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09-07-2010, 05:52 PM   #1
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Pentax and Lens question

I have a pentax k-x and a pentaxf3.5-6.3 18-250mm lens. I am finding that alot of the shots I take at zoos and stuff is at max zoom and at F6.3 the best shutter speed i can get is around 1/20-1/50 of a second.

I am thinking of purchasing the sigma 70-200mm F2.8 lens. At it maximum zoom of 200mm since it is f2.8 what sort of shutter speed would i get in the same situation as above, example if i am getting 1/20 at f6.3 what would i get at F2.8? Is there a rule that can be followed for shutter speeds at different apatures??
Off course this is assuming I use the same ISO each time as well

I am trying to work out if I would be able to shoot faster in low light with this new lens

thanks

David


Last edited by isusdfr; 09-07-2010 at 06:14 PM.
09-07-2010, 06:14 PM   #2
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I think that's an extra 2.5 stops of light, so if you're in manual mode, center the light meter, and then increase the shutter speed until the light meter reads -2 and that will give you a close estimate

What ISO do you use? if you get the cheaper DA 55-300mm f/4-5.6 you could basically have a 300mm f/4 by bumping up the ISO 2/3s of step (two clicks of the dial), at least that's what my K7's light meter said, and the Kx's high ISO performance is very impressive, you could definitely use it

Last edited by future_retro; 09-07-2010 at 06:23 PM.
09-07-2010, 06:44 PM   #3
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Buy the fast lens, sure. Or bump the ISO. That's what the Kx is great at, higher ISOs. Or go into TAv mode, set A= f/8 and T= 1/100 or faster, and let the ISO float.

Do the zoos you visit have anti-flash policies?
09-07-2010, 06:50 PM   #4
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Unless they added it in a recent update I don't think that the Kx has a TAv mode

09-07-2010, 07:39 PM   #5
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The difference in "aperture" is easy to check by setting the camera to "A" mode, pointing toward a light source and simple changing the aperture from 6.3 to 2.8 to see to complementary shutter speed :-)

Either the Sigma or Tamron 70-200 is a nice lens, though a little heavy due to the larger maximum aperture. A monopod may help your arm muscles, as well as allow use of a slightly slower shutter.

All lenses are bit bit sharper when you stop down, so those lenses will be a bit better at, say f5.6, which is still an big improvement over the wide open f6.3 you now have.

The 55-300 is a decent lens and will give you more range which is helpful at the zoo, at the trade off of speed.
09-07-2010, 08:29 PM   #6
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I'd suggest doing a web search for the terms iso, aperture, and shutter speed, to learn how they relate. Short answer: increasing/decreasing ISO or shutter speed by a factor of two matches increasing/decreasing aperture by a factor of the square root of two (rounded to 1.4 for convenience. Most people just memorize the sequence of apertures that corresponds to doubling/halving shutter speed or ISO: f/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.
09-08-2010, 06:16 AM   #7
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QuoteOriginally posted by Marc Sabatella Quote
I'd suggest doing a web search for the terms iso, aperture, and shutter speed, to learn how they relate. Short answer: increasing/decreasing ISO or shutter speed by a factor of two matches increasing/decreasing aperture by a factor of the square root of two (rounded to 1.4 for convenience. Most people just memorize the sequence of apertures that corresponds to doubling/halving shutter speed or ISO: f/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.
Here is a camera simulator that lets you do that.

Aperture, shutter and ISO value | SLR Camera Simulator

Tim
09-08-2010, 06:29 AM   #8
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QuoteOriginally posted by isusdfr Quote

I am thinking of purchasing the sigma 70-200mm F2.8 lens. At it maximum zoom of 200mm since it is f2.8 what sort of shutter speed would i get in the same situation as above, example if i am getting 1/20 at f6.3 what would i get at F2.8?

Shutterspeed 1/100 at f 2.8 equals 1/20 at f 6.3 (approximatly).

