Lens speed

Dolby

Dolby

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Is there a difference between max aperture on dSLR lenses and P&S?

I see many P&S are F2.8 with some compacts F2.0 - but dSLR lenses in my price range are typically over F3.

Does this mean they'll be dull? Or is the way they measure different?
 
Why would they be dull? :confused: The dslr lenses should be sharper if anything because they're better made and they're not trying to cram 20x worth of zoom in one lens.

The way I see it is that the cheaper lenses can get away with being slower because a dslr has a higher usable ISO range. I've taken many a shot at 3200 with my 400D that has less noise than my aP&S at 800.
 
I assumed they'd be dull (dark) cause less light goes through?

ISO thing makes sense though.
 
Dolby said:
I assumed they'd be dull (dark) cause less light goes through?

ISO thing makes sense though.
Click to expand...
The aperture on a P&S is rarely fixed at f/2.8. So your SX1 might be 2.8 at its widest but as you zoom that drops to f/5.7.
 
F stop is calculated, based in certain physical lens measurements. I'm not sure of details, but if a lens has a lot of crap glass (cheap zooms) in it, it may actually let less light through, than a good lens (good primes) with the same F stop. Also, it's a ratio, so if you've got a tiny CCD, you need less lens diameter than with a DSLR sized CCD to achieve the same ratio, but as bwana said, DSLRs have bigger CCD pixels, so they can catch more light per pixel, with less noise.

I know, I'm making no sense. Let me put it this way, 35mm needs (roughly) a 200mm lens to make a 4x magnification, but a P&S might get away with a 10mm lens at the same magnification.

And to conclude making no sense, picture quality has got to do with the sensor size, the sensor pixel density, the quality of the lens, the sensitivity of the sensor, etc. And that's just the 'hard' factors, then there are things like focus accuracy and so on that make a DSLR superior to a P&S, even if the specs seem similar.

Edit: Wikipedia makes more sense. F stop is:

the diameter of the entrance pupil in terms of the focal length of the lens
Click to expand...
. So if you convert everything to total lumens landing on the CCD, you'll see that your P&S CCD has fewer lumens to work with than your DSLR (for the same f stop), and hence, has a poorer image quality.
 
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lol, i half tried typing a message, but I'm sure it made less sense than yours Drunkard. I think the gist is that an F number is only one factor to determine the quantity and quality of light that is eventually captured on the sensor. The physics and mathematics remain the same to calculate the F number - but as bwana and drunkard stated, the photons captured by the sensor on a DSLR (due to the size of the sensor) is much more usable.

The optics of DSLR lenses are also much better and (for the most part) present less aberrations and distortions than a P&S zoom.
 
SubliminalThought, I've got a headache just thinking about it. :)

Maybe it'll make sense if I try a numerical example. Assume 2 cameras:

1. P&S - 30mm lens (f/2.8) giving a 6x magnification on a 10 megapixel CCD measuring 2.4x3.6mm

2. DSLR - 300mm lens (f2.8) giving a 6x magnification on a 10 megapixel CCD measuring 24x36mm

From the f stop equation, the entrance pupil of the P&S lens is 7.1mm, and 71mm for the DSLR. Assuming the same light intensity (lumens/m²), the DSLR gathers 100x more photons than the P&S (entrance pupil surface area is 3957mm² vs 39.6mm².) So each DSLR pixel gets 100x as many photons (each DSLR pixel is of course 100x bigger, so the light intensity on the DSLR pixel is the same as on the P&S pixel). This allows a DSLR pixel to operate in a more 'relaxed' fashion, giving a better quality picture, not a brighter picture. Why? Because a change of 1 photon on the P&S will yield the same change as 100 photons on the DSLR (the DSLR sensor is more 'forgiving'.)

Which is why, even if we ignore the other factors which differ between DSLRs and P&Ss (and we really can't ignore them), the DSLR produces better pictures.

That's the best I can do. If someone can explain it better, please do.
 
I actually understand a little of what you're saying - so there's more to it than simply F stop number.

I think I also wouldn't be able to do Macro shots without a diffenet lens?

So like 3 different lenses are needed - ouch!
 
Dolby said:
I actually understand a little of what you're saying - so there's more to it than simply F stop number.

I think I also wouldn't be able to do Macro shots without a diffenet lens?

So like 3 different lenses are needed - ouch!
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Macro lenses are multi purpose lenses - ie a 100mm macro lens is also going to make a mighty fine portrait lens. Existing lenses can also be used in conjunction with expansion tubes.

Then again you can always cheat like I do and just turn the 50mm f/1.8 around and use it that way. :)

A lens should also last a lifetime.
 
Thanks bwana.

This one is particular I'm looking at says minimim focusing distance of 31cm. Isn't macro a few cm max?
 
Dolby said:
Thanks bwana.

This one is particular I'm looking at says minimim focusing distance of 31cm. Isn't macro a few cm max?
Click to expand...

Nope. Macro refers to 1:1 ratio. The minimum working/focus distance is what you are talking about. My 24-105 has a 41cm and a magnification factor of about 1:0.21 (or something). The Canon 100mm f/2,8 Macro has a minimum working/focus distance of about 15cm and a magnification factor of 1:1. The Canon 180mm Macro has a minimum working/focus distance of about 48cm for it's magnification factor of 1:1. So working/focus distance depends on the focal range and macro just means a magnification ratio of 1:1.

Did I get that right bwana?
 
As MMM said, true macro lenses are 1:1. Some manufacturers (notably Sigma) designate some of their zoom lenses as macro. AFAIK, their are no zoom lenses which are true macro.

[MMM, a more important number is the minimum working distance, which is the distance from the front of the lens to the subject. For the Canon 100, it's 15cm as you said, but for the 180mm it's 24cm (source). AFAIK, the min focussing distance is the distance from the "film plane" to the subject.]
 
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Ok ... note I'm not comparing, but asking to learn :

Why do P&S go 1cm away from the subject, and its clear?
 
Dolby said:
Ok ... note I'm not comparing, but asking to learn :

Why do P&S go 1cm away from the subject, and its clear?
Click to expand...

I imagine it's all to do with the diameter of the lens, as well as the geometry of the lens elements.
 
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