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09-17-2009, 11:20 AM   #1
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fast lens for lowlight

i obviously have to do some research on fast lenses and what they do/how they work.

but my big questions i: im using a pentax istd for nightlife photography. it takes so long for my camera to focus or even want to take the pictures-so frustrating!!

i wasnt sure if this meant that i just need to go ahead and invest in a better camera, but i want to explore the idea of a faster lense first.

i'm currenlty just using the kit lense that came with it, and a fisheye lense i have acts the same way.

anyone know if this would improve my situation??

09-17-2009, 12:21 PM   #2
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When we speak of "fast" lenses, we are not talking about fat they are at focusing. We are referring to their maximum aperture. that is, a lens that can do f/1.4 is "fast" compared to one that can only do f/4, because f/1.4 allows a much faster shutter speed. Meaning you have a much better chance of getting a picture without motion blur or camera shake. But the f/1.4 won't necessary focus any faster. Focus speed is more about the camera than the lens, although some lenses *do* focus faster than others. And kit lens is not one of the faster ones. The DA40 is relatively "fast" in the maximum aperture sense (f/2.8), and it is about the fastest-focus lens Pentax has ever made, because it is so small and hence easy for the camera to turn. But in very low light, the camera with struggle with any lens. The K-7 offers a focus assist lamp, and that can make a big difference. Aside from that, some other mdoels might have slightly improved low light AF over your *istD, but nothing that's going to make the kit lens perform magic for you.
09-17-2009, 12:45 PM   #3
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I agree with Marc on the DA40mm. Bump the iso to 800 on the K line of cameras and you will get very fast focus in low light with this lens. (faster with the K7)
09-17-2009, 03:48 PM   #4
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Hi yinzerparty,

I'm going to disagree with Marc's explanation from this perspective -- he left something out:

A faster lens (wider max aperture) allows more light into the camera and gives the AF system more light to work with, so a faster lens will allow better AF performance when light levels get low enough to start effecting the AF speed, all other things (lens FL, DOF, sharpness and contrast) being equal (which they're usually not).

Don't forget, an f2.8 lens allows twice the light as an f4, and an f4 allows twice that of an f5.6. Your kit lens at @ 50-55mm has a max aperture of f5.6, f4.5 from @ 35-50, f4 from @ 20-35, and f3.5 from @ 18-20 (at least as far as the camera's meter is concerned). This is a pretty reasonable spread, so you can test for yourself whether a faster lens might help. If you aim at a scene with little contrast, that will give you essentially the same Ev at 18 and 55mm like a horizon with no light sources other than the sun at the break of dawn or the latest dusk, you manually focus to minimum focus distance between shots, and you see a significant difference in AF speed between 18 and 55mm, then the difference is most likely due to the max aperture of the lens. Keep in mind that DOF covers a wider spread as you get wider in FL (even at wider apertures), so this also helps the AF system to gain a lock, but at a significant distance, this should not matter that much. Also remember that f3.5 is only about a half stop slower than f2.8, so there is not much to gain from lens speed if you mostly encounter your focusing problems at the shortest FLs of the kit lens.

If you're experiencing AF problems at the longest end of the FL range, then a lens like the FA 50/1.4 can make a dramatic difference in AF speed because of the 4 stop difference between f5.6 and f1.4 (the 50/1.4 will allow @ 16x the light into the camera for the AF sensors to use). Remember that the camera uses the max aperture for AF regardless of the Av chosen for the exposure. Also remember that this speed can cause it's own set of problems with very shallow DOF making greater demands on the AF system's accuracy and calibration.

When the light levels get low enough, all cameras struggle with AF, though some more than others. The K2000 and K-7 especially use different algorithms in the AF system that allow them to lock faster, especially in more difficult situations. I shoot mostly long teles with TCs, so the advantage in AF sensitivity is possibly more apparent to me, but I see close to a 2/3 stop difference between the K20 and K-7 in sensitivity. The K20 would start to struggle in bright sunlight when the lens/TC combo max Av started approaching f8, with the K-7, it's closer to f10 Av. If you can translate that directly to low light sensitivity (which I'm not sure you can) then a 2/3 stop Ev advantage in low light AF sensitivity is not that minor a difference between bodies, IMO.

There are other factors which can effect AF speed. Cameras are naturally limited in processing power because of size. The use of Automatic modes, for metering (green, P, Av,and Tv), and Auto AF point selection, will all slow down AF perfromance to some degree. Also, since the AF sensors are oriented to either vertical (for the far outside side sensors) or vertical or horizontal for the cross type sensors in the 9 point central rectagular grid. Contrast borders within the AF sensor areas that more closely align to either horizontal or vertical will allow for faster AF lock.

A lot to consider, so there is no easy answer. . .

