My own 'body vs. glass' dilemma

Necronomicon

If it wasn't important I'd go out and get a 24-70L without a moment's hesitation.

Put my query this way - will my EF 50mm lens give me the same field of view as an EF-S lens zoomed to 50mm? My understanding was that the image would be cropped using the EF, but not on the EF-S, therefore giving me a wider field of view.

DaireQuinlan

Necronomicon wrote: »

Put my query this way - will my EF 50mm lens give me the same field of view as an EF-S lens zoomed to 50mm? .

I can't think of many more ways to say this, but yes, yes it will. The focal length of the lens is 50mm. 50mm is the focal length of the lens. 50mm = 50mm. So assuming they're mounted on bodies of similar format, they will have the same FOV.

FoxT

From a photographers point of view, the focal length of a lens is an indicator of the angle of view, or Field of view.

A telephoto lens will have an angle of view of say 10 degrees or less, a 'normal' lens has an angle of view of about 50 degrees, and a wide angle lens will have an angle of view of 80-150 degrees. (These are approximate).

For reasons unknown to me, camera lens manufacturers have always specified lens' focal lengths & not their angle of view.

Back in the 35 mm film days, everybody just got used to it. On a 35mm camera, everybody 'knew' that 40- 60mm gave 'normal' view angles ( similar to the human eye) with 24mm was wide, 100mm was long, etc.

When digital came along, one thing that happened was that a variety of sensor sizes became available in camera bodies that accepted the same lenses.

If you have a bigger image sensor, then it will always give a wider angle of view than a smaller one, all other parameters being kept equal. For 35mm film camera users transitioning to digital it was desirable to come up with a rule of thumb to explain this. Thus, the 'crop factor' was born.

(Of course, for the growing number of people who never used a 35mm film camera, the 'crop factor' issue is confusing & unhelpful. )


So, when people say that a 50mm lens on say a 7D is 'really' 85mm - this is shorthand for stating the following:


"This lens has a 50mm focal length. The angle of view that it delivers on a 1.6x crop frame sensor is the same angle of view that an 85mm lens will give on a full frame sensor. "


The focal length of a 50mm lens is ALWAYS 50mm - Doesn't matter if it is EF, EF-S, FD, or whatever. But the angle of view varies depending on sensor size.


When designing a lens, one of the factors to consider is the sensor size. All other factors being equal, it is more expensive to make a lens that performs well with larger sensors. The lens needs to be physically larger, because it needs to produce a larger, focused image. ("Image circle")

Another design factor is view angle. A lens that gives a 10 degree angle of view for a crop sensor camera will be smaller & cheaper than a lens that gives a 10 degree angle of view on a 35mm camera.

Here's an example: A 200mm lens gives a 10 degree horizontal angle of view on a full frame sensor.

On a crop frame sensor, you will need (200/1.6)mm for the same effect. This works out at about 135mm.
A 200mm f/2 lens is GBP 5 grand at Wex, while a 135mm f/2 lens is about GBP900


The point? for a given angle of view, telephoto lenses for crop sensor cameras are cheaper and/or faster aperture, and/or physically smaller.

So, Steve , your EF 50mm lens will give you the same field of view ( or view angle, as I have referred to it here) as an EF-S 17-55 set to 50mm, regardless of what camera body you use. (But see warning below)


The EF-S lens is different from an EF lens in the following 2 ways that I know of:

1 - The EF-S lens gives a smaller image circle.
2 - The EF-S lens MAY (some will, I dont know if all do) interfere mechanically with the mirror if you put it on a FF body. This is a bad, bad, thing.

Summary:

For a given camera sensor, & lens focal length the Field of view is always the same.

Lenses designed for crop sensors do not work well on FF bodies, and in some cases may lead to damage.

Lenses designed for FF sensors can be used on crop sensor bodies ( At least on Canon)

Hope that helps!

-FoxT

FoxT

...always the same length, but your field of view will vary depending on the size of your partner's body!

Necronomicon

FoxT wrote:

So, when people say that a 50mm lens on say a 7D is 'really' 85mm - this is shorthand for stating the following:


"This lens has a 50mm focal length. The angle of view that it delivers on a 1.6x crop frame sensor is the same angle of view that an 85mm lens will give on a full frame sensor. "


The focal length of a 50mm lens is ALWAYS 50mm - Doesn't matter if it is EF, EF-S, FD, or whatever. But the angle of view varies depending on sensor size.

Cheers. I think what I was basically imagining was that such a thing as 'cropped lenses' existed, and that's what EF-S were. So using your explanation above, my assumption was:

"This EF-S lens has a 50mm focal length. The angle of view that it delivers on a 1.6x crop frame sensor is the same angle of view that an EF 50mm lens will give on a full frame sensor. "

This was obviously quite incorrect.

Necronomicon

Thanks to everyone for the advice (and for clarifying my incorrect assumptions about EF-S mounts). I just ordered the 17-55 2.8 IS, €826 delivered. Unfortunately the lens hood doesn't come with, so I'll have to think about ordering that soon. There was a definite draw towards owning my first 'L', but the EF-S just makes more sense. Looking forward to posting my first shots.