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06-16-2012, 10:27 PM   #1
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Sigma 50mm or Pentax FA 50mm

Hello all, this is my first post, My name is Daniel, and I have a few questions, and wanted to know if a few people out there could help me.. I have been using My KX now for around 1.5 years and in that time have got pretty familiar with all the settings and understanding F values and Shutter speeds etc... In this time I have I purchased one lens, a 55-300. And I got the 18-55 with my KX. Now my question is that Iím now looking for a new lens. I mainly use the camera taking photos of my kids, or just portrait pics. I was just about set on buying the Pentax SMC fA 50MM 1.4f or the Sigma 50mm 1.4 EX DG HSM, Now I know both if theses lenses are great for Low light and they produce great depth of field. But the only thing that Iím not sure of is the 50mm prime fixed focal. Is there any other lenses of there that someone can recommend to me that is not a prime lenses that still can give me great depth of field, and is great for low light? I was thinking about selling the 18-55 that come with the kx and buying another, high end lenses that would cover a focal length of 17 to 55 of something like that and would still give great sharp pics? I know theses types of lenses would cost a little more, But if there is a few people that can recommend some that would be greatly appreciated...


Thanks very much

Daniel



Last edited by dannydza; 06-18-2012 at 04:48 AM.
06-17-2012, 12:47 AM   #2
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Welcome aboard, Daniel! I've been photographing for a long long time, but the K20D I bought 4 years ago was my first dSLR. Its first kit contained the DA10-17 fisheye zoom, DA18-250 superzoom, and FA50/1.4. I now have 240+ lenses but those are still my basic three.

Any 50/1.4 will have very thin DOF when wide open, much thicker DOF when stopped down. We call this effect DOF CONTROL. The faster a lens is, the wider its aperture, then the more DOF control is possible. Slower lenses have naturally thicker, deeper DOF, but less DOF control. By the way, I think of my FA50/1.4 as my gotta-get-the-shot lens. It is GOOD!

Here are the basic guidelines for DOF control:

* For thinner DOF, use a longer lens and/or a wider aperture and/or a closer lens-subject distance
* For thicker DOF, use a shorter lens and/or a tighter aperture and/or a further lens-subject distance

Wider, shorter lenses also have thicker DOF and thus offer less DOF control. We can offset this by shooting closer. I have MF (manual focus) Vivitar 24/2 and 28/2 primes and can achieve thin DOF only with close shots. I get a bit more DOF control with a 35/2 lens and LOTS more with an 85/2.

Zoom lenses for low light with good DOF control are a problem. They are big, and fairly costly, and not really fast. There are NO available zooms faster than f/2.8. None. The best DA18-55 replacement is probably the Tamron 17-50/2.8 or maybe its Sigma counterpart. For thinner DOF with longer focal ranges, see the Tamron and Sigma 70-200/2.8's -- but those REALLY aren't cheap.

I'll say that your best option for a fairly fast zoom is the Tamron 17-50/2.8. You can't go wrong there.
_______________________________

PS -- Something to consider with low light: A major concern is stopping motion. Is an old ROT (rule of thumb) to use a shutter speed that's the reciprocal of the focal length: S= 1/FL. (It's actually more complicated, but this is a good place to start.) So to stop a certain motion with a 50mm lens, set the shutter to 1/50 second. But the rule tells us that the same motion can be stopped with a 24mm lens at 1/25 second!

A lens with an FE (fisheye) projection has a wider AOV (angle of view) than a rectilinear lens and can shoot even slower. My slightly fishy Zenitar 16/2.8 has the AOV of a 12mm rectilinear lens, so could take that same motion-stopping shot at 1/12 second! This is why I like to use wide and UWA (ultrawide) for low light. Such very wide lenses also have very thick DOF, thus little DOF control. If you want infinite DOF, go wide!

Last edited by RioRico; 06-17-2012 at 01:02 AM.
06-17-2012, 03:24 AM   #3
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Thanks so much for your reply, I very much appreciated it, I will have a look at the Tamron 17-50/2.8 like you had said.. I have so much to learn

Thanks


Daniel
06-17-2012, 04:42 AM   #4
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QuoteOriginally posted by RioRico Quote
PS -- Something to consider with low light: A major concern is stopping motion. Is an old ROT (rule of thumb) to use a shutter speed that's the reciprocal of the focal length: S= 1/FL. (It's actually more complicated, but this is a good place to start.) So to stop a certain motion with a 50mm lens, set the shutter to 1/50 second. But the rule tells us that the same motion can be stopped with a 24mm lens at 1/25 second!
But that rule only works with 135formatt right, for aps-C you need to do 1.5x to get the correct value.

