Probably a silly aperture question!

[Simon Devlin]

Do all aperture values for any lens let in the same amount of light?

The answer is probably no!

For example, a Pentax f/1.4 lens at f/4 allows a greater quantity of light than say a mobile phone lens @ f/4?

Surely this is what we pay the extra money for and not just the ability to have a higher aperture, blurring out the foreground/background and bokeh benefits!

I just need the clarification!

 

[mike703]

Simon Devlin wrote:

Do all aperture values for any lens let in the same amount of light?

The answer is probably no!

Amount of light? No. Light intensity? Yes.

For example, a Pentax f/1.4 lens at f/4 allows a greater quantity of light than say a mobile phone lens @ f/4?

Yes but the sensor is much larger in the former case, so the lens has to let more light in to cover the much larger surface area of the sensor (i.e. the image circle is much wider). But for a given amount of sensor (per mm2) the light hitting each sensor will be the same (total amount of light divided by area = intensity) so the exposure needed will be the same

Surely this is what we pay the extra money for and not just the ability to have a higher aperture, blurring out the foreground/background and bokeh benefits!

Sure. An an f/4 lens that covers a FF sensor will be bigger and more expensive than an f/4 lens that only needs to covers an APS-C sensor, and so on down the food chain.

Best wishes

--

Mike

http://www.pentaxphotogallery.com/artists/mikeward

 

[Simon Devlin]

Amount/intensity is what I meant. So the more intense light requires a shorter shutter speed for a given exposure.

Thanks Mike

 

[DAVID MANZE]

Hi,

The aperture is a function of registry distance and the diameter of the lens opening.

 

[LDBOK]

Assuming the same ISO, the EV required for proper exposure will be the same regardless of sensor size.

http://larrydbrown.com

 

[Tan68]

f/4 is always f/4 for the sake of calculating exposure.

the aperture will be physically bigger for longer focal length lenses. a 100mm lens at f/4 will have a larger aperture/opening than a 50mm lens at f/4. the opening in the 100mm lens, at f/4, is 100/4=25mm. the opening in the 50mm lens is 50/4=12.5mm.

i guess the 100mm lens lets in more light, overall, but the intensity of the light... the brightness of the light that hits the sensor is the same between the lenses.


so, the little cell phone lens might be a 5mm lens. at f/4, the aperture is 5/4=1.25mm. it will let in a lot less total light than either the 50mm lens or the 100mm lens. however, the intensity of the light hitting the little cell phone camera sensor is the same. because all the lenses are set for f/4. and, the rest is mike's explanation.

 

[Gerry Winterbourne]

Simon Devlin wrote:

Amount/intensity is what I meant.

Which? You get a bigger amount of light through a bigger hole. As the sensor gets bigger the hole gets bigger too - and the rate at which it gets bigger means that a given f-stop gives the same intensity although the total amount gets bigger.

So the more intense light requires a shorter shutter speed for a given exposure.

Well, yes, as far as it goes; but of itself that doesn't mean much. If you have the same ISO and f-stop set on two cameras, one a tiny-sensor phone camera and the other a FF (D)SLR and you shoot the same scene you'll get the same shutter speed from both.

 

[awaldram]

The real answer is no every lens lets different levels of light in for the same F-stop.

F-stop is only nominally how much light hits the sensor.

T-stops are considerably more equalized allowing for absorbance of light by the lens and coatings.

better lens have low t ratings for the same F-stop.

The bubbled plastic on a cell phone is rubbish

 

[SRT201]

awaldram wrote:

The bubbled plastic on a cell phone is rubbish

Please tell me you aren't suggesting that a DSLR is better than an iPhone!

 

[Tan68]

you're right, there are variations in the amount of light that passes through lens elements. better lenses, better coatings, and other better stuff let more light through.

even if two lenses have the same transmittance, there can be slight variations in the size of the aperture.


both of these circumstances do affect how much light passes through the lens. however, they both fall into the realm of 'finer point'.

while both are true, maybe either are a bit esoteric for this thread..?

 

[Tan68]

SRT201 wrote:

awaldram wrote:

The bubbled plastic on a cell phone is rubbish

Please tell me you aren't suggesting that a DSLR is better than an iPhone!

a dSLR isn't better than an iPhone. the iPhone does apps and things like that. the iPhone is always with you and you can send your pictures right to some image hosting site. you can send all of the pictures. 28 pictures of the cat sitting on the window sill. i can listen to music while the pictures upload.

 

[Simon Devlin]

Tan68 wrote:

f/4 is always f/4 for the sake of calculating exposure.

the aperture will be physically bigger for longer focal length lenses. a 100mm lens at f/4 will have a larger aperture/opening than a 50mm lens at f/4. the opening in the 100mm lens, at f/4, is 100/4=25mm. the opening in the 50mm lens is 50/4=12.5mm.

i guess the 100mm lens lets in more light, overall, but the intensity of the light... the brightness of the light that hits the sensor is the same between the lenses.

so, the little cell phone lens might be a 5mm lens. at f/4, the aperture is 5/4=1.25mm. it will let in a lot less total light than either the 50mm lens or the 100mm lens. however, the intensity of the light hitting the little cell phone camera sensor is the same. because all the lenses are set for f/4. and, the rest is mike's explanation.

Good explanation! Hence the massive diameter of the Canon 400mm f/2.8... more like a cannon!


Although you can always select a much lower shutter speed than the average point and shoot given the same ISO.

How long will it be until cell phones have more mega-pixels than my K-30? I see some have over 8MP already!

 

[Marc Sabatella]

Simon Devlin wrote:

Amount/intensity is what I meant. So the more intense light requires a shorter shutter speed for a given exposure.

