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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 03 Aug 2007 (Friday) 19:15
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Question re: Longer Focal Lengths and More Light

 
Mum2J&M
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Aug 03, 2007 19:15 |  #1

I've been using the 70-200L 4 IS at the long end and have noticed I need considerably more light than I'd imagined - even in very bright sunlight. Is this pretty normal?


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Tumak
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Aug 03, 2007 19:27 |  #2

Should be f4 all the way. However 70 needs about 1/60 sec and 200 needs about 250 depending on skill lever. Then probably 2 or 3 stops less for IS. However IS does not stop action, only lots of light for a fast shutter speed will do that. If you are trying to catch that cute little critter in your avatar, good luck.




  
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Hermeto
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Aug 03, 2007 19:28 |  #3
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I don’t understand the question.

Focal length has nothing to do with exposure (amount of light as you call it), except indirectly - that you need faster shutter speed to avoid camera motion blur with longer lenses, and therefore wider aperture or higher ISO to keep exposure the same..


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rammy
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Aug 04, 2007 08:19 |  #4

Tumak, that is to do with getting a sharp picture, not on metering, which is what I think the OP question is about.

I think what is happening is that at 70mm you have more of the scene in view and probably more light if some sky or bright areas are framed and so eval-metering (I guess you are using that?) will give you fast speed. When you zoom in to 200mm you may be zooming into a darker more specific area (compared to the 70mm view) and so not so many bright areas meaning you have to slow the shutter or open the aperture.

Have a look at some threads on POTN on metering scenes and should make some sense. Also, try spot-metering and meter at 70mm and 200mm, without reframing and you will get the same readings.


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Mum2J&M
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Aug 04, 2007 09:09 |  #5

I was using spot metering mostly since I knew I didn't want the whole pic to be overexposed. I guess that's right - the farther away, the more camera shake, the higher the shutter speed, the more light needed via aperture and/or ISO. I was mostly shooting at 200 and having trouble, so that would make sense. Less trouble with the wider FL i.e. 70-ish.


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Mum2J&M
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Aug 04, 2007 09:33 |  #6

I was using an ISO of 400, which just seemed very high to me... I guess I didn't want my aperture too wide since I didn't want the blur and my shutter speed had to be on the higher side for no camera shake/to freeze the action. So the ISO was just seeming really high, but because of what I was trying to do, it makes more sense.


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Tumak
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Aug 04, 2007 22:02 |  #7

I am sorry Rammy, but I have reread all this about three times and I do not understand what was wrong with my answer.




  
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Jim ­ G
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Aug 04, 2007 22:20 |  #8

Mum2J&M wrote in post #3667446 (external link)
I was using an ISO of 400, which just seemed very high to me... I guess I didn't want my aperture too wide since I didn't want the blur and my shutter speed had to be on the higher side for no camera shake/to freeze the action. So the ISO was just seeming really high, but because of what I was trying to do, it makes more sense.

400 isn't high - 800 is very clean on a 20d/30d! I regularly shoot at 1600 and with a proper exposure the noise is minimal :)


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pagnamenta
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Aug 04, 2007 22:31 |  #9

If I understand correctly, you find that at the wider end, the image seems brighter and at the long end, it looks darker through the viewfinder. If you use the same shutter speed for both shots, the wider shot is brighter. Is this right? If not, sorry.


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gooble
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Aug 04, 2007 23:41 |  #10

Mum2J&M wrote in post #3667446 (external link)
I was using an ISO of 400, which just seemed very high to me... I guess I didn't want my aperture too wide since I didn't want the blur and my shutter speed had to be on the higher side for no camera shake/to freeze the action. So the ISO was just seeming really high, but because of what I was trying to do, it makes more sense.

What were you shooting? What "blur" did you not want? Blur is typically a function of motion and slow shutter which the aperature will have no effect on. If the images are exhibiting motion blur that you don't want, you need to open the aperature wider and increase the shutter speed. However, the increased OOF areas due to narrower DOF will happen when opening the aperature up. Anyway I'm not sure if you're complaining about motion blur or OOF due to narrow DOF but maybe this helps.




  
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Mum2J&M
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Aug 04, 2007 23:47 |  #11

I'm speaking of the out of focus areas due to narrower dof. I guess I can't really get my head around that concept. Why does this happen? I know the slower shutter speeds will cause blur, but why would there be increased oof areas with a narrower dof? Are you just talking about a smaller focal plane in general due to a larger aperture opening? Sorry. Now I guess I'm confused.


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Hermeto
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Aug 05, 2007 00:32 |  #12
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Smaller focal plane?
No such thing, I’m afraid.

Yes indeed Mum, you seem to be confused a bit..


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gooble
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Aug 05, 2007 00:38 |  #13

Hermeto wrote in post #3670845 (external link)
Smaller focal plane?
No such thing, I’m afraid.

Yes indeed Mum, you seem to be confused a bit..

Right. There is an area in front of and behind the focal plane which is encompassed by the DOF.




  
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bieber
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Aug 05, 2007 00:49 |  #14

Mum2J&M wrote in post #3670713 (external link)
I'm speaking of the out of focus areas due to narrower dof. I guess I can't really get my head around that concept. Why does this happen? I know the slower shutter speeds will cause blur, but why would there be increased oof areas with a narrower dof? Are you just talking about a smaller focal plane in general due to a larger aperture opening? Sorry. Now I guess I'm confused.

DOF simply refers to the amount of depth before and past your focus point your lens can have in focus. This gets smaller as you zoom out, because DOF is proportional to aperture value, and inversely proportional to focal length. i.e., the smaller the number you get by dividing [aperture value]/[focal length], the less DOF you have.


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SkipD
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Aug 05, 2007 02:13 |  #15

This link to Canon's web site (external link) may help with a basic understanding of the concept of depth of field. Make sure you scroll all the way down and carefully look at the whole page.

The menu at the left side of the page will take you to other topics with a lot of basic photography information.


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Question re: Longer Focal Lengths and More Light
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