DSLRUsers.com

Hello all...

I'm not sure how to frame this question... here goes:

Is f2.8 the same for all lenses?

For example, if you had a 50mm f1.8 set at f2.8 and compared it to say a 17-80 f2.8 set to f2.8 @50mm - would it be the same amount of light passing thru the lens? I guess the prime lens would be sharper(?) but otherwise would the resulting image be identical?

Thanks
Gecko

gecko wrote:Hello all...

I'm not sure how to frame this question... here goes:

Is f2.8 the same for all lenses?

Gecko

No

For example, if you had a 50mm f1.8 set at f2.8 and compared it to say a 17-80 f2.8 set to f2.8 @50mm - would it be the same amount of light passing thru the lens? I guess the prime lens would be sharper(?) but otherwise would the resulting image be identical?

What's the 17-80?, the amount of light will be the same at 2.8 but each glass has difference characteritics same as quality.

The same amount of light would be passed by each lens and both settings would give an identical exposure value.

can i also add, if nikon did have a 18-70 with 2.8, it would let the same amount of light through the aperture, but the important factor is quality of 'bokeh'

Gecko,

An f/stop is a mathematical expression which is based on the ratio between the diameter of the aperture of the front element and the focal length of the lens. See here for A Tedious Explanation of the f/stop

So yes, at f2.8 on various lenses they should transmit an equivilent amount of light to the film plane and thus all the lenses should achieve the same exposure in the same light.

Rob.

Edit: Wretched autoclosing tags
Edit2: Words in red to improve statement and hopefully make Mike happy.

NO! Aperature, expressed as an f-number is a ratio based on the size of the opening based on focal length. So, f2.8 on a 50mm lens is smaller than f2.8 on a 200mm lens. The amount of light passed to the film/sensor is same ratio for a 50mm and a 200mm which makes the exposure the same because of the optical physics involved.

regards
Mike Parker
Frederick, MD

Thanks for the swift responses

What's the 17-80?

Sorry Birddog, I should have said 17-55 f2.8 (just as an example)

Would most consider the 24-120 VR a 'faster' lens because of the VR? Would it be as useful as an f2.8 lens in a low light situation?

All these questions about fast lenses and low light are a result of me trying to take some pics at a concert over the weekend - images will be posted soon...

Thanks
Gecko

genji wrote:can i also add, if nikon did have a 18-70 with 2.8, it would let the same amount of light through the aperture, but the important factor is quality of 'bokeh'

The important factor?

Or simply an important factor?

Or perhaps, depending upon one's needs of any particular image, totally irrelevant, and simply nmo more than a factor.

I love this: we have about three answers in each camp thus far; surely they cannot all be correct? :0

The reality is that f2.8, from a metering perspective, is f2.8, and it will always permit only the same quantity of fixed light to pass, from the subject, to the film or sensor.

Never any more, and never any less.

Otherwise, no form of light meter would work. Ever.

There are certainly a number of other characteristics, such as Bokeh and DoF that will vary relative to the shape of the aperture blades and focal length of the lens in use, and these can and will have an effect on the image being produced, bat at f2.8 you will always get a consistant quantity of light passed through the lens in order to create the exposure.

gecko wrote:Would most consider the 24-120 VR a 'faster' lens because of the VR? Would it be as useful as an f2.8 lens in a low light situation?

Faster?

No, not really.

It works in a different manner, and permits you to work with a slower shutter speed at its fastest aperture.

As would a 2.8 VR.

The compromise is that you are using a slower shutter speed, and at 2.8 you might be using 1/60, which might be enough to freeze movement, say, on the stage. Drop that back to 1/20 (say) and subject movement might become an issue.

So, working with a slower shutter speed might be advantageous, or it might not, depending upon the circumstances in play at the time you're making the exposure.

So, what about variations in the properties of the glass between different lenses? ie The amount of light that is actually transmitted, instead of being reflected or absorbed as it passes each element. Does this come into play in the design of the lens and location/design of the aperture mechanism?

I would have thought that a lens design with fewer elements and/or better glass characteristics would transmit more light than a lesser design. And... that it is in the engineering of those lenses that light transmitted at a given apertures become equal... Theoretical vs Actual (or effective) Aperture

Matt

cameraguy21773 wrote:NO! Aperature, expressed as an f-number is a ratio based on the size of the opening based on focal length. So, f2.8 on a 50mm lens is smaller than f2.8 on a 200mm lens. The amount of light passed to the film/sensor is same ratio for a 50mm and a 200mm which makes the exposure the same because of the optical physics involved.

You are correct, though I would suggest that my original statement was just generalised, rather than incorrect. Which is why I included a link for further reading if he was interested in persuing it further.

The size of the opening (aperture) of the front element is used (via the formula) to suggest the max f/stop of the lens.

For example, the 200mm/f2 Nikkor's front element should be around 100mm. Its actually more like 115mm due to having to take focusing into account, amongst other things. Large format guys often have to take this sort of thing into account using bellows lenses, as do serious macro photographers.

The diameter of the DIAPHRAGM aperture (which controls the effective aperture and thus the amount of light hitting the film plane) will typically be much smaller due to generally being located near the rear element of the lens.

MattC, this is why you end up with the same exposure despite the vaugeries of # of elements and glass quality etc. I imagine that they calibrate their final diaphragm aperture sizes based on light transmission to the film plane.

gstark wrote:

genji wrote:can i also add, if nikon did have a 18-70 with 2.8, it would let the same amount of light through the aperture, but the important factor is quality of 'bokeh'

The important factor?

Or simply an important factor?

Or perhaps, depending upon one's needs of any particular image, totally irrelevant, and simply nmo more than a factor.

I love this: we have about three answers in each camp thus far; surely they cannot all be correct? :0

your're right gary i shouldn'e generalise, i meant, "imo i consider quality of 'bokeh' to be one of the important factors..."