# what does focal length atcually mean?



## just x joey (Jan 24, 2007)

say i have a lens with a focal length of 85mm, on 35mm camera, does that mean it can focus on sumthign 85mm away? or what?


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## firemedic0135 (Jan 24, 2007)

I always thought it was the distance from the lens to the sensor/film but I could be way off.I will watch and see what the pros say about it...........


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## table1349 (Jan 24, 2007)

http://www.paragon-press.com/lens/lenchart.htm


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## just x joey (Jan 24, 2007)

yah, that's what it is, but i dunno what it means. i jsut read sumthign in this book i have that says at 28mm u have a 76degree angle of veiw, at 50mm u have about 46degree angle of veiw... lol i dunno :-\ i guess its jsut the higher the focal length the smalelr the "crop" basically... i think? haha im confused. basically i wanna know with a 100mm lens, how far away can the subject be ( liek a persons face) to fill the frame?


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## auer1816 (Jan 24, 2007)

firemedic0135 said:


> I always thought it was the distance from the lens to the sensor/film but I could be way off.


 
You're right -- mostly.  It's the distance from the lens' optical center to the sensor/film plane when the lens is focused to infinity.  The optical center is the point on a lens where the light passes through unrefracted.  Refracting means bending or changing direction.  Hope this helps, but I don't think I simplified anything.

Basically, bigger numbers mean more zoom.  That's all.


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## just x joey (Jan 24, 2007)

crap. ne ways, i have a new question, if i have a 50mm lens on a film camera, shuld i get a 30mm lens to use on a digital camera to make them the same?


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## auer1816 (Jan 24, 2007)

Assuming you have an APS-C sized sensor, you'll end up with a 45mm equivalent.  You just take the lens focal length and multiply it by the sensor's crop factor.


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## markc (Jan 24, 2007)

auer1816 said:


> Basically, bigger numbers mean more zoom.  That's all.


Sorry to be picky, but bigger numbers means the lens is more _telephoto_. Zoom means a changeable focal length.


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## fmw (Jan 25, 2007)

When we talk about lens focal length we are generally talking about the back focal length or distance between the rear element and the film or sensor plane. Wide angle lenses for SLRs use retrofocus to keep the rear element far enough away from the sensor to stay out of the way of the mirror but, for most lenses this is the measurement.

Lenses with longer focal lengths act something like telescopes by magnifying the image and foreshortening perspective (reducing the appearance of depth in the subject.) Lenses with shorter focal lengths work in the opposite manner. Here are a couple of examples.







The dunes at Death Valley were photographed using a telephoto lens - one with a long focal length. The dunes were a couple of miles from my lens and the mountains you see in the background were quite a few miles beyond the dunes. The long lens magnified the dunes to fill the frame and made the mountains beyond look fairly close. That is the foreshortening I was talking about.






This is a wide angle shot of Death Valley made with a short focal length about 20 miles away from the first image. Those same mountains you saw in the first shot are visible in the upper right portion of the frame. Here the perspective is very different. The lens tends to stretch out distances. You can see those mountains many miles away and the rocks just in front of where I was standing to make the shot. The lens also tends to make things look smaller - the opposite of the magnification you get from a long focal length lens.

These are extreme examples but it helps show you the difference focal length makes on visual perspective.


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## just x joey (Jan 25, 2007)

well you see, i know the longer the focal lenth the further away the subject can be to fill the frame, BUT im wondering if i should get a 60mm lens or a 85mm lens, and depending on wich on i get, how far away for each can i be away from a persons face to fill the frame?


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## markc (Jan 25, 2007)

If you are talking about a 35mm film camera, the 85mm is good for head and shoulder portraits. You can fill the frame with their face, but that's starts getting a little close. The 60mm would be out for strict portraits. On a 1.5 or 1.6 crop digital SLR, the 60mm would give you the same field of view as a 80mm or 85mm lens on a film camera, so the same thing applies as the 85mm. The 85mm would be around 130mm, which is on the other end of a typical portrait lens. It would be great for head shots, and you could just back up for head/shoulder shots.

What camera are you using now? Do you have a zoom? Just set it to the focal lengths you want to try out.


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## just x joey (Jan 25, 2007)

im gettign a 30d, but im getting it with a sigma 17-50mm, i want a little longer prime, but i dunno how long... crap lol


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## markc (Jan 25, 2007)

Well, the 85mm/f1.8 is an awesome lens. Since you have up to 50mm covered with the zoom, I'd go for that. For reference, I use that and the 50mm/f1.4 for over 99% of my shots. I do all portraits and environmental portraits with a 10D.


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## just x joey (Jan 25, 2007)

i was looking at getting the sigma 30mm, because that basically is a 50mm on a 35mm camera. and thats what i been using only on my canon FTB, so... i think it will be a good choice. i hope i don't end up being too far away from my subject... im also getting the really cheap canon 50mm lens, thats like $75.


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