09-08-2010, 09:04 AM   #9
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QuoteOriginally posted by isusdfr Quote
Is there a rule that can be followed for shutter speeds at different apertures??
The short answer is, "Yes...there is a rule." It used to be that each step up or down in aperture was equal to a step up or down in shutter speed. It was a halving or doubling of the exposure. These days, it's become less clear because so many of our cameras use variable apertures and shutter speeds. Here's a link to a Wikipedia article that shows the common apertures: F-number - Wikipedia, the free encyclopedia
If you're now shooting at f6.3 at 1/50 of a second, as future_retro said, you're going to gain about 2.5 stops by going to an f2.8 lens. So...do the math. One f-stop wider would put you at 1/100 in shutter speed...a second f-stop wider would put you at 1/200 in shutter speed..plus 1/2 stop makes it about 1/300 in shutter speed. You can also adjust your ISO to make these same gains. Again, starting at f6.3 at 1/50 as a starting point...and assuming you're shooting at ISO 100, bumping your ISO up to 200 would let you use a 1/100 shutter speed. Doubling that ISO up to 400 would let you shoot at 1/200 shutter speed. Moving on up to ISO 800 would let you shoot at 1/400 at f6.3. Make sense?
09-08-2010, 02:39 PM   #10
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QuoteOriginally posted by isusdfr Quote
if i am getting 1/20 at f6.3 what would i get at F2.8? Is there a rule that can be followed for shutter speeds at different apatures??
The F-stop notation means the width of the aperture is the focal length of the lens divided by the number. So at 200mm, f/2.8 means the aperture is 200/2.8 = 71mm; and f/6.3 means the aperture is 200/6.3 = 32mm. So f/2.8 is 2.25 times as wide as f/6.3. When you are comparing two F-stops, the focal length cancels so that's can be worked out more simply as 6.3/2.8.

Because the lens is a circle and lets in light over its entire area, a lens twice as wide lets in four times as much light. So f/2.8 lets in 2.25*2.25 = 5 times as much light as f/6.3. The sensor responds in a linear way, so to receive the same amount of light, the shutter needs to be open for 1/5th as long. So going from f/6.3 to f/2.8 should reduce the shutter time from 1/50s to 1/250s to give the same exposure.

Some people prefer to think in terms of stops, and they know all the magic numbers, f/1.4, f/2.8, f/4 etc, and they know the effect of moving up a stop or down a stop to the next magic number. I find it helps to know what the numbers mean and then just do the maths. The numbers aren't really magic. They arise out of the geometry, and are mostly based on the fact that the square root of 2 is 1.4. So in going from f/2.8 to f/4 you are multiplying by the square root of 2, which means letting in 2 times as much light, and halving the shutter speed.
09-08-2010, 03:30 PM   #11
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There is more to this than just lens speed. The Sigma 70-200 f/2.8 weighs almost 3 lbs, and there is a reason a tripod mount is built in to the lens. This is a large, heavy lens in the $900 range. Sports or nature photography come to mind, and lugging it around for a few hours at the zoo or at the playground won't be much fun. This isn't a casual shooting type of lens.

But before buying a new lens, just bump the K-x ISO up to 400, 800 or even 1600, and shoot whatever mode you are shooting now. Push the ISO button on the back of the camera and select a higher, fixed ISO speed. You might be surprised how good the K-x is at higher ISO, and might even not have to buy a new lens. (You can save higher ISO settings between powering up/down the camera through the "Custom" menus...)

That said, take a look at the Pentax DA 55-300. It's $350, a fraction of the weight/cost, and is highly regarded. A little faster than your Sigma 18-250, and I'd guess the Pentax DA 55-300 has better image quality in the 200-300mm area. Even the Pentax DA 50-200 is pretty good, very inexpensive and lightweight.
09-08-2010, 03:51 PM   #12
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Thanks for the info everyone. I realise that the lens is heavy I would be using at the zoo for those shots that i want to make into a poster or print for the wall. I suppose I should have said that in the situation when i find that i am shooting at 1/20-1/50 of a second that I usally am at ISO 1600.

Oh and bTW I am in Australia the 50-300mm pentax lens is around $700 that is only $200 cheaper than the 70-200 F2.8 that I want to get!!!

Last edited by isusdfr; 09-08-2010 at 04:07 PM.
09-08-2010, 04:28 PM   #13
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Be sure they allow lenses like that at the zoo you are interested in, and tripods too - neither is a given.
09-08-2010, 06:09 PM   #14
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They allow the lenses, no tripods just monopods.
09-08-2010, 06:58 PM   #15
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QuoteOriginally posted by isusdfr Quote
They allow the lenses, no tripods just monopods.
I have the Tamron 70-200. It is a bit heavy but I think that actually helps with stability. It takes a bit more energy to get to to move around. The much lighter Pentax 55-300 is all over the place.

I do use a monopod, not just for stability, but because I have a bad shoulder, and I'm frequently hauling a 150-500, too.
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