How to work out FOV with apsc sensor

jambredz

jambredz

Veteran
Ok maybe i'm just confusing myself. But since the 7d sensor size is almost identical to 35mm film i dont care about crop factor and 50mm being 85mm and that jazz. I think in terms of if i'm shooting film (i never have but its nice to think i have a sensor of the same size :) ). So i have a supertak 50mm and a yashica 28mm (for full frame stills) which gives the saem FOV as if i was shooting film with a 50mm or 28mm. Now i hav a 17-50 tamron apsc lens which effectively is NOT the same FOV at the same focal length. How would i work that out in 35mm film terms when i have that lens on? Am i even making sense....
 
Take your 17-50 and zoom it to 28mm. Look through the viewfinder at the field of view. Then take the lens off and put on your Yashica 28mm and look at the field of view.

They're the same.

A millimeter is a millimeter.

Any 28mm lens, on any given camera, is going to look exactly the same as any other 28mm lens when viewed on that same camera.

28mm = 28mm.

Take your 17-50 and zoom it into full telephoto - 50mm. Then compare that, against mounting your 50mm. They'll be the same. A millimeter is a millimeter.

(note: some lenses will have very tiny variations in their marked focal length, but this will be a minor variation, not a "crop factor").
 
thanks Barry... but i know ive done that already...i took stills though. Def seemed different to me. i'll do it again and post something if needed.
 
Your depth of field is going to be more shallow on your prime lens and sharper because there is less glass between the front of the lens to the sensor. The zoom has more glass and it will not look the same as in depth and clarity. Is that what is different?
 
no i'm not talking about image quality ...just field of view. Just wanted to be able to match feild of views on the lenses cuz they dont match up at the same focal lenght. I'm not saying the focal length is different....just field of view. So essentially i'm asking at what focal length tamron 17-50 will give the same FOV as my yashica 28mm.

will post in a couple minutes an example of the 50mm comparison.
 
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Some lenses will offer minor variations in FOV, but they should be definitely quite comparable. There's nothing about it being an EF-S or EF lens or whatever that will make any difference at all in terms of FOV.
 
test

http://www.youtube.com/watch?v=MjTFO4Xh1xc

Both lenses at f 2.8 iso 1250
7d


Now i couldnt do it with the 28mm because on the tamron i have markings for 17, 24, 35, 50 ....i'd have to guestimate...didnt wanna do that. its not like hindering me or anything...was just curious though. u can see the different more on the right hand side where the white box gets cut off. Also the base of the frame with the coaster and desk.
 
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Yes, that's about within the margin of error. You will find variations like that among different lenses. There's nothing to do for calculating the field of view, that's just... different lenses will render slightly differently.
 
kool...thanks for the verification. I do wonder why thats the case. if a mm is an mm is a mm on the same sensor.
 
jambredz said:
kool...thanks for the verification. I do wonder why thats the case. if a mm is an mm is a mm on the same sensor.
Click to expand...

Because those who make the lenses, aren't terribly consistent or accurate in their markings. I think it's more a case of that the zoom you have isn't reaching technically a full 50mm, it's probably more like a 16-48 lens or something like that, and they just labeled it 17-50.
 
Could there be in this mix the fact that the Yashica and Supertakumar lenses were made to resolve 35mm still film and the 17-50 is made specifically for the Canon sensors? Given that 35mm still film is, what, 4 times larger than 35mm motion picture emulsion/APS-C sensors?
 
No, it doesn't matter what format the lens was made for, regarding its image magnificattion -- any 50mm lens (within the aforestated margin of error) will deliver the exact same image magnification.

A lens being made for a different format will only affect its circle of coverage. Put a smaller-format lens (like a c-mount 25mm) on a larger-format sensor (like a 35mm still film camera) and you'll see a whole lot of black vignetting and a small circle of illumination in the center of the frame. But, that illuminated center circle, will look exactly the same as if you'd used a still-film 25mm lens on that same camera.
 
Barry_Green said:
No, it doesn't matter what format the lens was made for, regarding its image magnificattion -- any 50mm lens (within the aforestated margin of error) will deliver the exact same image magnification.

A lens being made for a different format will only affect its circle of coverage. Put a smaller-format lens (like a c-mount 25mm) on a larger-format sensor (like a 35mm still film camera) and you'll see a whole lot of black vignetting and a small circle of illumination in the center of the frame. But, that illuminated center circle, will look exactly the same as if you'd used a still-film 25mm lens on that same camera.
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No doubt there. My comment was a shot in the dark that the still photography lenses were engineered with a margin of error that, when compared to other still photo lenses on the same film body, would be marginal. But on a digital SLR, that margin of error could be exaggerated.

Just a thought. I doubt it's applicable, but this topic intrigues me.
 
I would actualy say it is an issue of sloppiness by different still lens manufacturers. You have no idea how much slop they will allow, something like 10%. Same thing with lightmeters. Accurate to within 3/4 of a stop either way, so two lightmeters could read 1.5 stops apart!

There is a mathematical formula for determining EXACTLY what angle of view you would see on a given focal length to a given imager size (cosine tangent anyone?), but they allow all that slop in there. One of the many reasons why cine lenses are so expensive, as they are designed and built to a tighter tolerance.
 
jambredz said:
kool...thanks for the verification. I do wonder why thats the case. if a mm is an mm is a mm on the same sensor.
Click to expand...

Because what is printed on the barrel as Xmm, may be Xmm +/- some tolerance, and so, while for a given focal length the FOV is fixed by calculations, practical lenses have slightly different focal lengths from their 'nominal', given the manufacterer's tolerance, and price.

If the variable of interest... 'f-stop', 'Dof', or 'FoV' is critical, then what is in order is detailed tests with the exact equipment that is going to be used. Back in the olden days I would have my shutters for stills calibrated, and the 'marked' speed was frequently never 'dead on'. So, I'd use these adjusted shutter speeds in my 'head' calculations for adjusting f-stops... the f-stop was 'calibrated', in the sense that I would take data on exposure, development, printing, then get an 'overall' calibration from my meter reading, to final print.
 
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