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02-05-2010, 08:21 AM   #1
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Newbie question - Prime f1.4/1.8 or small tele zoom f2.8

Hi,

I bought my first SLR a couple weeks ago (Pentax k-x) and I have a few doubts about the lens I should buy.

I've worked with some fast primes (from other brands) f1.4 and f1.8 and I really liked the low light capabilities and the shallow DOF.

My question is if a choose to buy a f2.8 small tele zoom lens like the tamron 17-55mm or 28-75mm, will I significantly loose DOF capability and is the f2.8 a fast enough to use indoors without flash using ISO up to1600 or even 3200?

Thank you for help.

02-05-2010, 08:44 AM   #2
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I'm sure others may respond in a bit more technical way than I. However, f2.8 is plenty fast for most situations. My DA*16-50 is way fast and very adaquate DOF for a lot of low light situations. I have several lenses in the f1.4, 1.8, 2 range. While they add stops and provide a shallow DOF, they are harder to focus too. I view those lenses as suitable for special stiuations, while my f2.8 zoom is nearly as good and much more useful in a wider range of situations.
02-05-2010, 09:08 AM   #3
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There is a online calculator you can use for DoF:
Understanding Depth of Field in Photography

I *think* if you really like shallow DoF pictures, you want a f2.8 zoom lens with longer focal length like 70-200mm/2.8 -- i am not sure if pentax makes one like nikon does? both tamron and sigma has a 70-200/2.8 for pentax.
02-05-2010, 10:02 AM   #4
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Another dof calculator can be found at http://www.dofmaster.com/; versions for windows, iphone etc, tables for printing and the online version

At 50 cm subject distance, the dof is 6mm (for f/1.8) and 10mm (for f/2.8)
At 5m subject distance, it's 71cm versus 113cm

Just some examples so you can draw your conclusions.


EDIT: just realised that I forgot to mention the focal length of 50mm


Last edited by sterretje; 02-05-2010 at 10:58 PM.
02-05-2010, 10:37 AM   #5
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Thank you for your replies.

And what about indoor shooting without flash...I know the difference is 2 f stops...but can I still take indoor pictures taking advantage of the high iso capabilities of the k-x?

How far do I need to push the ISO to take good indoor pictures without flash?
02-05-2010, 10:39 AM   #6
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QuoteOriginally posted by Pretender Quote
I've worked with some fast primes (from other brands) f1.4 and f1.8 and I really liked the low light capabilities and the shallow DOF.
Was that film or digital? And if digital, was that with stabilization or not?

QuoteQuote:
My question is if a choose to buy a f2.8 small tele zoom lens like the tamron 17-55mm or 28-75mm, will I significantly loose DOF capability and is the f2.8 a fast enough to use indoors without flash using ISO up to1600 or even 3200?
Small is a relative thing; those lenses are pretty darned big compared to typical primes in that range. But of course, small compared to, say, a 300/2.8.

Anyhow, here's why i wondered if your experience was film or digital. If film, f/2.8 probably seems rather slow, because you were probably not wanting to go beyond ISO 400. With digital, you can safely use ISO 1600 or 3200 with no more noise than ISO 400 film, so even with f/2.8, you'll be able to get faster shutter speeds than you used to get with f/1.4.

If your previous experience is with digital, then you may have been accustomed to usign ISO 1600 already, and f/2.8 is two stops slower than f/1.4. But that's just about the benefit you can expect from SR, so it kind of evens out. Except for moving subjects - where DOF would have been so shallow at f/1.4 that you'd have had little success anyhow - f/2.8 and ISO 1600-3200 is fine for "most" situations. And you may actually find yourself better off in those situations than you would have at f/1.4 with a faster shutter speed but shallower DOF.

Of course, when you specific *want* shallow DOF (these are rarely the same situations when you need fast shutter speeds), you won't be able to get as shallow. If you really like taking those kind of pictures, it might be worth picking up a cheap used manual focus prime just to get that capability when you want it.
02-05-2010, 10:46 AM   #7
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If you really like the shallow DOF given by a 1.4 or 1.7 aperture, then 2.8 isn't a good substitute. However, 2.8 is a more usable aperture for broader use.

