Different lenses, same aperture, different speed. What's this?

Some days ago, I looked through my F4 with 2.5/105 MF-Nikkor and

later through my 3.5-4.5/28-105 AF-Zoom-Nikkor. At the same aperture

set on both lenses (zoom at 105mm) the camera shows different shutter

speeds. The difference is about 1 EV. I tried both lenses with my FM -

the same. I then tried another 2.5/105 - the same. Then another 28-

105 - the same. That means, lenses and cameras work ok. I then

compared other lenses - always the same. There are differences

between manual primes an autofocus zooms. I see no difference between

lenses with similar design, e.g. with my 2/50 MF-Nikkor and my 1,8/50

AF-Nikkor. BTW I know that the aperture of zooms like the 28-105

varies over the focal length, I considered this. My question is: does

a worse "filter factor" zooms have cause such differences? E.g. the

AF 28-105 has 16 elements, the MF 2.5/105 only 5. What do you think?

I've noticed the same thing. f stops are derived by calculation and don't represent true light transmittance. I am not surprised that a good prime transmits more light than a decent zoom does at the same indicated f-stop.

<i>I know that the aperture of zooms like the 28-105

varies over the focal length, I considered this. </i>

<p>

How did you consider it? On a 28-105, if you set f/4 on the lens at 105, you'll really get f/5 (2/3 stop less). Are you saying your experiments show it's a full stop worse than that? 1-2/3 off?

At 105, the real apeture of the zoom is 2/3 stop less than the marked apeture on the apeture ring. If you hook the lens up to a body that shows apeture in the LCD display, you'll notice how the apeture changes as you zoom. I don't know if the F4 has this display or not.

Thanks so far. Todd: at 105mm I use the yellow index, at 28 mm the green. Pete: the F4 only shows the f-stop mark in the finder window.

Whoa. wait. What aperture did you place both at? I would suggest putting both lenses at f5.6 or f8 at 105mm and focusing at infinity and testing. I would not be the least bit surprised if your zoom ate up a whole stop more light. There are many more surfaces to reflect and absorb light. This is true with any zoom. Compare a 200mm 2.8 to a 70/80-200 2.8 @ 200mm. The zoom will most surely need more exposure time because it will be gobbling up some light.

 

This is one of the biggest issues with zooms for some people. The fact that they are so much more complex and have so much more glass that they can eat as much as a stop of your light.

The aperture setting is just that: the size of the aperture. It does not measure

the amount light that passes through a lens. At the surface of each lens

element a small percentage of light is reflected. Lenses with more elements

generally transmit less light. Zoom lenses usually have more lens elements

and therefore generally transmit less light than prime lens.

 

Typical reflection at a glass-air surface with uncoated glass is generally

around 4%. A good lens coating will reduce the reflections to under 1%.

Note that when lens elements are grouped (cemented or otherwise optically

coupled) it reduces the number of surfaces for reflections.

 

For example, on a lens with five elements, if you have 99% transmission at

each lens surface, you would expect roughly 90% of the light to be transmitted

( 0.99 ^ 10 ). For a lens with 10 elements and 99% transmission you should

get roughly 82% of the light through the lens ( 0.99 ^ 20 ).

Thanks Carl and Dale. I made the test at f5.6 and f8 and hold the camera into the sky, Carl. What you mean is the same thing I suggested and called "filter factor". The zoom has 16 elements the prime 5. That MUST cause a difference in light transmission. But some people I talked to said, the difference can't be 1 f-stop with modern MC-lenses. One guy tested it with Leica and Canon equipment, he noticed nothing. But I can't say what lenses he compared. Then I thought, maybe this is a weakness of the Nikon system.

The important question for me is, does Nikon's meter compensate for the actual amount of light entering the lens?

 

I don't see how it could compensate, given that the lens isn't stopped down until you press the shutter.

 

I would love to hear how the meter can compensate.

I'm sure you have shot a couple of comparisons of the two lenses. How do they look?

The results on slide or neg film don't show any differences. I think the TTL-meetering of the camera compensates the different light transmission of the lenses. That's why I get different speeds. I have no other explanation.

Naji Massouh wrote:<p>

<I>The important question for me is, does Nikon's meter compensate for the

actual amount of light entering the lens? </I><p>

 

Essentially all modern SLR cameras use through the lens (TTL) metering. As

the name implies the meter measures the amount of light that passes through

the lens. This means the meter compensates for any light losses in the lens

and for most filters in front of the lens. Polarizing filters are the exception

because polarized light interferes with the beam splitters used to divert light to

the meters of modern SLRs.<p>

 

<I>I don't see how it could compensate, given that the lens isn't stopped down

until you press the shutter. <p>

 

I would love to hear how the meter can compensate.</I><p>

 

The f-stop of the lens is provided to the camera by either mechanical (i.e. AI &

AIS lens) or electrically (i.e. G lens) means. The f-stop for the exposure is set

by either the body (electronic) or on the aperture ring (mechanical).

Regardless of how the f-stop is set, it should be a simple calculation to

determine how many f-stops there are between the lens wide open and the

requested f-stop. Knowing this the meter can calculate the correct exposure.

Hope this helps.