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01-14-2010, 08:08 AM   #1
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what's considered fast enough?

i have a 50 1.4 and even as a total beginner can appreciate the lens. but i've also played with it enough to know what else i might need in the future. i would eventually like a wider option for indoor shooting while keeping the sharpness and speed of my 50.

with my limited background i'm basing the speed of the lens, its capability to shoot in low light, and its sharpness on its aperture rating (f/1.4). is this correct?

when i'm looking at other lenses though....i seldom see an aperture rating that low. especially in the 18-40mm range. so what should i be looking for in lens? are f2 ratings enough?....even some of the lenses in the 70mm onward range are much higher. does this mean they are not as fast? i get the feeling there is a formula to determine all this based on focal lengnth aperture rating and even prime vs zoom.

thanks for all the help.

01-14-2010, 08:18 AM   #2
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sigma's 20mm f1.8 and 30mm f1.4 are awesome primes.

my general rule is no zooms slower than f2.8, no primes slower than f1.8.
01-14-2010, 08:24 AM   #3
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Aperture is a function of the opening of the lens (the size of the diaphragm in the lens; the aperture iris) over the focal length.

So figure the 50mm lens' aperture opening (when set at f/1.4) is 70mm wide. 70/50 = 1.4

Maximum aperture has nothing to do with sharpness, although often lenses with larger apertures are indeed sharper simply because they are more expensive lenses, and generally more expensive equals better when it comes to lenses.

I would suggest shooting your 50mm in aperture priority mode as a test case. See if a lens with an f/2.8 opening is fast enough for you. If so, try it at f/4. The larger the f/number the less expensive the lens. You won't find a zoom lens with f/1.4, but you may find 28mm fixed focal length lenses, 85mm, etc. at f/1.4. But they will be crazy expensive.

Speed is a non-technical term. We say a lens is fast if the maximum aperture is f/1.2, 1.4, 1.8, 2, 2.8. "Speed" or a "fast lens" refers to how much light the lens lets in, and thus how fast a shutter speed you can use all other things equal.
01-14-2010, 08:34 AM   #4
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So, lets' take this example. Let's say we have a 16-50 2.8 DA* lens. At 16mm, the aperture is around 45mm wide. But at 50mm, the aperture has to be 140mm wide to let in the same amount of light. This makes the lens expensive (much more glass is required, and coatings are required to cut down flare from these big hunks of glass, and good focus mechanisms, and etc etc). That's why we don't generally see anything faster than an f/2.8 zoom.

That's also why prime lenses (fixed focal length, like your 50 1.4) tend to be faster and cheaper, but also have excellent optics.

01-14-2010, 08:43 AM   #5
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QuoteOriginally posted by brofkand Quote
Aperture is a function of the opening of the lens (the size of the diaphragm in the lens; the aperture iris) over the focal length.

So figure the 50mm lens' aperture opening (when set at f/1.4) is 70mm wide. 70/50 = 1.4
Just a little nitpicking. While everything you said about sharpness and speed is correct, your math is backwards, or should I say, upside down. A 50mm, f/1.4 lens has a maximum opening of 50/1.4 = 35.7mm, not 70mm. Just look at a lens. Seventy mm is almost three inches. The opening on my 50mm, f/1.4 is no where near three inches.

The formula is focal length/opening size = f-stop. 50/35.7 = 1.4.
01-14-2010, 08:45 AM   #6
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Understand that you'll get the effect raising the ISO of your camera from say 200 to 400 (or any full stop ratio) as you would using an f1.4 lens instead of an f2 lens. Cost of the lens is inversely proportional to it's f ratio - ie the lower the number, the higher the cost. And as was mentioned before, there is generally a correlation between the cost of the lens and its sharpness.
01-14-2010, 09:00 AM   #7
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There's this thing called the exposure triangle.

For every increase in F-stop, you need to compensate by either raising the ISO 1 stop (400 -> 800) or double the shutter speed (1/30s -> 1/15s), in order to keep the correct exposure. Hope this helps

And yes, f/2 is usually fast enough. Have you noticed yourself stopping down the lens at all when shooting people indoors? I typically find that f/1.4 gives a SUPER shallow depth of field, so shallow that only 1 eye of my subject will be in focus and the other one out of focus - so I end up stopping it down to f/2 or f/2.8, which solves the problem nicely.

As posted earlier, you can get the sigma 30mm f/1.4 for like $399 these days.
01-14-2010, 09:07 AM   #8
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Ah, thank you for the correction. The validity of those numbers didn't shine on me...70mm doesn't really make sense!

Thanks.

01-14-2010, 09:14 AM   #9
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understanding the mechanics of what a zoom is doing at different focal lengths doesn't really matter.

what does matter is the apertures available to you, and the mtf perfomance which measures contrast and sharpness.

learn to read an mtf chart before you buy a lens.
01-14-2010, 12:08 PM   #10
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i shoot my jobs on 2.8 lenses but i never open em all the way except in emergency. i generally shoot at 3.2 unless I have too much light or want more DOF.

at times i wish i had a couple of faster lenses but im doing fine on what i have....
01-14-2010, 01:17 PM   #11
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Right. 'Fast' is a term which is actually kind of relative in common use: one lens relative to another, and one lens relative to others of its general focal length. For a 50mm, f2.8 is 'slow:' for a zoom lens, it's 'fast.'