Scott

09-17-2009, 03:55 PM   #5
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QuoteOriginally posted by snostorm Quote
Hi yinzerparty,

I'm going to disagree with Marc's explanation from this perspective -- he left something out:

A faster lens (wider max aperture) allows more light into the camera and gives the AF system more light to work with, so a faster lens will allow better AF performance when light levels get low enough to start effecting the AF speed, all other things (lens FL, DOF, sharpness and contrast) being equal (which they're usually not).
Scott
Scott, you and Marc are both right.
Your explanation is essentially correct, however practically doesn't make much difference at night particularly when focusing on a poorly contrasting object, which is the first thing I would critique when someone says "Focusing is bad/frustrating at night" - why wouldn't you manual focus in these conditions anyway? Or pont the active AF point at a subject in the frame that is at least somewhat lit for the camera to be able to work on? Technique goes a long way in knowing how to use your equipment properly.

An f/2.8 lens is only one stop faster than an f/4 lens, but at night we're taking stops and stops of light less than during daylight hours...
09-17-2009, 04:12 PM   #6
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Hi Ash,

I was responding considering the OP's mention of "nightlife" subjects. I assumed we were talking about considerably more light and contrast than "night" photography, in which case I'd agree that MF is about the only way to go.

As with most questions, an example pic would make the question a lot easier to answer specifically.

Scott
09-17-2009, 06:19 PM   #7
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yinzerparty, I'm just wondering what night photography are you talking about. is it the regular walk-around street photo-shooting or scenic views or cityscapes? if it's the former, I would suggest either 50 1.4 or 1.7 and even a 35/2. but if it's the latter, I don't think that the existing wide lenses speed would be sufficient enough to be shot handheld. 2.8 seems slow for night photography.
09-17-2009, 10:22 PM   #8
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QuoteOriginally posted by snostorm Quote
I'm going to disagree with Marc's explanation from this perspective -- he left something out:

A faster lens (wider max aperture) allows more light into the camera and gives the AF system more light to work with, so a faster lens will allow better AF performance when light levels get low enough to start effecting the AF speed, all other things (lens FL, DOF, sharpness and contrast) being equal (which they're usually not).
You're right; I left that out - deliberately, because of that last phrase ("they're usually not"). That is, while maximum aperture is one factor that affect focus speed, it really doesn't in itself have a big enough effect that you can assume a faster lens will actually focus much faster in practice. But the kit lens is indeed one of the slower focusing lenses, and most "faster" lenses will also focus faster. I mentioned the DA40 because it's a lens where virtually all the factors that contribute to fast AF come together - relatively large maximum aperture, very sharp and contrasty wide open, small size, short focus throw, excellent build quality. But realistically, it too can still sometimes hunt in very low light.

09-18-2009, 07:54 AM   #9
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so...

im pretty new to photography and certainly don't know every word used here, but i'm trying!

im talking nightlife in clubs, bars, concerts, where sometimes there is almost no light to work off of other than a lamp the dj is using. therefore, it's really hard for me to use manual because i can't even see anything...

am i concluding correctly that investing in something like the DA40 would help to focus a little bit faster, and that my kit lense is pretty slow with focusing?

i was standing for about 4 full seconds before my camera decided to take this picture.


in another part of the club where the light was behind the subjects, my camera didnt work at all

and in a better part where everything is lit, it's no problem



i assume that with a better camera, this wouldn't be as big of a problem. but i also think that i have more to learn with the istd and that nightlife photography shouldn't really be as hard as this is seeming to be.
09-18-2009, 09:11 AM   #10
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QuoteOriginally posted by yinzerparty Quote
therefore, it's really hard for me to use manual because i can't even see anything...
Part of that, though, is because your lens is "slow" in the traditional sense of having a small maximum aperture. An f/2.8 or better lens would produce a noticeably bright viewfinder image, with shallower DOF too, which makes manual focusing easier. And you can't be fast at something you don't even try - practice does help, a lot. I'll bet I could have gotten those shots in focus manually in *way* under 4 seconds.

QuoteQuote:
am i concluding correctly that investing in something like the DA40 would help to focus a little bit faster, and that my kit lense is pretty slow with focusing?
Yes, but I think the key phrase here is "a little bit faster". Focusing in extremely low light without a dedicated focus assist lamp like the K-7 has is going to be slow, period.

Since those pictures appear to have been taken with flash, you don't actually need a "fast" lens (large maximum aperture) - as I said before, that term refers to the ability to get a faster *shutter* speed, and you don't need faster shutter speeds when using flash. So all you need is to focus faster. The DA40 will help a little, but not nearly as much as a focus assist lamp. The DA40 is also not wide enough for most of those kind of shots - I note both of those were taken at 18mm. So instead of a lens, you might consider investing in a flash unit that *has* an AF assist lamp (no, I don't which might have that feature, but I'm sure some do).

You would be better served with a "faster" (larger maximum aperture) lens, though, to make manual focusing easier, and to give you a chance at shooting without flash. I'd consider the Pentax 16-50 (or one of the similar lenses from Sigma or Tamron), or perhaps the Tamron 28-75, all of which are constant f/2.8. But none of them on their own will help that much with focus speed.
09-18-2009, 09:13 AM   #11
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Well, the previous posts have contained good information about this subject.