06-17-2012, 05:13 AM   #5
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QuoteOriginally posted by Anvh Quote
But that rule only works with 135formatt right, for aps-C you need to do 1.5x to get the correct value.
I left that out to avoid seeding confusion. That's why I said, " (It's actually more complicated, but this is a good place to start.) " I didn't want to spend another paragraph explaining crap.factor. Even without factoring-in the format difference, the 1/FL rule holds for *relationships* of shutter speeds. A 24mm lens can stop the same motion as a 50mm lens at 1/2 the shutter speed. A 12mm lens (or its FE equivalent) stops the same motion at 1/4 the shutter speed of a 50mm. Longer lenses need faster shutters to stop the same motion. Wider lenses allow shooting with slower shutters. Et cetera.
06-17-2012, 05:37 AM   #6
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S= 1/(FL x 1.5) is the correct formula for the Kx.
No "crap" factor explanation is needed.


As for lenses, lets make a list it isnt that long.
You want lenses with the lowest f/ number and f/2.8 is as good as it get with zoom lenses but f/4 is still pretty decent

Tamron
17-50 f/2.8

Sigma
17-50 f/2.8
18-50mm f/2.8
17-70mm f/2.8-4.0 (this one has variable f-number it has f/2.8 at 17mm and f/4 at 70mm)

Pentax
16-50 f/2.8
17-70 f/4

This should interest you.
DA* 16-50mm vs. Sigma and Tamron 17-50mm F2.8 Comparison - Introduction - PentaxForums.com
06-17-2012, 03:31 PM   #7
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QuoteOriginally posted by Anvh Quote
S= 1/(FL x 1.5) is the correct formula for the Kx.
No "crap" factor explanation is needed.
As I said in PM, "1.5" here is a magic number, and I hate magic numbers. Also, notice that I was NOT discussing the optimal shutter speed for handheld shooting, but the RELATION between different focal lengths and shutter speeds for stopping some arbitrary motion. This is independent of frame size and crap.factor. The merits of 1/FL vs 2/(3FL) are irrelevant and issue-confusing.

Last edited by RioRico; 06-17-2012 at 03:37 PM.
06-17-2012, 05:27 PM   #8
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I have the SIgma 17 - 70mm and although I've not used it extensively thus far, I've been pleased thus far with it's performance. It's not that expensive, relatively speaking. JMT

06-17-2012, 07:29 PM   #9
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QuoteOriginally posted by RioRico Quote
This is independent of frame size and crap.factor. The merits of 1/FL vs 2/(3FL) are irrelevant and issue-confusing.
It's relevant because the CoC is differnt but lets not discuss that here.
06-18-2012, 04:20 AM   #10
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O.k guys thanks for the replies, Im now more confused!!! lol.. I still might think about the 50mm 1.4, But I'm not 100%, why does there have to be such a selection of lenses! I just want to make the correct decision with my next purchase..


Thanks again

Daniel
06-18-2012, 04:54 AM   #11
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Are you talking about the sigma 17-50mm F2.8 EX DC OS HSM? or the 17-70mm F2.8-4 DC Macro OS HSM?
06-18-2012, 05:54 AM   #12
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My understanding of the 1/FL rule of thumb is that it prevents blurring due to camera shake. If 1/25 @ 50mm FL is too slow to freeze your child on a swing, zooming out to 25mm focal length isn't going to make her swing more slowly. Longer focal lengths make the effect of shake more pronounced. Shake resistance gives you some leeway with regards to the 1/FL rule. You can generally get away with 2/FL, maybe more.

As Rico says, ignore "crap.factor". It just adds unnecessary confusion.

However, due to that crap.factor, 35mm is a more generally useful focal length than 50mm (normal rather than short telephoto). Good prime candidates in order of cost are:
  • FA 43mm Limited F1.9
  • Sigma 30mm F1.4
  • DA 35mm Macro Limited F2.8
  • DA 40mm Limited F2.8
  • DA 35mm F2.4
The FA 31mm F1.8 Limited would be a very good choice but costs stupid money, unfortunately. Your K-x performs well in low light so F2.8 is probably good enough with a little bump in ISO. I have the 35mm Limited. It's not a bokeh monster by any means but is more than capable of subject isolation.

