Focal length & aperture comparison question

[Quilb]

Hey,

I asked this in one of my other threads but i think because of the change it got lost and thought i would ask where its on its own so:

I notice on my S3 IS lens it says 6.0-72.0mm 1:2.7-3.5
Now from what i have seen its a 6x crop sensor.

Are the apertures comparable to that on proper lenses of an SLR?

eg.

10mm focal length on S3 IS (35mm equiv of 60mm) @ f/3.5
compared to
40mm lens on D200 (35mm equiv of 60mm) @ f/3.5

would one image be brighter then the other?
only reason i'm thinking it would be is because of the larger sensor on the D200?

[DaveB]

The apertures are equivalent, as the "f-stop" value is the ratio of the focal length to the diameter of the aperture within the lens.
Thus as the f-stop doubles (e.g. f/4 -> f/8 ) the area of the internal lens aperture reduces by a factor of 4, and f/4 -> f/5.6 (an increase of 1.414x: the square root of 2) involves a decrease in area by a factor of 2 (and thus a halving of the amount of light).
While a 100mm/2 lens will have an internal diameter of 50mm, and a 20mm/2 lens will have an internal diameter of 10mm, by involving the focal length of the lens in the calculation of the f-stop we're left with just a measure of how much light the lens lets through.

The size of the sensor behind the lens doesn't affect the amount of light involved (per unit of area) and only really affects the field of view (the larger the sensor the wider the field captured by the sensor).

[Yi-P]

Hey,
Are the apertures comparable to that on proper lenses of an SLR?

In some way, yes. As mentioned above, aperture is just a ratio between your focal length and diameter of your diaphragm blade. I believe you get same exposure given the f/stop and shutter values are somewhat equivalent, but is very hard to tell on a P&S anyway.

[Quilb]

The apertures are equivalent, as the "f-stop" value is the ratio of the focal length to the diameter of the aperture within the lens.
Thus as the f-stop doubles (e.g. f/4 -> f/8 ) the area of the internal lens aperture reduces by a factor of 4, and f/4 -> f/5.6 (an increase of 1.414x: the square root of 2) involves a decrease in area by a factor of 2 (and thus a halving of the amount of light).
While a 100mm/2 lens will have an internal diameter of 50mm, and a 20mm/2 lens will have an internal diameter of 10mm, by involving the focal length of the lens in the calculation of the f-stop we're left with just a measure of how much light the lens lets through.

The size of the sensor behind the lens doesn't affect the amount of light involved (per unit of area) and only really affects the field of view (the larger the sensor the wider the field captured by the sensor).

This has just made me more confused. Is there a way you can simplify what you wrote?

sorry maybe i should wear a dunce hat

[Andyt]

This has just made me more confused. Is there a way you can simplify what you wrote? sorry maybe i should wear a dunce hat
Hi! Quilb,

If you are trying to compare the performance of two lenses it is necessary to have an understanding of the relationship of a number of factors, such as aperture, f stop (focal length, as stated by DaveB), ISO, shutter speed etc.

It took me a while to get the gist of it, and only after having it explained to me a few times by a number of people and reading the numerous articles that abound in the world of Google did I feel confident that I understood the "basics". But not enough to write an explanation here

As a starting point suggest a look at http://photo.net/learn/making-photographs/exposure

And keep at it until you get there, it will amaze you the difference it makes to your photography and the way you "create" pics

My 2c worth.............

[Quilb]

Well i have a basic understanding, but the way i understood the above was like so:

40mm lens on D200 (35mm equiv of 60mm) @ f/3.5
will let in more light then the
10mm focal length on S3 IS (35mm equiv of 60mm) @ f/3.5

As the 40mm lens has larger opening to let more light in @ f/3.5


So essentially i would get same picture but the one taken with 40mm lens would be brighter?

This is not how i thought it worked but kinda makes sense to me. Is this correct or did i read it all wrong?

[ATJ]

The f/stop basically tells you how much light you will get in. At f/16 you will get exactly the same amount of light regardless of the lens being used.

The whole point of f/stops is you don't need to know anything else about the lens.