As mentioned, f/4 is always f/4 as far as figuring out what shutter speed is necessary at any given ISO level. That's the whole point of using apereture ratios like f/4 instead of stating aperture in millimeters or the like - by using ratios, we know f/4 always means the same thin in terms of expousre no matter what camera you are using.

The advantage of a DSLR over a small sensor camera / cellphone here is that you can use higher ISO levels with less noise. So while a cellphone might mak out at ISO 800 and look terrible there, thus limiting you to a 1/2" expousre at f/4 (for example), a DSLR might allow you to shoot at ISO 3200 with no more noise than the cellphone at ISO 800, thus giving you a shutter speed of 1/8" at that same f/4 aperture. Plus, a DSLR gives you the possibility of using an f/1.4 lens, which you'll rarely if ever find on a cellphone or small sensor camera.

As others mention, there is also a matter that different lenses at f/4 might let through *slightly* different amounts of light due to different thickness of glass or whatever, but that's largely irrelevant - it makes only a tiny difference in exposure and most photographers pay that no heed. fFor all practical purposes, f/4 is f/4, even if differences between two lenses are such that it's more like f/3.96 versus f/4.07 or whatever with respect to amount of light that is actually transmitted through the lens.

 

[JacobSR]

Tan68 wrote:

SRT201 wrote:

awaldram wrote:

The bubbled plastic on a cell phone is rubbish

Please tell me you aren't suggesting that a DSLR is better than an iPhone!

a dSLR isn't better than an iPhone. the iPhone does apps and things like that. the iPhone is always with you and you can send your pictures right to some image hosting site. you can send all of the pictures. 28 pictures of the cat sitting on the window sill. i can listen to music while the pictures upload.

The car isn't better than a bicycle. The bicycle is safer since I'm mostly off the road, is healthier because I'm getting my exercise and the fresh air. It's more comfortable even on dirt roads, and more fun because I get the feel of rolling on the ground, the feel of the wind striking my face, and. I do all that while I meet different people and mingle with them.



Jacob

 

[LDBOK]

Don't worry. Even if they cram 16MP into a tiny cellphone sensor, your K-30 will still have much better IQ.

 

[alanr0]

Simon Devlin wrote:

How long will it be until cell phones have more mega-pixels than my K-30? I see some have over 8MP already!

It's already happened: http://www.dpreview.com/news/2012/02/27/Nokia-808-PureView-with-41MP-sensor

Reviewed here

Cheers

 

[James O'Neill]

Simon Devlin wrote:

Do all aperture values for any lens let in the same amount of light?

The answer is probably no!

For example, a Pentax f/1.4 lens at f/4 allows a greater quantity of light than say a mobile phone lens @ f/4?

Surely this is what we pay the extra money for and not just the ability to have a higher aperture, blurring out the foreground/background and bokeh benefits!

I just need the clarification!

OK. I didn't read every answer but it seems no one has explained why it is f/



f/ is focal length divided by. When lenses were simple the distance of the apperture from the film was the focal length (at least when the lens was focused at infinity and close enough the rest of the time).

If you had an 18mm lens the apperture would be a lot closer to the film than with a 250mm lens, so it needed a bigger hole. If it was twice as far away the hole needs to be 4 times the area (because the area it spreads out increases as a square law), but the twice the diameter. In other words if you keep the ratio of the focal length to apperture diameter the same for all your lenses you have and identical level of light reaching the film.

Early lens appertures had a scale but didn't have fixed positions, later ones got fixed stops at multiples of square-root 2, hence 2, 2.8, 4 , 5.6, 8 etc which is where we've come to talk about halving or doubling of light as a "stop"

So far as the the phone question is concerned there is an general equation

(1/f^2) * Time * ISO = k / (Reflectance * Illuminance)

where f is the apperture, Time is the shutter speed, ISO is the speed rating and K is a constant. There is actually a fixed value of K if illumance is in Lux. Generally the apperture is correctly calculated and the ISO fiddled to give correct exposure.



With mobile phone cameras the focal length is so short that f/4 means a very small diameter of glass where f/4 on a 300mm lens is really quite wide.

 

[Cideway]

Make three rolls of black cardboard, on 10cm long with a diameter of 2.5cm, one 20cm long with a diameter of 5cm and lastly one 28 cm long with a diameter 7cm. Fit a torch (flashlight) on to the length of cardboard and observe the intensity of the light coming out the end, the spots out the end should all appear to be the same brightness.




When you think about it the light going into a 50mm lens with a 12.5mm opening it is the same as the light entering a 100mm lens with a 25mm opening and the light coming out the other end is the same, so what changed, well the light going through the 50mm lens only had to travel half the distance but it had to go through a hole half the diameter, while the 100mm lens the light travelled twice the distance but the counter that it went through a hole with twice the diameter.

 

[wildkat2]

Simon Devlin wrote:

Do all aperture values for any lens let in the same amount of light?

The answer is probably no!

For example, a Pentax f/1.4 lens at f/4 allows a greater quantity of light than say a mobile phone lens @ f/4?

Surely this is what we pay the extra money for and not just the ability to have a higher aperture, blurring out the foreground/background and bokeh benefits!

I just need the clarification!

 

[awaldram]

Tan68 wrote:

SRT201 wrote:

awaldram wrote:

The bubbled plastic on a cell phone is rubbish

Please tell me you aren't suggesting that a DSLR is better than an iPhone!

a dSLR isn't better than an iPhone. the iPhone does apps and things like that. the iPhone is always with you and you can send your pictures right to some image hosting site. you can send all of the pictures. 28 pictures of the cat sitting on the window sill. i can listen to music while the pictures upload.

The iphone will do exactly what apple tell you it'll do.