If you look at Pentax's FA limited series, I think you'll find these a pretty good indicator of what you'd likely find 'fast enough' for whatever given job you might want to do with a digital.

Most makers are very good at making small and fast 50mm lenses: for nearly anything else, a 1.7-2 sort of lens ought to be quite sufficient. They're gonna cost, depending how realistic that is to want, for the focal length.

I've got a Sigma 28 1.8 that I'm loving, here: quite affordable and is just lovely. Sufficiently fast for me on digital, anyway. Hyooge, though: it's made to be a wide angle on full-frame. My only real lament about the size (It's not tremendously heavy, just bulky) is that it's kind of imposing to point at people.

The 30 1.4 is smaller, (still rather chunky) obviously that bit faster, a shade less wide, made just for digital, and can be had for not too much more.

In general, you will want to try to stay off your lens' widest setting for sharpness' sake, anyway: the speed can help you see and focus and it can be very nice to have, though. When we say 'This lens is sharper wide open,' it kind of refers to these tradeoffs.

You can always say, 'What's fast enough for doing this kind of job this kind of way under these types of conditions?"
01-14-2010, 01:36 PM   #12
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For my general type of photography, birds and wildlife, I like my DA*300 F4 but that is often not "fast" enough. A 300 f2.8 would be great, at a much higher cost and much higher weight.
For other photos I do, such as family stuff, scenery, etc., I have a DA*16-50 F2.8 which I find fast enough for most situations. I am looking for a "faster" lens, such as a F1.2, but that is for special effects that those lenses are capable of, i.e.: bokeh. So, really, a fast lens is relative to what you are using it for, as most people mentioned earlier in this thread.
Bottom line is, and this is a recall from another earlier post: prime: nothing less than f1.8, zooms, nothing less than f2.8. Then gain, it is all relative to what you'll be using for.

JP
01-14-2010, 03:08 PM   #13
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As some have said, it's kind of relative. A 50mm lens for a typical 35mm film camera was not only about the easiest lens to build, but also the most useful, so a lot of them were made. there was a lot of pressure to make them "fast", but it was also easy to achieve just because that focal length happened to work out that way. Other focal lengths tended to be harder to make - or at least, to make "fast" - and not as much in demand. Therefore, you just don't see nearly as many fast lenses at other focal lengths. Which is fine, really, because while f/1.4 is nice to have sometimes, really, DOF is so shallow at f/1.4 that for "most" purposes, you're better off shooting stopped down a bit. And given that high ISO capabilities on digital SLR's is *way* beyond what film could do, the need for such wide apertures just to get a fast enough shutter speed has also greatly decreased (and SR has further had an effect on that). So while on film, you might realistically limit yourself to ISO 400 and need f/1.4 to get a shutter speed of, say, 1/30", on digital, you can easily shoot ISO 1600 and get results as good if not better, which allows you shutter speeds of 1/30" even at f/2.8. And SR would allow you to shoot with slower shutter speeds in many (but of course not all) cases. Combine that with the fact that few people buy primes any more compared to zooms (because zooms have become so much better than they used to be), and that's why you just don't see people putting out many new faster primes.
01-14-2010, 03:35 PM   #14
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QuoteOriginally posted by erickallemeyn Quote
And yes, f/2 is usually fast enough. Have you noticed yourself stopping down the lens at all when shooting people indoors? I typically find that f/1.4 gives a SUPER shallow depth of field, so shallow that only 1 eye of my subject will be in focus and the other one out of focus - so I end up stopping it down to f/2 or f/2.8, which solves the problem nicely.
Exactly. At close distances, a 50/1.4 at f1.4 has so little depth of field, many subjects don't look right, especially headshots.

The design of the camera mount means lenses around 50mm can be pretty fast without much trouble. It is more difficult to design a good fast lens at other focal lengths, so the cost begins to escalate.

IMO, f2.8 is fast enough from about 24mm to 135mm, if the lens is good at f2.8. Lenses almost always are better when they're not wide open, so a lens faster than f2.8 can have better image quality at f2.8. The viewfinder is brighter for easier focusing, both manual and auto.

Above 135mm, f4 starts out being pretty good and by 300mm is great. Beyond that, faster lenses than f4 would really be very useful. They would allow more reasonable shutter speeds. But they have such large prices and sizes that they are hard to afford or carry.

Below 24mm, shutter speed is much less of an issue, and more stuff will be in focus anyway with a wide angle field of view. So since subject isolation is trickier and not as necessary, f4 is fine here too.

I'm not saying anything faster than these is useless, just that you can get by without it.
01-16-2010, 12:54 AM   #15
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I have the FA50/1.4 and almost NEVER shoot wit it at 1.4...no real advantages to the way I shoot. I usually have it at 2.0 and above and find it's much more pleasing. I also use an FA35/2 and find it is much more pleasing and usable wide open than the FA50. I also use my Tamron 17-50/2.8 very regularly and find it's widest setting is very very nice....but, I almost always set my lenses at at least 4.0 to 5.6 when just shooting regular shots, good light, etc. I find that is the sweet spot on most of my lenses and unless the light is poor, I keep them there.

Having fast glass has it's advantages for sure, but shooting wide open all the time may not be the best way to utilize those lenses or be the best shooter you can be.

Jason
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