If you want a zoom, then the Tamron 17-50mm is a good choice (and, as it happens, I have one up for sale right now). The Pentax and Sigma equivalents are a lot bulkier. I liked the Sigma 17-70mm F2.8-4 a lot. However, it is also rather bulky.

You might consider the F35-70mm F3.5-4.5. It's cheap as chips ($50 or thereabouts) with very good optics and a macro mode. I personally found that I didn't like it due to handling issues and that it wasn't quite bright enough for me to get good use out of it. YMMV.

Consider flash for low light. A little bit of bounce flash makes available light problems go away. In other words, rather than buy a lens, buy a flash. I have a Pentax 360 with a Sto-Fen Omnibounce and it works very nicely for me.
06-18-2012, 07:15 AM   #13
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Returning to the OP: The question was, what kit.zoom replacement (not a prime) is good for low light and has "great depth of field"? The latter is tricky -- what's wanted, thick DOF, or great DOF control (thin DOF)? Without venturing into Circles Of Confusion, we can say that:

* Longer faster lenses have thinner DOF and thus more DOF control.
* Wider lenses of any speed (aperture) have thicker DOF and may be quite suitable for low light, as shown by the 1/FL motion-stopping comparison.

I think most of us agree that the Tamron 17-50/2.8 is a great candidate. It's in the same focal range as the DA18-55, is much faster overall, offers better optics and adequate DOF control, and doesn't cost a mint. I could suggest all sorts of primes, and zooms in other ranges, but those don't address the question. If primes had been asked about, I wouldn't hesitate recommending some 24/2 or 30/1.4.

QuoteOriginally posted by dannydza Quote
O.k guys thanks for the replies, Im now more confused!!! lol.. I still might think about the 50mm 1.4, But I'm not 100%, why does there have to be such a selection of lenses! I just want to make the correct decision with my next purchase.
So many choices! Not quite as bad as wandering down the breakfast cereal aisle at any supermarkup, but still daunting! Learning the differences between lenses takes time. But, some points:

* Is no perfect lens. Each lens is a different tool for different tasks.
* Even a perfect lens for somebody else might not be perfect for you.
* Good old manual-focus lenses can cost a fraction of new AF lenses.
* We're not married to new lenses. If it doesn't suit you, send it back.

In other words, don't sweat it. Buy a lens that seems appropriate. If it works for you, great! If not, be sure the seller has a no-questions return+refund policy.
06-18-2012, 07:41 AM   #14
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All these replies are great information.
The thing I can think of adding is this

Like all things in life, most times you have to give something up to gain something else

A "fast" lens like the 50 1.4 will indeed let you shoot in lower light. The trade off it that in order to get that performance, you get a very shallow depth of field. In some conditions, it might only be inches (but can be a good thing as well, as long as you understand F stops)
My wife bought the Pentax 50 1.4 and with no real knowledge of how F stops effect DOF, it was very frustrating to a point she hated the lens. For example taking a portrait with window light she would only get a few inches in focus which was not that flattering, depending on the effect she wanted.
She is getting use to it, but as you tell us you know a lot about F stops, then it should not be as hard for you.
I would classify a lens like that as not one for a beginner.

That said, for the price and low light performance, you can't beat the 50 1.4. it will be 75mm on your camera, which is just right for portraits. At around F 4 is is very sharp as well.

good luck with your choices

randy
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06-18-2012, 07:56 AM   #15
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QuoteOriginally posted by dannydza Quote
I know both if theses lenses are great for Low light and they produce great depth of field. But the only thing that I’m not sure of is the 50mm prime fixed focal. Is there any other lenses of there that someone can recommend to me that is not a prime lenses that still can give me great depth of field, and is great for low light?
Is it the 50mm focal length that worries you or primes in general? Is 50mm a focal length you use often?
If you went through your pictures using the kit could you pick out a 'most-used' focal length? If they're pretty much an even mix of focal lengths then a zoom will most likely suit your style more than a prime...

I can recommend the Tamron 17-50mm 2.8 as being a pretty good 'all-rounder'... That said... I much prefer to shoot with primes myself so my Tamron sees little use these days (and my shortly be up for sale)...
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