Note that by "same amount of light" I am referring to the amount of light hitting a fixed area rather that the whole "film plane" or CCD area. (I hope this doesn't further confuse you...) If you were to compare a true 35mm camera to a 6x6 with lenses at f/16, there will be more TOTAL light reaching the film plane of the 6x6 simply because the film plane is larger (60x60mm versus 36x24mm) but (and this is the important part) you would get EXACTLY the same amount of light in any 36x24mm area on the 6x6 film plane as you do on the 35mm camera (assuming the overall exposure of the image is the same).

If you were using a point and shoot, a DX, a FF and a 6x6 camera with different lenses to shoot the exact same scene in exactly the same conditions with each camera using the same ISO, you would use the same f/stop and shutter speed to get the same image with the same overall exposure. Similarly, if you were to use the same camera but changed the focal length of the lens and moved closer or further from the subject to frame it the same, you would use the same f/stop and shutter speed for all the shots to get the same exposure.

[gstark]

So essentially i would get same picture but the one taken with 40mm lens would be brighter?

Given the same shutter speed and ISO was used, no.

I think that where you may be getting some confusion, is that I suspect that you're trying to relate a given aperture value to a the physical "opening" that any aperture value might represent.

Let's go back a step, if we can, and just understand that, in any given situation, there's a finite amount of light available for you to shoot with. Doesn't matter if it's outdoors, indoors, under water, sunny, raining .... or you're going to be using flash.

The deal is that, at the moment that you're going to make your actual exposure, there is only a finite amount of light.

To make a good exposure, you, as the photographer, need to understand exactly how much light that might happen to be, so you need two things: a way of assessing how much light there is, and a means of expressing the result of your assessment.

Most cameras today have built-in light meters, or you can buy a handheld one. That takes care of the assessment part of the process.

The problem lies in how the results of this assessment are expressed: mostly, it's expressed in terms of a combination of a set of values, those values being a shutter speed, and aperture, and an ISO.

What's happened is that everything has been laid out on a platter for you: "here's the settings you need and you'll get a great photo" is what the camera seems to be saying to you.

Wrong!

What the camera's actually saying to you is "Here's a set of values that seems to be reasonable for the current available light value. Please use these as a starting point, and now make some intelligent judgments about the actual scene you're shooting, your composition, and how you want the final image to look, and then you can make an image".

The problem is, as I said, the camera is giving you one (of many potential) final result. The step of telling you the actual EV (Exposure Value) has been omitted.

Mostly, that's probably ok, but if you're anting to understand this stuff .... well, you need the data, don't you?


And an EV is exactly that: it's just a measurement of light. THAT is the means of expressing your assessment of the value of the available light in any given situation. It's always expressed in the same way, and it always has exactly the same meaning.

EV 5 is always EV 5, just as saying you're driving at 45kph expresses a certain constant, so too does saying that the current light is EV 5. Or EV 11. Or whatever.

Now - and this is where it gets tricky - there's no need to know too much about specific EVs or how much light they represent. What's more important is to understand the concept that, like driving at a given speed, any given EV simply represents the amount of light available to make an image.

Hand in hand with that concept goes the simple fact that any given aperture and shutter speed combination results in the passage of a certain amount of light. No more, and no less.

And, for any given combination of shutter speed and aperture values, there's a myriad of other combinations of shutter speeds and aperture values that will result in exactly the same passage of exactly the amount of light.

No more, and no less.

This is all very easily expressed mathematically, and quick look at the range of shutter speeds available on a camera will show you some of this: 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, and so on.

The only other major factor is that of emulsion/sensor sensitivity. This tells you how receptive to light the medium that is going to record your image is, it's it's measure in ISO.

So .... we can take a particular value of measured available light, apply to it the sensitivity of the recording medium, and we can then deduce a set of shutter speeds and aperture values and (hopefully) come up with a well exposed image.

And this is true regardless of the camera we're using (coming back to the point of your question).

I can use a handheld meter, see an EV of ... whatever ... maybe then set ISO 100, 1/250 @ f/8, and apply those values across a broad range of cameras, and obtain a set of correctly exposed images from each of those cameras.

