How do i know how wide my lens is??

[shorty6049]

So i was talking to a friend and we were discussing lenses (wide angle more specifically) and i came apon the realization that the focal length doesnt necessarily determine the "wideness" of a lens. for example, my sony 18-70 is not as wide as a 20mm Sigma lens... the difference is about 45 degrees to 90degrees , respectively, so can anyone explain how to figure out how wide a lens is? or do i just have to look in the specs for each lens to get that info?

 

[Big Mike]

What do you mean by 'wideness'? The field of view?

It is determinded by the focal length and your 18-17mm should have a wider view (at 18mm) than your 20mm...(assuming both are on the same camera).

 

[shorty6049]

yeah, by wideness i meant field of view, sorry i forgot to specify that... I ask that because on sony's website, it says that my 18-70 has a FOV of 45 degrees at its widest, and the sigma site says thier 20mm has a FOV of 94.5 degrees.... it says its made for digital, so i would assume that means that the FOV that they give is for a common digital sensor? i'm so confused

 

[shorty6049]

i emailed Sigma , so they should help me get this straightened out, if anyone else knows anythign about this please give me your advise though, thanks!

 

[HASHASHIN]

What do you mean by 'wideness'? The field of view?

It is determinded by the focal length and your 18-17mm should have a wider view (at 18mm) than your 20mm...(assuming both are on the same camera).

:thumbup:

 

[shorty6049]

is there any crazy way that that could be wrong though? i dont get why sony's site would say their lens only had a 45 degree FOV if all of sigmas lenses in that area are around 90 degrees.... its not common for them to give FOV isn one direction from the normal (perpendicular) is it?

 

[Big Mike]

I see the confusion...the Sony is designed for digital camera with a smaller sensor (APS-C)....while the Sigma 20mm is designed to be used on 35mm film SLR cameras (but of course it can also be used on the digital cameras as well).

So actually, the Sigma does have a much wider field of view...but you won't see that on your camera because your camera (with it's smaller sensor) will effectively 'crop' the image that comes though the lens.

That's why I said "assuming both are on the same camera". So to figure out the effective field of view of the Sigma, on your camera, you have to figure in the 'crop factor'...it's 1.5.

 

[shorty6049]

well 94.5/1.5 is still 63 , so that still would suggest a larger FOV ....

 

[Big Mike]

The 1.5 factor is applied to the focal length, from there you should be able to calculate the field of view. The 1.5 is the comparison factor...it compares 35mm film to your camera.

Really, I don't think it matters...the fact is that a shorter focal length has a wider view (on the same camera).

 

[shingfan]

well 94.5/1.5 is still 63 , so that still would suggest a larger FOV ....

the crop factor is not a linear factor to all aspect of a lense.....dont overuse it.....it is merely use as a represention of the focal lenght on digital cameras compare to 35mm film cameras......as mike said.....18mm should provide a wider FOV compare to a 20mm on the same digital camera......

if you compare a picture taken at 100mm and a picture taken at 200mm.....you wont see a 2x dimension on the 200mm.....instead.....what difference you see would be the change in overall area....i suspect the area is 2 times as big compare to the 100mm (i have not verify this)

 

[ksmattfish]

Google "FOV calculator" or "field of view calculator", and look for one that allows you to choose various formats or cameras. You also have to pay attention to whether it is giving you the field of view horizontally or from corner to corner.

 

[shorty6049]

ok, thanks everyone

 

[shorty6049]

so would that be normal though for a lens to cut the FOV in HALF on a DSLR as opposed to a 35mm?? it seems like thats a huge crop factor in field of view... also, why does sony's site have the FOV listed for just the digital camera , and sigma would list it for a 35... shouldnt there be some sort of standard that everyone would follow? (i can understand i guess that they would do that because its made only for digital, but still it just confuses me)

 

[shingfan]

so would that be normal though for a lens to cut the FOV in HALF on a DSLR as opposed to a 35mm?? it seems like thats a huge crop factor in field of view... also, why does sony's site have the FOV listed for just the digital camera , and sigma would list it for a 35... shouldnt there be some sort of standard that everyone would follow? (i can understand i guess that they would do that because its made only for digital, but still it just confuses me)

because the sony lense is designed for digital camera as Mike pointed out (it doesnt work on film camera)......the sigma is designed for both film and digital

i'll give you an example......i have 2 persons....one is able to carry 10 apples at a time.......the other is only able to carry 5 apples at a time.......if an apple maker (if there exist such a device) is able to make only 5 apples each time......does it matter how many you can carry???....no.....because you can only carry 5 each time....even if you spec. that you can carry 10 each time...but i only have 5 available for you to carry....so your efficiency is only 5 each time....same for the sigma case...it is able to support higher angle for film camera....but when you attach it to a digital camera...the limitation factor becomes the camera...not the lense (but when you advertise it....you would put up the better specification.....not the lower one for use with digital camera)

 

[shorty6049]

yeah, i suppose, but the sony lens still works on a film camera, but the image circle is just a little too small so you get wicked vignetting at the edges.... so even if on the sigma site says "for digital", it still doesnt give digital specs for FOV?