 BetterPhoto Member |
About the Numbers on My SLR Lens My SLR lens has a specification; 50mm 1:1.8. What does 1:1.8 refer to and what does it mean?
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John P. Sandstedt |
The f/number on your lens reflects its speed - that is, the relative amount of light it passes. The f/ratio is the ratio of the focal length of the lens to the widest diaphragm opening. The SMALLER the f/number, the faster the lens. As you close down (stop down), you'll lose half the light for each full stop (f/1.8 (f/2.0,) f/2.8, f/4.0, f/5.6, f/11, f/16). Note you're halving the light as the f/numbers increase by multiplying any value by the square root of 2 (1.414). Of course, values are rounded off.
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Maria Melnyk |
Unless I'm mistaken, I have a feeling that the 1:1.8 is not the speed, or aperture of the lens. It is a ratio. Is this a macro lens? This 1:1.8 refers to the image size of the subject on your film. In other words, if the lens had a 1:1 ratio, your image could be taken life-size on the film. For example, if you took a close-up photo of a bug, the bug on the film would be the same size as the actual bug. But with a 1:1.8 lens ratio, you could not get that size image; 1.8 is roughly a 2, so the image on the film would be half of life size, or 1:2. Does this make sense? If not, I'll try to explain it another way. Now, in case I'm wrong and the numbers do actually refer to the largest aperture your lens is capable of, I'm then wondering what the "1" is doing in front of the 1.8; this doesn't make sense.
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Tom Walker |
Maria, it is the speed of the lens, the f sstops are recpricals
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Kerry L. Walker |
John is correct. 1.8 is the maximum aperature.
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Christopher A. Vedros |
Maria, I've always wondered why they write the max aperture on a lens that way. It does make it look like a ratio, but when you see it on the front of a lens, it always refers to the max aperture. Anyone? Anyone?
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Maria Melnyk |
If anyone cares to laugh at me, go right ahead. I just pulled out my 85mm lens, and it says 1:1.8 also. (And I once took - and passed - an exam on this stuff.) Yes, Tom, I had forgotten that f-stops are reciprocals. That, Chris, is the answer to your question. F-stops are reciprocals, so there is actually a "1" over that aperture number. It refers to the focal length of the lens divided by the diameter of the opening.
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Raghurama Hegde |
The f-number or f-stop as it is usually called is in general the ratio of the focal length to the diameter of the aperture opening you are using for a particular shot. And since it is a ratio it is expressed as 1:f-no., but in practice people usually give the f-no. as 1.8, 3.6 etc with the implicit understanding that it is a ratio. When the specification of lens is given the f-no for the widest aperture opening is usually given and this also indicates how fast the lens can go. If you've noticed the nos on a zoom it would be something like 1:3.5-4.5 which would arise because the focal length changes as you zoom in or out but the diameter of the aperture opening remains the same.
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John P. Sandstedt |
Hey Gang - get your mathematics straight. As I said earlier in this thread, the f/number is the ratio of the focal length to the maximum opening of the diaphragm of the lens. There "ain't" no reciprocal involved. The calculation: a 50 mm lens has a 25 mm diaphragm opening - therefore 50/25 = 2 = f/number of the lens. If you add a 2X converter [this will double the normal lens's focal length] the mathematics goes 100/25 = 4 = f/stop. Yes, it's crazy and would make more sense if the math were diaphragm/focal length. But, who said things should be logical in photography? After all, the camera says the shutter speed is 125 [in this case it is the reciprocal 1/125 sec.] Catch??
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Tom Walker |
john, you're just stating it backwards yes 50mm lens devided by 25mm opening is f2, or as it's generally known 25 is 1/2 ( reciprocal of 2) of 50
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Justin G. |
I love photography because I hate math!!! And you guys gotta bring it right back don't ya!! Good lesson though I'm learning a lot here.
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John P. Sandstedt |
Tom ???? What are you implying? We must get back to basics. The speed of a lens is defined as ratio of the focal length of a lens, focused at infinity, divided by the widest opening if the diaphragm. Got that? Now if one stops down the lens, s/he close the diaphragm and the f/number is calculated by dividing the focal length by the diameter of the diaphragm. Hence, stopping down one stop closes the diaphragm from 52 mm to ~12, the f/number becomes f/4. The use of the slash in dewscriptor f/XXX doesn't mean to divide - as in ratio. Lens openings have been described as f:number for example and, since that's the fact, Man, how is this a reciprocal. Sorry, but the definition IS backwards and gives folks agita - but, that's the way it is and has been forever.
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Tom Walker |
thats why I love this site, john we're saying the same thing but putting it differently
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