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PostPosted: Tue Sep 13, 2011 7:25 pm 
Just getting my head round a couple of principals and have some dumb questions to ask.

Because wider angle lenses 'see' more and therefore have more light available to them does this make them faster for the same f stop or would say f4 on a 24mm lens require a near as damnit similar shutter speed/iso as f4 on a 200mm lens when pointing at the same place with even lighting?

So is 14mm f2.8 lens as fast as a 50mm f1.8 lens say?

Ian


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PostPosted: Tue Sep 13, 2011 7:48 pm 
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Hi Ian,

I think the word you were looking for is "principles", unless you are very friendly with your teachers. :oops: :P :lol:

How to think of this? Take a scene with a lot of "stuff" in it, all pretty much the same and all illuminated equally. Then, for a constant ISO and shutter speed, the f-number doesn't have to change to get the correct exposure whether you have a 24mm lens or a 200mm lens in use.

The trick is that the scene being photographed is composed of extended objects. But try using the same exposure, shutter speed and f-number when photographing a star and you will see a big difference between the 24mm and 200mm lenses. For a point source, in an ideal case, all the light is being concentrated on a single pixel no matter what the focal length of the lens. What matters is the diameter of the entrance pupil and you can do the math for yourself.

Hope that way of analysing the question makes sense, helps you finish off the analysis yourself and will be a useful tool in your arsenal.

Bob.

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PostPosted: Tue Sep 13, 2011 8:55 pm 
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Another way to look at it:

You're taking a photo of a couple on a beach - bright sky
Focal length for a head/shoulder shot is 50mm, with very little background in the shot

Matrix Metering - the subject will be evenly exposed

Stand in the same spot, slap a 14-24mm lens on at 24mm. All of a sudden the scene will include a lot of background, and the bright sky.

Matrix Metering - the subject will be under exposed as the camera averages out the exposure with the large area of bright sky.

Spot Metering on the subjects in both instances should result in a similar exposure for the same aperture as you're metering off the focal point, as long as you're at the same ISO and aperture.

Where a wide angle can become 'faster', if you like, is when you're hand holding. If you want acceptible sharpness, look for a shutter speed that's reciprocal to your focal length (with a 1.6 multiplier for Canon crop sensors)

So shooting at 100mm, you'll need a min shutter speed of 1/160th, without the aid of IS or VR, depending on your flavour of camera. However shooting at 14mm, you'll need a min shutter of 1/20th as long as that doesn't introduce motion blur in the scene.

Therefore in lower light you can get away handholding for longer with a wider lens before upping your ISO. So in a very roundabout way, it's 'faster', as long as you get acceptable results with the lens at or near wide open.

For example, with my 85mm, I'm shooting on a min shutter speed of 1/80th or 1/100th before the auto ISO kicks in to up the ISO up to a max of 6400. That's with a max aperture of f1.4. With my 14-24 at night, I've got a minimum shutter speed of 1/30th before the auto ISO kicks in. Even though it's 2 stops slower at f2.8 max aperture, I'm able to shoot at the same ISO as the 85mm f1.4 and get acceptable sharpness.


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PostPosted: Tue Sep 13, 2011 9:04 pm 
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So after all that:

A wide angle can be 'faster' if you're shooting in matrix or centre weighted metering (but not with acceptable results)

Correctly exposing for the subject with spot metering will result in the same exposure (shutter,ISO,aperture), irrespective of the focal length, and to the detriment of the background (potentially blown highlights)

You can potentially 'get away' with hand holding for longer in low light or at night with a wider angled lens, purely because of the focal length and min shutter speed required for acceptably sharp shots. This can result in shooting into the dark for longer than a faster but longer focal length lens.


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PostPosted: Wed Sep 14, 2011 5:35 am 
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The whole point of the f-stop is that for a given combination of shutter speed and ISO two lenses with different focal length will give the same exposure if the f-stop is equal. As you say, a wide angle lens sees more. It sees more light which it can gather compared to a longer lens with smaller field of view. That's why the entrance pupil needs to be larger on longer lenses to obtain the same light-gathering capabilities, i.e. the same f-stop

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PostPosted: Wed Sep 14, 2011 11:55 am 
Thanks for your replies guys, much appriciated.

janern- Thats what I thought but there was something not making sense too.

Phil- Thats what was not making sense and now its clarified. Wider angle lenses arent faster but are easier to hand hold at lower speeds making them feel faster.

Shukkran


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