One thing that you should be aware of is that if you can't see well enough to focus manually, it's quite likely that your camera won't be able to "see" well enough to auto-focus. Of course, a faster lens than the kit lens (that is, wider aperture, as Marc mentioned) will allow more light in while you're focusing and will also provide more light for the auto-focus system (as Scott mentioned). This could help if your problem is related to the camera having trouble focusing at all (although there are other factors too, as Marc pointed out), but it will not make the focus mechanism shift focus faster. If the light is low enough, though, this might only really be remedied by an auto-focus assist lamp (as the K-7 has).

Another thing that you should know if you are looking at really fast lenses is that if you use the largest aperture available for the actual photograph, it makes the depth of field much shallower, and focus becomes more critical. This usually isn't too bad at f/2.8 on APS-C (although it can be problematic for group shots), but really becomes a factor at wider apertures than that, so if you have the wider aperture available for focusing purposes, that doesn't mean you have to use it for the actual photo.

All these things combined would seem to support Marc's example of the DA40 as being a good lens for fast focusing. I only see two real issues possible with using this lens. One is that you may not be ready to spend that much money on the lens (it goes for around $475 US, so not terribly bad, but not the cheapest either). The other is that you may find the angle of view too narrow, as this lens is considered just slightly on the long side for APS-C. If you have an issue with the angle of view, one of the shorter Limited lenses might be an alternative, although some options (*cough* FA 31 Limited *cough*) are downright expensive (lavishly expensive in my view ).

Edit: I was typing when Marc made his last reply, so I didn't see it, but he's right. If you are going to use flash anyway, then you might as well invest in a better flash with an auto-focus assist lamp as change lenses or camera, unless having a flash on top of the camera makes the camera "too big" for you to feel comfortable using it for this purpose. My post was more relevant to available light shooting.

Last edited by CFWhitman; 09-18-2009 at 09:20 AM.
09-18-2009, 09:34 AM   #12
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QuoteQuote:
you may not be ready to spend that much money on the lens (it goes for around $475 US
It's actually available for quite a bit less than that - $340 from B&H, for example. But after seeing the pictures posted - both taken with flash at 18mm - I'm thinking that's not the right lens. Perhaps the DA21, or 16-50, but a flash with focus assist should be first priority.
09-18-2009, 10:45 AM   #13
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QuoteOriginally posted by Marc Sabatella Quote
It's actually available for quite a bit less than that - $340 from B&H, for example. But after seeing the pictures posted - both taken with flash at 18mm - I'm thinking that's not the right lens. Perhaps the DA21, or 16-50, but a flash with focus assist should be first priority.
Yes, you're right. I'm sorry. I don't know why I didn't remember this from a few weeks ago when I was looking at this lens and comparing it to another lens, and I did look at the B&H price. Just now I quickly looked for a price on the net and saw one of those artificially elevated ones without realizing it. It used to be you could pretty much count on finding a normal price at Amazon (along with some high prices), but lately sometimes they just have sources listed with elevated prices. I'm sure that's related to the economy craziness somehow (probably either because of shortages of lenses or shortages of sources for lenses available to Amazon), but it is annoying.
09-18-2009, 11:08 AM   #14
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Hi yinzerparty,

OK, with some example shots, this problem is a bit easier to get a handle on.

As I stated before, if you find yourself shooting at the widest end of the zoom, a faster lens will not help much since your kit lens has a max aperture of f3.5 at 18-20mm. The DA21 Ltd is an f3.2, so if you were to spend the money on this lens, you'd not really get any advantage in light gathering.

There is a technique that you could try without buying anything. Single shot AF (AF S) mode is focus lock priority, so it won't let you take the shot if the camera hasn't locked focus. Continuous Focus AF (AF C) is shutter button priority, so it allows the photographer to take a shot, even if the AF system has not found a focus lock.

The Pentax AF usually works by getting the focus close at first try, then it tries in very small steps to find a precise focus lock. In very dim light, this process is slowed down considerably as you have experienced, and the camera won't allow the shutter to be tripped until a lock is established.

You can cheat the system a bit by using AF C and choosing to trip the shutter just after the initial "getting close" AF step.

Switch the camera to AF C.

Activate the AF, then wait until the focus motor stops. If the scene appears to be in reasonable focus at this point, then full press the shutter to take the shot. If the scene is obviously out of focus, then release the shutter button and try again. In very dim light, it's important to give the AF system as much to work with as possible, so try to get a contrast border somewhere inside the AF sensor's area. By using Center Point AF mode, this area is pretty much defined as the central circular area of the grid on the standard focusing screen. You want to try to get a contrast border within this area.

With the kit lens at any of the shorter FLs, the DOF (especially when stopped down a bit and using flash as in your examples) should be deep enough to give you a reasonably focused shot, even if the AF system isn't totally satisfied. Your subjects are moving slightly anyway, so you'll have to expect some misfocused shots, but I think that you'll get a lot of shots that you were previously missing.

I found this technique a little difficult at first, but if you keep trying, you'll get used to it.

I'd try this before spending money on an external flash, or another lens. I realize that my description might not be easily understood, so if you have any questions, feel free to ask.

Scott
09-18-2009, 11:45 AM   #15
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