Those cameras could range from a simple compact film PHD, a more complex but still simple digital PHD, a consumer DSLR, a pro DSLR, a medium format SLR or DSLR, and then even a LF sheet film camera.

Your confusion lies in that fact that, for each of these cameras, because of their actual sizes, and because of the relative size of their componentry (image sensing area, lenses and focal lengths etc) the physical size of some elements discussed, such as the aperture, will be very different.

That's the nature of the beast: aperture is based upon a number of complex relationships, and while its physical attributes are governed by those complex relationships, the end result - its expression as a simple value - is, to all intents and purposes, constant across all the different cameras that we're likely to encounter.

[DaveB]

I'll try again.

Well i have a basic understanding, but the way i understood the above was like so:

40mm lens on D200 (35mm equiv of 60mm) @ f/3.5
will let in more light then the
10mm focal length on S3 IS (35mm equiv of 60mm) @ f/3.5

As the 40mm lens has larger opening to let more light in @ f/3.5

Well, it depends how you measure "more light". Think in relative terms rather than absolute terms...
For each "unit" of light hitting the front of the lens, an f/3.5 lens passes 1/3.5 of that out the back.

The whole point of f/stops is you don't need to know anything else about the lens.

Bingo!

The size of the sensor behind the lens doesn't affect the amount of light involved (per unit of area) and only really affects the field of view (the larger the sensor the wider the field captured by the sensor).

If both lenses have the same aperture (f/3.5 in your example) then focal length (e.g. 40mm on your D200 example, and 10mm on your S3 IS example) does not affect the "amount" of light passed through.

[Quilb]

Ahh i see.

I believe i understand what you are saying. Mmmmm, So in the end the picture will be the same (exposure anyway) except on the DSLR it will in theory be a better image (Sharpness) due to better lens quality, sensor, etc.

I would get less noise at same ISO because of the larger sensor of the DSLR though. is that correct?

Also that would mean i would need to buy the more expensive lenses in order to take pictures in the same amount of light with same shutter speed and ISO that i do on my current P&S as the built in lens is apparently f/2.7-3.5

[ATJ]

I would get less noise at same ISO because of the larger sensor of the DSLR though. is that correct?

I don't believe so. The amount of noise is not related to the size of the sensor itself, but rather the electronics of the sensor and the software behind it. For example, I believe Canon and Nikon have the same size sensors and yet at high ISO, Canon have far less noise.

Also that would mean i would need to buy the more expensive lenses in order to take pictures in the same amount of light with same shutter speed and ISO that i do on my current P&S as the built in lens is apparently f/2.7-3.5

No. Definitely not! It has nothing at all to do with the cost. f/stop is f/stop. Full stop. Same f/stop + same shutter speed + same ISO = same exposure regardless of the cost of the lens.

[DaveB]

I would get less noise at same ISO because of the larger sensor of the DSLR though. is that correct?

Not exactly.

In general it's true that the larger each pixel is (not the sensor as a whole) then the more photons it can capture and proportionally lower the noise. But sensor technology keeps progressing, and at the same pixel size the noise levels keep getting better and better.

[Quilb]

lower f-stop lenses are however more expensive is what i meant. Generally a f/2.7-3.5 lens will be a lot more then the usual f/4-5.6 kit lens you find.

[ATJ]

lower f-stop lenses are however more expensive is what i meant. Generally a f/2.7-3.5 lens will be a lot more then the usual f/4-5.6 kit lens you find.

Yes, a lens with a larger maximum aperture (smaller f/stop) will generally be more expensive, but you generally won't take all your shots at that aperture. An f/2.8 lens will require the same ISO and same shutter speed as an f/5.6 lens when both lenses are used at f/5.6, f/8, f/11, etc.

So it will only every make a difference when you are shooting wide open or close to it. Anything smaller and the lens will be no different. Additionally, a DSLR lens will allow for much smaller apertures that a point and shoot. My Nikon Coolpix 4500 can only go to f/10.5 whereas all my (D)SLR lenses go to at least f/22 and smaller. The smaller aperture gives much more depth of field and also allows for slower shutter speeds in high light situations.