At ISO 1600 or 3200, f/2.8 is very usable w/o flash. I've found that indoors at night in my rather poorly lit living room at f/2.8 and ISO 800 I get barely usable shutter speeds (for shooting my 2-year old) in the 1/20-ish range. Manual focus is difficult, though. From what I've seen of the K-x (I don't own one, but I've followed it pretty closely), the high-ISO capabilities would fare quite nicely giving you an extra 1-2 stops of light for a faster shutter. The DOF will vary with focal length, of course. For shooting people in lower light, 2.8 is as wide as I'd want to go to increase my likelihood of keeping my subject in focus.

I don't know how well the K-x autofocuses in dim light, but it will be better with a 1.4 or 1.7 lens than a 2.8 one. Personally, I'd rather have a fast 50 and either the 35 or 40 limited (both 2.8 lenses) than a 2.8 zoom. Or maybe a used 35/2. Also, keep in mind that a stopping down a 1.4 lens to 2.8 will likely give greater IQ than a shooting a 2.8 lens at 2.8.

I recommend you researching the Lens Clubs here on the forums for shots with the various lenses you're considering to see what can be done with them before taking the plunge.
02-05-2010, 10:49 AM   #8
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I should also mention that Marc has shown me the capabilities of shooting his K200D with a 40/2.8 Ltd. at ISO 1600 effectively pushed to 3200 through PP. I was impressed, although I still haven't managed to buy the lens yet. Still, your K-x will do a better job than his K200D in low light capture.

02-05-2010, 11:34 AM   #9
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21mm f/3.2 and 40mm f/2.8 did not give enough speed and DOF indoors,
so I got a Sigma 30mm f/1.4.

In my -limited- experience ,
yes, you will significantly loose DOF capability,
and no, f2.8 is not fast enough to use indoors without flash, but I use a K20D and don't usually bump ISO over 800.

You may use f/2.8 with ISO 1600-3200
Even you have enough speed with that, I'd suggest a f/1.4-1.8 lens for indoors -especially portrait/object shots- just for DOF.
There is a critical point for DOF at f/2 , transition from f/1.4-f/2 to f/2.8 is significant - to me.
I could not imagine 2 stops differ that much.
Just my 2 cents.
02-05-2010, 11:43 AM   #10
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Also, take a look at the bokeh of these zooms, so that you don't get a nasty surprise in a critical situation.

24-70 f2.8 vs. 24-105 IS f.4 cont. - FM Forums (Tamron 28-70; looks OK)
Strange bokeh with Tamron 17-50mm f/2.8 - The Photo Forum - Photography Discussion Forum (Tamron 17-50; looks nasty)
02-05-2010, 12:10 PM   #11
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I have a 50 1.4 with my K200 and I love it for shooting the grandkids in dim interior light without interfering with their play or sleep. I also like the bokeh doing close ups of flowers hand held in the field. The zoom is nice for framing your shots but if you post process you can take care of that later. I understand that your k-x does a lot better than my k-200 with ISO so my experience may not apply.
02-05-2010, 12:43 PM   #12
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According to the ISO numbers, the K-x should do well indoors with a Tamron 28-75:2.8 lens, especially for electronic display. I just received a K-x so will know soon if it is a practical combination.

I'm not yet going to throw my F50:1.7 though!

A similar & important related question is about high school gyms; is f:4 fast enough with the K-x for basketball, theater, etc? Again the numbers imply it should be fast enough - if so a whole bunch of good, long lenses become viable!

Dave in Iowa
02-06-2010, 11:17 AM   #13
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Thank you all for sharing your experiences...
When a said I had experience using primes, was just shooting a couple of hours with friend's cameras...I'm really an amateur.
Right know I don't to spent a lot of money on lens so I'm going to think a little bit on everything you said!
02-08-2010, 05:12 AM   #14
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If you're just getting started with photography and looking to progress, I'd recommend you get a fast, cheap prime like the 50mm f/1.4. By walking around to frame your shots rather than (lazily) zooming you'll think and learn more about perspective and composition.

OTOH, if you're just looking for the point-and-shoot experience, get a 16-50mm f/2.8 zoom and be done with it.
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