Focal lengths!

dM-Photography

dM-Photography

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David
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Hi there,

Been studying photography for a year now and heading onto my HNC in august. Doing great and producing some great images but one thing that's always confused me are focal lengths when it comes to full frame or cropped sensors.

For example, when I buy an EF-S lens for canon and say its 50mm prime. Does this mean its 96mm? but EF-S lenses are made for cropped bodies are they not? so why would they not advertise the true focal length?

To clear things up, could you tell me the true focal lengths for my lenses which are listed below, I have a canon 1100d 1.6 crop:

EF-S 18-135mm
EF 50mm 1.8 mkII
Sigma 10-20mm

Thing that confuses me is that the sigma will not work on a full frame, so I take it the true focal length is 10-20mm?

I know that the 50mm prime is an EF model made for full frame so I know this is round about 96mm, but what happens with EF-S lenses......

Thanks for any info and clearing my confused head :)
Cheers
David
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Don't get bogged down with EF or EF-S 'fitting'

In terms of focal length they're the same.
 
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The true focal length of each lens is exactly what is printed on the lens.

So the 10-20mm lens has a focal length of 10-20mm, the 18-55mm is exactly that too - 18mm to 55mm. It doesn't matter what size sensor the lens is designed to be used with, the focal length printed on the lens is the focal length of the lens.

If you stop thinking about focal length and start thinking about field of view, things get easier.

Take the 50mm lens. On your 1100D with a 1.6 crop sensor this lens will have the same field of view as an an 80mm lens would on a full frame camera. It is still a 50mm lens, that never changes.
 
If a lens says it is 50mm, it is 50mm.

It is the body that changes the effective focal range.

50mm on a full frame is 50mm
50mm on a crop sensor (1.6) is 80mm

all 50mm lenses are 50mm lenses :)
 
Thanks for clearing that up. So when people say 100mm is a good portrait lens, i should buy a 100mm? i was under the impression that my 50mm prime was great as its equivilent is 96mm.

I think the whole perseption of this cropped stuff confuses alot of people, for example "i want a really long telephoto lens, hmm ill just get a 400mm and use it on a cropped body for more magnification" but from what im reading, 400mm is 400mm..
 
This got me confused for ages and yes the focal lengths are the same but what the camera takes is not the same, or at least if you printed the images out the crop sensor would print a cropped Image of what the full frame takes...well that's how I found it easier to understand after a lot of head scratching.
Now the bit that confuses me is if you have to crop the image on a full frame would it be the same quality as a non cropped image on a crop sensor camera! :thinking:
 
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No a 50mm on a 1.6 is similar to a 80mm on a full frame when you frame the subject at the same physical distance away
 
The point to member is the word crop.

Take a photo that's displayed on your screen, crop it and display it on the screen again and it's like you've zoomed in.

The crop ca,era does just that.
 
A lens is always the same, full frame or DC 400mm is 400mm

Take a photo with a 50mm lens on a DX and a 50mm lens on a FF and it will look different.

The FF will be at 50mm and the DX will be at 80mm.

I use a DX and just think in terms of lens values, I don't care that my 200mm is actually 320mm, I see it as 200mm.

If I use a 500mm lens on a crop sensor, it will appear closer than FF, but if I used a FF I could always crop down to the same image.
 
so the only difference is how much arround the edges is seen? distortion on lenses will always be the same

Thata what im wondering aswell martyn hehe, for example a 12mp camera is a 12 mp camera no matter what, so you would think the cropped image would be better quality as its more pixels into a smaller space, where the pixels are spread out more on the full frame, then croppes out. hmm hehe
 
Byker28i said:
The point to member is the word crop.

Take a photo that's displayed on your screen, crop it and display it on the screen again and it's like you've zoomed in.

The crop ca,era does just that.
Click to expand...

This has totally cleared it up for me.

Focal length is always the same.
Distortion is always the same.
Field of view is the thing that changes.

Correct? :D
 
dM-Photography said:
so the only difference is how much arround the edges is seen?
Click to expand...

That's the easiest way to describe it yes.

With a crop sensor you get the shot of the bird for example.

With a full frame sensor you get the shot of the bird plus more around the outside.

Put those two images together (at the same size) and the bird will appear further away on the full frame shot.

You can obviously crop images afterwards but then you have to start thinking about the resolution of the image.
 
dM-Photography said:
so the only difference is how much arround the edges is seen? distortion on lenses will always be the same
Click to expand...

Where did "distortion" come from?

If you mean the nasty stuff towards the edges of the frame... Sometimes lenses aren't so good as you get towards the edges and if that's the case they may perform better on a smaller format as you simply don't see the crappy bits towards the edges that would be visible if the lens was used on a larger format camera. Note though that if you used a lens with crappy edges on a larger format camera cropping the edges out would achieve the same result as using a smaller format in that the crappy edges wouldn't be seen and the lens looks to be a good-un again :D

If you mean perspective distortion... That's a product of camera to subject distance and if you use the same lens on different formats as the field of view alters with the larger formats you may move closer to keep the subject the same size in the frame and if you do that you'll see more perspective distortion as you move your 50mm from MFT to APS-C to FF.
 
As more of the central area of the lens is used with a crop camera this does reduce the softness/distortion closer to the edge that could be seen if using a full frame body. However the downside is that more pixels are 'seeing' through this central area, so the lens quality at the centre may need to be very high to get equivalent results.
 
stu.artd said:
As more of the central area of the lens is used with a crop camera this does reduce the softness/distortion closer to the edge that could be seen if using a full frame body. However the downside is that more pixels are 'seeing' through this central area, so the lens quality at the centre may need to be very high to get equivalent results.
Click to expand...

But for many peope who only produce images for web, screen and reasonably sized prints (maybe whole images up to and including A3) this may not be an issue. Personally I've found that even quite humble lenses can produce more than acceptable results if you'd don't print the size of a barn or pixel peep at 200%+ :D
 
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