a crop body will not give as flattering a photo as 50mm lens on a FF body. With both the above cameras, distance to subject would be identical(ish) to retain the same field of view.

The 35 lens however would accentuate the distance between foreground and background elements in the scene - the nose would be slightly larger in ratio to the ears for example. This is how wideangle lenses work.
Click to expand...
Surely that is not the case, though?

If the distance to the subject is the same in the two cases, then the relative distances of the nose and ears would be the same. So how can one lens "accentuate the distance"?
 
StewartR said:
Surely that is not the case, though?

If the distance to the subject is the same in the two cases, then the relative distances of the nose and ears would be the same. So how can one lens "accentuate the distance"?
Click to expand...

Stewart R is right. The only factor that affects apparent perspective is the distance from lens to subject, or more properly from the lens to the various parts of the subject.

Just like life really, perspective depends on viewpoint...
 
Doing a bit more research, Stewart is right, I'm wrong :bonk:
 
StewartR said:
Surely that is not the case, though?

If the distance to the subject is the same in the two cases, then the relative distances of the nose and ears would be the same. So how can one lens "accentuate the distance"?
Click to expand...

That'll explain my misunderstanding and the common asumption about 85mm being more flatering. I assumed it's a product of the lens and it just appears to be as by it's very nature the longer lens forces you to be further away.
 
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