MegaSack DRAW - This year's winner is user - rgwb
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So I've been reading a few of the photo threads, and have gathered that the coverage for a given focal length varies with sensor size. But I'm not sure if I have my head around how this works in practice.
I suppose the key to the whole thing is understanding what, say, 50mm really means in a lens, anyway?
Assuming that the answer to that will make comparative zoom calculations across sensor sizes straightforward, and will also answer the question of whether, ignoring the sensor size issue, the 5.8mm lens on my compact equivalent to a 5.8mm lens on any other camera.
I'm trying to figure out what focal length I'd need on a Sony A55 to reproduce the same visible area in the final picture as my Canon PowerShot SD1000 with a 5.8mm lens, just to have a reference point.
I suppose I could go visit a photography site, or perhaps even a shop. But given molgrip's thread about boredom...
[url= http://en.wikipedia.org/wiki/Focal_length ]Focal length[/url]
[url= http://www.dpreview.com/products/canon/compacts/canon_sd1000 ]According to this [/url] the SD1000 has a lens that's equivalent to a 35-105mm zoom lens on a full frame camera.
The a55 has an aps-c sensor which is 2/3 the width of a full frame sensor, which is usually expressed the other way up i.e. 3/2 or 1.5x crop.
So... the equivalent of 35-105mm on the a55 is 24-70mm.
i know the answer, but I'm unable to explain it.
basically, focal length and sensor size are directly related to take the 'same photo'. I think the metric you compare is the area of the sensor, to the length of the lens. A compact camera may have any size of sensor - the bridge style cameras normally have a slightly bigger sensor than the true compacts, then you have normal dslrs, which typically run at 75% of a 35mm frame, then full sensor slrs, then up to large format cameras, things like hasselblads.
I'm a bit hazy on the details, but - a traditional 35mm camera has a diagonal of nearly 50mm, so a 50mm lens is considered 1:1. Meaning that what you see on the sensor is the same size as the real thing would appear if you had your eye down where the camera lens is - I THINK!
So 100mm means twice life size, and so on.
Divide by your crop factor to get the equivalent for your DSLR.
i know the answer, but I'm unable to explain it.basically, focal length and sensor size are directly related to take the 'same photo'. I think the metric you compare is the area of the sensor, to the length of the lens. A compact camera may have any size of sensor - the bridge style cameras normally have a slightly bigger sensor than the true compacts, then you have normal dslrs, which typically run at 75% of a 35mm frame, then full sensor slrs, then up to large format cameras, things like hasselblads.
Yeah, that's all it is.
[url= http://en.wikipedia.org/wiki/Image_sensor_format ]Have a look at this...[/url]
A focal length of 50mm is a focal length of 50mm regardless of sensor but when the image is focused on the sensor the image circle is big enough to cover a 36mm wide FF sensor. That same lens projects the same image onto a 24mm wide aps-c sensor but the image beyond the sensor isn't recorded, so the image is cropped from 36mm to 24mm. Hence the 1.5 crop.
The resultant image, if printed the same size would show the same scene as a 75mm lens would on a FF sensor.
You can use the same math with any sensor size.
It only gets used with smaller sensors though. It's a marketing thing and has no relationship to physics or anything else. It just happens that 35mm film (35mm being the diagonal of 36x24mm film) was 'normal' for most people.
50mm lenses are arbitrary too. It just happened that lens manufacturers could make fast lenses cheaply in the 40-60mm range (50mm being the more common) for 35mm film. It doesn't correspond to human field of vision or anything else, not deliberately (or by chance).